I'm planning on having the rear end rebuiltthis springand I'm thinking about doing away with the original 3:38 gears. Here are the particulars:

1969 350/350 M21 4spd. Convertible (Convertible=Heavy)

Rebuilt OriginalEngine:
Still has 11:1 compression
LT-1 intake
Holley 650 carb
Cam is slightly bigger than '70 LT-1 with solid lifters
MSD ignition and distributor
Headers
2.5" Dual exhaust with H pipe
Trans is original

Needless to say its built for the higher end and not for low end torque. It's a dog out of the hole but really screams over 3500rpm. I've found that I have to run at least 25% 110 octane fuel to keep it running right which I have no problem with since we don't drive these cars for economic reasons.

Any thoughts on gear ratios? I'm thinking about 3:55's or 3:73's. I'm not concerned about fuel economy so much but I'd like to be able to drive 70 mph without feeling like the engine is screaming. We live in the city and we don't take it on long trips since there aren't a lot of gas stations to buy race gas at anyway.

Also, as a sidebar topic, how do the trans gear ratios of an automatic (350hydromatic?) compare to a manual?

Any input from those that have experimented with gear ratios would be appreciated.

Thanks!
Rick.

 

If you go to the 3:73 gear you will not enjoy highway cruising any more. I had 3:73's in my '79 and the RPM's were just too high and the car was too noisy. I changed over to a 3:24 ratio ( I know it's an oddball #) and am much happier with the lower highway RPM's. I do have a manual trans so I can't tell you about the automatic gearing. My choice has always been a stick - just for the pure enjoyment. My opinion - keep your rear gear ratio the way it is.

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It would seem to me that you wouldn't want small gears as you say that your engine doesn't have alot of low end torque, so why waste the engine and "bog" it around town. My opinion is; go with an overdrive transmission. With that extra gear, you have the best of both worlds. The more I read about the reworked 2004R for a small block, the more I think that it's the one to go with. Just gears won't give you everything you want. Either you have a good hole shot (w/ bigger gears)or a big top end (w/smaller gears). I know first hand that it sucks turning over 3000 rpm on the highway for any period of time.

 

3.08's, 3sp manual stick w/od ( still waitin' for rebuild ), 400+ horse 454, 75 mph @ 2300 rpm.

( correction, it's a 3.07 )

 

Experiment with slipping the clutch a bit more comimg off the line at higher RPM's, this should give you a better shot off the line and still maintain your cruising RPM's.I would think that with 11:1 compression you should have plenty of grunt!

Why are you considering a rear end rebuild, is yours going bad?

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I agree w/ DV8. I've got 3.55s and w/ 255-60/15s, get 22mph per 1,000rpm. No problem on launch cuz I've got the M20 w/ a 2.52 1st vs your's w/ a 2.20 1st. Just another option for you.

 

And for automatics 350 =2.52, 400 = 2.48. PG.

 

Plus on top of that you get the inherent torque multiplication of the convertor at launch.

 

At this point I guess I don't NEED to have the rear end rebuilt. However, to my knowledge it's never been opened up or cleaned out. I have no idea of the condition of the lube inside or if there is anything inside that shouldn't be there. The bushings where it mounts to the frame are disintegrating quickly as are all of suspension rod bushings. Since I'll already be paying for the labor (this is one job I think I'll skip doing by laying on the garage floor) to get far enough into the rear end to replace the mounts it makes sense to, at the very least, open up the rear end, inspect it, clean it, and replace the fluid. At that point, if I can gain some out of the hole performance by changing the gears I think its worth doing, provided of course the sacrifices aren't too great.

73shark, you mentioned that you have the 3.55's and you are turning 1000rpm's for every 22mph. So is 66mph at 3,000rpm's accurate? (that doesn't seem bad) Also, I'm assuming that 4th is the same (direct) for both the rock crusher and the M21.

Last but not least, changing the transis not an option.

 

Yes 4th gear is 1 to 1 on both transmissions, heres the #'s
M20 2.52 M212.2
1.88 1.64
1.471.27
1. 1.

Heres a ball park of rear ratios:
gear ratio mph per 1,000 rpm rpm at 70 mph
4.1118. 3800
3.70 21. 3200
3.55 22.3100
3.08 25. 2500

There's a lot of labor involved to get to the rear end so while your at it you might consider having the trailing arm bearings and bushings replaced, that way everything will be new and good for many years. PG.