I have a 1988 iroc z28. And was wondering how would I tell if my car came with the G92 performance option? The closes code I have is G80. Thanks.

 

G80 is posi. The SPID would say G92, plus the associated rear gear and oil cooler codes.

 

In 1988 you should have RPO (G92) for the performance gears along with RPO (KC4) the external oil cooler which came with the G92 gears.

 

I have G80 And KC4 but know G92 code. Also J65 But don't have N10. Guess that means I don't have the performance gears? Thanks for the help guys.

 

The N10 became available in 1989. KC4 would indicate a G92. Post a clear pic of your SPID and we'll look it over.

 

Here they are Thanks.

 

No G92.

If you had G92 it would be in the list right after G80.

Your gear ratio code is GH3. Not sure what # that means. My first guess would be 2.73.

 

Thanks. I believe it's 2:77 Gears? Not sure if there are go for take off or top end lol.

 

Yes 2.77

 

That would make sense: almost all of the 9-bolts (7.75" ring gear) had posi (G80), but quite a few came with terrible gears. 2.77 is the 9-bolt equivalent to the 10-bolt's 2.73 (7.625" ring). It's a grocery cart / highway / gas mileage gear, TOTALLY anti-performance. (unless you're into Bonneville type stuff) Definitely NOT a G92 ratio. Which would have been, 3.27 for an auto car, or 3.45 for a stick.

 

I bought the car from the org owner. He had passed and left it to his son. The car had 5,500 miles on it when I got it. The son told me that he had just rebuilt the top half of the engine and trans also added a shift kit. I did not get into the details of why he rebuilt a 5,500 mile car. I was told his father order the car new. And they had built a garage just for the car before he ordered it. I was told the car set for years in the garage. I guess I should have asked the son why he rebuilt a car that only had 5,500 miles on it. Maybe he also changed the gears from 2:77 to something else. Guess the only way to know is to check. Thank you all for your time and help.

 

Sounds like a nice car! Lets see some pics.

 

It's not impossible that the gears have been changed, but also not likely...

2.77 is a 2 series gear. (duh) In that model of rear, the break between 2 series and 3 series, is between 3.08 and 3.27. The difference is the location of the ring gear flange on the carrier: since "ratio" is NOT ONLY the ratio between the numbers of teeth on the gears BUT ALSO the ratio of their diameters, then as the ratio increases, the pinion gets SMALLER since the ring dia is always the same. This means that either the ring has to get thicker, or, it has to be moved closer to the pinion. But making a thicker ring becomes quite difficult (expensive) when it gets too thick, because it will tend to warp (become scrap) during heat-treating. OTOH making too many variations of carrier also drives up expense. Therefore in any given model of rear, the factory will vary the ring thickness over some limited range of ratios, then at some point will step to a different location of the ring flange on the carrier and use that for another range of ratios, then perhaps step it again.

Photos of the same thing, in a 7½" 10-bolt:



Aftermarket gear mfrs make "special" thick gears, at a significantly higher cost than "normal" ones, for some rears, particularly those with high demand. In the 9-bolt there are only the 2 & 3 series though, there's no 4 series such as there is in the 12-bolt, Dana 60, Frod 9", etc. that are used in trucks. 3 series carriers are HARD to come by for the 9-bolt, except out of wrecked cars, and that makes them VERY uncommon. AFAIK there are no "special" thick gears for 9-bolt. Therefore the end of the ratio road for your rear is pretty much, 3.08; and that's so little different from 2.77 that it's not likely that someone would go to the trouble and expense of changing it.

The most likely way it would have got changed is not by "replacing the gears", but rather, by changing the whole rear. Which again is not impossible, but since there never were many 3 series 9-bolts in the world to begin with (ones that will fit these cars came only in certain 85-89 Firebird and 87-89 Camaro models), it's not likely either.

Only way to know is to pop the cover off and look. Which if you haven't changed the fluid lately, you need to do anyway. So go outside and do that, and feel free to let us know what you find.