Hi all, Been thinking about swapping out the 3.90 lsd for a 3.09 lsd unit on my 2020 M6. Have heard all kinds of things like it will fry the clutch to it will grenade the T6060 trans but have not been able to find any one that has done it. I had a 1968 GTO M21 4 spd with a 3.08 LSD, 1968 Firebird coupe 400 CID with a TH400 AT and a 2.97 LSD, 1976 Pontiac T/A 455 SD M22 4 spd and a 3.08 LSD, and 1979 T/A Pontiac 400 6.5L Borg Warner T-10 4 spd and a 3.23 that i changed to 3.73 LSD and never had any problems with the drive lines, just they were not jumpy off the line. The 392 has plenty of torque for the 3.09 and should boost the MPG when in 6th cruising. Just looking for the real world cons if any to do the swap and not much info out there for going backwards but seems that the AT's are getting big jumps with the 3.90 LSD units. I know M6 are a small group but i love the M6's. Thanks all
1st question is WHY? I guess it would make 1st gear more usable, but what TOP SPEED are you looking for? You would never use 6th below 145mph. I was actually thinking of 4:56's so I could start off the line in 2nd. Why would changing the gears in the rear FRY the clutch? You obvious know how to drive a manual. Was this just INTERNET dweebs telling you this? When I ordered my V10 3/4 ton, I ordered it with 3.54 gears instead of the stock 4.10, then put 36" Super Swappers on it. I NEVER USE 5th gear below 70 mph now. Just curious, not slamming you.
The swap you're considering will dramatically negatively impact the car's launch and beyond performance. The car's current diff ratio is thoughtfully married to it's manual trans gearing.
A drastic change like that, you probably won’t be able to use 6th for normal driving and will be constantly down shifting for the slightest incline.
I have a 6M car, and I had the Getrag with 3.91 gears and swapped to a 9 inch with 3.70 gears. Even this small change was very noticeable. I have a 5.7 with a bigger cam and in my case the 3.70 gears are as low numerically as I can go. With a 392 that has a stock cam, I bet you could get away with a gear in the 3.50s. But I would say that's about it IMHO.
I miss the old MOPAR 8 3/4" rearends. Could drive during the week with 3.23 gears, and swap in 4.56's for the track. and 2.73's for LONG Trips. Had it down to well under an hr to swap Pumpkins & Tires
This is the best time of year to swap out pumpkins! LOL Sorry, I couldn't resist, you set yourself up.
