TAC Tech > Number Crunching

1978 L78 T/A 6.6 Original Block?

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tom420:
Okay. I've been up, down, and around, this 1978 Pontiac 400 4BBL Trans Am issue for three weeks now and I still have no idea what block is supposed to be in my dad's 1978 Trans Am bought new from the dealership.

My dad died a few years ago and left a 1978 Trans Am, which he bought brand new from the dealership to my brother. Just before my dad died, my brother managed to blow up the original engine. My sister, worrying that my dad would find out and do unspeakable things to my brother, took the car to a "backyard" mechanic to get it back on the road before my dad found out.

The "backyard" mechanic pulled the original motor and slapped in a worn out Pontiac 301 with an adapter to bolt up the TH 350 auto which my brother promptly blew two years later, thankfully after my dad died.

Now, I've been tasked to return the car to the original numbers matching beast it was.

Unfortunately, the original block/motor is long gone so I don't have original casting numbers etc.

I need help identifying what exact block was in the original car.

The build sheet lists the motor as "L78" which some sites say was the 500557 block. Other sites say it was a 568557 block. To throw massive confusion into the mix the car has the T/A 6.6 decals on the hood scoop which I have been told ONLY came on the W72 1978 T/As. Granted my dad could have added the decals after he re-painted the car in the late '80s but I don't remember. My sister is trying to find some original pics from the '70s when he bought it.It also has the "WS6" performance package.

I'll attach the original build sheet. I need to return the car to numbers matching so I would appreciate if an expert could guide me in the right "block" direction at least.

aussieta:
the L78 is the 400 engine,
W72 is the improved engine, which does not appear on your build sheet
someone else should know more about casting numbers

5th T/A:
Referring back to Rocky Rotella's book, page 175. Your car with a L78 400 as noted on the build sheet would have had a block number of 500557. Mid year 1975 Pontiac switched the 400CI block number from a 481988 to a slightly lighter weight 500557. In 1977 they used the 500557 block with the W72 engine but experienced problems with main journal bulkhead failures, so for 1978 Pontiac switched back to the stronger 481988 casting.

You should be able to fin a 557 block, but unless you find the original block, it will not be a matching numbers car because the original block has the sequence number of the Cars VIN stamped on it.


A couple of other things worth mentioning.

The Turbo 350 trans that came in your 1978 TA will bolt up to a Pontiac 301 without any adapter. Both engines have the same BOP Buick/Olds/Pontiac bolt Pattern.

Why someone would go through all the work to install a Ponitac 301 is beyond me. While engine mounts are the same and the trans will bolt up. The Power steering and alternator brackets are different. Most the AC compressor brackets are different. The 301 engine has a lower deck height and intake manifold requiring a much taller shaker baseplate. All these items will have to be changed back if you wish to install a Pontiac 400. The reason I know all thois is I swaped the stock 301 out of my 1980 TA and replaced it with a Pontiac 400.

FormTA:
Tons of valuable information there. I have unfortunately always had 301 cars so my knowledge is very limited on the 400 cars. Very interesting!

Wallington:
I think Tom is also asking about the differences between the 500557 and 568557 blocks used on base 400's.  I don't have anything definitive, just side notes and from what John's W72 write-up said.

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