Guys,

I recently swapped my 700R4 for T-56 trans. I drove the car a short while on the 32b code that was dyno tuned many years ago with the automatic. There were a bunch of things about the auto tune that I didn't like with the manual trans. So I switched the code to 6E APYP which is a corvette 6spd tune. The car didn't run right at first so I pasted over the timing and fuel from my old 32b into the APYP code. Now the cars drives pretty decent and pulls hard.

My only issue is that there is a surge when letting out the clutch from a stop. Just as the clutch becomes fully engaged the car surges up to about 6-7 mph and doesn't like driving any slower than that in first. More surging if I try to go slower. This makes it a real pain to drive in traffic.

So I swapped the APYP timing table back in, and the surging seem to be about 50% better. But of course the car doesn't run right everywhere else.

Now here is the funny thing. When the engine was dyno tuned several years ago, I was running a 2 cycle engine, burning tons of oil due to a crappy machine shop. As a result, my timing table is extremely conservative. See below. Engine has since been fixed and runs well.

Any good ideas how to smooth this thing out?

Engine:
L98
Trickflow 195cc heads
ZZ4 cam
Edelbrock TPI intake manifold
AS&M runners
TPIS 58mm throttle body
SLP 1 3/4" headers

Chassis put down 280hp / 370ft-lb on the conservative timing table while running lean. I have since enriched the fueling.

My plan is to slowly increase the timing in the low rpm / low load areas and see if that helps.

Thanks,

John

 

Check a data log to see what the IAC is doing. If the RPM dips below the desired idle speed the ECM will open the IAC.

It may also be opening from throttle follower (TF). This is when the IAC opens in proportion to the TPS%.

RBob.

 

Thanks RBob.

I did datalog a few starts. It does seem as I just touch or go under the desired idle speed just as the clutch is coming all the way out. In those cases the IAC did flare up a little bit, about 30 counts. I have driven my Dad's C6 Vette with T-56 among other sports cars quite a bit and do not have the same problem, so I'd like to think its not a driver issue. Other factors for my car include a larger bore master cylinder that is really nice to shift fast, but a little harder to modulate considering the shorter stroke. Also the T-56 has a shallow 2.66 first gear combined with my tall 3.27 rear gears, it means I will probably never be able to inch along at 5-6mph.

My commanded idle is 850rpm at operating temperature. My 58mm throttle body makes it a little tough to have a idle lower than that. Right now my idle IAC counts range from 0-12 counts. I need to play with it some more.

Next I am going to test setting the commanded idle to 650rpm. The car won't idle that low due how I have minimum air setup. But it should give me a little buffer for letting the clutch out before the IAC intervenes.

 

RBob, you are the man. I dropped the commanded idle to 650 rpm and added some timing to the low RPM high load area. Worked like a charm. Car drives great now. Let me know if you ever make it out to the Seattle area. I owe you some beers.