89 Corvette Performance Issues
Thread Starter
Heel & Toe
Joined: Jul 2023
Posts: 18
Likes: 0
89 Corvette Performance Issues
I've had a 89 Auto for 2 years now and I'm learning more about the car everyday. Recently took car to mechanic so he could weld a header leak and replace my valve cover gaskets. While doing so he claimed it was "Built like a brick sh*t house." Basically whole top end is built. Aluminum heads, springs, lifters, etc. The car is also cammed with a 3.44/.75 rear end and a shift kit and possibly more transmission work. It's someone else's project, so I am finding new things done to it every few months.
My problem with the car is that it is pretty much just as slow as my stock 89 Iroc Z. It's a tiny bit quicker to 60, but nothing that is even hardly noticeable. I feel like with all these thousands of dollars that the last owners poured into it, it should be able to be atleast decently quicker than stock? I would hope for maybe 5.5-6 second 0-60 atleast but its closer to 7 for sure. I would hope id be in the 250-300whp ballpark but if anything its 200 or less.
The only weird quirks about the car is that it will go from a stop to 30 mph and upshift from first without any throttle. It has some (gear?) whining on the highway when pushing gas. The car has good brakes until you try to stop when in first it takes forever and wont actually stop till brakes are to the floor and the tires lock up.
Also at launch it seems to be pretty sluggish (compared to iroc). Iroc will burn the tires off from a launch even when you dont want to, corvette wont even come close, it kinda seems bogged. Corvette has 315's with decently new NT555G2s, iroc has stock 245s with old but low mile regular summer tires, so i understand that it grips better, but launch still seems lack luster.
I've posted before that I think it might be something to do with the torque converter and I was wondering if anyone has any thoughts on that? People seemed to disagree with that. I'm not sure if I should get it tuned or get a compression test or something along those lines. It's hard to find a performance shop around here and probably twice as hard to find one that will tune an older car like mine.
Sorry for rambling, any thoughts are appreciated.
My problem with the car is that it is pretty much just as slow as my stock 89 Iroc Z. It's a tiny bit quicker to 60, but nothing that is even hardly noticeable. I feel like with all these thousands of dollars that the last owners poured into it, it should be able to be atleast decently quicker than stock? I would hope for maybe 5.5-6 second 0-60 atleast but its closer to 7 for sure. I would hope id be in the 250-300whp ballpark but if anything its 200 or less.
The only weird quirks about the car is that it will go from a stop to 30 mph and upshift from first without any throttle. It has some (gear?) whining on the highway when pushing gas. The car has good brakes until you try to stop when in first it takes forever and wont actually stop till brakes are to the floor and the tires lock up.
Also at launch it seems to be pretty sluggish (compared to iroc). Iroc will burn the tires off from a launch even when you dont want to, corvette wont even come close, it kinda seems bogged. Corvette has 315's with decently new NT555G2s, iroc has stock 245s with old but low mile regular summer tires, so i understand that it grips better, but launch still seems lack luster.
I've posted before that I think it might be something to do with the torque converter and I was wondering if anyone has any thoughts on that? People seemed to disagree with that. I'm not sure if I should get it tuned or get a compression test or something along those lines. It's hard to find a performance shop around here and probably twice as hard to find one that will tune an older car like mine.
Sorry for rambling, any thoughts are appreciated.
Burning Brakes
Joined: Aug 2008
Posts: 845
Likes: 220
From: Tampa, FL
Your 89 IROC has what... a 305 TPI in it I think? That's probably pretty decent, like a 0-60 in low 7s, and a 15 second quarter mile stock?
From what I understand, even the 84 Corvette is supposed to achieve 0-60 in ~7 seconds (or less) with the 4+3 manual. But if it's built like you say it should be a little bit quicker. You have .7 liters more engine, and I think the Corvette weighs about the same. Are you sure the rear end is 3.44:1? I was trying to figure out what the gear ratio in my Corvette was, and it seems all the 84s got a 3.07:1 for the automatic, but later year Corvettes got something really lame like a 2.59:1 or something painful.
That's always the problem when you buy someone else's project, you don't really know what's done if it's not documented well. Often times people will buy a bunch of parts and the combination never works how they think it will. I'd be willing to bet that tuning is probably not great... but I can only guess. Is it a stock ECM?
Where are you located?
From what I understand, even the 84 Corvette is supposed to achieve 0-60 in ~7 seconds (or less) with the 4+3 manual. But if it's built like you say it should be a little bit quicker. You have .7 liters more engine, and I think the Corvette weighs about the same. Are you sure the rear end is 3.44:1? I was trying to figure out what the gear ratio in my Corvette was, and it seems all the 84s got a 3.07:1 for the automatic, but later year Corvettes got something really lame like a 2.59:1 or something painful.
That's always the problem when you buy someone else's project, you don't really know what's done if it's not documented well. Often times people will buy a bunch of parts and the combination never works how they think it will. I'd be willing to bet that tuning is probably not great... but I can only guess. Is it a stock ECM?
Where are you located?
Thread Starter
Heel & Toe
Joined: Jul 2023
Posts: 18
Likes: 0
iroc is a 350 l98, same engine and trans as vette. Vettes rear end isnt stock, i did the tape test on the drive shaft and just eyeballing ti seems to be 3.75 or 3.44. Definitely not the stock 2.59.
Im in rural mid nebraska area
Im in rural mid nebraska area
Last edited by hazeC4; Aug 5, 2025 at 08:13 PM.
Thread Starter
Heel & Toe
Joined: Jul 2023
Posts: 18
Likes: 0
Burning Brakes
Joined: Aug 2008
Posts: 845
Likes: 220
From: Tampa, FL
My guess is tuning, like others have said. You do those kinds of mods with a stock tune, and it just won't take advantage of it... or there's a bottleneck somewhere that's not being addressed which is resulting in those changes doing absolutely nothing.
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Id check spark plugs, plug wires, cap rotor, fuel pressure, and timing before going into tune.
