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

[Brandon T.]

I ask this because I have a l36 with a m21 and a 3.08. I always thought it was a 3.36 or a 3.55 but to my surprise according to my rpm's etc it's a 3.08 which makes a great cruiser on the hwy.

anyway I'm still after the perfect 69 for me and I want it to have a 427 (perfer the l36 actually) and a m21 close ratio 4sp. Since a lot of you guys have probably driven a lot of 427 m21's with different rear end ratios what is your fav combo?

I think a 69 427 l36 or any other 427 and a m21 car could have had a 3.08, 3.36, 3.55 or 4.11 rear end.

thoughts?

[Terry M.]

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

[Mark K.]

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

I have a 67 L71 with a 3.70. It's a great acceleration gear for the street. Highway manners are OK, but perhaps a bit high in revs. I love driving the car on twisty roads where I can blast through the straightaways. Not a long trip kinda car though.

[Edward J.]

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

Brandon,

I think to have the best of both worlds is the 3.36 ratio, great off line take off and still manageable rpms at 65mph. the 3.55-4.11 great 1/4 mile gears, if you drive your car alot, the higher rpms tend to get to loud in car when cruising and the radio is drowned out, and your companion driving with you may not here you talking.

Just my two cents, maybe I getting old.

[Brandon T.]

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

I have a 67 L71 with a 3.70. It's a great acceleration gear for the street. Highway manners are OK, but perhaps a bit high in revs. I love driving the car on twisty roads where I can blast through the straightaways. Not a long trip kinda car though.

Is that with an m20 or m21?

[Ronald L.]

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

Agreed, my 1st L36 427 was a 336 and it was much nicer cruising that the 3.70 configuration I have now...

[Brandon T.]

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

thanks for the thoughts guys, please include what trans with the ratio your talking about.

because a m20 with a 3.36 is about like a m21 with a 3.55 or 3.70.

Reason I'm getting ideas is because we're not in a perfect world where I can go around trying different combos so I want to narrow my search down even more for the perfect car.

so far I'm thinking a m21 with the 3.36 or 3.55 would be nice..maybe even 3.70

but a m20 with a 3.70 I think would be too high reving.

but I'm wanting a m21 so thats what I have to consider how a m21 does with each ratio. Again mine is a m21 with 3.08 so I know how that combo is.

[Mark K.]

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

Is that with an m20 or m21?

M21 - You couldn't get the L71 with the M20, only the M21.

[Mark K.]

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

thanks for the thoughts guys, please include what trans with the ratio your talking about.

because a m20 with a 3.36 is about like a m21 with a 3.55 or 3.70.

Reason I'm getting ideas is because we're not in a perfect world where I can go around trying different combos so I want to narrow my search down even more for the perfect car.

so far I'm thinking a m21 with the 3.36 or 3.55 would be nice..maybe even 3.70

but a m20 with a 3.70 I think would be too high reving.

but I'm wanting a m21 so thats what I have to consider how a m21 does with each ratio. Again mine is a m21 with 3.08 so I know how that combo is.

Not sure what you mean here, but both transmissions will have a 1:1 ratio in top gear. Only the low gears affect a difference in RPM - which shouldn't matter if highway gearing is what you are after.

[Jean C.]

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

Nothing better than my '68...L89, M21, 4.11 gear...the perfect setup for sure, certainly my favourite combination.
Best regards,

[Brandon T.]

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

Not sure what you mean here, but both transmissions will have a 1:1 ratio in top gear. Only the low gears affect a difference in RPM - which shouldn't matter if highway gearing is what you are after.

well a lot goes into the differences for startes m21 will cause harder starts which a lower ratio would help with etc..but not low enough to be a pin on top end.

[Steven S.]

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

Right now I'm running an M20 w/ 3.36, seems to be a good middle of the road combination, decent crusing RPM with a acceptable 1st gear. Not a fan of the M20's 3-4 RPM drop though.

Steve

[Jephrey S.]

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

It's not a 427, but my 70 LT1 is not exactly a docile engine and has plenty of torque. I have the M21 with 3.70 gears. I love that combination in the twisty bits, but it is a little too low (numerically high) for comfortable cruising in my opinion. With factory reproduction pipes at US highway speeds the noise can become an annoyance after about 30 minutes and you can forget about the factory radio. At autobahn speeds, expect to be deaf for an hour or so once you reach your destination and/or next gas station.

[Brandon T.]

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

Sound like a lot with the 3.70 and obviously the 4.11 seem to think it's not such a good ALL AROUND gear.

thinking the m21 and 3.36 or 3.55 would be the way to go and actually with an m21..probably the 3.55 would be the best. and 3.36 or 3.08 with the m20.

So, m21 and 3.55 is what I'm thinking on this so far.

[Chuck G.]

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