The Bus

My friend has a 87 C4 race car. Having trouble keeping from going lean from 4 to 5.5k RPMs. Car has wideband O2 sensors in each exhaust.

Engine : ZZ4 crate motor
Stock intake system except for 24 lb injectors.
Example. Yesterday 1st 20 minute outing at Nelson Ledges A/F ratio was 14 at 4 to 5K. Water temp 200, oil temp 220.
2nd outing A/F was 14 to 15 at 4 to 5K. water 200, oil 220.
3rd outing A/F went to 16,17 and developed a knock on backstretch at WOT. A/F went to 17. water 200, oil 220

Had a chip to increase air/fuel ratio. Helped obviously not solved problem

Does he need chip burnt for more fuel or what is wrong??????

NEED HELP. He knows nothing about engines and I'm just a parts changer.

You can E Mail me directly or just post suggestions. We need this solved before NASA Mid Ohio end of June.

THANKS

l98tpi

get it on a dyno and dyno tune. Burning chips in the office or home are a crap shoot.

crainholio

GM factory tuning is usually on the rich side in every Power Enrichment table I've seen including TPI engines.

But if your chip "increased the air:fuel ratio" then it leaned it out, if I'm reading correctly.

It's also possible that the 24# injectors are beyond their limit in terms of duty cycle at high revs. Also possible the fuel system isn't moving enough volume to keep the injectors supplied.

As has been stated, a dyno session is the best approach. Include a fuel pressure gauge to ensure the rail pressure isn't nose-diving at high revs. An old & tired Delphi turbine fuel pump will have a tough time keeping up with high demand in my experience, and you'll see the pressure fall off as the revs climb.

If the pump is weak, a Walbro GSS340 gerotor pump kit is a drop-in replacement that ensures no further problems. It also uses less amps at any delivery pressure I've used from 43 to 58psi, and the stainless steel gerotor isn't as prone to failure during starvation as the plastic turbines.

Also scan the injector pulsewidth and engine RPM so you can calculate duty cycle (some scanners can calculate this on the fly which is nice) and if you're above 85% it's time to go bigger on the injectors or go up on the delivery pressure. I liked Ford/SVO injectors when I ran TPI, but frankly with stock 350 displacement and running 45psi at the rail I've never needed anything bigger than their blue-top 24#. 383ci usually needed their red-top 30# units consistently.

edit to add: this was with street-legal cars running full exhaust and streetable cams...a full-out track setup will want more fuel and may call for 30# injectors. Put it on a dyno with wideband O2, scanning inj pulsewidth vs. RPM, and watching the fuel pressure gauge and you'll be able to diagnose whether it's:
a.) injectors
b.) pump
c.) tuning

blackozvet

after modding my engine it wouldnt rev past 4000, I could feel the knock sensor kicking in, first thought was a faulty knock sensor,
did some research on cf and the answer was fuel pump,
fitted a 255 lph pump, and straight away it would rev to 5500

mrc24x

Rudy,

I agree on the fuel pump being a likely cause. The 24 lb. injectors should be plenty big enough. (I run them and make 370 at the wheels without issue)