i own a 1990 iroc-z convertable tpi 305 car. Its an automatic. when i first got it i was somewhat impresed with its performence. It ran great, then all of a sudden it started running not so great. It idels fine,and runs, but is just missing the perfomance it had. I took it to the drag strip last week and was ashamed of the numbers it put up. It ran a 17.330 at 80mph. I think it should at least run a 16. I know my 2.73 gears aren't helping but thats pathetic.

So far ive tried adjusting the tps, i reset the tv cable, checked fuel pressure and its all seems to be good, replaced the o2 sensor, new fuel filter,and reset the computer, but nothing seems to make a diffrenece. I even had gm diagnose it and they said something was wrong but they didn't know what. i need to check the timing and put in new plugs, wires,cap,and rotor. But they all seem to be good, the car has 80,000 miles on it .

What else should i do?

 

If you need a tuneup then I would suggest doing that first. You'll be amazed at the difference if any or all of those parts are in need of replacement.

 

well it might be as simple as transmisson slipping??... do you take off with lots of back against the seat??.... what is you rpms at the start/launch....and though the traps??

 

at the start i held it at about 2500 to 3000 with the brake and then let go of the brake, in first gear then shifted manually into second at 5500, at the end of the run it would be just ready to shift into third at around 5500 maybe a little more i know this is not ideal you want to be in the upper part of the 1 to 1 gear when you reach the end of the track, but this is all due to the 2.73 gears, i ran 7 runs and a 17.330 was the best it would do.

 

This isn't the end all answer but you shouldn't be shifting at 5500 RPMs if you have the stock TPI. The peak HP comes at 4400 RPMs so 4800 RPMs is ideal for shifting, definitely not later than 5000 RPMs.

 

I also shift at about 5300 rpms, I know that the power band is at 4400, at about @5300 the car pulls very hard, and @4400 I can barly feel myself go to the back of my seat.
I would really give your car a great tune up, plugs, wires, cap, rotor it will make a difference. (hopefully!!)

 

Do you have the stock plenum and runners?
If so then you would be faster shifting at a lower RPM. Keep in mind your tach may not be accurate though so 5300 RPMs on your tach **may** be the best for you. Have you dynoed it or taken it to the track to compare?

 

running them times it doesn't sound as if it's putting you back in the seat at any point.

 

No I have never taken my car to a track, I really want to, and there are a few tracks around. And yes they are stock.

 

Zippy, what times are you talking about?, The other guys or mine, I didnt post any.

 

not your street. somewhat referring to the 5500 he's running. how did you get so much launch rpm????

 

his street. miller, how did you get so much launch rpm????

 

It should be the "simpler" things that should be checked first. You may just find that it needed a tune up ( you know that they are required periodically.....even if the parts still "look fine"). A best time of 17.3 is brutal ( sorry to say ).

 

I took my 88 Formula to the track and the car was shifting at about 5K with no pull at all. At 4500 I can feel the pull just stop.
With a TV adjustment, the shift points lowered to 4200 and it ran much better times, but I had to start in first myself becuase in D or OD it would shift to 2nd gear almost immediately.
Car shifting at 5K I ran a mid 16. Car shifting at 4200, I ran a 15.75 @87mph. I was launching at idle and cutting 2.1 60' times with the lousy tires I have. Brake stalling it to 2K resulted in me sitting on the line and spinning like I was trying to do a burn out.

 

Dude there is something terrribly wrong with that , shoot you should be running at least mid 15's. I had an 89 Iroc with a 305 TPI and a 3.42 , bone stock that ran 15.0. Try not manually shifting just to see how that works, oh and definetly dont rev it above 5000 , I ran my best run shifting at 4700.Good luck

 

Agree with those that have said your shifting too high. the few times I drag raced my mom's old '86 TPI 305 auto (3.08 gear) I found the best results by shifting at 4400.

And those results weren't too impressive: I think it ran like mid 16's. My '96 stock Chevy truck can match that...at 4500' elevation! lol!

Those things are slugs, but you should at least be in the high 16's IMO.

 

I bet your tach is wrong, like all the rest of them.

 

I got a new tach, and just hooked it up today, my other tach is not conneted, I think that my tach was off, I tried running it up to 5000, the car just bogged down, horribly, I have the new tach set up to the coil not directly to the old tach, I think that the other was stuck at a higher mark, which would expain why it said my car was idleing at like 1200, were now its at like 750-800.

 

i did the tuneup and it seemed to help some, but throttle response is still not that great, i need to check the timing still, but after that i don't know what to do, next time i go to the track, probably next week i will try shifting at a lower rpm and see what happens.

i did try letting the car shift for me, but it was even slower