M6 to M12 conversion -worth the trouble?
Thread Starter
Heel & Toe
Joined: Apr 2012
Posts: 23
Likes: 0
From: Smyrna Tennessee
M6 to M12 conversion -worth the trouble?
I have a 1999 coupe with 170,000 miles and the transission is starting to grind on 2-3 and 3-4 shifts, also hard to get in reverse and 6th. My thought is that a rebuild is in order at least. However, would I benefit to changing to the M12 from a Z06? I do occasional drag racing and autocross, but 95% of the time it is street driven. It has a cam, headers, exhaust, CAI and a 75 shot of nitrous. I'm guessing around 400 rwhp NA. How much improvement could I expect out of the lower Z06 gearing? I have someone who will do a complete transmission and clutch change for $850 if I provide the parts. A salvage Z06 tranny is about $1300. Would I be smarter to have the M6 rebuilt and maybe change the rear gearing? I like going quick, but also love the 30 mpg I get on the highway. I don't have unlimited funds, so asking for advise on how to get the most bang for the buck. Also, any problem with the conversion, like 10 mm to 12 mm torque tube bolts? Thanks guys for your advice.
Drifting
Joined: Mar 2011
Posts: 1,412
Likes: 15
From: Richardson TX
You can buy the gears individually.
I'm not sure if you can "mix and match", but I would think you'd be able to.
I personally would like to have Z06 1st, 2nd, and 3rd gears, and keep MN6 4th, 5th, and 6th gears (4th is 1.0 on both trannys anyway).
If anyone knows if someone has done this, that would be very interesting information.
I'm not sure if you can "mix and match", but I would think you'd be able to.
I personally would like to have Z06 1st, 2nd, and 3rd gears, and keep MN6 4th, 5th, and 6th gears (4th is 1.0 on both trannys anyway).
If anyone knows if someone has done this, that would be very interesting information.
Thread Starter
Heel & Toe
Joined: Apr 2012
Posts: 23
Likes: 0
From: Smyrna Tennessee
I've done a quick look, and it seems the gears are over $200 each, and my transmission guy says it would be less expensive to buy a used tranny already set up and then I could resell mine for parts. If I can get a low milage tranny, then I shouldn't need a rebuild, just change the fluid and install. I'm hoping to do the tranny swap, clutch package and master and slave cylinders plus any small misc. pieces for around $3000 installed.
Thread Starter
Heel & Toe
Joined: Apr 2012
Posts: 23
Likes: 0
From: Smyrna Tennessee
I've also seen a few posts about changing the rear gearing to 3.90 from the stock 3.42 and all the posts seem positive. I wonder if this may be a more cost effective performance boost than a M12. Since I do quite a bit of travelling in it, how bad does the 3.9 hurt gas milage on the highway? At what RPM would that be at 75 mph in 6th?
Pro
Joined: Jun 2009
Posts: 552
Likes: 4
From: Mukilteo WA
I have 400rwhp and put in a z06 trans thinking it would be the better way to go than an m6 since I road course the car.Even with 305,s first is useless. I wouldn't worry about it and get the best local trans you can find of either model.
Thread Starter
Heel & Toe
Joined: Apr 2012
Posts: 23
Likes: 0
From: Smyrna Tennessee
A very good point. However, do you find the wider gear separation on the M12 helpful on the road coarse, or do you think the closer ratios of the M6 better, or no real difference? I'm thinking the lower first gear might be good with drag radials on the strip, but how much should I expect to gain, ET wise?
Safety Car
Joined: Jul 2010
Posts: 3,999
Likes: 386
From: Chandler AZ
From what I read, the non Z06 6 speed is great if matched with 4.10 gears, you could send both your trans and rear to a guy here to have them rebuilt and built up, and 4.10 gears.
I have never used him, but from what I read here, he is the guy I will use when I have the $$$ RKT56.com I belive is the website, you are closer to him than I am, you might even be able to drive to him to have all the work done, if your car would make it.
I have never used him, but from what I read here, he is the guy I will use when I have the $$$ RKT56.com I belive is the website, you are closer to him than I am, you might even be able to drive to him to have all the work done, if your car would make it.
Race Director
Joined: Dec 2006
Posts: 12,389
Likes: 2,910
From: Fort Worth, Texas
Having run both trans as well as 3.15, 3.42 and 4.10 gears...the M6 and 4.10 is my favorite all around combo, because it has great gearing as well as shift extension. The M12 and 3.42 would be my 2nd choice, you should gain ~.2 from going to the M12 trans all else being equal in my experience. Keep in mind a '99 has the weaker single rib case, hope that helps. You can play around with this calculator to see differences... http://www.f-body.org/gears/
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Safety Car
Joined: Jul 2010
Posts: 3,999
Likes: 386
From: Chandler AZ
the M6 and 4.10 is my favorite all around combo, because it has great gearing as well as shift extension.http://www.f-body.org/gears/
Pro
Joined: Jun 2009
Posts: 552
Likes: 4
From: Mukilteo WA
Unfortunately I just got the car in track ready shape in November and now I'm laid up with an injury. Tracked a 91 Escort last year while vette down.
I've driven both trans on the street but with various motors in front of them which made a bigger difference to me.
I've driven both trans on the street but with various motors in front of them which made a bigger difference to me.
Thread Starter
Heel & Toe
Joined: Apr 2012
Posts: 23
Likes: 0
From: Smyrna Tennessee
Thanks everyone for the information! The gear calculator is a real help. Seems a 3.90 rear gearing would give a nice improvement in ET as well as retain good highway manners with a M6 or change to a M12 for about the same effect. I should gain about 2 tenths with the change to an M12 is exactly what I wanted to know. Thanks again everyone!
Supporting Vendor
Joined: Apr 2011
Posts: 3,332
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From: Frederick Maryland
St. Jude Donor '08-'09-'10-'11-'12, '14-'15-'16-'17-'18
I have a 1999 coupe with 170,000 miles and the transission is starting to grind on 2-3 and 3-4 shifts, also hard to get in reverse and 6th. My thought is that a rebuild is in order at least. However, would I benefit to changing to the M12 from a Z06? I do occasional drag racing and autocross, but 95% of the time it is street driven. It has a cam, headers, exhaust, CAI and a 75 shot of nitrous. I'm guessing around 400 rwhp NA. How much improvement could I expect out of the lower Z06 gearing? I have someone who will do a complete transmission and clutch change for $850 if I provide the parts. A salvage Z06 tranny is about $1300. Would I be smarter to have the M6 rebuilt and maybe change the rear gearing? I like going quick, but also love the 30 mpg I get on the highway. I don't have unlimited funds, so asking for advise on how to get the most bang for the buck. Also, any problem with the conversion, like 10 mm to 12 mm torque tube bolts? Thanks guys for your advice.
You can buy the gears individually.
I'm not sure if you can "mix and match", but I would think you'd be able to.
I personally would like to have Z06 1st, 2nd, and 3rd gears, and keep MN6 4th, 5th, and 6th gears (4th is 1.0 on both trannys anyway).
If anyone knows if someone has done this, that would be very interesting information.
I'm not sure if you can "mix and match", but I would think you'd be able to.
I personally would like to have Z06 1st, 2nd, and 3rd gears, and keep MN6 4th, 5th, and 6th gears (4th is 1.0 on both trannys anyway).
If anyone knows if someone has done this, that would be very interesting information.
From what I read, the non Z06 6 speed is great if matched with 4.10 gears, you could send both your trans and rear to a guy here to have them rebuilt and built up, and 4.10 gears.
I have never used him, but from what I read here, he is the guy I will use when I have the $$$ RKT56.com I belive is the website, you are closer to him than I am, you might even be able to drive to him to have all the work done, if your car would make it.
I have never used him, but from what I read here, he is the guy I will use when I have the $$$ RKT56.com I belive is the website, you are closer to him than I am, you might even be able to drive to him to have all the work done, if your car would make it.
I had every combination under the sun on my personal FRC and I think the MM6 with 3.90's was my favorite set up. Now going to TR6060 and C6 Z06 diff so it will be MM6 1-4, .80/.63 overdrive with 3.42's.
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Drifting
Joined: Apr 2008
Posts: 1,687
Likes: 86
From: De Pere, WI
I am swapping my M6 for a M12 and keeping my 3.42s. With all the highway driving I do I don't want to take the mileage hit. I plan on rebuilding and keeping my M6 in case I want to swap it back.