sick_tight

So im thinking about either rebuilding my M12 on my Z06 and upgrading it or just buying a new built trans from RPM. My 3rd and 4th gears are grinding so im thinking its bad synchros. (previous owner did this) and If im buying a whole new trans im thinking about getting the M6 gearing because the previous owner already installed 3.90 gears and i feel like 1st and 2nd gear with the M12 and 3.90 gears are pretty worthless.

I would eventually like to autocross/road race the car, and i did read that the some racers prefer the M6 with the 3.90 gearing. but i would like to know what you guys think which is the better option.

SNLPerformance

iTrader: (15)

Having run both the M6 and M12 trans gearing in my FRC as well as 3.15, 3.42 and 4.10 rearend gears you really need to consider what you plan to do with it, I recommend playing around with this calculator to see the differences... http://www.f-body.org/gears/
I've driven a Z06 with 3.90s and honestly couldn't tell the difference between mine with the 4.10s at the time. More gearing is always better for acceleration if you can hook and aren't having to make an extra shift, so tires can be a limiting factor as well as the speed you race to.
I like the better shift extension of the M6 as it has better rpm drop. In my case the M6 and 4.10s were geared perfectly for the 1/8, the M12 and 4.10s yielded the best 60' and 1/4 time, and the M12 and 3.15s work best for the mile and better for the street because I won't run around on DR as I won't sacrifice the handling. Also don't forget to consider the 5th/6th gear of the MZ6 C6Z51 ratios.

sick_tight

much to ponder, I am not very interested in 1/4 times with the car and i would like a drawn out ratio. so if youre saying the M12 with 3.15s for your mile run was a good combo then im thinking since i have the 3.90 it may be best to get the m6 1-4 and now im curious about the 5th and 6th OD gears, never thought about the MZ6 ratios, what would be the benefit of that? lower gearing (think thats right) to stretch out the top speed? right now my highway cruise in 6th ill be doing 70 and im a little above 2000rpm as opposed to my 4th gen Z in 6th at 70 i was around 1600rpm

I am pushing close to 600hp to the crank so i think the M6 gearing will help me get the tires planted better. If its worth anything im running nitto 555's on all 4 corners, I always heard the BFG KDW type II were one of the best summer tires but that was a completely different forum.

thanks for your input

SNLPerformance

iTrader: (15)

Agreed the M6 would be better, it has a .74/.50 5th and 6th vs the MZ6 with .71/.56 and the M12 .84/.56. So the M6 has a taller final gear and the MZ6 has the tallest 5th gear of the bunch, hope this helps.

sick_tight

yeah i think ill go with the M6, especially since i found a used one for a good price i just have to get it shipped and i might have that done for free. ( a buddy i know has a hook up) while i install that M6 ill actually tear apart my M12 and see what the issue really is and if its a simple fix ill sell that especially since ill have the time to open it up and learn to work on a transmission.

$750 a good deal for a M6? i thought so, (not finalized) but its out of an 01 with 30k

SNLPerformance

iTrader: (15)

Sounds like it, if it's in good shape.

sick_tight

just bought it last night, looks like MM6 or M6 whatever you want to call it is whats going in next.

now i just need to build some ramps to get this high enough to tear it out. once its on the ramps i can jack it up an inch and put my jack stands on since you need to remove rear wheels and cradle to lower the trans

SNLPerformance

iTrader: (15)

Yeah I've done it a dozen times on my car, but fortunately never on jack stands...good luck as it's not going to be easy

sick_tight

yeah once i make my ramps and get under it ill be able to see where to place the stands. the ramps im building should be 14" high to the bottom of the tires so ill have room to maneuver but im sure this will still be a PITA. looking at everything you need to disconnect just to get the trans out is ridiculous haha. and im doing this alone so ill be taking pictures so i dont forget anything. im just worried about cracking my FAST intake because thats a pretty penny to replace. I might as well just pull it to be safe, easy enough to do.

and pointers or steps to emphasize from your experience?

SNLPerformance

iTrader: (15)

You can let the trans tunnel brace support the torque tube, trans and diff after removing rear cradle. Then I recommend removing enough bolts in it to get the diff off and then the trans as it is going to still be heavy doing it piece by piece even if you have some assistance which I recommend even if you ate your wheaties fwiw I bench 315. Before you pull the trans tunnel completely off and the torque tube be sure and support the back of the engine as to not accidentally crack the intake if you don't pull it. Hope that helps, Chris

sick_tight

yeah im going to do it in as many pieces as i can since im doing it alone, and ill need to remove the read diff anyway since im preplacing the whole trans. ill be buying/renting a trans jack or 2 for the process, i read on cf.com that they recommend 2 jacks for each piece. shouldnt be too bad if i remove the intake and take my time. thanks for your help