M39 Vs. MK6 5 Speed
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Car: 1989 IROC-Z 100% Original
Engine: 5.0 TPI
Transmission: T-5
Axle/Gears: 3.08
M39 Vs. MK6 5 Speed
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??
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??
#2
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Re: M39 Vs. MK6 5 Speed
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.
Last edited by InfernalVortex; 03-25-2011 at 09:36 AM.
#3
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Re: M39 Vs. MK6 5 Speed
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 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??
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 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??
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.
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.
#4
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Re: M39 Vs. MK6 5 Speed
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.
#6
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Re: M39 Vs. MK6 5 Speed
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.
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.
#7
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Car: 1989 IROC-Z 100% Original
Engine: 5.0 TPI
Transmission: T-5
Axle/Gears: 3.08
Re: M39 Vs. MK6 5 Speed
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.
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.
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?
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#8
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Car: 1989 IROC-Z 100% Original
Engine: 5.0 TPI
Transmission: T-5
Axle/Gears: 3.08
Re: M39 Vs. MK6 5 Speed
Those looks like the 1LE PBR aluminum dual piston calipers and the larger rotors. Yeah thats what I want, where could I get them??
#9
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Car: 1989 IROC-Z 100% Original
Engine: 5.0 TPI
Transmission: T-5
Axle/Gears: 3.08
Re: M39 Vs. MK6 5 Speed
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.
#10
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Re: M39 Vs. MK6 5 Speed
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.
#11
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Re: M39 Vs. MK6 5 Speed
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
https://www.thirdgen.org/forums/tran...-10-bolts.html
JamesC
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