Actual effect restrictor plays in power

[StevenL]

When it comes to airflow through the restricter, when does the restricter start playing a role in limiting power. Is it throughout the entire powerband or does it mainly effect ultimate hp?

Thanks
Steven

[Adambomb]

When does the restrictor start playing a role? At idle. Seriously. Has anyone ever heard a formula car that idled smoothly below 2000 rpm? Especially given that the engines used, in their factory configuration, idle smoothly around 1000-1500 rpm. It's also a lot harder to get them to start nicely. The restrictor has noticeable effects across the entire operating range of the engine.

[StevenL]

Thanks, wasnt sure. Anyone have a dyno comparing and engine with and without a restricter with the same kind of intake manifold and setup?

[A Richards]

Originally posted by Adambomb:
When does the restrictor start playing a role? At idle. Seriously. Has anyone ever heard a formula car that idled smoothly below 2000 rpm? Especially given that the engines used, in their factory configuration, idle smoothly around 1000-1500 rpm. It's also a lot harder to get them to start nicely. The restrictor has noticeable effects across the entire operating range of the engine.

I think you will find that the restrictor has absolutely nothing to do with engines starting and idling. It's the large plenum volume that causes that. When your engine goes to start your throttle is restricting almost all airflow anyway, in this instance the relatively large 20mm hole is going to be negligible, yeah???. I took our plenum off the engine once and ran the restrictor straight into the injector housing. I've never seen an engine start so easily and idle so low, down to about 850rpm on a KTM 525... hard too believe but true. This is coming from an engine that is a pig to start and cant idle below 3000rpm with the plenum installed. With this setup however you could hold WOT and the engine could not rev past 9K. I would be interested to hear from someone who has removed the restrictor from their engine but left their plenum installed. I believe that engine hunting and high idle would still be the result. I believe that a large plenum makes it difficult to maintain low pressure on your inlet port at low engine speeds.

[Hector]

Originally posted by Adambomb:
When does the restrictor start playing a role? At idle. Seriously. Has anyone ever heard a formula car that idled smoothly below 2000 rpm? Especially given that the engines used, in their factory configuration, idle smoothly around 1000-1500 rpm. It's also a lot harder to get them to start nicely. The restrictor has noticeable effects across the entire operating range of the engine.
I don't agree with this.

We have a 600 cc engine that idles beautifully, starts easily, and idles at the same rpm as our base engine.

I'll say the same thing that I said in an earlier post: if your engine doesn't start easily and idle well, it's your fault, not the restrictor's. Most teams focus on 100% throttle tuning. Where you set your idle (mass flow rate at 0% throttle) is up to you, and how your car idles at that position is also up to you. Most ECUs come with temperature and throttle-based enrichment - add a little fuel and remove some timing during cold start, and your engine will start on the first crank

Do you think your restrictor really cuts off mass flow rate at idle rpm? If so, why not just open the throttle a bit more and tune properly? You'll still get the same mass flow rate as a stock engine, just with a wee bit more throttle actuation.

P.S. I once got to listen to our car idle next to U.W.A. at a test event. Sounded like two content kittens purring away...

[Jimmy01]

I agree with both of the last two posts. A large intake manifold makes tuning more difficult at low rpm but certainly not impossible. But I think there is more to it than just tuning (although this is probably the most important part). A very important factor to consider is fuel preparation. If your injector is spraying fuel onto the runner walls, dribbling, operating at a very low duty cycle, pointing at a bad angle or spraying at the wrong time you will more than likely have big issues with starting and low speed running.

The other issue with high performance motorcycle engines and low speed running is cam timing. At low speeds fuel and air can often be forced out of the cylinder at the start of the compression stroke and back into the plenum causing irregular fueling and hunting. In a stock engine the carb/throttle bodies are quite close to the valves and generally have a flat sliding plate as well/instead of a butterfly. So at idle and low throttle openings there is effectively a barrier reflecting the pressure waves which helps to minimise the effect I talked about earlier.

The restrictor has an effect on engine performance once there starts to be a descent pressure drop across it. At low engine speeds it will have a very small effect and become increasingly more annoying as engine speed increases. So yes it will effect your whole torque curve at WOT but to varying degrees. Remember that for an NA engine, maximum torque at any speed will be when the pressure inside the manifold is atmospheric (ignoring pulse tuning effects). When you throttle an engine it decreases the pressure inside the intake manifold thus decresing power.

The magnitude of the effect of your restricter at any point will be proportional to the mass or volume flow rate of air through it.

[A Richards]

Originally posted by Jimmy01:

Remember that for an NA engine, maximum torque at any speed will be when the pressure inside the manifold is atmospheric (ignoring pulse tuning effects). When you throttle an engine it decreases the pressure inside the intake manifold thus decresing power.



I believe that the answer to the original post is in the above statement. Could we not say that for a given engine at WOT at any particular operating speed, that when the manifold air pressure is lower in a restricted intake than that of an intake with no restrictor that the restrictor “starts playing a role in limiting power”. Depending on the design of said engine and manifold combination, the magnitude of and operating speed at which these variations occur will be affected.

Getting an engine to start and idle is all about correctly managing the manifold pressure, fuel and ignition. IMO I would say that this is easier to achieve when you have a small plenum and multiple cylinders. I would also go as far to say that it is easier to make more power with a smaller intake when you have multiple cylinders.

[BuckeyeEngines]

The starting and idling is all related to tuning. If we are running something thats not tuned well, it wont start for shit. But we spend alot of time with starting compensations and the car starts right up. The biggest thing we see with the restrictor is limiting of RPM. We do not make power above 12700 rpm. However we are making more than stock torque from 6-10k rpm. This is from a direct comparison of a stock F4i with stock airbox/ecu and the same engine with our duel plenum restricted intake, which isnt fully tuned in.

[Adambomb]

D'oh! OK, I stand corrected, and take back everything I said. Actually, I read the first couple replies right before I went to bed, and couldn't get to sleep until 5am (and subsequently slept through my first class of the day). The "big light bulb" went off. Honestly I'd say I've underestimated the importance of plenum volume; now that I think of it, I've never even played with plenum volume independent of other parameters, and attributed low-rpm unhappiness with primarily cam timing and intake port size. Naturally this didn't explain how a sportbike could idle nicely at 1000 rpm and still pull like a raped ape to 15 grand; this also explains why bikes use ITBs or individual carbs (honestly that confused me, seemed uselessly complex). I know plenums can work magic up top, but never realized how much they hurt down low. I never gave it too much consideration because FSAE plenums typically aren’t any bigger than the airbox on the bike they came out of, but now I see the key factor is that the near-port throttling masks the big plenum down low, while at WOT you still get to keep your Helmholtz system and all its pulse-charging goodness.

One example of this is with my beloved ’88 Ninja 600. The first thing I noticed when I got it (while cleaning and giving some much needed TLC to the ancient carbs) was how gawd-awful the airbox was on it. The air had to change directions about 7 times and pass through all these tiny nooks and crannies before entering the airbox through these tiny little triangles no bigger than your thumb before it got to the plenum. Any sane person would have just put individual filters on the carbs, tossed in a jet kit, and called it a day. But NOOO, I wanted to keep the plenum. So I hacked all the stock filter accommodations off the airbox and grafted on the biggest K&N filter I could cram in there. It was “effective”…so much so that it would no longer start or idle without massive amounts of ether. And that’s where I originally developed my theory that the restrictor messes up your idle. Ended up having to turn the idle fuel screws out a whopping 1.5 turns to get it to start and run perfectly; although in hindsight the fact that I could completely restore factory idle quality and startability with basic tuning pretty much debunks that theory.

But that system definitely worked well up top…so well, in fact, that I put in a Stage 3 jet kit and then proceeded to burn the valves. Ended up having to go something like 4 main jet sizes up from the Stage 3 kit to get it to run perfectly, and it may well respond to even further jetting (I just haven’t had the desire to torture myself by wrestling those damned carbs out again…another reason to go with individual filters!).

+1 to Jimmy.

But it does make perfect sense, a lot of the bad attributes of a big-plenum are the same as having a bad vacuum leak (and interestingly enough, I’ve found that vacuum leaks are slightly trickier to diagnose on FSAE cars, mainly because they don’t pull much vacuum anyway, and the net result is it just acts “slightly crappier than usual”), and from a dynamic standpoint, pretty much the same thing is going on with low vacuum. And while low vacuum can play real hell with a carb, I thought FI was fairly insensitive and could effectively compensate for it…not the case I see.

However, I will say that in all of our cars, idle quality and starting could only be improved so far with just tuning. With a lot of tweaking we managed to get our idle from 3000 to 2000, and it only requires a moderate amount of cussing and only a slight magic touch to start when cold. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif (although E85 may play some role in that too)

I have also found a new level of appreciation for UAS Graz’s multi-volume intake plenum. http://fsae.com/groupee_common/emoticons/icon_cool.gif

[DonMolina]

To put it simply, the restrictor deteriorates the hydraulic losses after the throttle valve (which is not the #1 issue with venturi designs and a lot of CFD work) all over the range.


The #1 issue with it is of course choking the flow. When the incoming air reaches Ma=1 the flow chokes and you simply cannot add any more to the cylinder. No more air, no more fuel --> no more power.

After all that's the point of its introduction. Limiting power to about 100BHP and as a result no killed 20yo and no lawsuits http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

I think under the critical RPM at which choking occues, there is no significant effect on performance, taking that you have a good intake system designed.

[Steve O]

I think there is merit to both sides of the argument here... the restrictor is affecting you at the WOT or when you demand more air than can go through the restrictor in a NA situation AND during any points at which you have resonant interference from keeping air from flowing in. This is something that exists in the stock configuration without the restrictor but the resonant points are changed with the restrictor; therefore, your runners must be tuned in conjunction with your restrictor to maximize positive interference. So yes its a tuning issue but the restrictor affects tuning and horsepower curves across the entire RPM range. I know the original question is when does it limit power so to tie that in, you may be sacrificing something like idling to have less of a loss in the high end.

[SamB]

Originally posted by DonMolina:
The #1 issue with it is of course choking the flow. When the incoming air reaches Ma=1 the flow chokes and you simply cannot add any more to the cylinder. No more air, no more fuel --> no more power.


I don't think its so much choking the flow because assuming standard temp, gas const etc a flow of M=1 would have a high enough volumetric flow rate to fill all cylinders entirely at 14,000rpm so surely if you can achieve M=1 at the throat of your restrictor then the engine will behave as if it were unrestricted.

The problem we have here is that we have no faesible way of doing this and that air is primarily only drawn into the intake due to the prssure difference between plenum and throttle. I am of the opinion that the restrictor does not effect you until you go past a throttle area of 100pi which can be seen in the power curves our cars have produced.

Sam

[Marshall.Hagen]

We found restrictor choke really takes affect after peak torque.

Both setups mapped using a DTA S80-Pro for MBT and identical lambda maps:
http://i107.photobucket.com/albums/m296/Marshmph/RestrictedVSunrestricedjpg.jpg

[Jimmy01]

The graph above explains the restricter effect quite well. You can see at low rpm there is a small drop in torque right across the map, this will be due to increased pumping losses in the restricted engine as the air flows through the smaller orifice. Around peak torque the torque values get a bit closer, at first this was confusing but I think I might know why: As the engine is at its most efficient, the airflow is more stable reducing the inefficiencies caused by the restricter. Anyone else have any ideas?

After peak torque there is an obvious drop off in performance as the flow rate through the restricter becomes too high, resulting in an increasing pressure drop across it.

The power curve drops away slightly too, but this could be contributed to greater mechanical losses as the engine spins faster.

[VFR750R]

Originally posted by Marshall.Hagen:
We found restrictor choke really takes affect after peak torque.

Both setups mapped using a DTA S80-Pro for MBT and identical lambda maps:


Just so it's clear, is this with everything else identical? (just the restrictor removed from the FSAE intake system?)

[Mikey Antonakakis]

Originally posted by VFR750R:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Marshall.Hagen:
We found restrictor choke really takes affect after peak torque.

Both setups mapped using a DTA S80-Pro for MBT and identical lambda maps:


Just so it's clear, is this with everything else identical? (just the restrictor removed from the FSAE intake system?) </div></BLOCKQUOTE>
+1

[Marshall.Hagen]

Originally posted by VFR750R:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Marshall.Hagen:
We found restrictor choke really takes affect after peak torque.

Both setups mapped using a DTA S80-Pro for MBT and identical lambda maps:


Just so it's clear, is this with everything else identical? (just the restrictor removed from the FSAE intake system?) </div></BLOCKQUOTE>

Non-restricted curve represents the standard unit individual throttlebody system and standard exhaust manifold mapped with the DTA EMS.

Restricted was still with the factory exhaust manifold, but with FSAE intake (volume & length tuned), and mapped with DTA EMS.

Longer runners were able to gain 2-5% over the factory-spec F4i torque curve in some areas in the low revs, but with much less top-end power. Of course, those numbers are for the design judges http://fsae.com/groupee_common/emoticons/icon_smile.gif

I do not have the graphs on this laptop, but the best method of determing (quantitatively) restrictor affect is to overlay manifold pressure vs RPM, restricted vs standard.

[Simon Dingle]

Originally posted by SamB:
I don't think its so much choking the flow because assuming standard temp, gas const etc a flow of M=1 would have a high enough volumetric flow rate to fill all cylinders entirely at 14,000rpm so surely if you can achieve M=1 at the throat of your restrictor then the engine will behave as if it were unrestricted.
Sam

The isentropic mass flow rate of a 20mm restrictor is about 74g/s and this equates to the air flow rate of a 600cc engine at around 12,500rpm with a volumetric efficiency of 1. Therefore the engine will never be able to get the kind of air that it normally gets unrestricted, and that's assuming that it only operates at a volumetric efficiency of 1 which is fairly unlikely.


I am of the opinion that the restrictor does not effect you until you go past a throttle area of 100pi

I don't really understand this comment. By 100pi, do you mean 314mm^2 (a 20mm diameter throttle)? If so, then of course your 20mm restrictor won't play an effect as you engine is also being primarily restricted by your <20mm throttle.

I do however agree that the direct cause of the lower torque in FSAE cars is the low pressure in the plenum. We found that the plenum pressure begins to drop below atmospheric from as low as 4,000rpm (at WOT of course).


@Jimmy01

I would suggest that the restricted torque curve gets closer to the unrestricted torque curve at the higher speed tuning peak (around 10,500rpm) and the lower speed tuning peak (around 8,000rpm) due to the ram air effect caused by the FSAE intake manifold.

Also, the restricted power curve drops off because the torque drops off due to the restrictor as you mentioned. The reason that power does not stay flat from this point onwards is because of the increased mechanical engine speeds at higher engine speeds.

[Jimmy01]

Since the intake manifolds are different between the two graphs, any detailed comparison is reasonably pointless. It would be great to see some graphs of a true comparison. i.e. only the resrtrictor changing. Would be really easy to do, I can't think why I have never done it actually.


Of course, those numbers are for the design judges

I assume that is a design graph as well then http://fsae.com/groupee_common/emoticons/icon_wink.gif

[DonMolina]

Originally posted by Simon Dingle:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by SamB:
I don't think its so much choking the flow because assuming standard temp, gas const etc a flow of M=1 would have a high enough volumetric flow rate to fill all cylinders entirely at 14,000rpm so surely if you can achieve M=1 at the throat of your restrictor then the engine will behave as if it were unrestricted.
Sam

The isentropic mass flow rate of a 20mm restrictor is about 74g/s and this equates to the air flow rate of a 600cc engine at around 12,500rpm with a volumetric efficiency of 1. Therefore the engine will never be able to get the kind of air that it normally gets unrestricted, and that's assuming that it only operates at a volumetric efficiency of 1 which is fairly unlikely. </div></BLOCKQUOTE>

+1 man
For about 600cc it's more like 12,000 rpm.
Of course if you run a 450cc the rpm that choking happens will go up to 15k.

[ZAMR]

You are assuming (or approximating) steady-state conditions though. While yes, if you run the numbers choking for a single (with less volume) will occur at higher RPM than a four cylinder.

However, what about the unsteady state condition? A 450cc single must draw 450cc (operating at 100% VE) in ONE stroke whereas a 600cc 4cyl must only draw 150cc in a single stroke. Therefore the four cylinder will operate "closer" to steady state condition than the single (drawing 150cc/stroke constantly, 1 cylinder for each stroke). The single essentially draws 450cc from the plenum in a third of the time taken by the 4cyl (150cc * 3strokes = 450cc = 450cc * 1stroke).

Thus the 450cc essentially sucks from the plenum and waits a cycle to refill, then sucks again (I know it doesn't work exactly like this, but air is elastic at these speeds and the general concept, I believe, is applicable).

If you can get a single to operate at 100% VE, I believe choking will take effect below 15K. You would have to draw 74g/s CONSTANTLY at 15K to obtain 100% VE. The only way to do this is run forced air induction.

You will see this elastic effect in a 4cyl engine as well, but less pronounced since it draws, by design, at a "steadier" state than a single. We run an Ape 550cc twin, and WAVE claimed that last year's restrictor choked on one cylinder's intake stroke at 9K while the other cylinder did not choke until at least 12.5K(rev limit), likely due to the extremely uneven firing intervals.

This brings me back to the topic of the forum: the effect the restrictor has on power. The unsteady state condition is why the restrictor has the ability to reduce VE below the mathematical steady state choke condition. It is why no team has a noticeable power drop at exactly the same place in their dyno charts. Each intake has its own properties eg. plenum volume, restrictor pressure drop, and pressure wave propagation in the runners, all of which have a huge effect on VE, power, and total mass flow. Each engine configuration has its own air drawing properties (4cyl, twin, single).

Therfore, the real effect the restrictor has on power is inevitably linked directly to engine selection and intake geometry, and will come into play at different times on every single car. It is a HUGE engineering challenge to get them to work together, larger than many may think!

If any of this is up to debate or outright wrong, post it. I'll admit that I'm wrong a lot of the time and dyslexic all the time.

[Scrappy]

I agree with what you said Zach. I have thought about the effect of the unsteady flow through the restrictor and in the plenum for the 4 cyl vs 1 cyl in many of the same terms. In my mind, calculating the engine RPM at which you are able to attain the theoretical max flow and give 100% VE is nothing more than a quick and dirty way to gauge how far off the engine geometry is from being optimal to run restricted without lots of modification. By “geometry” I mean the number of cylinders, bore, stroke, size of the ports, size and number of valves, camshaft profiles and overlap, etc. The combination of some of those things dictate the OEM redline, and you can see where that falls in comparison to what your calculated choke RPM is. Just for the record, I think the term “choke RPM” is used pretty loosely most of the time, hence why some new teams might think its impossible to make any power past that RPM number. Don’t need to go any deeper into that, its already been covered in the earlier posts on this thread.

For the 4 cyl, the 12k RPM figure falls under all the newer 600cc 4cyl engines OEM redlines. (Don’t the newest 600cc bikes rev out to 16k from the factory?) For the 450cc 1 cyls, the 15k RPM figure is way above what all the OEM redlines are. (10-11k?)

So going back to the plenum and restrictor, the single cylinder teams from my memory have been saying that they don’t choke all the way out to redline. To me, this is saying that they don’t see any appreciable reduction in the manifold pressure due to the restrictor. I would like to see a plot of manifold pressure vs. RPM at WOT for a single cylinder team, I imagine it has to be a lot flatter than a typical 4cyl plot. Any single cyl teams care to comment?

I guess the point of my rambling is to build on what Zach said, that even though the flow through the restrictor and plenum of a 1cyl is far more unsteady than a 4cyl, the net effect on power isn’t as dramatic. I know this might seem very apparent, but I guess I was thinking more in terms of what power and torque stock 450’s make unrestricted, and what teams are able to make restricted without lots of effort into changing stock engine geometry. Its nice to have no overlapping intake events and more time for the plenum pressure to recover after an intake event, your ports and cams become much less of a hindrance to making power than they are on the 4cyl. We all know we want to choke the restrictor over as much of the operating range as possible, but at the same time it is nice running an NA 1 cyl largely unrestricted and not having to go “backwards” with the cams and the ports.

[Mikey Antonakakis]

Good discussion! Hopefully this semester I'll be able to get plots of manifold pressure (or maybe pressure just downstream of the restrictor?) vs crank angle... or maybe in reference to pressure at the ports, or both. Assuming I had this capability, and you were in my shoes, what would you look at?

[Simon Dingle]

+1 to Zach and Scrappy

The 74g/s is is an extremely large asumption, especially for a single cylinder. For steady state it is assumed that for a single, there is 450cc of air drawn into the cylinder evenly throughout the entire 720° crank-angle. However the reality is that that there is 450cc drawn into the cylinder in the first 90° and then nothing for the next 540°. I'd be interested to know at what engine speed a single begins to choke the restrictor due to the peak instantaneous intake mass flow rate, I'd imagine that it is much lower than a 4-cylinder when normalised against the steady-state choking speed.

Am I right in thinking that this could be investigated using the setup that Mikey has by measuring static pressure at the restrictor throat? Providing you had the temperal resolution to measure pressure vs. crank-angle, you could see the pressure rise and fall (indicating velocity fluctuation). I'm guessing (most of this is guess work) that you would see a platau in static pressure for part of the intake stroke (indicating maximum air velocity and therefore maximum mass flow rate) at an average mass flow rate well below the steady-state estimation for choked flow. I imagine that this would be more prominant in a single than a 4-cylinder.

I'm really wishing that we had a single cylinder to test some of the theories that I'm currectly thinking through in my mind. Although I also wish that our dyno was working so we could do some testing on our 4-cylinder engine http://fsae.com/groupee_common/emoticons/icon_frown.gif

[Mikey Antonakakis]

To expand on what Simon is saying, perhaps a comparison of data from a pressure sensor with the resolution to measure vs crank angle and say a coupled 1D-3D engine model?

[Adambomb]

In a recent development, it appears we may have cylinder pressure data this year for our YMF 565 (that would be a Cyclone Racing spec for what used to be a YFZ 450 that is now "a little bigger" and running 14:1 on E85).

As for achieving "slightly more" steady state flow with a restrictor on an NA single, until we have the leisure of getting our shiny little roots blower working that has been collecting dust for a couple years, we may have a bit of a solution hidden somewhere else in our intake. Mum's the word on that though. Especially if it works as well as our resident Aero guru predicts. http://fsae.com/groupee_common/emoticons/icon_wink.gif At any rate I would be interested in collecting some MAP data, and don't see a problem sharing that.

[t21jj]

Originally posted by Adambomb:
In a recent development, it appears we may have cylinder pressure data this year for our YMF 565 (that would be a Cyclone Racing spec for what used to be a YFZ 450 that is now "a little bigger" and running 14:1 on E85).

As for achieving "slightly more" steady state flow with a restrictor on an NA single, until we have the leisure of getting our shiny little roots blower working that has been collecting dust for a couple years, we may have a bit of a solution hidden somewhere else in our intake. Mum's the word on that though. Especially if it works as well as our resident Aero guru predicts. http://fsae.com/groupee_common/emoticons/icon_wink.gif At any rate I would be interested in collecting some MAP data, and don't see a problem sharing that.

Ok Adam you lost me how did you come up with YMF and what does it mean?

[StevenL]

Wow! Thanks guys for all the info! I was half expecting to get flamed on here. had my flame suit in hand. Anyone doing anything this year to try and beat the restrictor?

[Jimmy01]

Originally posted by Simon Dingle:

The 74g/s is is an extremely large asumption, especially for a single cylinder. For steady state it is assumed that for a single, there is 450cc of air drawn into the cylinder evenly throughout the entire 720° crank-angle. However the reality is that that there is 450cc drawn into the cylinder in the first 90° and then nothing for the next 540°. I'd be interested to know at what engine speed a single begins to choke the restrictor due to the peak instantaneous intake mass flow rate, I'd imagine that it is much lower than a 4-cylinder when normalised against the steady-state choking speed. http://fsae.com/groupee_common/emoticons/icon_frown.gif

You are right when you say that the cylinder is only drawing air for a portion of the engine cycle but remember it is closer to 150° of the crank cycle not 90°. And although the cylinder is only drawing air for 20% of the cycle, the plenum is likely drawing air constantly through the restrictor. However this flow will not be constant, it will fluctuate a lot (especially at low engine speed). A plenum helps to damp the pressure fluctuation and I like to think of it acting as a mini atmosphere. A bigger plenum will act as a better damper, thats why we get more top end power with a bigger plenum. I can vouch from experience that plenum volume makes a huge difference in restricted singles, a 1 litre plenum almost halves max power and reduces torque right across the map when compared to a 3L plenum.

It would be interesting to look at pressure fluctuations in the plenum over a cycle. Also, pressure at the throat of the restricter, you could probably get an idea of the change in air speed through the restricter. I assume you are talking about a high speed pressure transducer Mikey?

[ZAMR]

originally posted by Jimmy01
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> quote:
Originally posted by Simon Dingle:

The 74g/s is is an extremely large asumption, especially for a single cylinder. For steady state it is assumed that for a single, there is 450cc of air drawn into the cylinder evenly throughout the entire 720° crank-angle. However the reality is that that there is 450cc drawn into the cylinder in the first 90° and then nothing for the next 540°. I'd be interested to know at what engine speed a single begins to choke the restrictor due to the peak instantaneous intake mass flow rate, I'd imagine that it is much lower than a 4-cylinder when normalised against the steady-state choking speed.



You are right when you say that the cylinder is only drawing air for a portion of the engine cycle but remember it is closer to 180° of the crank cycle not 90°. And although the cylinder is only drawing air for 25% of the cycle, the plenum is likely drawing air constantly through the restrictor. However this flow will not be constant, it will fluctuate a lot (especially at low engine speed). A plenum helps to damp the pressure fluctuation and I like to think of it acting as a mini atmosphere. A bigger plenum will act as a better damper, thats why we get more top end power with a bigger plenum. I can vouch from experience that plenum volume makes a huge difference in restricted singles, a 1 litre plenum almost halves max power and reduces torque right across the map when compared to a 3L plenum. </div></BLOCKQUOTE>

No one here is saying the restrictor is an on off switch. Of course it draws air throughout the entire cycle. But everyone here is curious as to what pressure a throat will fall to at a certain rpm at a certain crank angle for a certain engine (kind of turning a restrictor into a flowmetre, although I'm told accurate measurement of negative gage pressure of an elastic fluid at high speed is very hard to accomplish). This information could tell you when a restrictor instantaneously chokes and comes to significant role in power restriction, as Simon Says (haha).

@Adambomb: I am intrigued by this magical steady state device http://fsae.com/groupee_common/emoticons/icon_smile.gif Care to share any more pretty please?

[t21jj]

Originally posted by ZAMR:
@Adambomb: I am intrigued by this magical steady state device http://fsae.com/groupee_common/emoticons/icon_smile.gif Care to share any more pretty please?

While I can't speak for Adam the odds of that are slim. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Simon Dingle]

Originally posted by Jimmy01:
I can vouch from experience that plenum volume makes a huge difference in restricted singles, a 1 litre plenum almost halves max power and reduces torque right across the map when compared to a 3L plenum.

Interesting, I knew that the plenum acted as a damper between the intake port(s) and the restrictor but I wasn't aware that the plenum volume made that much difference to performance. I ran a simulation in Ricardo WAVE and it made a max difference of about 1%. However, it does make sense that it would have a larger impact on singles. Also, I didn't bother with volumes below 3L as you would be choking the trumpets for a four-cylinder.

@Adambomb: Have you been implementing some of the ideas from the NOS topic? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Scrappy]

Originally posted by Jimmy01:

You are right when you say that the cylinder is only drawing air for a portion of the engine cycle but remember it is closer to 150° of the crank cycle not 90°. And although the cylinder is only drawing air for 20% of the cycle, the plenum is likely drawing air constantly through the restrictor. However this flow will not be constant, it will fluctuate a lot (especially at low engine speed). A plenum helps to damp the pressure fluctuation and I like to think of it acting as a mini atmosphere. A bigger plenum will act as a better damper, thats why we get more top end power with a bigger plenum. I can vouch from experience that plenum volume makes a huge difference in restricted singles, a 1 litre plenum almost halves max power and reduces torque right across the map when compared to a 3L plenum.



I would say that is pretty good evidence right there that the small plenum is effectively allowing the restrictor to become instantaneously choked during the intake stroke. Bump that volume up, and voila' he doubles his top end max power and increases torque across the range. You dont need crank angle sensors, pressure transducers, and a sim to figure that out! http://fsae.com/groupee_common/emoticons/icon_smile.gif Never underestimate the power of testing, especially janky dyno setups.

That being said, it would be really cool to get all that data and correlate that to your model, I think that would allow you to strike the best balance between power and throttle response on a single. I have never driven a 1 cyl FSAE car before, but I have driven 4 cyl cars with different sized plenums, and the plenum volume does make a perceptible difference at least to me. I can only imagine the throttle response on a 1cyl car with a 3L plenum compared to a 4cyl with a 3L plenum........yikes.

[Mikey Antonakakis]

Yeah, high speed pressure transducer (20kHz response I think, which corresponds to a data point every 4 degrees of crank revolution at 14k rpm, if my calcs are correct).

We're using the AT-Power FSAE throttle body, which has a relatively small change in diameter after the restrictor, so pressure fluctuations at the inlet to the plenum would be at least similar to what they are in the neck. I don't think I'd even really need to correlate the data to crank position to get insightful data, maybe just rpm. I was thinking that if I had two of these sensors, I could set one up at a port just to compare the amplitudes I guess. And if I had three, then at the inlet to the runner and the port (even though it'd be most useful at the valve). If anything it'd be cool to watch the resonance tuning happen.

[Adambomb]

@T21jj: YMF 565 = Yamaha MF 565.

@Zach: It's a pretty simple device, no moving parts...again, not sure of how effective it is yet. I will say that it made intake construction a large pain though. I'm hoping we should have some numbers to compare its performance to that of a much simpler manifold shaped like some sort of Tiki totem, aka the Tiki MANifold.

@Simon: LOL, hell no! Aside from the perils of getting caught, I'm thinking the ol' YMF 565 is probably "ragged edge" enough anyway! Besides, pulling the plenum apart to refill the bottle just sounds like too much work to me anyway. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[t21jj]

Originally posted by Adambomb:
@T21jj: YMF 565 = Yamaha MF 565.


Nice figured it out in the email, good call on the abbreviation.

[Wesley]

There are quite a few threads about plenum volume, and a few of them I've been involved with address this very concern - the effect of a large plenum is to allow for a more consistent pressure drop through the restrictor by maximizing available drawing volume for each cylinder.

This is also why spherical plenums are often used on singles and doubles, and log-plenums used on 4-cyls. By putting the majority of the plenum volume directly over the inlets, and elevating the inlets off of the intake floor, the air has to move a lot less linear distance to fill the area emptied by the intaking cylinder.

This quicker plenum emptying was one of the reasons I advocated against attempting a big-bang motor on an FSAE car (among other reasons) because the effect of intaking two cylinders at a time gives you the increased instantaneous choked flow of a two cylinder with the increased pumping losses of a four.

The restrictor plays a role at almost every engine speed, simply due to flow loss, as was said, but that role increases with the square of the velocity through the throat, hence the low impact at idle (mostly due to plenum volume) The actual point at which choked flow begins to occur (even if it is oscillating) can be determined by knowing if you can empty the plenum in one intake event. As intake pressure drops with increasing RPM, this obviously becomes easier and easier for a single cylinder to completely evacuate the plenum.

[BrendonD]

Good thread here. I have a question...

What sort of idle speeds are you guys with single cylinder engines getting? Ours (LTR450) starts every time with no drama on the Motec M400, but won't go to less than 24-2500rpm thanks to the 2L plenum sitting on top of it creating a (more) constant pressure differential through the restrictor, giving us the effect of a more open throttle plate.

I'm thinking let it be and move on. Thoughts?

[Charlie]

Originally posted by BrendonD:
Good thread here. I have a question...

What sort of idle speeds are you guys with single cylinder engines getting? Ours (LTR450) starts every time with no drama on the Motec M400, but won't go to less than 24-2500rpm thanks to the 2L plenum sitting on top of it creating a (more) constant pressure differential through the restrictor, giving us the effect of a more open throttle plate.

I'm thinking let it be and move on. Thoughts?

I think there are some serious flaws in the conclusions you are making.

I don't see how a big plenum affect idle speed, at least not steady-state idle speed.

I have not done the math but I can't imagine that the restrictor has any affect on the airflow at idle for a single, or even a 600cc engine for that matter.

If your throttle plate is almost closed that is your 'restrictor'... the 20mm orifice is irrelevant.

[Nick Renold]

Originally posted by Charlie:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BrendonD:
Good thread here. I have a question...

What sort of idle speeds are you guys with single cylinder engines getting? Ours (LTR450) starts every time with no drama on the Motec M400, but won't go to less than 24-2500rpm thanks to the 2L plenum sitting on top of it creating a (more) constant pressure differential through the restrictor, giving us the effect of a more open throttle plate.

I'm thinking let it be and move on. Thoughts?

I think there are some serious flaws in the conclusions you are making.

I don't see how a big plenum affect idle speed, at least not steady-state idle speed.

I have not done the math but I can't imagine that the restrictor has any affect on the airflow at idle for a single, or even a 600cc engine for that matter.

If your throttle plate is almost closed that is your 'restrictor'... the 20mm orifice is irrelevant. </div></BLOCKQUOTE>

I believe Brendon was referring to the discussion between A Richards and Jimmy01 at the beginning of this thread, regarding large plenums making it difficult to tune the engine for a good idle, especially with single cylinder engines.


Originally posted by Jimmy01
A large intake manifold makes tuning more difficult at low rpm but certainly not impossible. But I think there is more to it than just tuning (although this is probably the most important part). A very important factor to consider is fuel preparation. If your injector is spraying fuel onto the runner walls, dribbling, operating at a very low duty cycle, pointing at a bad angle or spraying at the wrong time you will more than likely have big issues with starting and low speed running.

The other issue with high performance motorcycle engines and low speed running is cam timing. At low speeds fuel and air can often be forced out of the cylinder at the start of the compression stroke and back into the plenum causing irregular fueling and hunting. In a stock engine the carb/throttle bodies are quite close to the valves and generally have a flat sliding plate as well/instead of a butterfly. So at idle and low throttle openings there is effectively a barrier reflecting the pressure waves which helps to minimise the effect I talked about earlier.

[BrendonD]

Nick is correct in what I was referring to. I originally wanted to know what people are ending up with as idle speeds on their single cylinder motors with large plenum volumes, as ours is near 2400-2500.

Charlie, you bring up a good point about the throttle plate having more of an effect vs the restrictor at idle.

After giving this some thought, here's what I've come up with:

Perhaps it's time to re-evaluate our throttle body setup and move to something that has a better throttle plate "seal" at the idle position, and doesn't flow as much air. This could allow the engine to pull enough vacuum on a larger plenum to get the idle down closer to stock.

The problem is that on the original setup the throttle bodies were much closer (read: right on the head) to the intake port, so there was less air available on the intake stroke at idle due to a smaller available volume to pull from. With the larger FSAE plenum that's not the case, and the throttle body is essentially allowing too much air to flow at idle.

Am I on the right track or way off in left field here?

[Mbirt]

Awesome thread. I just stumbled upon it today and it directly relates to the GT-Power simulations for our crf450x I've conducted the last several days.

For our thumper simulated making 50hp from 8-11k rpm with a 3 liter plenum, 10hp is lost at 8000rpm and 5hp is lost at 11000rpm when the plenum volume is reduced to 1 liter. Our car used a 1l plenum volume last year and, sure enough, the simulated power curve matches the dyno curve.

I've linked a .jpg of average and minimum plenum pressures vs. rpm for the 1 and 3 liter volume simulations. Hopefully it'll be approved sometime this year.

http://i56.tinypic.com/2usu7ih.jpg

[Mbirt]

My reply to this thread included a picture of some plenum pressure graphs from GT-Power, so it's going to be a while before it's approved.

In regard to your idle speed... how many degrees of ignition advance are you running at idle? Have you tried lowering it? With less timing, you'll need more throttle opening to maintain the same idle. Although, IMHO, if you have a steady idle now at 2500rpm which doesn't hunt and the engine responds well off-idle to throttle input, it's not worth messing with. Sure, you could tune it hybrid alpha-n at low rpm to have a nice steady idle, but it would just complicate matters one step further. Singles run considerably more aggressive cam profiles than their multi-cylinder bretheren.

[BrendonD]

Originally posted by Mbirt:
My reply to this thread included a picture of some plenum pressure graphs from GT-Power, so it's going to be a while before it's approved.

In regard to your idle speed... how many degrees of ignition advance are you running at idle? Have you tried lowering it? With less timing, you'll need more throttle opening to maintain the same idle. Although, IMHO, if you have a steady idle now at 2500rpm which doesn't hunt and the engine responds well off-idle to throttle input, it's not worth messing with. Sure, you could tune it hybrid alpha-n at low rpm to have a nice steady idle, but it would just complicate matters one step further. Singles run considerably more aggressive cam profiles than their multi-cylinder bretheren.

I'll give the idle timing a shot, but IIRC we're not running much at the moment. That still doesn't solve the problem of too much air available for the engine in an idle scenario. Definitely a variable to play with though.

Cool graphs! the plenum is totally necessary for a good power/torque curve but messes with tuning in other areas.

[Jimmy01]

That still doesn't solve the problem of too much air available for the engine in an idle scenario.

Brendon, I think you're still missing the point here, the throttle body controls the volume of air into the engine until the restricter allows no more. In the case of a large plenum, air is more consistently flowing through the throttle (which is why you can get more power when restricted) so for a given throttle position it will flow more air but the solution to this is simple - close the throttle more! A throttle with no idle adjusted into it should stall the engine.

However, this does not mean you can adjust your throttle and you will now magically bring the idle down to 1200rpm, instead your engine will probably cough, hunt and stall. If you removed the plenum and put the throttle in the intake runner, tuning idle at 1200 rpm would be easy.

As Mbirt said, if its idling well at 2500, i would just be happy with that and spend your time doing something more important. We had trouble idling below 3500rpm last year (3 L plenum) but we had really bad fuel prep and spent almost zero time trying to tune idle. This year we are looking at a behemoth 4L plenum and starting/idle has improved significantly due to other improvements we have made.

[Mbirt]

Originally posted by BrendonD:
That still doesn't solve the problem of too much air available for the engine in an idle scenario. Definitely a variable to play with though.

It does in practice, I've done this many times in tuning our idle. As you lower your timing at idle, your BMEP decreases to the point that the motor cannot keep itself spinning. To compensate, you must increase the idle throttle opening or put some advance back in it. So there was not enough air available to the engine at the low spark timing which caused it to die.

Jimmy, I'm interested in this scenario where you saw power double from 1-3 liters of plenum volume. I wish this were the case for us--we made 44hp on a 1l plenum last year. An 88hp naturally aspirated single would be awesome. We did have a super-long 16.5" runner length to the plenum, however. The capacitance due to runner volume must have helped some.

[Jimmy01]

I was probably exaggerating a bit there but we have seen a significant difference between a 1 litre and 3 litre plenum on several occasions. It may have been the no plenum setup that halved power, cant quite remember now and don't have the data on me. Our setup was by no means excellent though so I'm sure there are much more gains to be found with the 1 litre plenum. If your whole setup is pretty good to start with, the difference between the 1 and 3L plenum is not going to be nearly as significant as what we have seen. MBirt, You will have at least another 0.6L in your runner alone so its not a true 1L!
Our engine guys are up to 55hp at the moment with standard cams and a 3 or 4 L plenum.

[L B0MB]

A bit off topic, but I reckon taking 10 degrees of overlap off the intake cam would help a lot once these engines are restricted

[BrendonD]

Originally posted by Jimmy01:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> That still doesn't solve the problem of too much air available for the engine in an idle scenario.

Brendon, I think you're still missing the point here, the throttle body controls the volume of air into the engine until the restricter allows no more. In the case of a large plenum, air is more consistently flowing through the throttle (which is why you can get more power when restricted) so for a given throttle position it will flow more air but the solution to this is simple - close the throttle more! A throttle with no idle adjusted into it should stall the engine.

However, this does not mean you can adjust your throttle and you will now magically bring the idle down to 1200rpm, instead your engine will probably cough, hunt and stall. If you removed the plenum and put the throttle in the intake runner, tuning idle at 1200 rpm would be easy.

As Mbirt said, if its idling well at 2500, i would just be happy with that and spend your time doing something more important. We had trouble idling below 3500rpm last year (3 L plenum) but we had really bad fuel prep and spent almost zero time trying to tune idle. This year we are looking at a behemoth 4L plenum and starting/idle has improved significantly due to other improvements we have made. </div></BLOCKQUOTE>

I was more trying to state that the throttle body we have on our dyno (stock) was designed to flow more in the idle scenario on the stock setup than is desired for a FSAE setup with a plenum. We have that sucker closed as far as it will go without modification... hence my comment about potentially replacing/modifying the hardware. I get what you're saying about the restrictor vs throttle plate, the throttle body hardware we have now may just be "leaking" too much air into the plenum. 0% throttle position was the first thing I checked when we got it running reliably.

Again, you're right that this is a largely trivial matter since I'm not going to start replacing hardware on an engine that starts and idles every time with no drama. We'll be developing the maps for a solid torque curve first. The original motive of my post was to see what other people's singles were idling at, if they were leaving it in that range or spending the time to try and reduce it.

Thanks for the replies guys!

[Mbirt]

Originally posted by L B0MB:
A bit off topic, but I reckon taking 10 degrees of overlap off the intake cam would help a lot once these engines are restricted

Our aftermarket cam, which a certain world champion team has also run in the last 2 years, has 248 deg intake duration and 270 exhaust with 42.5 degrees overlap. I've heard from some bike engine builders that this may be overkill even for many unrestricted moto-x bikes. I'll definitely be testing other cams this winter.