I understand that the 5 speed IROC's came with two different transmissions.

What is the real difference between the M39 and MK6 5 speed transmission?

I know the LG4 motors came with the M39, so did the L69 come with the M39 as well? I know the L69 motors came with the 3.73 disc rear-end.

I'm asking because I stumbled upon a 1989 LB9 5 speed for sale, I want to jump on it but it has the 3.08 drum rear-end. It upset me I hate drum brakes. I can't believe they didn't make all IROC's 4 wheel disc, they should have. And it has the 15'' Wheels not the 16'', but I can easily change.
I know the 3.08 drum came with the M39 trans; and the G92 Performance 3.45 and/or 3.42 disc came with the MK6 trans. So say I bought it and swapped out the rear-end to a 3.45 or 3.73 disc, would it pull any less being the M39 then the stock MK6 trans? Is the MK6 closer geared? As anyone driven both the M39 and MK6 5 speed and felt any difference?
I know the M39 has a higher 5th gear, so if you had both side by side, with the same rear-end, Floored in 5th gear the MK6 would pull harder... right? But by how much?? But the M39 would get better MPG... This car in particular has low miles. Would it be wise to buy it and swap out the rear-end, I would just leave the M39 in it, or should I wait till a Car comes along with a MK6 trans and the performance disc rear-end??

 

If it makes you feel any better, the 82-88 rear discs were a pretty awful system. Most people you'll find say they doubt they ever worked at all. The 89-92 discs were much, much better for the most part. Dont sweat the drums... Id just be annoyed at the gear ratio, I think that might still be a 2-series carrier. The 89-92 rear discs worked much better but were known to have bad proportioning valves, so it's still a crapshoot.

 

do the math.
2.95 * 3.73 = 11.0035 (easier launch than the 3.08 rear by far)
2.95 * 3.08 = 9.086 (sluggish / more clutch slippage required off the line)
.63 * 3.73 = 2.3499
.74 * 3.73 = 2.7602 (too much highway rpm IMO)
.63 * 3.08 = 1.9404 (not enough highway pull in 5th)
.74 * 3.08 = 2.2792

You could buy a car w/ 3.73 rear, .74 fifth gear and swap fifth gear.
Or you could find a rear for a car w/ 3.08 rear, .63 fifth gear.

Usually a rear is more costly than swapping 5th.

Someone on here (ebmiller's site?) sells disc swap stuff and I'd rather have that on a drum rear than the 82-88 stuff.

 

For the record, these are the rear disc brakes you want. They were available from 89 until 92 in this iteration, and slightly redesigned in 93 and used until 97. If you get an aftermarket rear for a 3rd gen they often come set up for these disc brakes.

 

In my years on the board, I've never heard or read that about 89+ combination valves. May I ask where the info comes from?

JamesC

 

I was actually doing some research to verify my info in this thread, and found that the prop valves go bad and fail to supply adequate pressure to the rears over time. The fix is to use a 1LE prop valve.

https://www.thirdgen.org/forums/brak...akes-weak.html

Note posts # 6, 13, 14, 22

Note the OP in that thread has an 89 and is actually mistaken and has the good calipers, not the awful iron ones since those werent around after 88.

 

Ok I think I understand.
The car I'm looking to buy has the M39, which has the .63 5th gear. I'm not touching the trans at all. I would take out the drum 3.08 rear-end, because I hate drum brakes; Now should I go with a stock disc rear-end -OR- Go with an aftermarket disc rear-end? Brands?? And what would be better gears with the M39? 3.45 or 3.73 gears?
.63 * 3.45 = 2.1735
.63 * 3.73 = 2.3499
I'm thinking 3.73 gears with M39 trans. What do you guys think?

 

Those looks like the 1LE PBR aluminum dual piston calipers and the larger rotors. Yeah thats what I want, where could I get them??

 

From my understanding Borg-Warner made the G92 High Performance 3.73 disc rear-end for the 5 speeds in 1984-1986. In 1987 -1989 it was the BW 3.45 disc rear-end . But in 1990 BW was dropped and the old 7.625 rear-ends were brought back with the 3.42 gearing until 1992, the last year of the 3rd gen.

 

They are PBR 1lE aluminum calipers, but they're only single piston. There were no dual piston rear calipers until 1998. You can get a 98+ 4th gen rear and have dual piston ones, though.

In 1989 GM made the PBR 1LE rear brakes the standard rear disc brakes. Cars that didnt have the 1LE package but DID have rear discs came with rear rotors bigger than their front rotors. Pretty funny if you ask me...

But you can get those off any rear disc brake car made from 89 until 97. The LT1 calipers look a little different but are extremely similar and I believe they interchange with no issues.

 

Check the following FAQ, toward the end, for info on masters, combination valves, parking brake cables, speedo gears, etc.

https://www.thirdgen.org/forums/tran...-10-bolts.html

JamesC

 

some 88 cars had the rear pbr calipers as well.. its like alot of things brought into production mid year