Motor oscillation

[Randy Van der Ree]

We are running our GSXR-600 with a Performance Electronics ECU, and we are noticing some oscillation in the performance of the motor at low RPM (1000 - 1500) and it normalizes above 2000 RPM, is this normal?

[Bill Kunst]

What year? This may give some idea as to the expected redline. A 15k rpm motor isn't going to run well at 1k rpms. I have a Honda Superhawk, redlining at 9500, and any loaded throttling below 2500 causes the motor to become very unhappy. I would guess that this has a lot to do with overlap inthe valvetrain. Try not running it at sucha low rpm! Just a thought,
Bill

[SpdRcr]

we ran a GSX-R motor last year with the performance electronics ECU. I am assuming you guys are running the restrictor on it.

we couldn't even get ours to run that low. our idle was at about 3200 rpm after tuning. we could go below 3200, to about 2500, but there is just too much lag. Plus you should never see those low RPMs in race situations.

Why are you running such a low RPM?

[Chuck Dean]

The GSX-R 600 in its stock configuration will idle below 1500 RPM, no problem.

For a long time we had an idle above 4k but after some recent idle tuning we have brought it down to 2000 rpm. Try using the ignition maps to set your idle point. Use a point of low ignition advance to hold your RPM. The natural tendency of the engine is to try to to rev higher - but it can't with the low timing value.

Keeping your idle low can help with driveability issues - it'll make 1st gear much more useable. This is important when you are driving in the pits and pulling up to a staging line.

As far as the "engine" oscillations go, continue working with the ignition timing and fueling in the area of oscillation. Also, watch for harsh transitions. A sudden change in the fuel/ignition map can cause these oscillations.

Lastly, if you can't fix it through tuning, check for intake leaks. Intake leaks are common causes of high idle RPMs and surging idles.

[Bryan Hagenauer]

Low rpm oscillation is really common on fsae cars. It's not a good thing, but it happens. A good idle can be difficult to tune properly.

[Kirk Feldkamp]

Originally posted by Chuck Dean:
Try using the ignition maps to set your idle point. Use a point of low ignition advance to hold your RPM. The natural tendency of the engine is to try to to rev higher - but it can't with the low timing value.

As far as the "engine" oscillations go, continue working with the ignition timing and fueling in the area of oscillation. Also, watch for harsh transitions. A sudden change in the fuel/ignition map can cause these oscillations.
idles.

The man speaks the truth. Ignition timing and a smooth fuel map are key in your idle pocket.

-Kirk

[Cody the Genius]

Another reason for a surging motor at low RPM is a vacuum leak. These are normally common on F-SAE cars as well because the intake manifolds are all student made and assembled. I think that is one of the major reasons very few people can get a car to idle well below 2500 rpm. It's gonna be interesting to see how many teams can pass the "tennis ball" test of the restrictor.

Though it is a good point that racing engines are not made to taxi, idle, or cruise the pits- They are meant to race. If you are seeing 4,000 Rpm on the track than there is a big issue with gearing or you need to hit you driver until he down shits. I mean, Top fuel engines rarely even idle below 4,500 Rpm.

[KU_Racing]

Top Fuel motors definately idle waay below 4500 rpm. Anyways, everybody is right- no vacuum leaks+smooth fuel curve+low timing at idle+smooth idle advance = good low quiet idle.

[Erich Ohlde]

I'm assuming your are talking about rough idle, but without being able to hear it I'm just guessing. You are getting this rough idle most likely because of low exhaust back pressure (stock exhaust = restrictive muffler) and because bike motors tend to have a rather aggressive cam.
It's normal, a buddy of mine had an R6 with stock exhaust, it had a nice idle about 1200rpm. He then put a yoshimura aftermarket race pipe on it and it developed a sweet rough idle.

[Cody the Genius]

I know that this is not directly related to the topic but.....


Modern top fuel engines do idle usually between 4800-5000 RPM. Top Fuel, not pro stock and they aren't exactly your average big block. The 48 volt starters they use can spin the motor to nearly 5000 RPM, with blower, and this is done for two main reasons.

1. Under 4000 RPM there is a larger risk of hydrolocking because of the lower port velocities as well as a leaky blower that is not helping to clear out the cylinders as much. Seems odd but is very common when running a ~1.25:1 AF ratio. They really do have the equivalent of a fancy fire hose for fuelling.

2. The second reason for this is hydrodynamic lubrication. Even when running ~125-150 psi oil pressure there is still enough power to punch through the oil film on the crank and rods unless serious hydrodynamic pumping (ie. RPM) is keeping the the rods off the crank. This is also the reason for high cranking speeds as oil pressure is completely built up and stabilized before the ignition switch is even looked at.


Nitromethane makes for a very different animal and don't forget that until this year (rules) 500 CID engines were breaking 10,000 RPM as they went out the back door. Amazing that they only see ~1000 revolutions under full load before rebuild but that is what it takes to make 7500 Hp.

[kwancho]

Haha, wow, the more I hear about Top Fuel, the more I understand how ridiculous it is. 1:25:1. Jeez. Hydrolocking. Hah. They shouldn't drive their cold air intake-equipped car through a deep puddle, clearly.

[Garlic]

And you can also see how a Top Fuel car's idle can be a good example for our cars, since they are so similar!

[james17]

Originally posted by Garlic:
And you can also see how a Top Fuel car's idle can be a good example for our cars, since they are so similar!

It wouldn't be hard to Justify the statement that our engines serve a purpose much more similar to Top Fuel engines then that of a production car. And his facts are pretty much dead on to what I know.

As far as idle, I've tuned more then a few cars, and had a hand in two SAE cars, and there are things that cross to other engines and things that don't.

The first thing is to make sure you have the proper amount of fuel at idle. Too much fuel and the engine will load up while idling and bog, to little fuel and the engine will surge and die.

Timing is obviously next, to much timing will cause a very "lumpy"� idle, much like a vehicle with a large degree of camshaft lobe overlap. To little timing will cause what I've always known as a weak idle, which sounds like what you may be experiencing, where the engine can't sustain idle.

The next thing, and harder to fix, is backpressure. To little back pressure and the engine basically tends to act lean and under timed. You will have surging, a weak easy to kill idle, and the response will be slow off idle.

Obviously if you have non-stock cams you have to watch out for overlap, which will cause a lumpy overly rich idle. The flip side of that is that your dealing with dual cams, in which case with stock cams overlap is typically not an issue.

A few other things, obviously a vacuum leak will cause surging and a rich to lean pattern at idle. Watch for engine temp, too high or low and the engine won't idle.

As a rule, when tuning for idle on a carbureted vehicle I use just off idle to know what the idle condition really is. If when you get on the throttle the engine bogs and then picks up a slightly higher RPM, think lean, if it does the opposite, rich.

We have our higher compression, non-stock cam, slightly touched up head CBR idling at 1500 rpm like it's a slightly cammed street vehicle; it is possible with some work.

[VFR750R]

Don't forget the fact that we are fuel injected and have the ability to adjust the fuel fairly precisely.
A top fueler flows 1.5gallons per second at WOT. What's the resolution with a fuel system that can flow that amount of fuel. What happens if you run really big injectors and then use the ecm to control them. Very small changes in pulse width cause large changes in air fuel ratio. It's a percentage thing.

And no, a pushrod, 2-valve, 58psi supercharged, nitro burning big block is not going to tell you what a 600cc 4 cylinder restricted gasoline burning engine will idle at. What the stock bike will idle is the closest comparison. Even with higher compression or cams, it won't change it that much.

[72_chev_truck]

I also had a similar experience with idle quality with a f4I and a performance electronics ecu. i will have to try these tips. also i have been having some trouble in the mid range. it sounds like its misfiring at steady load but when you step on it it takes a second and then wild tire spin. any ideas?

[Ben Inkster]

I thought I would add a factor in to this discussion that I don't think anyone has broached yet.

I did some research on this topic after putting a variable intake on the dyno for the first time in 2004. We noticed that the different runner lengths affected the idle oscillation from high amplitude rpm oscillation to little to none (although I doubt the engine was optimally tuned for idle at this stage but it still suggests that the intake has an affect). After this i decided to simulate on Wave at the idle speeds and it was evident that some runner lengths had a more efficient airflow at the idle speed and were causing a sharper torque gradient surrounding the point of idle. I theorise that this is very similar to an unstable gain on a PID controller and the system overshoots and gets out of control. We would often see the idle oscillation get out of control and eventually stall itself.

I think that the three main factors of air, fuel and ignition are all important tuning factors for idle just as the P the I and the D are in a PID controller.

Anyway, i think this is a valuable discussion worth a look at because a low engine idle can contribute to a low fuel consumption, which is becoming more and more valuable with all these single cylinders around.

Cheers
-Ben

[james17]

Originally posted by 72_chev_truck:
I also had a similar experience with idle quality with a f4I and a performance electronics ecu. i will have to try these tips. also i have been having some trouble in the mid range. it sounds like its misfiring at steady load but when you step on it it takes a second and then wild tire spin. any ideas?

I would look towards a lean condition, but do what your telling me it does few times, then don't rev the engine or let it go back to idle, just clean cut it right after it bogs, then pull the plugs and check them. That method has been used in racing for years, before 0-2 data aq., which would be the easiest way to diagnose such an issue.

As far as our engines versus top fuel, I know they are not a direct comparison. What I mean is that a Ford Taurus' engine system is designed to work from 400rpm (idle) to redline (say 7200rpm) and it has to work well and make decent power and mileage all the way through that band, but at no point in that system are the engineers tuning for absolute max power, they are tuning for max drivability, low NVH and such. A FSAE engine, ours at least, is designed for the largest area under the torque and H.P curves possible and having those curves as smooth and close as possible while also obviously tuning for max power. When we designed our headers, our intake, our cams and such we designed them for those purposes, much like any top fuel team would do, which makes the ideals behind our engines and a top fuel engine much more similar then the ideals behind a Ford Taurus. Most of the tuning techniques I learned I learned from an old drag racer, and they made for a very fast daily driver that idled at 1200rpm with a 500 rpm surge and clouds of black smoke, when I began tuning on my own for my daily driver I did so in a fashion much more akin to what a OEM engineer would do, tuning for idle and drivability and power as a afterthought. FSAE engines don't belch clouds of nitro-methanol and consume fuel in the gallon's per-mile range (it would be cool if they did thought, ya?), but they also aren't designed to idle at 400rpm, cold run and be killed, or drive down to the 7-11 for smokes. That's all I was trying to say as far as comparison.

[Cody the Genius]

Thank you james17 I am glad you get the point I was trying to make. A true race engine has made no compromises to power or drivability outsides its own rev range. I would definately trade 1/2 ft-lb of torque at 10,000 RPM for 25 ft-lbs off idle. I would also more effectively spend my time working towards any sizable gain where the engines revs while on the track rather than be concerned over a 3000 RPM idle. That is the point I was trying to make- only valid compromises should be made.

The mention of a top fuel motor was only an example of compromise-less engineering. The motor only needs to last for 1/4 mile, so it lasts 1321 feet and thats it. The long post about why they idle that high was written to refute what KU_Racing said about them idling way lower, which upon further research I found some teams that do idle closer to 3800-4000 RPM, though that is not the norm.

All other points criticizing the similarities are kind of moot. No, a vacuum timed mechanical fuel injection system that can flow 8 gallons in 5 seconds is not a close comparison to four 15 cc/min injectors controlled by an ecu. I figured most people could understand that but maybe I should have made my points more clear. I apologize.


-I always found it funny that when a structure is compromised it means that it lacks integrity and is going to fail.

[Chuck Dean]

Let me reiterate something I said in my first post, before everyone began discussing top fuel cars.

Our idle used to be around 4000 rpm. We ran it that way for a long time. However, after I did a little bit of tuning, I was able to "push" the idle down using ignition timing. It's not that complicated - you just to spend a day playing with the fuel/ignition maps until you're happy.

I've pushed the idle down as low as 1100 rpm.

Additionaly, lambda control helps as it can rapidly sweep above and below stoich to help maintain consisten fueling - resulting in smoother idle.

Also, there's no reason to not aim for a low idle. The benefits of a low idle should be obvious.

[VFR750R]

Are the benefits of a low idle that obvious. The higher your static idle, the lower your engine braking. Engine braking can upset your corner entry, which makes you setup your car to offset this force, which in turn compromises your mid corner and corner exit grip. An ideal idle in my mind for speed would be the lowest rpm you see on the track so going through the slowest corner-no brake or throttle input is needed to balance the car.

[Chris Boyden]

All other points criticizing the similarities are kind of moot. No, a vacuum timed mechanical fuel injection system that can flow 8 gallons in 5 seconds is not a close comparison to four 15 cc/min injectors controlled by an ecu. I figured most people could understand that but maybe I should have made my points more clear. I apologize.


Not planning on producing much power w/ 15cc/min injectors....unless you have 10 or so per cylinder.

[Garlic]

Engine braking is good! You want engine braking on a autox / road course. Yes it can upset the car but that's something that the driver needs to learn (downshift points, etc). He can even use the throttle if he must to balance the car. The alternative (too much idle) the driver can do nothing about.

The reason it's so good is because the alternate is bad- the car will power itself into corners when you are trying to slow it down. You brake mostly in a straight line... when you are turning, although there are plenty of times you'll still be braking, it's unlikely that you'll have engine braking.

In all the racing I've done I've had drivers complain that the car is driving into corners... never complain about too much engine braking.

In drag racing idle matters not at all... in road or auto-x racing the car is at idle a suprising amount of the time... it's an aspect worth tuning on.

[VFR750R]

In Nascar, drivers perfer less engine braking. Cup cars are so loose as it is however that they may feel more sensitive to it. They also never shift so when they get to the end of the straight they basically just let off and have 9500rpm engine braking. It's not a big deal at the bigger tracks but when we're using a low rear end gear like at a flat track like martinsville, loudon, and even pheonix, it makes a difference. We've had cases where at smaller tracks (were HP is less significant)we'll run a taller gear then necessary to settle the car.

[Biggy72]

is that still the case on road courses? It seems like that would be a more equivalent comparison to our cars. I was kind of wondering if anyone from here went on to work for any cup teams, so even though I'm only one of two nascar people in our club, it's good to know there are others out there.