hey, I have a 1989 iroc-z 350 TPI

Laster year I was running

14.6/14.7's consistent

with Stock 2.77 gears

MSD goodies

Aluminum drive shaft

Poly trans mount

SLP runner ported.

Magnaflow 3in cat-back

This year I added

3.70 gears
24lb injectors
adjustable FPR

Cars running 15.0's

WTF I am upset and about to sell this thing.

 

Did you burn a new chip for it? Adding injector and fuel pressure will only make the WOT PE super super rich unless you tell the computer about it. It's already really rich from the factory. Get a chip burned with the proper injector constants.
Going from an A/F of probably high 11.9's (from the factory) to now probably low 11's maybe 10's will slow any engine down.

Bigger injectors are not an "upgrade" if they aren't needed, and if the computer doesn't know about it.

 

3.70 is pretty high gears for TPI. I wouldnt go over 3.42 with TPI. Now if it was a stick thats a differant story.

 

yep you need a custom chip... those 24's are hurting you right now without the tables being changed in the ECM

 

You need to take more of a systems type approach for modifing a car. If the engine doesn't rev high enough to need 3.7s, then they'll never work right for you. You have to decide what you want to do with a car HP/TQ/RPM wise, and then work on achieving your goals. If you're running a 700R4, it has a terribly low (high numerically) first gear, and you don't need to carry alot of rear gear to get the car moving. I was happy with 3.42s with my mild 355 combo, FWIW.

If your going to get serious about running fast, then I'd suggest you start looking at scan tools, and chip burning.

 

Unless you have about 300 rwhp, a tpi doesn't need and will not respond to any rear gear over 3.42. Your gears are way too deep.

 

The gear is not what is making you slower though.

It is TOO much fuel from lack of tuning and the fact that you really need more cam for those SLP runners.

If this tells you anything most engines make max SAFE power with the A/F in the 12.6-12.8 range. As others stated factory w/ 22 lb/hr injectors PE is around 11.9:1 to keep the cat from overheating. With the 24 lb/hr injectors will give you 9% more fuel even with the stock FP setting. That puts you around 10.7:1 in PE. I am guessing you adjusted your fuel pressure to more than you had stock. Lets say you bumped the pressure from 43.5 to 50. You are now up to 25.73 lbs/hr that is 16.95% too rich. That means you are basically at 10:1 a/f ratio which is going to prevent complete combustion and rob you of power. Depending on where your fuel pressure is try reducing your fuel pressure by several psi and see if you get faster. I would try to shoot for around 37 psi of fuel pressure and see how that makes it run.

 

I Roc AZ, remember, when modifying your TPI, that the approach we took in tuning carbs, doesnt always work for our engines. Alot of gear, fuel, and timing, stall, are not going to give improvements, and as you found out, made your cars performance take a step in the wrong direction. You must make your mods to keep the engine in its "sweet spot", where it makes its max. power and torque. EFI engines that have a lower BSFC generally does not need alot of fuel to make power, especially when the eprom is not tuned for the fuel.

 

With those larger injectors, you'll have to use that AFPR you've got to dial it in and go from there.

Good Luck!

 

Try, a bit lower fuel pressure, but then it defeats the larger injector idea altogether, and the injector will deliver less atomized fuel. A tune through the eprom is what is needed.

 

I beg to differ..... it responds REALLLLY well . But I agree the 3,42 's are probably a better choice . I have the 3.73's now . I can idle along at about 40 km/h in second gear ...mash the throttle and the tires break loose a bit. But also I can hit about 100km/h ( 60mph) and I am at barley 2000rpm ( thank *** for overdrive) Also hp rateings don't dictate gear sizes needed .It's more to do with cams , etc

 

Thanks for the comments. I had woundered about the ECU needing ot be chipped.

It alos seems like my tranny does not respond as well weither with the new gears.