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

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.

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

quote:

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.

quote:

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...

This message has been edited. Last edited by: Hector,

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.

quote:

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.

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.

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. (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.

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

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.

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.

quote:

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

We found restrictor choke really takes affect after peak torque.

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

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.

quote:

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?)

quote:

Originally posted by VFR750R:

quote:

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?)

+1

quote:

Originally posted by VFR750R:

quote:

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?)

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

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.

quote:

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.

quote:

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.

This message has been edited. Last edited by: Simon Dingle,

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.

quote:

Of course, those numbers are for the design judges

I assume that is a design graph as well then

quote:

Originally posted by Simon Dingle:

quote:

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.

+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.