when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

[Brandon T.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

FWIW, I have a 69 L68 400 car. It has the THM 400 automatic, but the final drive is 1:1.

Original rear was a 3.08. When I rebuilt it a few years ago, I put 3.36 gears in it.

It actually ran faster at the drag strip with 3.08's, about 1-2 tenths.

Those hydraulic lifter BB's run out of oomph at about 5300 RPM. They're "torquey" motors, not hi revvers.

Chuck

Yeah my 68 m21 with 3.08's on the hwy would be a hand full to mess with. Overall I like the 3.08 with the m21 but would perfer a bit more gear next car I buy so thinking the 3.36 or 3.55 would be nice.

[Erik S.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

Brandon - I am struggling with the same issue; have a 68 L71 with M21 (obviously) and 4.11. In Europe fuel is about 9 bucks a gallon so want to get the refs down. Considering either M21 with 3.55 OR M20 with 3.36 OR M21 with 3.36 and burning the clutch a bit

[Chuck G.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

Yeah my 68 m21 with 3.08's on the hwy would be a hand full to mess with. Overall I like the 3.08 with the m21 but would perfer a bit more gear next car I buy so thinking the 3.36 or 3.55 would be nice.

If you want the best of both worlds, keep the 3.08 rear and go with an M20 wide ratio trans.

1st gear is 2.56/1 in an M20 as I recall, whereas the M21 is 2.20/1

More "grunt" off the line with a wide ratio. BTW, I recently learned that complete gear sets and complete reproduction transmissions are available.

Tore out the Muncie in my 56 Chevy to rebuild it a month or so ago. Was amazed with all the "stuff" that is now available.....cheap too.

Chuck

[Michael J.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

Brandon,
I have a L68, M21 close ratio with 3.55 posi. I like this combination-still has good bottom end torque and great for highway driving as well.
Terry

Totally agree, mine is an L71 though, but same same. And it works best for getting those other two carbs open as well.

[Steven S.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

I'd agree, with an M21 I think a 3.55 would be a good all around choice. I have to ask though, isn't a 3.08 and a M21 a clutch slipper?? I always imagined that it would be... has to be a bit sluggish out of the hole if nothing else.

Steve

[Brandon T.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

I'd agree, with an M21 I think a 3.55 would be a good all around choice. I have to ask though, isn't a 3.08 and a M21 a clutch slipper?? I always imagined that it would be... has to be a bit sluggish out of the hole if nothing else.

Steve

Actually no, infact until I ran numbers and checked rpm's and mph I thought it was a 3.36 or 3.55.

I was surprised to find it is a 3.08 however, I knew it wasn't a 3.70 or 4.11 because it's pretty tame and nice on the hwy.

I'm sure a 4.11 is much more car out of the hole but the 427's grunt must take care of that because it's not sluggish out of the hole at all.

I did hear from a guy on here that had a SB and 3.08 with a m21 and he said it wasn't too fun but that was with a SB.

[Robert G.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

3.55 with side exhaust is just too much on the highway. An overdrive gear would be best, but I would recommend 3.36 over 3.55 any day. 3000 rpms at 65 and I can just feel the engine wearing out, especially knowing other cars can turn about 1800-2000 rpms at 70.

[John L.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

I have a 66 with L72, M21 and a 3:36 rear. It performs well at takeoff and cruise. I feel the power of the engine makes this combination wirk so well.

[Timothy B.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

Brandon,

I have 327 300hp cars with wide box transmissions and 3.36 rear gears. They are great for the street with the low torque range of the 300hp camshaft.

I would not even consider a gear less (numerically) than 3.55 with a close ratio box. If you have your heart set on a big block and it's got a higher performance (solid lifter) camshaft the start off will even be harder because the power curve is shifter higher up the RPM range.

If you find a nice 69 big block the gearing would not keep me from the purchase, that can be fixed very easily..

[Joe R.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

Brandon,

When I got my '66 427 (L72) with M21, it had a 3.55 rear. It was strong out of the hole and on the highway when I punched it. But with side pipes it is too loud for more that 20 - 30 minutes of driving, plus you can't hear the radio (not that that's very important!). Also, the RPMs at 60 MPH in 4th gear were about 2800 if I remember correctly, too high for me. I've heard from several folks that the 3.36 would be the perfect ratio for street and strip. I have a restored tranny waiting to be installed that is setup for a 3.36 ratio. I plan to have the rear end restored and while we're in there, change the ring and pinion to give a 3.36 ratio.

Joe

[Patrick N.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

per an older post of mine, I am settling on the 427 /400 with M20 and keeping the 3:08 per many good points of view from the group for this combo.

My car originally had the 3:36. Someday I may set it up that way. I am very curious to experience how this (M20+3:08) combo will pull out of the hole-hope it does not disappoint, but I really want to drive it and it sounds the 3:08 is preferred for the highway.

Pat

[Brandon T.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

per an older post of mine, I am settling on the 427 /400 with M20 and keeping the 3:08 per many good points of view from the group for this combo.

My car originally had the 3:36. Someday I may set it up that way. I am very curious to experience how this (M20+3:08) combo will pull out of the hole-hope it does not disappoint, but I really want to drive it and it sounds the 3:08 is preferred for the highway.

Pat

My l36 has a m21 and 3.08 it does fine, I actually thought it was a 3.36 or 3.55 until I actually took note of the rpm and mphs.

[Duke W.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

With the prodigious torque of the L-36/68, the WR/3.08 is probably the best combination unless you are a serious drag racer or live in San Francisco in which case I would recommend an automatic.

But then again, I doubt if anyone with a vintage Corvette would want to live in San Franciso regardless of transmission type.

Politics aside, the last time I visited a buddy in Frisco (self described as the last white, straight, conservative adult male left in the city and since moved to Vegas), I never saw so many cars with such thoroughly beat-up bumpers/doors since the last time I was in New York decades ago.

Multiply 760 by the lavailable axle ratios, and the closest to the tach reading at 60 MPH is what you have assuming the correct speedometer gearing is installed for the installed axle.

Duke

[Brandon T.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

With the prodigious torque of the L-36/68, the WR/3.08 is probably the best combination unless you are a serious drag racer or live in San Francisco in which case I would recommend an automatic.

But then again, I doubt if anyone with a vintage Corvette would want to live in San Franciso regardless of transmission type.

Politics aside, the last time I visited a buddy in Frisco (self described as the last white, straight, conservative adult male left in the city and since moved to Vegas), I never saw so many cars with such thoroughly beat-up bumpers/doors since the last time I was in New York decades ago.

Multiply 760 by the lavailable axle ratios, and the closest to the tach reading at 60 MPH is what you have assuming the correct speedometer gearing is installed for the installed axle.

Duke

I would agree, which for the m21 in your OP? I'm thinking for a l36 m21 combo the 3.36 or 3.55

[Duke W.]

Re: when talking 427 and m21 close ratio 4sp, what rear diff ratio is your fav?

Consider that my SWC has the somewhat torque shy 327/340 with a CR/3.08 drivetrain and spent the first five years of its life in Seattle...

It was neat to do 75 MPH in first gear (and over 150 a couple of times in fourth), but I had to be careful that all races I engaged in were from a 40 MPH rolling start, which was perfect for first gear. It was like a five-speed with no first.

There's only a five percent difference between a 3.55 and 3.36, and you will probably spend much more time cruising on the highway/freeway than doing drag racing launches, so I recommend the 3.36, which will yield about 2600 revs at 60 in top gear - not unreasonable unless you spend a lot of time above 80.

The combination of more torque and a ten percent shorter gear than my SWC means it will be much more lively off the line - more than adequate if you aren't the type that needs to beat everyone else across the intersection when the light turns green.

Duke