M-20 with L-78 or L-72 & deep gear -How?

**Paul_S** · #1 07-09-2004, 08:04 AM

What are the book/order form limits for '65-'69 Passenger, Chevelle, Nova and Camaro L-78 or L-72 cars with an M-20? As far I know it depends on the rear gear ratio, but what are the "rules" and are there documented exceptions? If you ordered a 4-speed in place of the standard HD 3-speed and a highway gear (say 2.73-3.31) then you were given the M-20, but if you ordered a deep gear (say 3.55-4.88) then you were given an M-21. I think the limits I listed are correct for '65-'66 passenger, but for '66 Chevelle the limit is 4.10 with M-20. Why? What are the limits for the other years/cars? One of the main things that made me ask/question this was the post of the first '67 BM Chevelle. The car originally came with an L-78, M-20 and 4.56:1 rear. Did they change the option limits for '67 Chevelles? Is M-20 listed on the original paperwork for the car? As far as I know the P.O.P. does not specify M-20 or M-21.

I'm just trying to figure out why and how things were done/ordered. If you have any info, original documentation with the option details for either car/year or think I'm off on something, please post.

Thanks, Paul

**COPO** · #2 07-09-2004, 03:01 PM

I own the 67 BM Chevelle you mentioned. The POP lists the L-78 and the 4.56 gears (later changed by Motion to 4.88's) The original magazine article states it had an M-20 and the original owner confirmed this, however you are correct the POP only indicates a muncie, not which one, and the original trans is no longer with the car.

Another interesting one is the former Capt. Carl '69 Garnet Red COPO Camaro that received an M-20 from the factory and has original paperwork to prove it.

**Jeff H** · #3 07-09-2004, 04:15 PM

I thought you had to specify if you wanted the wide ratio or close ratio(M20 vs M21) when you ordered the car. I know there are quite a few 69 Z28's out there with M20's in them with 3.73 gears like mine. If there is a general rule that GM used to determine when a car got the M20 vs the M21, then I'm not aware of it.

**firstgenaddict** · #4 07-09-2004, 04:57 PM

On Pontiac GTO's 68-70 you recieved the M-20 with all gear ratios EXCEPT 3.90 and 4.33. With those ratios the higher (numerically) first gear ratio would have made 1st gear virtually unuseable.

**MosportGreen66** · #5 07-09-2004, 05:15 PM

I don't know of this counts but I know of a 1967 Corvette
L-71 (427/435) with an M-20 with a 3.73 rear? Thought this would pretain to the subject

-Dan

Don_Lightfoot · #6 07-09-2004, 05:46 PM

[ QUOTE ]
I know there are quite a few 69 Z28's out there with M20's in them with 3.73 gears like mine.

[/ QUOTE ]

The 69 RS/SS 350 I did up in the mid 1990's had the original drivetrain front to back and it was an M-20 with 4.10's.

I'm wondering if there were different circumstances between BB and SB

Schonyenko2 · #7 07-09-2004, 05:53 PM

My 70 nova L78 had a M20 with a 3.55 rear Schonye

**Salvatore** · #8 07-09-2004, 06:16 PM

Most small blocks respond better with a lower first gear. (M20) Especially a Z/28. They have no torque. I have the original M21 in my Z. Fortunally it has 4:10's or it would have a hard time just pulling out. I have a fresh M20 ready for it, just to lazy to change it. BB's have more torque so the M21 is a little better in mid range I believe. I think you could probably get what you wanted back then.

**firstgenaddict** · #9 07-09-2004, 07:32 PM

I remember alot of older magazine articles stating that they got the best 1/4 mile ets by revving to about 4000 and starting in 2nd gear. It would cause the engine to fall from the peak torque range there by allowing the tires to hook up. Then the engine would come in to the fat part of the torque curve with the car already hooked and rolling so you wouldn't have to baby it off the line.

berger · #10 07-09-2004, 09:37 PM

tony(mrmotion) how is it checked off on your original canadian doc'd jan 68 L78 M22 4.88 factory, on the dealer order form you got from the original owner professor Mas----.