End the Throttle Bounce

ctuinstra

Goblin Guru
We have always had a bit of throttle bounce that ends up in a vicious cycle of ON-OFF surging. Typically, it seem like it gets started (but not always) from a little bounce in the foot where you abruptly press the pedal and then quickly let off. The car then surges forward some and then back, this causes your foot to bounce again and the cycle continues and once it starts, you can't really stop it unless you let off completely, push in the clutch, or just floor it. At least this is what happens in our car. I've been able to reduce it by playing with the torque management, softening the shocks a bit and better control of the foot.

But tonight it was very cool and dry with the air density being especially high and it was just horrible. Every time I took off, I really had to be careful so as not to start the jerking again (especially with war-torn roads we have around here from this past winter). So this lead me to try a few things knowing it was a really good day for testing this.

I enabled the dynamic airflow all the time and reduced the predictive calculations. I believe the ECM is trying to predict the airflow and getting wrong and starting a bad oscillation. I set it as follows in the Airflow - Dynamic - Dynamic Airflow (for the P12 ECM):

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This way its using more of the VE tables and it's filtered and smoothed out more than the pure MAF. I think that there could be a reverberation in the airflow across the MAF when using such a short intake tube from the filter to the MAF sensor. Just my thought, I don't really have much proof. The logging data shows everything bouncing at that point and it's hard to say which came first.

Either way, I took the car back out for another drive and WOW! What a difference it makes in the drive-ability! It's so smooth. There was one large bump and I could tell that I bounce the throttle and it surged once and then is stopped. It's never stopped before. I could tell there were many times where it could have had the issue, but it didn't.

I checked the logging data and the ARF is spot on, my VE tuning is still very good. LTFT were at 0.8! And the STFT were always correcting up and down as they should be.

You can see here the White is the pedal and the red is the RPM. There is a dip here and a bad bounce in my foot to the right. The RPM never really moved.
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Very happy with the way it drives! I was wanting to do this for a long time but thought it was best to stay with the factory settings. But running off the VE table is so much better.
 

Gtstorey

Goblin Guru
My TC shows some of these symptoms when the MAF isn’t calibrated correctly, and the TC runs mostly on the MAF. With the location of my filter/intake, I have to retune my MAF every time I make a windshield change (I have 3, tall, short & driver only). I don’t know where the transition are on the SC for how much it runs on MAF or VE, but maybe your MAF needs a little tweaking?
 

Desert Sasqwatch

Bigfoot Goblin
Maybe one of these will help with the possible air reverberating in the intake pipe?

 

Gtstorey

Goblin Guru
Maybe one of these will help with the possible air reverberating in the intake pipe?

I have one in mine. I don’t know if it made any difference since I installed it prior to initial startup.
 

ctuinstra

Goblin Guru
Maybe one of these will help with the possible air reverberating in the intake pipe?

The one thing I wanted to try, but never wanted it bad enough, was to add a very long intake before the MAF to help smooth the velocity of the airflow. I can only imagine that would help a lot, but I didn't know the best way to do that without it looking ridiculous.
 

Gtstorey

Goblin Guru
I’ve thought about doing that also, but never really seemed to be worth the effort. Once I get mine sailed in after a change, it does pretty good. Not perfect but hasn’t been worth the effort.
 

Dsteinhorst

Well-Known Member
My 09 2.2 has had some of the mentioned surging since converting to supercharged. I agree it is a MAF tuning thing, as the LAP to my knowledge cannot be tuned for boost in VE mode. It is always in MAF mode above 1200 rpm. Mine is manageable at this point, an average person may not even notice.

I do think the springs in the clutch plate play a role. I've always wanted to try a solid plate, but I don't want the other drivability issues that come with it.
 

Chubbs

Well-Known Member
Forgive my ignorance here if I'm wrong....but in that screenshot, I think you disabled the VE tables. The high RPM disable is only 200 rpm, so anything above that disables Dynamic airflow. Doesn't that mean you have disabled the VE tables, and are forcing it to run on the MAF?

I was going to post about the throttle bounce issue (which I have really bad!). I figured since it's a physical problem (your foot actually bounces) I would fix it with a physical solution. I put a damper on the throttle pedal. And it works (mostly).

I bought a 1/8 scale RC car shock absorber off of amazon for $20 for a pair. Filled with 1200 cSt oil. It fixed my throttle bounce about 70%. It still happens in some situations, but it's rare. No issues with hanging throttle or anything like that. I think with slightly thicker oil, I may be able to get rid of my throttle bounce completely.
 

Attachments

Rauq

Goblin Guru
Forgive my ignorance here if I'm wrong....but in that screenshot, I think you disabled the VE tables. The high RPM disable is only 200 rpm, so anything above that disables Dynamic airflow. Doesn't that mean you have disabled the VE tables, and are forcing it to run on the MAF?
That's exactly what this is, above the Dynamic Airflow High RPM Disable threshold it runs on 100% MAF for fueling.
 

OptimizePrime

Goblin Guru
If you guys have messed with the torque management ([ECM 12801] table on an LNF) and maf calibrations and still hunt at idle, it's likely the interplay between the optimum torque table and your timing tables at idle rpms and load (800-1000rpm and 20-40% load).

You maybe be able to bandaid it by setting your idle to 900-925.
 
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ctuinstra

Goblin Guru
That's exactly what this is, above the Dynamic Airflow High RPM Disable threshold it runs on 100% MAF for fueling.
I'm not going to pretend to be an expert on this. I believe my first post was wrong as you all stated. This is allowing Dynamic Airflow on, but that in itself does not necessarily mean that its MAF only, from my understanding. Here is a good write up about it to help understand this a bit more

"It's easy to get tripped up on the terminology. I find it helpful to clearly define the two modes of airflow calculation: Dynamic and Steady State.

Dynamic by definition means moving, changing. Steady state means steady, consistent. Mass Airflow meters are very good at measuring airflow that's nice and steady, like a constant 2000 RPM for example. Sudden changes in airflow are not so well detected, because that airflow change in the motor has to travel thru the meter first, so there is a bit of lag in the response time from the actual MAF measurement to the actual engine.

MAP sensors are very sensitive to changes in manifold vacuum. The instant you open the throttle up, the manifold vacuum changes, and the MAP sensor detects this right away. So the PCM quickly knows changes in MAP, but not so quickly for changes in MAF.

For this reason, GM runs a blended model. At lower RPM, GM pays close attention to the MAP sensor. At higher RPM, the engine is breathing in air so fast that a change in airflow is much easier to detect, and faster to report back to the PCM. So at higher RPM, GM pays closer attention to the MAF.

With MAP/speed density, airflow is estimated based on VE, IAT, and displacement. With MAF, airflow is actually directly measured, which is even better than estimating. So for sudden throttle changes, or low RPM, we should be looking at the MAP (dynamic airflow). For higher RPM, or a nice steady airflow amount that doesn't vary, we should be looking at MAF (steady state). And this is exactly what GM does with their blended model.

The High RPM disable scalar simply tells the PCM at what point to permanently quit looking towards MAP. It's that balance point between high rpm consistency and low rpm fluctuation in Mass Airflow.

But bear in mind, it's not an on/off switch. It's a blended model. So even at low RPM, the airflow calculation still is weighted more towards MAF than MAP. But as you suddenly change manifold pressure, it quickly looks more towards the MAP/VE calculations so it can estimate airflow.

In short, this is why GM has such killer throttle response. "


I think because the VE table can allow for a much better throttle response, especially down low RPM, as compared to MAF; by enabling the Dynamic Airflow this is causing it to prefer the MAF over the MAP (VE) and thus actually slowing down the throttle response, or dampening it a bit. Does that make sense? With using the SD (Speed Density), or VE tables, the throttle is TOO responsive with our setup and needs to be dampened or softened a bit.

I just don't understand why the OEM tune basically had DA turned off (5800 RPM).
 

Rauq

Goblin Guru
I too will not pretend to be an expert here, but I will speak from my experience.

You're right to clarify that below the Dynamic Airflow High RPM Disable, the computer is blending VE and MAF fueling, and while above it, it is only using MAF fueling. However, the computer will still run VE calculations above that point and can throw CELs for MAP/MAF/TPS disagreement.

I'm not sure why the factory tune disables Dynamic above 5800. The GM Stage 2 tune disables at 3500. I don't think this is the reason why, but I can say my setup can exceed 215kPa (limit of the VE table) by 3500, so I have mine set at 3000. The sensor should read up to ~250kPa and I believe the computer has some interpolation calculations it can do if it exceeds the readable table in HP Tuners, but I'd rather not dabble way out there.

As far as throttle bounce goes, mine definitely gets worse if I'm running a tune for datalogging that doesn't idle well. My MAF is in a larger housing and idle airflow is right at the bottom of the usable range of the sensor. If I'm MAF tuning and Dynamic is turned off (so no VE contribution to fueling), it bucks pretty dang bad at very low RPMs. Once I got my tune ironed out enough, I regained the ability to idle around in 1st gear. I would say that's an important defining feature of a good tune, at least as far as that aspect of performance goes.

If you log Volumetric Efficiency Airflow, Dynamic Airflow, and Mass Airflow, you can see Dynamic bounce around (mostly) between VE and MAF. Ideally they'd all wander in lockstep, but I've been able to see generally that VE and Dynamic correlate better at lower RPM and airflow and MAF and Dynamic correlate better at higher RPM and airflow. MAF does also track better at low load than VE does at high load, which makes sense as the difference between MAF and Dynamic above the Dynamic High RPM Disable point should basically be the same plus whatever modifiers the P12 throws at it that you may or may not be able to see.
 

ctuinstra

Goblin Guru
I’m going to have to do that logging to see that correlation. I’m going to have to spend some. Time on it to make more sense as to why this works so well for me. Just wish I had more concrete information. It’s seems to be pretty scarce when it comes to this subject.
 

ctuinstra

Goblin Guru
I drove the car for quite a while again today. I cannot be more happy with this car right now! I drives soooo much better. The throttle is so much more forgiving. I also notice that there used to be this on/off throttle gear-lash slop when trying to coast or cruise at idle or just above. In other words, while coasting down a slight grade and you let off the throttle there would be this backlash and then you barely press the throttle it would jerk and repeat. Now it doesn't do that at all. When you let off the throttle, it slowly starts to slow down and then you get on the gas again it smoothly rolls back in. Don't get me wrong, there is still lots of throttle, but it seems much more fluid.

I cannot recommend making this change enough. This is assuming the rest of the car is tuned well. It made a world of difference!
 

SmsDetroit

Goblin Guru
I have an LNF and I was hoping to make these changes but I’m not seeing it in HP tuners. What am I missing?
 

Ross

Goblin Guru
The LNF is Chevy generation 4 technology in the engine control module. I haven't heard anyone complaining that it has throttle bounce.
It is different when it comes to tuning the engine.
The LSJ is generation 3 tech in the ECM... and it has throttle bounce once you take away half the weight of the Cobalt.
 
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