Gearing?
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Joined: Sep 2006
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From: Whiteland Indiana
Gearing?
I have a 1968 - 427cu in, M21 close ratio trans with 4:11 rearend. The car was an original 435 HP but is now a 390 HP with a crane cam. The 4:11 gears are great for local street driving and drag racing but not to good at 70-80MPH. Am looking for a set of gears that will meet both local and highway driving. Am thinking about a set 3:70 or 3:55 gears. Anyone have any experience with these in this configuration?
Le Mans Master
Joined: Apr 2007
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From: Graceland in a Not Correctly Restored Stingray
Once had a 4.11:1 behind one of my 427's (still have it). Changed it to a 3.55:1 and have been quite satisfied with it. As long as you're hooked up, your 0-60mph could actually improve as you'll spend less time shifting to get there.
Le Mans Master
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
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Big Blocks Rule!
Two 427's with 4:11's. It must be like driving a rocket!
You guys make me sick, my 190 hp auto is way too slow!
Leave it as is and show the ricers what real cars can do. Pete
You guys make me sick, my 190 hp auto is way too slow!
Leave it as is and show the ricers what real cars can do. Pete
Team Owner
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From: Sorrento Italy
4.11 hands down for me, not only for a quick start but also to get the best out of every gear at any speed you get into it...
Of course there is a reverse side and it's that any people carrier passes over you on the motorway at half of the speed of your engine...
Of course there is a reverse side and it's that any people carrier passes over you on the motorway at half of the speed of your engine...
Team Owner
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From: Springfield MO
St. Jude Donor '07
I think 3.55s would probably be just fine behind a BBC. You'll lose some acceleration, but it will be much more friendly on the highway.
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From: Sorrento Italy
[QUOTE=I'm Batman;1562337441]What's that high-pitched buzzing assaulting my eardrums?
....maybe a tiger mosquito flying around your pistonhead......?.......
....maybe a tiger mosquito flying around your pistonhead......?.......
Last edited by panic; Oct 17, 2007 at 04:14 AM.
Race Director
Joined: Dec 1999
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From: Forked River NJ
3.55s would be OK but with a close ratio trans your first gear is going to be pretty long. You have the torque to take off but your forst gear ratio is 7.81. That is pretty weak for a 1st gear ratio
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Team Owner
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From: Springfield MO
St. Jude Donor '07
It's significantly more money, but an overdrive manual transmission might be a better way to get highway manners while still maintaining performance. Close ratio transmissions are great for racing, not always so great for the street.
Le Mans Master
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From: Graceland in a Not Correctly Restored Stingray
Melting Slicks
Joined: Mar 2002
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From: Beringen
A 3.55 with a short ratio trans will be asking for trouble in first and good take off, although i'm not sure how it will perform with a BB. With a SB it will certainly fall onto its face.
I once had a 3.08 with a short ratio, it was a real bitch to take off and the first was so long that i could hit 75 mph at something like 6000 rpm.
I bought a 3.70 and i'm still considering going to 4.11. I think a 4.11 can't be beaten in performance terms. Highway rpm (75 around here) would be around 3800 so that is indeed really scary.
However i have an engine that can turn 7000+ so there is lotts of room....maybe a 5-speed could help things a bit, but at a cost.
What i want to say with this is, there is lotts of things to take into account, and one car is not the same as the other, one driver is not the same as the other, and one engine is not the same as the other.
I once had a 3.08 with a short ratio, it was a real bitch to take off and the first was so long that i could hit 75 mph at something like 6000 rpm.
I bought a 3.70 and i'm still considering going to 4.11. I think a 4.11 can't be beaten in performance terms. Highway rpm (75 around here) would be around 3800 so that is indeed really scary.
However i have an engine that can turn 7000+ so there is lotts of room....maybe a 5-speed could help things a bit, but at a cost.
What i want to say with this is, there is lotts of things to take into account, and one car is not the same as the other, one driver is not the same as the other, and one engine is not the same as the other.
Drifting
Joined: Sep 2006
Posts: 1,259
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From: MS
Chevy's recommendations for the 427/390 with M21 were 3.36 standard, with 3.08, 3.55 and 3.70 optional
so don't see where 3.55's should be a problem unless the cam is really radical with no low end torque.
so don't see where 3.55's should be a problem unless the cam is really radical with no low end torque.
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From: Nashville NC
Thread Starter
Advanced
Joined: Sep 2006
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From: Whiteland Indiana
Good Stuff
Thanks, I will compare the RPM to MPH calculation on both the 3:55, 3:70 and 4:11. I run about 3000 RPM now at 55 MPH. Rear tires are 27.3 inches tall (BF Goodrich T/A's).
Last edited by james.frazita; Oct 17, 2007 at 07:40 PM.
Racer
Joined: Dec 2006
Posts: 403
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From: Swansea IL
You might consider going to a wide ratio trans and install the 3.55 in the rear. If you do the math, the first three gears have the same multiplication and then you drop into fourth for cruising on the highway.
Might even consider going with 3.36 final drive with that trans.
Bob
Might even consider going with 3.36 final drive with that trans.
Bob