VF Commodore surge on 2nd to 1st downshift

Hi guys,

This has been bugging me for while. When coming to a stop I'm feeling an RPM surge between 2nd and 1st. Car has been tuned (road) over a period of a few weeks and is running extremely well.

Car details

2016 VF2 Commodore Black Edition that has had an V6 to V8 conversion

L77 with DoD delete

203/219 .551/.556 114LSA (baby cam)

TCE 2500 stall (was installed when the car had a larger cam)

1-7/8 headers

High Flow cats

VCM OTR

2bar SD tune but have the MAF enabled. Tune was done in 2 part, SD first with MAF being turned on later

It seems to me that timing is being pulled then added on downshift causing the surge??

I'll include my current tune and a log for anyone who'd like to provide some info. As i said above, the car runs beautifully, fantastic idle with lots of low to mid power with excellent economy.

Cheers and thanks for any input.

Phill

Anyone??

Phil,

Do you have a log including gear so we know when you 2-1 shift since that's the concern?

I'd also log idle airflow and spark if this is happening coasting.

Thanks Mike,

I'll get another log tomorrow night with gear and Idle air included.

Thanks,

Phill

Hi Mike,

Please see attached. I hope I have everything logged.....time stamp 08.24 shows what's happening

Thanks Phill,

First I agree it looks like it's driving well, so that's wonderful.

I checked the first three instances of 2 to 1 downshifts and they all occurred when you transitioned from coasting to driving by moving the accelerator pedal from 0% to something above zero. Ignition timing reduction while coasting is part of torque control and programming generally assumes you want less engine output while at 0% pedal, and more than that when you press the accelerator, so that's what the system is doing.

If there was a different instance where the thing occurred, please let me know what to look for.

Do you feel you're getting more engine response than expected in 1st gear? Anything you can do to describe the behavior you're wanting to change would help me help you more. You called it surge earlier, so I'm trying to understand what that means to you.

Talk to you soon,

Hi Mike,

It feels like when you coast to a stop they'll be a a noticeable RPM surge on each downshift. If I come to a stop using a firm brake it's not very prevalent. Maybe I'm too picky but shouldn't auto downshift be pretty much undetectable??

Obviously the RPM will need to change with the gear ratios. The question, is does the change in RPM result in more torque (pushing the vehicle forward) or less torque (slowing the vehicle) that what you want?

As David mentioned, downshifting does involve revving the engine up so the engine speed matches the new gear ratio, but that's really only needed when the engine and transmission are well coupled.

In your case, with an auto tranny, while coasting, your engine is sitting at idle speed in the log at 21 minutes 30 seconds for example, showing me the torque converter is unlocked. By decoupling the engine from the drivetrain, the downshifts occur without the engine revving up. I see no change in throttle position or engine RPM during those 0% accelerator pedal input downshifts, and vehicle speed decreases likely due to braking as this happens, so that looks normal.

Where I do see engine speed jump up, is at the time of 2-1 downshift, but only because you initiate the shift by press the accelerator pedal from 0% ot a higher value, causing not only the shift, but a throttle opening increase, which increases engine output. As engine output and speed increase, the torque converter starts coupling too, and the rev matching David mentioned comes more into play, and all those things will feel they are driving the vehicle further more.

Sticking to the behavior while coasting which I believe is your focus, it looks great, but I do understand how different your new cammed v8 feels compared to the old V6. As the old V6 approached a stop in 1st gear, it wasn't driving the vehicle forward against the brakes as significantly. Some of the extra output comes from the larger engine and greater inertia, and a bit more is added due to what's required for idle stability with the cam. Reducing airflow during decel will reduce engine output in that state, but you do need to use caution to avoid stumbling or stalling. Throttle follower torque and airflow final minimum under the idle airflow section are a few tables you may want to investigate further, as well as the follower tables under the idle RPM section.

Hi Guys,

Doing some reading on the Hptuners forum, I've been led to believe that the Torque Coefficients should be left stock (for my cam size). So I'm setting them back to factory. I'm also going to drop the the airflow down a bit (.1 or .2 G/S) under 1000rpm in the Airflow Final Min table.

Thanks for your input guys. Hoping that I can get this dialed in

Great, please let us know how it goes.

Phil,

Hopefully you've had success, but please let us know.