JL re 1978 Muncie & B/W 4-speeds

[Tom R.]

Started doing some research on 78 four-speeds and believe three 4 speeds were available in 78-79 depending on engine ordered. If the buyer ordered L48, they got the economy Muncie MM4 or the muncie version of M20 with a 2.85:1 ratio. L48s couldn't get the the heavy-duty M21 transmission.

If the buyer order L82 but with base M20 four-speed, they got the B/W RPO M20 with a 2.64:1 ratio. But if the L82 was ordered with M21, than RPO M21 got installed with the B/W 2.43:1 ratio transmission. The 2.64:1 tranny was coded with HS while the 2.43:1 tranny was coded with HW.

I used Colvin to arrive at much of this but the challenge is finding a 78 L82 with base 4-speed who can confirm the RPO used to identify transmisson type. I believe this is the "mystery" four-speed for 78-79. Curious what you might have.

It was the B/W wide ratio four-speed transmission that was the last to evolve out of Corvette lineage in 1981. The M21 was last available in 79.

[Joe L.]

Re: JL re 1978 Muncie & B/W 4-speeds

Tom-----

I don't see much of a mystery here. For 1978 and part of 1979, the "Muncie" 4 speed was the standard transmission used for Corvettes with L-48. This was not the same Muncie as that used for the 63-74 model years, though. Basically, this transmission was pretty much the same as the old Saginaw 4 speed transmission used for lower performance GM applications during the later 60s to early 70s. Production of that transmission, modified somewhat, was transferred from the Saginaw, MI transmission plant to the Muncie, IN transmission plant and the old Saginaw became a "Muncie".

The 1978-E1979 Muncie used for L-48 Corvettes also used an odd-ball clutch set-up. It used a 14" flywheel with a 10-1/2" clutch. It also used a 10 spline input shaft unlike virtually all other 1971+ four speed transmissions which used 26 spline.

Sometime during the 1979 model year (I don't know when), the L-48 standard transmission changed to the wide ratio Borg Warner Super T-10. The clutch and input shaft changed, too.

For 1978-79 L-82s, the standard transmission was the wide ratio four speed, M-20. This was a Borg Warner Super T-10 transmission. If you ordered L-82 without specifying any other transmission choice, this is what you got. You could also optionally specify, without additional charge, either M-21 or M-38. The M-21 was the close ratio Borg Warner Super T-10. The M-38 was a THM-350.

After the demise of the "Muncie" transmission (I believe that it went out of production in either late 1978 or early 1979), the Borg-Warner Super T-10 became the only 4 speed transmission available for a Corvette. For the 1980 and 1981 model years it underwent minor revisions, including a change in the gear ratios, and became known as the M-18.

[Tom R.]

Re: JL re 1978 Muncie & B/W 4-speeds

So than your sources confirm MM4 was the Muncie while the B/W was the M20 L82 base.

Do you have production numbers?

[Joe L.]

Re: JL re 1978 Muncie & B/W 4-speeds

Tom------

I have conflicting GM-sourced information regarding the 78-79 "Muncie". Sometimes, it's referred to as "MM4" and sometimes "M20". It would seem that the latter designation would cause confusion with the Borg Warner transmission. It would be interesting to see what someone with a Corvette equipped with that transmission had listed on their "service parts identification label".

I have no information on production quantities.

[Tom R.]

Re: JL re 1978 Muncie & B/W 4-speeds

I've corresponding with a couple of L82 M20 owners...I'll let you know what they find...and thanks.

[Joe L.]

Re: JL re 1978 Muncie & B/W 4-speeds

Tom-----

It's not the 1978 L-82 owners that will have the info; it's the 1978 L-48 with 4 speed owners. If the MM4 designation was used, it was used only for the "Muncie" transmissions.

The L-82 with 4 speed will be either M-20 or M-21.

[Tom R.]

Re: JL re 1978 Muncie & B/W 4-speeds

Ok