I have a 69 with a stock 455 and a 4 speed. Recently changed my rear gear from 3.08 to 3.42. It appears in the manual that 3.42 is on the borderline of getting a close or wide ratio tranny. Mine is an m20 since I had a 3.08. Which one would be better for a street driven car?

 

M-20. 3.42 is not deep enough to provide a decent launch with a 2.20 low on the street. With 500 lb-ft, you won't have to slip the clutch too much but the overall ratio with a 2.52 will make you happier.

 

I have a 455 with an M20 and 3:42 rear in my 68 and love it. When I bought the car had a 2.93 rear and it could barely start out in first.

 

I will say again "if all of the M21 Muncies were thrown in the ocean the world would be a better place"

 

Nice, sounds like a perfect combo to me. Car should feel great coming off a line.

Most definitely the M20, M21s are racing transmissions. Your car will feel sluggish with the M21 and a 3.42. The factory standard pairing for 1969 Convertible 442 400 CID with M21 was 3.91.

Its all about SLR ( starting line ratio ), even if you don't race. We all give our cars gas at a light or a highway merge etc...

Assuming your rolling on factory height tires here are all SLRs in the mix. ( A transmissions 1st gear ratio times its rear gear ratio. )

M20 and 3.08 ( how your car was before the rear gear swap ) = 7.76
M20 and 3.42 ( factory standard paring for your car when new ) = 8.62
M21 and 3.91 ( factory standard paring for your car when new ) = 8.6

M21 and 3.42 ( factory optional pairing for your car when new ) = 7.52 ( means I think you are going to be PISSED)

Just for reference when it comes to original muscle, an SLR of between 9 and 11 is what you would likely want for maximum (insane) off the line acceleration.

For example a 1966 W30 has an SLR of 9.53. Its a freaking bullet and you really need to know how to "drive"...

 

That’s great news, and great info. Didn’t want to tear it all apart if I didn’t have to. Thanks

 

I suppose that’s why the standard gear for 4 speeds in that year was 3.08.

 

Why?

 

Please explain what a 2.20 first gear is good for?

 

^^^ THIS! I've never understood why people want a close ratio trans, unless you have a very peaky motor that needs to stay in it's very narrow RPM range.

 

Great information Phil/69CSHC. The SLR on my '70 W-31 calculates out to 9.85 with M20 and 3.91 rear.

 

Does anyone here have experience driving a stock ‘70 W-30 with an M-21 and 3.42’s ? What was your driving experience ? If you had the tranny out of the car to “be gone through” would you change any of the internals if you were not satisfied with the stock setup ?

 

The theory behind using a M21 is the gear drop between 3-4 is less than a M20. These cars are heavy and need all the help getting moving. The Olds is a torque engine and the gear drop doesn't hurt using the M20. Think of it this way,drag racers use the steepest low gear they can get. I have no experience with the car you're asking about but plenty of experience using the Muncie trans. If it were me,I would change the gear set to the M20.

 

Daily driving to work and grocery store? Drag strip? Road race track? All would be very different experiences with different opinions on if it "worked well" or was lacking.

 

I would say taking it out for some nice Sunday cruises with some highway driving and some stoplight to stoplight traffic. I don’t know if I would take a W30 to the grocery store.

 

I converted my 65 442 Auto with a 3.23 rear end to a M-20 4sp. that was what the Service Manual info said was compatible to the 3.23

 

That 2.20 1st is a real pain in traffic. As soon as you let the clutch fully engage, you're going 30mph.

 

Your saying the two are not compatible?

 

My Rallye 350 was original 3.42 and M-21 and I did not like it. I had an M-20 in it for a while and liked it much better. I ended up going back to the M-21 just because it was the original transmission. If I was to do it over I would have put M-20 guts in my original case.

 

GM mandated that a M20 was to be used in cars with a 3.23 or higher rear axle.

 

That's what I would have done, use the original case for originality and put the 2.52 gear set in.

 

X2.
M21 with a 3.42 gear set was not a lot of fun.

 

That what I did with the M-21 I had from a 65 442 parts car, I converted it to a M-20 by changing the input and cluster gears and kept the original casing.

 

Very nice ! More aggressive than a 66 W30...

I think you will be pleased to know that HotRod 1968 tested a W31 engine in a F85 with same gearing as your ride and saw a 13.61 @ 103.21 MPH with headers and 27 inch slicks. They pushed it even further and saw a 13.20 @ 105.75 MPH with headers, bigger 29 inch tall slicks, and the rear end swapped to 4.66.

Incredible engine. A factory masterpiece.

I believe it.

Cars like this are an extra handful. I mentioned in a previous post about really needing to know how to drive. I should of also mentioned wanting to take on the task. You'd need to be Bill Hickman like to make these cars manageable in everyday use. There purpose built race cars/parts. M21 is a junior M22, same gearing minus the ridiculous additional strength. But if I had a car born with one of these transmissions I would not swap it out. Just learn to deal with it.

 

There's a reason why AutoGear made the M22W.

 

Must have been a lot of disappointed new owners of W-30, LS5, LS6 and GTO vehicles with M-21’s in 1970.

 

I made some test passes drag racing a friend's '70 W30 4 speed car... it was a factory M-21/3.91 gear but someone had put in 3.42 gears. Launching the car was a nightmare, and really hard on the clutch.

 

Am I correct in saying that the only 4-Speed manual transmission available on a 1970 W30 was the Muncie M-21 Close-Ratio with the 2.20 first gear OR was the M-20 Wide-Ratio with the 2.52 first gear also available ? With the M-21 Close-Ratio transmission 3.42 rear gears were standard and 3.91 gears were optional ? From what I have read it’s not exactly clear cut.

 

I often wondered why Olds used the M21 in 3:42 70 w30 cars, I can see it in 3:90 geared cars but not 3:42’s

 

I bought a 1970 Corvette with the Special High Performance 350 and had it for 45 years. It came standard with an M-21 and 3.70 axle (SLR 8.14). It was difficult to get a good launch without trashing the clutch. Even a clutch dump at 5500 rpm resulted in only 100 ft of wheelspin and a 13.70 at 107 mph.

I learned from that experience. And when I built my Vista Cruiser I paired a 2.66 first gear with a 4.10 axle for an SLR of 10.90. The Rocket lives up to its name in first--even leaving at idle.

And, yes, an M-20 was not an option on the 1970 W-30. An ex-brother-in-law had one with gearing of 3.42 (SLR 7.52). It was lifeless from a start; above 40 it was unbeatable. A 4.66 axle would have woken it (SLR 10.25).

 

The whole problem is balancing 1:1 highway manners. Sure, you can put enough gear out back to compensate for anything, but you're limiting your 4th gear cruise. My M22W has a 2.52 1st, and I have 3.70s out back. It's not the greatest out of the hole, but I can hold 65-68mph on the highway right around 3000-3300rpm.

 

I just had the m21 trans rebuilt 3 weeks ago, I should have had them change that first gear but didn’t think about it..

 

Absolutely. That's why it's surprising it took so long for overdrive manuals to appear.

 

It’s pretty bizarre that you could only get an M-21 in a ‘70 W30 when you had the option of getting a M-21 or M-20 in a ‘70 W31, Cutlass Supreme and some other models. Both tranny’s were equally capable - I wonder what the reasoning was ?

So if you had an M-20 in a ‘70 W30 with 3.42 gears would the 1/4 mile elapsed times differ from a similarly equipped car with an M-21 ? Would trap speeds differ ?

Would 4th gear Highway cruising RPM’s differ at 60 mph between the two transmissions with the same rear gearing ? How much ?

 

The M-21 was considered the ultimate for racing because it had equal percentage rev drops between gears. It was expected that you would order a numerically-high rear gear to create a suitable SLR.

The stick shift W-30 was considered a racing vehicle.

The M-20 had roughly equal rpm drops from 1 to 2 and 2 to 3, but had a much bigger drop between 3 and 4.

It would be engine dependent and you'd need to test them. With the same rear gear, the M-20 would have higher acceleration off the line but the shift from 3 to 4 would drop you into a black hole, acceleration-wise. The M-21 would leave the line slower but wouldn't have that black hole.

Both transmission have a 1:1 4th gear, so cruising rpm would be identical.

 

I am guessing that guys who run their cars in “Stock” classes with an M-21 run shorter rears like 4.33’s but do they also “experiment” with the gearing in the transmission ? Would that be the optimum solution to better E.T.’s or is running the shorter gear enough ? For example if the car was a ‘70 W30 with an M-21.

 

Ideal 1/4 mile performance puts the car at redline in high gear at the finish line.

 

"Redline" or do you mean "peak power" ?

 

Your question applies in every gear.

The go fast principle requires choosing a shift point such that the torque to the wheels is equal at the end of one gear versus the beginning of the next. That keeps your average torque at maximum throughout your run.

Effectively, this means that you should run past peak horsepower in each gear before shifting. If you don't run past peak, then your shift puts you much lower on the power curve at the start of the next gear--reducing average torque to wheels in every gear.

And, with a gearbox that has narrow, equal ratio drops between gears (looking at you, M-21), you can keep the shift rpm the same in every gear and you don't need to run as far past peak HP before shifting. This allows the average torque to wheels to be higher in each gear.

Now, look at the two possibilities in your question...

Through the traps at max power: Your 4th gear runup of revs to peak gave the tires some specific average torque.

Through the traps at redline: Your time in 4th gear is longer, but it gives the same average torque as your first case (runup to peak and and rundown to redline have equal average). However, this case requires you to chose a deeper rear gear. That deeper gear multiplies torque to the wheels throughout the entire run. And you cross the line quicker versus your first case.

 

I wonder what the integral function would be to show when you want to hit peak power. You're losing power as you go past it, but you also lost power while you took time to approach it.