davege

I am looking at a stock C4 for SCCA autocrossing this year.

Which car in stock trim would be faster and handle better? I know the 1990 is rated with a little more HP and the 1989 had the 3.54 gear. It is also my understanding the 1990 weighs a little more.

So which would it be a 1989 Z-51 with 3.54 rear gear or a 1990 Z-51?

Thanks,
Dave :chevy

scorp508

89 has a 3.33 gear.

davege

scorp508,
The 89 Cars built in the first couple of months had the 3.54 gear. In SCCA Stock class rules, cars with Z-51 option are allowed to update the 3.33 gear to a 3.54 due to it's avialability.

Dave :chevy

SpedRacr93

Early 89s did have 3.54s.

I'm not at home, so I don't have my suspension spreadsheet in front of me, but 89 Z51 cars came with 30 mm bars up front, and 24 mm bars in the rear. I know the 91 Z07 came with a 26 mm rear, but I'm not for certain if the 90 Z51 came with it or not. Spring rates however, are the same.

As far as weight is concerned, I'm sure that it is a minimal difference. The only difference is the interior.

90 Corvettes had 10.25:1 compression and used speed density engine control management, where the 89 had 9.5:1, and used mass air flow control.

All of these differences are so minute, that it won't make a difference one way or another in my view. Differences between the cars on a track would probably come down to drivers.

dgoodhue

84-89 have a stiffer structure. (The interior bracing is better, because of where the front speakers are in the 90up)


[Modified by dgoodhue, 1:19 PM 2/1/2002]

Scoob

I have an '89 Z51 (late production thus the 3.33 ratio) and I autocross it and I love it. Can't say how it compares to a '90 as I've not driven one.

Kale

Hello, Neighbor! Roseville resident here.

pablocruise

Darn, mine has the 3.33's.

scorp508

Well how about that!!! :) I thought that was in later years that happened.

PeteL

I like the 3.33; 4th is good for a little over 120 which is just about right for the north course at Pocono and an L98. I hate to have to grab 5th as I can't heel toe worth a darn. :rolleyes:

Only issues I see; (without looking at the spring chart also) is airbag on 90 and no bag on 89. I don't know if the sport seats are the same, but the 89 sport seats kick butt. Also 90 has two fans on the engine side of the radiator and the different slant on the radiator, while 89 has a single large fan, the straight up and down radiator and the aux fan used as a pusher. If I had a car that might be checked I would want the 89 for the MAF. In 90 you might have to tweak programming a little more for some certain, um hidden changes like passing steel ***** through the runners or maybe a little challange cheater cam or something like that.

*89x2*

I think there are some 89 auto cross Vettes for sale on the C4 Cars for sale section. I would buy an 89 due to a better steering wheel (stock w/ out bag) and the 89's actually had a better cam than other L98's. Someone said the compression ratio was higher however, that didn't happen until the reverse cooling LT1's in 92. IMO the 89 is a better car for all around including racing but then again, I may be partial :D

scorp508

It is true. My '87 is a 9.5:1 ratio, and it was upped to 10.20:1 (I think) later on.

*89x2*

:confused: When? Where?I have never seen an L98 advertised w/ more than 9.5:1c/r :confused: . I will add that one of my L98's is 7.5 and another one is 12.25 :eek: with another being 9.5 :D
FWIW, the LT1's touted higher compression achieved through reverse cooling... I got it on my L98 by shaving the heads and inst thinner gaskets...then there are the pistons... :jester

scorp508

I honestly can't remember what year was the switchover.

PeteL

Black book shows 5hp improvement in 90 due to camshaft and compression ratio changes. Does not mention how much change in CR.

*89x2*

C4's did have less c/r when equipped w/ Iron Heads...

scorp508

Right, but that isn't the swap I meant. The iron heads had a 74cc combustion chamber right? The L86-91 head had 58cc chambers. As stated above I guess it was 1990 that it was done along with the other things like the slanted radiator.

vader86

switch was in mid-late 86, about around 18,000 on the production line, and all verts had AL heads. I believe the compression in 86 with iron heads was 8.5-9:1

*89x2*

Scorp. I know < i was just adding about the 85/e86 C4's iron heads... If the L86-91's all had the same cc'd chambers, then where did the compression increase come from :confused: I know they claimed 5 hp more but that was due "in theory" to the removal of the MAF sensor when it changed to speed density... One of the performance mags had a cam article and it showed the 89's as the stoudtest w/ 88 next and 90 to follow...If I recall, I am pretty sure on the 89 then 88 part .
...Wouldn't take a 90 unless free, then only to re-sell... :cheers:

davege

89x2,
It was my understanding the hp improvment on the 90 engine was due to the lighter hyperutectic (sp?) pistons and possibly lighter rods. I need to check my parts books on this. Some people think the removal of the MAF going to speed density also contributed.

When you look at the Cam specs of a 90 compared to a 89 the 90 had a closer lobe center, less lift and less duration on both the intake and exhaust.

Dave :chevy