Hey guys,

So I ran the car today, I only got 2 good passes as 2 consecutive cars oiled the entire track and I wound up leaving.

My car is a close to stock 1989 Iroc Z with:

350TPI
9 bolt with 2.73 gears
3600 lbs w/ driver
Magnaflow 3" muffler
No cat
K&N filters
Radial Tires on stock 16" wheels



My first pass was as follows:

60' 2.275
ET: 14.93
MPH: 93.19

Second pass:

60' 2.201
ET: 14.87
MPH: 92.91

I really didn't have much trouble with traction rolling into the throttle. I am wanting to get this thing down to the high 13's with bolt-ons. What should my next mod(s) be? I am pretty much undecided between 3.42's or headers...

Gears. Those 2.73 gears are really hurting performance. Along with the gears, get some sticky tires.

Everything I read says that 3.42's are great gears for a TPI car. I just want the best performance, are 3.70's too steep? I can find plenty of new 3.70 gears on ebay for my car but nothing else.

Thanks!

I had a hopped up 383 through a TH350 and 3.27 gears and ran very high 11's. The gearing was wrong but I still ran good times with only 3.27 gears and 26" tall slicks.

Changed to a 10 bolt, 4.10 gears and 28" tall tires and got into the low to mid 11's.

3.70 may be a little too much with a 700R4. The starting ratio would be very low. On the highway, even with the overdrive, you will notice a loss of fuel mileage but stoplight to stoplight driving will be fun. The TPI doesn't like high rpms so gearing for higher rpms won't gain much performance.

Can't go wrong with gears or headers. Whatever you can get a good deal on first is what I would do. I know headers are possibly the single best mod you can do to the engine for instant 1/4 mile gains.

Gears first. With 3.73's you'll be around 2200 rpm @70 with a 26" tire. Thats not too bad. Then do headers/exhaust, 160 thermostat, pullies, 1.6 rockers, get a tune. In that order.

That's the kind of answer I was looking for . So you think 3.70's won't be too steep for a TPI motor? I don't care about RPMs on the freeway as much as 1/4 mile performance.

I'd do drag radials and a 2800 stall converter...then headers and a mild tune. Try giving it 2-3 deg more base timing or get the software/chip burner and really do some good work.

You'll gain MUCH more with a converter than any of the other mods at this time. 2.73's arent great but with converter you'll have 13's capable car. I went 13.89 at 97 with 2800 stall and 2.77 gears on street tires. Eventually added 3.42's and slicks and went 13.63 so not much more gained.

Your 60's should come down from 2.2's to 1.8's without much effort and likely hit high 1.7's

3.70's should be perfect for it then. 100mph in 3rd gear is right at 5000 rpm. Your 93 mph will put you at 4500-4700 crossing the stripe. Right in the peak of the power band.

Dont forget with headers/tune, you may start to approach 96-98mph plus! I know my car with full exhaust was in the 96-97 range but changed to 3.27 gears and it went 99-100mph in good air. Back to 3.42's and slicks it went back down to 97mph...not sure why

3.42's are best IMO but 3.70's/3.73's can work ok. They work better with siamesed runners or aftermarket intakes

Thanks for all the replies guys, sounds like my next mod will be a torque converter! Any recommendations on a budget converter? Anything I should do since my car has 90k miles on it as far as the transmission goes if I am upgrading the converter?

I've learned that budget converters are no good. Get a real good one and never look back. Vigilante and Edge come to mind.

I hear you, I had a Hughes in my TH350 and loved it. I have reservations about putting a $500-$1000 converter in a transmission with 90k miles on it though... Thoughts?

pm Fast 383 on this site. He sells torque converters and gives good prices the members.

I may have just scored a 2600 lockup stall locally, if all goes well I am picking it up tomorrow .

Do you have a 9 bolt or do you have 2.73s, because those don't go together. Definitely get better gears, 3.70 would be 9 bolt and 3.73 would be 10 bolt.

Also don't forget that you likely need a different carrier for higher gears since your stock set is the highway gears.

Yeah the carrier is really what is holding me back, 9bolt.com sells them for like $400, so it is something I will have to score used from someone on here or ebay. There are zero on ebay right now too. Oh well looks like I am going converter first now.

If it was a 2.73 10 bolt you can get 3.73's designed for 2 series carrier and run those. Thats what I did on my 99 TA.

But being an 89 iroc its likely a 2.77 9 bolt. Thats what I had. Gears are harder to find for those and carriers even harder. I thought the company in australia was recently re-started that imported 9 bolt stuff since it was still used over there. not sure tho

Here are my timeslips from Sunday: