A4 Corvette w/3.90s wont shift at WOT

[Chuck CoW]

Hey guys...

I could use alittle help on something. I'm having a problem with 3 different customers A4 Corvettes I built with 3.90 rear gears.

These cars are automatics and have NO motor work. Just Vararams exhaust...2 have headers. 2 have Pro torque converters. The common denominator seems to be a 3.90 rear diff ratio.

These cars run like crazy…..BUT, despite the fact that I’ve set the WOT shift RPM to 6,100. when at 100% full throttle….It seems the trans ignores the WOT shift rpm and revs right past the commanded shift point and hits the rev limiter. If you back off the throttle even the slightest bit it rocks into gear like a champion! One of the three cars (a 98 C5 auto) will sometimes shift great at WOT and then instantly the motor seems like it shuts off for 2 or 3 seconds and if I hold my foot to the floor, the motor suddenly “turns back on” and runs fine. For some reason…only at 100% throttle, it does not upshift and when the rpm’s exceed the WOT shift RPM the motor breaks up badly and continues to run like shit for about 30 seconds (even though the rpm’s are safely below the rpm limiter) ….at first it runs like it’s flooded….and then slowly “clears up” the more you continue to drive. Each car exhibits slightly different symptoms but bottom line is they (usually) don’t shift when commanded to at max throttle and if they do…the motor runs like garbage or “shuts off” while on the throttle . If you go 90% throttle they all BLAST into gear and run like wild. I’m lost and need some help…I’ve been staring that these tunes for 3 months now and despite the frustrated customers..I’m losing my mind.

I tune automatic cars and trucks every day and I do the same thing in every single tune…always works great…never a problem no matter what mods the cars have…..the problem seems to only occur when the car has a higher than stock rear axle ratio.

I have even thrown an original GM bin in the cars and only changed the rear axle ratio….and it does the same thing…..Additionally, I’ve even set the WOT shift rpm to like 5,000 where surely I’m not pushing the motor or running out of fuel and the same thing happens….So, I’m assuming it’s not the “tune”

But, I’ve been thinking…..could there be some issue with the 3.90 axle ratio and the other controllers in the car??? Or some Traction control/active handling/torque management nonsense not liking my WOT use of the 3.90’s and my tune or maybe the TAC module? I know very little about the background processing of the o/s and how values are passed back and forth between the controllers. As a programmer for 20+ years….I’m thinking there’s something happening behind the scenes that just isn’t obvious from what I see in the EFI LIVE editor.

As I said....I think the common denominator is the 3.90's with these 3 cars...Hopefully someone out there has got an answer for me....


Chuck CoW

[Tordne]

When using EFILive to change the diff ratio did you use the adjust feature to raise/lower the shift points based on the difference between the old and new ratios?

[foff667]

does it do it in all gears or just the 1-2? usually I use mph as my tag for the 1-2 shift and a mix of mph & rpm for the 2-3 shift...basically im asking if it could possibly just be the computer not being able to keep up with the new gearing?

[Chuck CoW]

When using EFILive to change the diff ratio did you use the adjust feature to raise/lower the shift points based on the difference between the old and new ratios?

oh yeah....I've been doing this for a very long time....I kmow this software cold.....That's what buggs me....It's just these 3 cars....

All normal driving is perfect....all heavy throttle driving is perfect....it's just 100% throttle that messes up...not 95% or 98%.....ONLY 100% throttle the pcm ignores the "shift at RPM" I'm lost.....

chuck

[Redline Motorsports]

I have had a similar issue with the A4 as well. What about mechanical delays vs. the electronics???

[Chuck CoW]

does it do it in all gears or just the 1-2? usually I use mph as my tag for the 1-2 shift and a mix of mph & rpm for the 2-3 shift...basically im asking if it could possibly just be the computer not being able to keep up with the new gearing?

well, with an A4 you can only max rpm shift 2 times....or you're goin too damn fast....

Yes, it is primarially the max throttle 1/2 although it dosen't do it every time.

and it has happened with the 2/3 but it is less frequent.

Chuck

[Chuck CoW]

I have had a similar issue with the A4 as well. What about mechanical delays vs. the electronics???

please explain


what it seems like to me is not a malfunction of the "shift at rpm" parameters, but the engine cruises right past the "shift PRM" and hits redline.....

and usually the engine in the 98 car just "shuts off" with my foot to the floor and then resumes afew seconds later.....the 2000 C5 begins to break up at the shift rpm.....hits the rev limiter and then begins to run like it's flooded or something i have never seen and slowly recovers...it sometimes takes 1 min to "clear out" and then runs fine......

chuck

[SinisterSS]

Here's something to try - Set the WOT shift speed in D0910 through D0922 at some rediculously low numbers that you know are easily attained in each of the gears (see example of 20, 38 and 60 mph). Next, set the WOT shift rpm where you want it to shift (5600 was used in this example). With these settings, it'll bang home a shift like clockwork at 5600 rpm. What ever rpm is set in the rpm table will control the shift point. 6500, 7000, 4500, etc. as long as the mph threshold has been exceeded.

To shift at WOT, both the speed and rpm must be "true". Think of the shift speed values as enablers that put the PCM into WOT upshift "mode" and the shift rpm values as the actual trigger that makes a shift happen.

[Chuck CoW]

Here's something to try - Set the WOT shift speed in D0910 through D0922 at some rediculously low numbers that you know are easily attained in each of the gears (see example of 20, 38 and 60 mph). Next, set the WOT shift rpm where you want it to shift (5600 was used in this example). With these settings, it'll bang home a shift like clockwork at 5600 rpm. What ever rpm is set in the rpm table will control the shift point. 6500, 7000, 4500, etc. as long as the mph threshold has been exceeded.

To shift at WOT, both the speed and rpm must be "true". Think of the shift speed values as enablers that put the PCM into WOT upshift "mode" and the shift rpm values as the actual trigger that makes a shift happen.

So far, I have actually set the WOT shift rpm to like 5,000 to make it shift..and it dosen't.....I see what you are saying.....I'll check it....

I use the same shift tables in almost every tune I write....NEVER a problem.....but, with the 3.90's installed....it's screwed up....Hopefully, it's something with the 3.90's messing with some internal calculation and not me losing my mind.....I do this every day for a living with out a hitch...Just these 3 cars.........Maybe it's time for me to retire?????

uh
Chuck

[GMPX]

Chuck, did you get anywhere with what was sent via Email from Paul?

Cheers,
Ross