The Bus

I'm trying to get some ideas to help a friend who has a C4 track car. Problem is engine knock on full throttle after a couple of hard laps. Does he need higher octane [ presently using 93 pump gas ], less timing ????? Air/fuel ratio is 17:1 at 5000 RPM.

Engine specs.
GM ZZ4 crate motor

rayk

AF at 17:1 is pretty lean isn't it? That might be the answer.

The Bus

I just talked to him and got more/better details. It only happens when the water temp goes up to 220 degs F. If the water temp remains below 200 F it runs fine. I know keep it below 200. I run my C4 at 220 F and oil at 240 F and no problems.

Zip Corvettes

Red Flag.

How are you determining it is 17:1 AFR. Second what ECU is running this engine. Who tuned it. STOP RUNNING THE CAR until you figure this out or he will be putting a new engine in it. WOT the car should be running 12:50 or AFR maybe even a bit richer on the track because of the heat. Then engine is probably getting hot because of the lean AFR. Really important to know what ECM is in the car for me to help.
Thanks

MarkBychowski

If the AFR is really 17:1, that is definitely a problem -- WAY too lean. Should be closer to 12.5-13:1 at WOT

Is he using a wide band O2 sensor to get that reading?

Those temps shouldn't be an issue and I wouldn't worry about timing until he gets the AFR right.

The Bus

It is a stock 87 computer with a reburned [ supposely for the ZZ4 crate motor and stock injectors ] chip. It is running the stock left side O2 sensor. There is A/F sensors in each bank. Runs 12.5 to 14 ratio up to near 5000. From the comments it sounds like the fuel system isn't supplying enough fuel. If so should he have the chip reburned or increase injector/pump size?? Any other suggestions will be appreciated.