I'm just wondering what kind of performance improvement to expect if I change from 2.77 to 3.42 gears in my 1990 LB9 iroc.

 

Should be good for about 4/10ths in the quarter, with good traction.
Seat of the pants effect will be even better.

 

2.77 are for a 9 bolt. 3.42 are for a 10 bolt. Are you swaping rear ends?

 

3:42 Gears will not work in 2:77 B-W 9 bolt rear. More trouble
then its worth to make changes on the 9 bolt.

 

I'm betting he meant to say 2.73, in which case the 3.42 will work just fine...

 

Why would you say that? The 9 bolt is about as simple as a differential gets and yet is stronger than most.

Parts use to be hard to find, but not anymore. I carry new gear sets (up to 4.10) and new posi carriers, install kits and more stuff is on the way soon.

Parts are more expensive than a 10 bolt, but overall unless you are making huge hp (ie 500-600+) a 9 bolt is a much cheaper option than a 9 inch or 12 bolt. I've been running my 9 bolt for over 3 years now behind my 500hp/520lbft big block (1.72-1.75 60 ft) with no problems. For example, I offer a new series 3 posi carrier for $445. 7.5" 10 bolt heavy duty carriers are approximately the same price, yet the 9 bolt is still much stronger. Remember, the 9 bolt was the high performance option for 3rd gens.

Lot's of 9 bolt information is on my page. I will be adding more during the next few weeks.

Miles

 

Miles, not bad mouthing the 9 bolt, I have one. the post sounded
like " I will put in some $200 3:42 gears from Summit". Not that easy.

 

I wasn't aware of the difference in rear ends. According to my RPO list I have 2.77 rear end gears. I did not know that I had the 9 bolt . I guess I'm out of luck.

 

With a 1990 305 TPI automatic you should have a 2.73 10-bolt rear end. Which RPO code do you have that tells you its a 9-bolt? GU2 is a 2.73 10-bolt and GH3 is a 2.77 9-bolt. Count the number of bolts on your differential cover to be sure. If you have a 10-bolt, the easiest way to upgrade is to buy the SLP take-off posi for $99, then get a set of 3.42 gears from a 4th gen car for $50 or so. If you really do have a 9-bolt, then get parts from Miles here.