camaro1185

So in the glove compartment, the RPO code says that I have the G80/G92 rear end, but when changing the rear end oil, the metal tag says 2.77. Which one should I trust?
On the highway, at 70 im at 2750RPM.
It def. feels like its more lively than a 2.77 rear.

AlkyIROC

G80 means you have a posi
G92 means you have a performance diff (9 bolt)

Neither say what gear ratio is in the diff. There's another RPO code for that. Since the tag on the diff says 2.77, there's a very good chance that's what's in the diff. There's no indication that the diff is still original. The 87/88 Iroc-Z L98 came with 3.27 gears.

If in doubt, count the teeth on the ring and pinion.

camaro1185

I have the L98, therefore then i do have the 3.27 gears. Plus i bought it from the 2nd owner and have been in contact with the first 2 owners. Nothing has been modified.

AlkyIROC

Well somehow a 2.77 tag got onto the diff. I wouldn't trust anything or anyone without counting the teeth to be sure.

Are you even sure it's an L98 and not a 305? Although tags and decals can easily be changed with enough effort, it's impossible to fake a casting number on the block. Well not totally impossible but if someone is willing to go through all that trouble, there must be a reason.

Since you already have a diff tag that contradicts what should be in the car, I'd start checking other things. Check the casting number on the engine. Check the suffix code on the engine. Throughout the car are VIN decals if you know where to find them. They're in places that can't be easily replaced like a center console could be. There should be one in the spare tire well attached to the outer fender. There's one under the deck lid but you have to remove the paneling to see it. They should all match the VIN on the cowl.

camaro1185

Where is the casting found on the engine?

Yes all the body panels have the same vin, I checked those before i bought the car.

1989IROC-Z5.7L

iTrader: (2)

RPO sticker should show the L98 code if it is a 350tpi. Also the VIN # XXXXXXX8XXXXXXXXX

It should also show what gear ratio it has.

http://www.chevy-camaro.com/chevy-ca...ar.asp?year=99

AlkyIROC

But the center console with the RPO sticker can easily be changed from another car. Even the cowl tag can be changed if you know what you're doing and have the proper rivets to make it look original. If someone did change everything to make it look like an L98 car, they would have had to do a lot of work.

Rechecking the TGO database, the L98 cars didn't come with 2.77 gears if they have the G92 RPO code so something is wrong. The G92 means the performance ratio of 3.27.

camaro1185

See this is why im so baffled, everything checks out that its a 350, g92 car. RPO's, vin, POSI, rear disk brakes, engine oil cooler, and the RPM while on the highway. If the car was a clone, then this is one hell of a clone.

I realize the car can have another console, but from what i have been told, the car was just driven for pleasure, and was NEVER modified. Other than the tires, brakes etc. in fact the only thing not orig. on this car is an aftermarket aluminum 3 core radiator.

Everything points toward a g92 car, but the tag on the diff says different.

Eitherway i dont care, i love this car! I just thought it would be cool to have a g92 car.

tony_cogliandro

iTrader: (3)

i have a stock rear end in my rs 90 and at 65 im running less than 2500, about 2250 if i remember correctly. you are running pretty high at 70 for a stock rear end ratio.

going to leave for work now but i will jump on the freeway and see what my stock 2.73 says. or is it 2.72. what ever.

tony_cogliandro

iTrader: (3)

well i was wrong. at 65 i am between 1600 and 1700 in a stock 305 rs with a stock rear end. i know its still stock because i bought the car new in 90 and i have not put the speed parts on it yet.
sounds like you have a 327 or 323 gear.

gregsz-28

iTrader: (1)

Do you not have an overdrive transmission? I think that is right where your RPM would be at 70 mph in drive, with 3.27 gears.
When I am in overdrive, with 3.42 gears, at 70 mph, I am at just over 2200 RPM.

camaro1185

Nope i got the 700R4 and put the selector in OD. Maybe od doesnt work, but im sure it does. Ill count how many gears it shifts and make sure its 4.

1989IROC-Z5.7L

iTrader: (2)

Quote:

Originally Posted by camaro1185

Everything points toward a g92 car, but the tag on the diff says different. Eitherway i dont care, i love this car! I just thought it would be cool to have a g92 car.

What gear ratio does the RPO sticker show it to have?

GH0 3.54 RATIO
GH2 2.29 RATIO
GH3 2.77 RATIO
GH4 2.92 RATIO
GK9 4.63 RATIO
GL3 6.17 RATIO
GM1 2.59 RATIO
GM3 3.45 RATIO
GM8 2.56 RATIO
GS4 3.70 RATIO
GT4 3.73 RATIO
GT5 4.10 RATIO
GT7 3.33 RATIO
GT8 4.10 RATIO
GU1 2.41 RATIO
GU2 2.73 RATIO
GU4 3.08 RATIO
GU5 3.23 RATIO
GU6 3.42 RATIO
GV2 5.83 RATIO
GV4 3.36 RATIO
GV8 2.72 RATIO
GW4 3.31 RATIO
GW6 3.27 RATIO
GW9 2.93 RATIO

And yes, I know these are not just camaro codes and gear ratios.

gregsz-28

iTrader: (1)

If your transmission is shifting into OD, you must have a rear end ratio close to 4.50, or your tachometer is WAY off.

tony_cogliandro

iTrader: (3)

i have to agree with you. i was going to put in a 323 or is it 322 - heck i cant remember now when i put in a new posi unit and i was told it would only be going at 2k for 65, or there abouts. he is way off point. it does sound like he is not shifting into OD.

you know if i stayed in 3rd and not od that is about where i would be at 65/70. maybe he only has a 271 or 2 or whatever.

memory sucks today.

redirocz28

I have an 86 iroc that had the 3.42 gear with the 5 speed T5 and at 70 was running around 2200 rpm. I recently installed 4.10 gears and at 70 mph Im running 2700 rpm so you either dont have overdrive or you dont have the original gears.

InfernalVortex

iTrader: (4)

Guys it's the tach taht's way, way off.

I dont know about those older tachs, but I know the 90-92 tachs are notorious for being optimistic.

I put an autometer tach in mine and wire the stock wire to it just to see how far off it was, and at 2000 real RPMs the stock tach says 2700. I bet it's the tach that's wrong.

Pull the back cover and count the teeth man. That's the only way to know. If you dont feel like doing that, then don't worry about it. I think I remember my 10-bolt gears having the gear count marked on the head of hte pinion gear. So if you can see that, it may answer your question.

camaro1185

Thanks. Maybe it is the tach. I posted in another thread that my tach showed the car shifting a little past the red line. maybe im off by a couple hundred rpm's.

1989IROC-Z5.7L

iTrader: (2)

Still curious what the rpo sticker shows for the gear ratio.

camaro1185

Ill figure it out and let you know. I had no time yesterday to look.

tony_cogliandro

iTrader: (3)

i have to hit the freeway today so i will drive in 3rd at 70 and see what i am at. if its 3k then your problem is not the tac its the trans. will report back later.

camaro1185

Thanks man, I appreciate that. Somethings def. off, I cant wait to figure out what.

tony_cogliandro

iTrader: (3)

ok here is what i did. did not put it into OD and drove at 70 - rpm was 2500 in 3rd.
if your tac is right - you probably have the higher ratio as you thought - that is the good news. the bad news is you probably have no overdrive.

InfernalVortex

iTrader: (4)

It's the tach. And the error in RPMs is going to get worse and worse as the RPMs rise. It maybe a hundred off at idle, a couple hundred off at 1200, and a thousand off at 3000 RPMs, and 1500 off at 5000 RPMs. The disparity gets larger as the RPMs rise.

I'd bet money it's the tach. Pay no attention to it.

If you have a nice timing light with a tach on it, you should compare the readings. The discrepancy you're seeing is pretty typical.

https://www.thirdgen.org/forums/post...33-post11.html

tony_cogliandro

iTrader: (3)

well you go the path of lest resistance. put a tac that you can read in the car and do a run to see what is happening. got to agree with you - need to run tac to tac.
but he should also see if its actually shifting into OD, which is simple too.
im for simple and easy and your suggestion fits that too.
good call.

daferris

iTrader: (8)

On '88's the std with L98 and the mandtory 700R4 trans option was 2:77 The G92 should have got the 3:27 ratio if you take the back cover off the rear it will be stamped on the O.D. of the ring gear what ratio it is. the rear cover uses silicone sealant for a gasket so you do not need to buy a new gasket just need a tube of silicone.
I have not had my IROC out this year do not remember what it cruised at for RPM's but think it was about 1900-2100 @70 (L98/700R4 2:77 245/50-16's)
Pretty sure '87 would have been the same as the L98 was a mid year option (think something like 1K built)

tony_cogliandro

iTrader: (3)

why not just turn the tire and see how many times the drive shaft turns to move the tire one revolution. a lot cleaner.

subroc

iTrader: (2)

I was just going to suggest that. Im surprised no one has said to do that yet. Its the easy way of not getting covered in gear oil and you still get to find out the gear ratio.

camaro1185

Thats what i plan on doing. I have no doubt its a correct G92 car. i got to replace the tran tail shaft seal anyway, so while im under there ill spin. Ill keep you guys up to date.

tony_cogliandro

iTrader: (3)

dont forget to post the answer - im really interrested in knowing too.

mcwille

i have a BW 9 bolt that was in my 87 trans am L98 i ran out of money last year and parted it out. i know my RPO sheet matched up from pulling parts and selling them i had and L98 G92 and my diff tag says 2.77 as well i just started pulling it apart to powder coat it before i sell it so i will count the teeth on it but i am sure that L98 G92 can have 2.77

tony_cogliandro

iTrader: (3)

if its 2.77 he has lost his OD or the tac.

camaro1185

Sorry it took so long. My RPO says its a GU5....according to your list thats 3.23 ratio, but its suppose to be 3.27.

tony_cogliandro

iTrader: (3)

ok but what about the tac or your overdrive?

camaro1185

Ya, its been rainin' like crazy here, and the car doesnt go out in the rain. Im going to try and find a tach to test out if mine is shot.