Go with the wide box. as it gets you rolling faster with a steeper first gear because I bet your car has a "rough idling" cam in it.

I personally ran a wide box in my Camaro with an L-88 427 (and I filled it every day with Sunoco 260 blue so I was running pump gas back then), and it was a joy to drive to work every day (I had a set of 4.56:1 gears for the street and a set of 5.13 or 5.38 (in another Camaro that looked just like my street Camaro) to race with.

Big Dave

 

I'm running a M21 behind my 10.15:1 427 with 3.31 gears. I don't remember the cam specs, but it isn't too rowdy. Works fine for me. Since won't be doing much highway driving, I'd opt for the M20. If you've got both transmissions already, why not put them both in, see which one works better for you, and rebuild the one you want. Those aren't too hard to swap.

 

With the 3.73 you are supposed to run the close ratio. Both trannys have the same final ratio so highway driving performance will be the same. This is the combo I went with when I built my BB 67 convertible. Lots of fun.

If you are going to swap out the rear gear in the future for mileage then go with the wide ratio for now.

BBs don't need the torque help that the M20 offers to get rolling from a standstill. Make sure you use a good, high capacity clutch (Centerforce DF II).

 

I have my factory 3.31 posi with an M21 in my SS. It's not bad on the highway at all. Any lower gears and I don't think I'd go on the highway much.

 

Either one will be fine with 3.73 gears but yes if you ever plan to go more toward 3.55, 3.42 or 3.31 then probably opt for the M20. The M21 should be just slightly stronger, mainly in low gear.

I run what is about an L88 cam in my 468 and it is fine with an M21 and 3.31 gears.

 

I believe since you have the option I would use the M20 with the lower first gear. I would think different if you were running 4.11 or 4.56 gears. Then as bmc66 says you have the option of going to the mid or low 3.xx gearing.
4th gear is the same either way, you might as well enjoy the lower first gear.

 

Use the 2.52:1 1st gear ratio.

 

M20 with 373's here. My 396 pulls pretty damn good with this set-up. Love the lower 1st gear.

 

M20 only, I ran mine with a 4.10 and a 4.30 in the vette, you could go from 1st to 3rd on the street.

If are ever thinking of changing the rear gear, then the wide ratio is the only choice. Plus the car will do better burnouts with the m20

 

My 70 Z28 with 3.73 rear came with a M20 from the factory. It has great gear ratios for the street. I wanted the noise of the M22 cause I have owned M22 rock crushers in the past and rebuilt my box with M22 gears. But I dislike the ratio of the M22 which is the same as the M21.

Go with the M20. It'll be a beast in 1-2-3 gears. Years ago I was thinking of rebuilding mine with M22W ratio gears which are almost the same as the M20. But the cost would not be as cheap as what I did.

 

Thanks to everyone - you guys always have great insights to share and I appreciate that. Seems like it's hard to go wrong with the 3.73, either box should work. I've been running the M20 close ratio and it's fun but I kinda feel like it hits 6k revs almost too quickly in first gear. I'm pulling the engine to do a couple other mods so I think I'll put it back in with the M21 2.20 ratio in and see if I like it. I see others are running 2.20 boxes with 3.31's and lower ratios so I should be ok with my 3.73.

 

Depending on your cam those higher gears you spoke of could kill your car, a 3.73 are a great all around gear

 

If you feel you are hitting 6k too fast you could try a larger tire size.

 

You would want a M20 on the 1/4 mile track. It will have a faster ET.

 

It's a street car that never sees the track.

It's a street car. :smile2:

 

The m20w I ran is the m20 with the maserio gears, i.e. Wide ratio rock crusher. The trans is a little louder in first gear, but not as loud as the original m22.

M20 is the best choice.

 

Having just finished this changeover I can say the M-20 is the way to go. I had a Saginaw that had a 3. something first and was like granny gear in truck. After researching info on this and the Corvette forum seems factory was M-20 on 3.08-3.55 and M-21 from 3.55 up. The driveability of mine changed greatly with the M-20. Just my 2 cents. By the way it's a 454 in a 69

 

Is anyone running a mild or stock bb with a 2.52 box and 3.31 rear? I'm thinking that might be a good combo for the street and be a little better for freeway speeds. I'm planning to change out my rear with a moser unit in the future when my budget allows.

If I run the 2.20 box with my current 3.73 that should give me a pretty good sense of what first gear drivability would be like with a 2.52/3.31 combo. If it seems sluggish maybe I go with a 3.42 or 3.55 instead.

 

and be a little better for freeway speeds.

Both the M20 and M21 have a 4th gear ratio of 1:1, so at any given speed, they will both cruise at the same rpm on the freeway.

 

Paul-

If you calculate the transmission first gear x the rear gear ratio you will get your final drive ratio when in first gear.

1st Gear, Rear Gear, Final Drive
2.20 * 3.73 = 8.21
2.52 * 3.31 = 8.34

You will note the ratio 2.52 first gear with 3.31 is very similar to the 2.20 first gear with 3.73. Maybe this will help you with your decision.

Andrew

 

They both have 1:1 final but I'd be running a 3.31 with the wide box versus 3.73 with the close box. 3.31 would be better for highway

 

If you buy an M22Z gearset wth its 2.98:1 first gear and run a 3.08 rear, you'll have the take off feel of 4.11's and lower highway RPMs. 1-4 gears are 2.98, 2.04, 1.47, 1.00.
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Whatever you do DO NOT use the M21/M22 with a gear ratio of anything lower numerically than a 3.73 or it will be a dog off the line, and you will go through a clutch faster. Don't even think about it.

 

Definitely not the case with a big block. Have been running an M21 behind my 468 / 12 inch clutch / 3.31 posi - for 8 years and have not had any issues. True it's not a rocket off the line, but that's more because the street tires won't hold. And it's a street car.

 

Lots of comments on gear ratio's, but that is mostly of interest to mathematicians and owners of small displacement small blocks.

But you are interested in is not your first gear to rear gear torque multiplications as street tires won't hold a big block should you feel rowdy. What is of more interest is in how the car behaves on the street.

I recall a 427 being thrown around earlier. The shorter stroke small BBC is going to rev faster with a slightly higher rear gear but but it will rev higher when cruising as well. Like I stated earlier, back when you could buy a 425 horse rebuildabe 427 four bolt core for $250 I used to run a 4.56 on the street and limit my cruise speed to double nickels.

With today's pump gas a BBC wants more cubes to make power, so I would only run 427 decals unless you have a solid cam and two fours sticking through the hood.

A 454 will make the power you need to drive on the street with a wide box and a set of 3.55 or 3.42 rear gears. The wider gap between third and forth gear with a wide box is a none issue with a big block that will be still enjoying it's broader higher torque curve. With a 302 it could be a problem; a 496 or bigger you won't even know there was a drop in RPM as you shift since you will have ample torque to break the tires loose just by flooring the engine.

Big Dave