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My question is I have a 65 396. It came with a 4:11 gears. I'm thinking about putting in a 3:55 or 3:36 for better driveability. Anyone out there with a big block that drives their car much have an opinion
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Re: C2 Differential Gear Ratio
I have a 67 390 HP with M20 & 308 gears. Great driver on the open road. Launches well, breaks the tires loose with the best of 'em. More than enough for me... but then I'm not out to race!!! just like to cruise and listen to the side pipes. -
Re: C2 Differential Gear Ratio
Hi Steve:
Isn't the M20 the "wide ratio" Muncie with the 2.52 first gear? The "close ratio" M21 has a 2.20 first gear. This makes a big difference in the effective gearing for launch. An M20 with a 3.08 rear has the same launch characteristics as an M21 with a 3.55 rear (2.52 x 3.08 = 7.76 and 2.20 x 3.55 = 7.81)
While I think that your combination is fine given the M20 and the torque of the 427/390, I think Mike needs to be careful, especially if he has an M21. The 1965 396/425 has a much more aggressive cam than the 427/390, so it has a lot less torque on the low end.
For the 396/425 engine, I would think that an M20 with a 3.36 rear or an M21 with a 3.55 rear would be about right as a compromise between first gear launch and highway cruising.
I've always thought that the M20, with its lower first gear, is a better all around transmission for street use than the M21. For a given effective first gear, the M20 allows a 15% reduction in cruisng RPM. I think it is a better compromise than the close ratio gear set of the M21, which was really optimised to keep the engine in the peak power band under racing conditions.Comment
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Re: C2 Differential Gear Ratio
i am in the middle of this situation right now. i have a 66 L72 that was a 4:11 switched to 3:55. i had an m21. when i went to rebuild it my trans man,one of the best in the country, said go with a m20 or you will not be happy with first. in a discusion about lt1's here not long ago they said GM engineers determined the a m20 with 3:55 was better tha a m21 with 4:11 in the quarter mile. i noticed that a lot of 65 BB's have the m20 trans. are you sure you don't? billComment
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Re: C2 Differential Gear Ratio
Thanks for the response. I do have the close ratio M21. Should I look at changing 1st gear. to the wide ratio. Or Iam I just creating more problems thans its worth?Comment
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Re: C2 Differential Gear Ratio
Mike
Over the years i have owned cars with M20,M21,M40 with
3.55 rear axle ratio.For general driving purposes
that combination was my all time favorite.Cruise RPM was
reasonable and they all pulled hard to red line.Comment
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Re: C2 Differential Gear Ratio
it really comes down to what you expect out of first and if you plan on taking any trips. if you have decided to change the gears i would try it with what you have and make the tranny change. if you are not happy with the launch. my car had a 327 when i bought it and i knew from the log it had the 4:11 swapped to 3:55 and had the receipts. it was a strong 327/350 hp and had a nice cam put first was a disapointment and now i know why. i wouldn't go higher than 3:55Comment
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Re: C2 Differential Gear Ratio
My L78 was originally a 4.56 rear axle car. My uncle couldn't STAND it (gear whine on the freeway + low fuel milage), so he changed the gear down to a 4.11. That's as low as you can go without pulling the transmission and changing it to match a '3-series' rear end...Comment
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"Base" diff was 3.36 with close ratio (M20 in '65)
Don't forget they called all Muncie close ratio 4-speeds M20 until start of 1966 production. In the power train table they call 3.36 with L78 "General Purpose" Only 6.5% of the L78's were so ordered. Most popular 396 rear end was 4.11 with 42%, with 3.70 next at 27%, and 4.56 third choice @ 15%.Comment
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Driveability? Are you kidding?
My 1969 L89 with 4.10 rear gets 8 MPG on a really good day, usually 6 or 7. My 1970 454 with 3.08 gets about 12 MPG at best - still not what I would call driveability. I'm going to swap in the Keisler TKO 600 trans, with a 4.10 rear it will launch like a 4.60 ratio, and cruise at 65 MPH at 2200 RPM. Better yet, swap the rear for a 3.36 and with the keisler, it will cruise at about 80 MPH at 2200 RPM.
MarkComment
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Re: Speedometer Corrections?
When you change the rear your speedometer is going to need correcting. I believe changing to anything below 3:70 rear will require changing the speedo drive gear. The speedo driven gear for rears 3:70. 4:11, 4:56 use a different drive gear. The diameters are different. When making this swap how are you guys correcting your speedometer.
Ralph#37280Comment
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Re: Speedometer Corrections?
Just a consideration, at what gear ratio does the case change?
HarryComment
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Re: Speedometer Corrections?
I believe 3:55 down to 3:08 uses the larger drive gear and 3:70 up uses a another.
Joe Lucia is the expert on this. Hopefully he will chime in.Comment
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Re: C2 Differential Gear Ratio
my 65 396 came with a 456.side exhaust & no radio. The 2nd owner drove it from St louis to Florida on vacation in the late 60s. Gas was cheap then . WHAT ! I SAID GAS WAS CHEAP ! what /speak up I cant hear you..What.Comment
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Re: C2 Differential Gear Ratio
Hi Mike:
I think you are the only one who can decide how to proceed, but you should probably try to be as informed as possible before you spend time and money making a change.
One option you could consider would be to change to a 3.55 rear and see if you are still comfortable with the launch characteristics in first gear. I think the cruising RPM with the 3.55 will be noticeably better (16% lower than your present 4.11), but again, only you can decide whether that is enough of an improvement.
If the M21 and the 3.55 give you a cruising RPM that you like, but the first gear launch is too sluggish, you can change to an M20 and get back to an effective first gear that is pretty much identical to what you have now.
Keep in mind that few, if any, of the internal gears used in the M21 were used in the M20. So, switching to the M20 gear set may be an expensive proposition.
As someone else pointed out, the "best" solution is probably the Keisler 5-speed transmission that has an overdrive 5th gear. With that transmission you could keep your 4.11 and still have a top gear cruising RPM that is 36% lower than you have now. Furthermore, your first gear would be lower than you have now, so the launch would be even stronger (possibly too strong from a traction point of view).
Lots of things to think about, but the planning is half the fun.....Comment
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