Dear USA Pros,

First, excuse me for my poor written english as I am French.
As you can imagine, corvettes in France are very very rare. You might see one or two every year on the streets....

I am a lucky owner for 7 years now !!

Never ever got any problem exept lately....

This is a 1991, L98 engine. 50.000 miles. Using it only during summer and race week end with Porshe s friends.

Before having your advices, first, let me give you what s inside:

Short racing ratio gear, Automatic transmition, relocated MAT, high volume MAF, platine spark plugs and competition wire, 330 mm comp brakes , Competition shocks with modified ride control system, handmade headers pipes, custom 2 ways silencers, no catalitic system, K and N filter with custom forced ram air box, polished admision air pipes, polished forged high compression pistons and chambers, high capacity oil pump, additional oil cooler and ultra fluid oil, high voltage alternator, competition Camshaft with titanium valves, modified EEC. No air conditionning system (we dont need it in France), deflectors for max fresh air on the heads. complete rigidified chassis.

It is tuned to use premium 98 and ultra 101 unleaded. No Regular 95.

This car is outputing more than 350 hp at 6500 rpm with 98 unleaded and 390hp with ultra 101 unleaded. 0-60 mph 4.6 sec.

The problem: When I am at cruising speed around 100-110 mph (I know) on the 3500/4000 rpm zone (our highways are more like speedways), and when I require more power in order to compet those late model diesel BMW or mercedes engine (outputting 250hp to 380hp for the last mercedes !!!! ), for some reason, the gearbox does not kick down the overdrive as usual to go up 4500/5000rpm zone but instead it keeps the overdrive on and the engine is hitting hard like a hammer !!!!! giving very few power or like so even if accelerating due to torque. More I push the accelerator and more it is hitting hard with creazy vibration....

I would like your advices for this problem that I am sure is somewhat a stupid thing for some reasons I cannot find due too much brain bull****ing dont knowing where I am anymore.

I really do not understand what is happening and I am pissed of not to be able to follow those diesel cars....grrrrr

Any ideas ???

1000 Thanks in advance.

Olivier

 

It sounds like the engine is experiencing knock at that point, due to the higher loads and relatively lower RPMs.

It seems like there are two issues: One - the transmission is no longer downshifting the way it used to. I am not at all familiar with automatics, so hopefully someone who is will chime in. Two - the engine knock (if that is what it is) as described above. Too much timing in that particular area of the spark table, a lean condition, bad gas, etc are some of the things that can cause knock.

Are there any codes? Do you have a scanner / laptop to hook up to the ECM to get some real time information?

 

Sounds like a question for Pete K.

I always thought the speed limit in France was 130 kph or 81 mph.

From your rpm at speed description, I guess you have 3.73 gears.

A downshift from 4th (od) -> 3rd at 4000 rpm, would put you at 5700 rpm in 3rd gear. This is likely above the normal 3 -> 4 upshift point, which could explain why its holding 4th gear.

Has your automatic transmission governor been re-worked to permit high rpm upshifts? I imagine this would influence downshifts as well, but 4th to 3rd may be peculiar due to the valving? Again a Pete K. question.

I agree with the other reply that it also sounds like your experiencing knock. Back off on the timing in your chip if you can for the rpm vs load cells in question.

You may get more response by re posting in the C4 Tech/Performance area.

 

Dear Pros,

Thank you for your answers.
Yes, the speed limit in France is 81 mph but 100mph is tolerated.

You are right, when at 3500rpm, it is downshifting to a little more than5000rpm.

Yes, this is exactly knocks !
I ckecked the air filter and cleaned it completely. Also, I noticied some kind of black durt just at the air ventury gas butterfly? Do not know the English term. Sorry.

What I do not understand is that this does not appear with other shifts.
Only from 4 to 3 at those rpms.

If I have to bring the car in germany for a tune up. it s gonna cost me 2000 euros.

I will post it in the tech forum.

Thank you all

 

If you have access to a timing light, check the initial timing with the EST wire disconnected.

6 degrees advance is the normal base value for L98s.

As a short term fix, you could simply retard the base timing a few degress until the knocking no longer occurs. If your currently at 6 degrees, try 4 degrees, etc.

My guess is that you have more intial timing than 6 degrees, but it also depends upon what has been programmed into your chip. Any idea what is in your chip? Do you have a copy of the bin file from whoever tuned it for high octane?

Retarding timing can be easily done, by rotating the distributor in the clockwise direction. To advance timing, rotate in the counter-clockwise direction.

Any mechanic should be able to do it easily for only a few euros, if you don't have a timing light, or don't know how to do it.

In 4th gear the load on the engine will be at its maximum. Knock tendency increases with load. At this rpm you will also be close to the peak torque, where cylinder pressures are also maximized.

Since the engine is also reving slowly in 4th gear, you will be exposed to these conditions for a longer time period, making it more likely to knock.

 

Dear All,

Thank you for your explanations,

I will apply your recommandations this week end.
However, I need to wire the EEC to a laptop in order to extract the code and work on it. So far, I only used the ODB2 system and it does not give me any infos related to this problem.

I know the chip is a protech that has been modified when the mods were applied. I have to contact a corvette pro in Germany or the only one in France to do the job.

I do not want to blow the engine for sure.

Thank you all,

 

I would retard/reduce the timing to 4 deg, drive it then adjust it a bit more. I have a new 383 rebuild with LPE cam and a Super Ram, reprogrammed chip etc. Moving the base timing just offsets the spark advance curve in the chip more positive or negative. Right now, I seem to be closer at 4 deg base versus the pinging I had at 6 deg base. My computer is an OBD1 (1991)

 

If its been fine and now a sudden problem, I'd check the simplist thing first-has the tv cable adjustment to the tranny slipped and is now loose???