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I have a 1970 LS-5 coupe with appr. 450HP, 500lbs-ft torque. I have a M-20 transmission and actually a 3.08 rear end. I would like to increase performance / accelaration in lower speed-range (up to appr. 60mph). I think to go with a 3.55. Does anybody has bad experience with this combination (M-20 & 3.55)? Are there any traction problems (BIG BLOCK)? Is the total nummerical ratio too high? Would be 3.36 far enough or even 3.7 a good choice? I would appreciate any comment from your experience?
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Re: Rear end ratio selection
Markus-----
For the drivetrain that you described, I think that the 3.08:1 is perfect. I would not recommend using anything numerically higher than 3.36:1 if you plan on doing any highway driving, at all.In Appreciation of John Hinckley -
Re: Rear end ratio selection
Joe, before I don't have mentioned that I have a complete LS6 setup (cam 4366, 3863144 (-> 3904362) 242/242, .496/.492, 0.24/.028, 114. It reves up to 6000rpm or more. With that in mind I would say the 3.08 is not the best choice. Most of the time I'm not able to use the high rpm range because even in 1. gear speed is too high above 5000rpm. That's why I though a 3.36 or 3.55 would be better for the car and the high rpm-potential of the engine. I drive not a lot on the highway.Comment
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Re: Rear end ratio selection
Markus-----
If you don't drive the car on the highway very much, then you could probably get away with a numerically higher rear end ratio. However, personally, I still wouldn't use a ratio numerically higher than 3.36:1. That ratio was the standard ratio for 1971 LS-6 and that was supplied with a close-ratio manual transmission. Your wide ratio transmission means that a numerically lower rear ratio should be used and that's what you have.
Keep in mind that the "standard" ratio supplied with any engine/transmission combination was the ratio that GM engineers determined would be the best for all-around driving. For LS-6 with M-21 one could order up to 3.55:1 for "performance" use. However, keep in mind that's with an M-21, NOT an M-20. For M-20, if it had been available with LS-6, I'm sure that 3.36:1 would have been the "performance" option.
LS-6 with M-22 could have received 3.70:1 or 4.11:1 ratios. However, the M-22 was really designed for off-highway, racing applications. The availability of 3.70:1 and 4.11:1 with that transmission was consistent with off-highway, racing applications, too.In Appreciation of John HinckleyComment
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Re: Rear end ratio selection
Markus —
About 30 years ago, my daily driver was a '71 454/425. The car might actually have been a ZR2, because it also had the c/r M-22 transmission, double-disc clutch, and H.D. brakes. That car had a 3.70:1 rear axle ratio. Redline was 6000 rpm, which was 60 mph in first. By a quickie calc on my 41cx, that engine would be turning about 3200 rpm at highway speeds. Your W/R would hit the redline sooner in first, but be the same at highway speeds.
Great car around town, but the Devil's own curse on the expressways during rush hour, which I drove morning and night.
Ol' GeezerComment
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Re: Rear end ratio selection
Sorry, Joe, for stepping on your post. Your post wasn't there when I started typing.
GeezerComment
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Re: Rear end ratio selection
I agree with the other posts that the m20 with 3.36 may be best choise for all around driving. In first second & third the response is like an m21 with 3.70 rear yet in 4th it cruses nicely on the highway. This is the set up I now have on my 64 365 and I quite happy with the all around performance. With your big block torque the ride should be awsome.
I had 3.70 with an m20 in my 66 300 hp. First second & third were like 4.11 with an m21. However, with the lower rpm 300 cam it was a bit of a push to cruse above 60-65 mph.
good luck.Comment
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Re: Rear end ratio selection
I think you'd be happy with either a 3.36 or 3.55. Generally you don't notice much of a change going from one rear end ratio to the next step up or down.
I have a 1972 LS-5 coupe with the original engine, M-20, and 3.08, and it is a marvelous driver. I have a sister car, a 1972 LS-5 roadster, (the only change to the engine is a 268 Comp Cams camshaft), M-20, and we replaced the 3.08 with a 3.55. The roadster gives more acceleration, and 70mph is about 2900 rpm (with P225-70R15 tires): a very entertainig ride.
Your car has a higher rpm engine and my feeling is the best performance and practicality compromise is the 3.55. 3.70 would be enjoyable around town, but it would be less fun on the highway. The lower (higher numerically) ratios do tend to whine more easily, unless set up right. Keep all your original parts. Enjoy your shark!Comment
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Big Block with 3.08 and 3.55
Well, I've done this, so I'll give you my thoughts.
I have a '68 427-390 hp with the M20 wide ratio four speed and a 3.08 rear. The car would fly, but it was a bit slow off the line. First trick, race someone more than stoplight to stoplight, and more than the 1/4 mile. Catch them on an open highway, pull up next to them around 90 mph, shove it back into third and leave them. That includes old police helicopters. Don't try it with new helicopters.
But my main fun was not so much highway travel as autocrossing and hillclimbs, so I picked up a 3.55 and that is as low as I would recommend going with the hydraulic cam big block.
With 3.08 With 3.55
at 55 mph => 19 mpg. at 55 mph => 14 mpg.
Top end 150 mph 130 mph
Now as to the LS6 you want to do, I would recommend you stay with the M20 wide ratio and go with the 3.55. The reason is this:
M21
2.20 first x 3.55 = 7.81
2.20 first x 3.70 = 8.14
2.20 first x 3.90 = 8.58
2.20 first x 4.11 = 9.04
M20
2.52 first x 3.55 = 8.95
2.52 first x 3.36 = 8.47
2.52 first x 3.08 = 7.76
The wide ratio M20 with the 3.55 will give you take off nearly equal to a close ratio M21 with a 4.11 rear. And running a 4.11 rear will get you about 6 mpg with that LS6, something even limited driving will find offensive with $2.50 premium.
A MPH Calculator from RPM and GearingComment
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