Hello, just wanted to know if someone could tell me what the Power Plant could of been in front of this 12 bolt, I have a factory 66 chevelle 12 bolt, stamp on housing is this: (KM) meaning 4.56 posi, date 5 27 plant b

396 325hp, 396 360 hp, 396 375hp,
powerglide or what 4 speed

not for sale, just wanted to know some info on it that is all.
thanks for your time...

As it's an ordered axle ratio and not standard for any engine or trans ,it could be in any 66 Chevelle.

This power team chart for El Caminos only shows the 4.56 as being optional on the 396 360HP and 375HP engines. I have a similar chart for Chevelle power teams and it doesn't show the 4.56 as being optional at all. I don't know why it wouldn't be listed. :confused: The Chevelle chart has a later date, maybe the 4.56 and 4.88 were discontinued by then? But, yours has a May 27 assembly date. What is the casting date and casting number?

The chart also indicates a 4.56 would only be mated with a close ratio 4 speed. The only engines to use close ratio 4 speeds would be the 360HP or 375HP.

I was also under the impression (don't have any evidence) that the K series codes were for the higher horsepower applications. The C series 12 bolts are commonly found on the 325HP 396s. I'm just throwing this out there; any comments?

http://www.chevelles.com/showroom/data/500/medium/07-13-2009_07_01_34PM.jpg
http://www.chevelles.com/showroom/data/500/medium/07-13-2009_06_48_39PM.jpg

Bruce:

Wouldn't you agree though.. and I am just throwing this out there... That one could ORDER any available Rear Gear Ratio that was available for a 66 SS Chevelle? I mean, aside from what was Standard application..

"Mr. Johnson...I really want this 66 SS with the 325Hp engine.. your basic 66 SS, but I understand there is a 4.56:1 Rear option.. That's what I want.. can you make sure that I get that rear and if so, I'll buy one of these new 66's! " :)

Bruce:

Wouldn't you agree though.. and I am just throwing this out there... That one could ORDER any available Rear Gear Ratio that was available for a 66 SS Chevelle? I mean, aside from what was Standard application..

"Mr. Johnson...I really want this 66 SS with the 325Hp engine.. your basic 66 SS, but I understand there is a 4.56:1 Rear option.. That's what I want.. can you make sure that I get that rear and if so, I'll buy one of these new 66's! " :)

but most of the time when your local dealer put that order thought to the central processing plant they would over ride the order and put the suggested option in this case the suggested rear end ratio. :beers:

I think there were practical limitations for warranty reasons. Too much chance of over revving a 325HP engine with 4.56 gears. The 360HP and 375HP engines had better valve springs for higher RPMs. There were additional limitations when air conditioning was ordered. Even when an optional 3.31 or higher gear ratio was ordered on a 360HP or 375HP the factory would replace the close ratio transmission with a wide ratio. You didn't have anything to say about it. The factory seemed to have a matrix of allowable combinations. If the dealer wanted to make you happy they would have to do the parts swapping.

"Yes sir, I've got a 4.56 rear in a L78 car out there and I can swap it for your 3.31."

I've got a story about my 66 El Camino concerning a rear end swap by the dealer related to me by the original owner. My El Camino was sold as having a 3.31 Posi (POP indicates a CF code). The original owner realized one day that he was only getting one tire mark and took it back to the dealer complaining that the posi didn't work. Turns out the EC didn't have posi, so the dealer swapped the CF coded rear out of another EC (67s were out by then). El Caminos are trucks and didn't have window stickers, so I guess the owner of the 67 never knew that his truck had a 66 rear in it and should have had a posi. Yes, I found that 67 housing in my EC.

I've got a story...

My dad ordered 4.56 with positraction (I have the original carbon copy of the order form), but according to mom and all my dads friends, when he picked up the car, he drove it out of the dealer's lot and down the street "testing" it, and he brought it right back in and complained, because it didn't have the gears he ordered... the dealer swapped the gears...

Sadly, it wasn't a Chevelle but a '58 BelAir with a 315 HP 348... "The" car to beat, according to all his racing buddies...

If anyone recalls or goes back to the Feature Stories section of Team Chevelle.. my original MYSTERY CHEVELLE story is quite similar to those told here.. feel free to go back to hoem page and read...

The MC rear axle coded for one thing but had different ( 4.56:1 gears in it ) b/c original owner refused to take car with the gears it came with.. and the dealer swapped in what he wanted.. he came back and got the car when it was done :) of course it was a 375hp car, so the gears he wanted were not questionable. :noway:

My 67 L79 has the 4.56:1 KM coded posi axel hooked to an M21. I have the POP with the KM code so it was factory installed. I also own a 1968 L78 camaro with factory 4.56:1 and M22. It also has the POP and window sticker. When I researched the rarity of the 4.56:1 axel combo on the camaro I was told that with special order drivetrain teams the order would require Chevrolet Zone approval before it would be accepted at the factory.

http://www.chevelles.com/forums/showthread.php?t=285465&page=3

Hello and thanks for everyones input....

the housing is a 3875745N cast date is D1?6

on the chevelle the trim tag have this one option that I'm not sure with
2Q someone once told me that for M-21

yes the car is a 66 SS

build date is 06A out of flint, mi

thanks for all your help.........

Anthony,

The 66 housing has a casting date of sometime in April 66. The 2Q can be considered like the 2L code for a 4-speed. I don't know how they differ.

Since the original drivetrain is gone and you have the 4.56, If I owned your car I would build it with the 4.56, M21 or new Autogear M22, and an L78 (using a late 4 bolt 3855961 block). It would be a lot fun!

Pat,
Thanks, for the info on your 67 L79. That was the other engine I thought of when I was making my replies in this thread. I wish Chevy had built some of those in 66 Chevelles; I would like to have one in an El Camino!

I might caution the use of the 4.56:1 ratio if you want to do anything else with the car besides hole shots & 1/8 mile runs. I took it out of my L78/M22 Camaro and substituted a 4.10:1 for much more reasonable drive-ability. You will have no 1st gear with the 4.56 and it will wind up and overheat on a 20 mile highway cruise. It will also cost you a tank of high octane for a couple hours behind the wheel.