Pontiac Block Size (vs SBC/BBC)? (2023 Update)

It's a medium block.

The rockers aren't adjustable. The intake manifolds are high rise style (pushrod cover (aka valley pan) is a separate part). The water pump goes through the timing chain housing. The fuel pump bolts to the timing chain housing. 3.75 x 4.12 on the 400. They have the torque of a Chevy BB, and rev like a Chevy SB.

all I can think of on a Sunday morn GW

James, all Pontiac V8's except the 301 are virtually the same physical size. It is hard for the untrained eye to even see the difference in the 301 though. Because of this, from 1965 on, all of the external and some of the internal components are interchangeable. As Geoff said, they have low end torque similar to a big block Chevy, but they do not rev like a small block chevy. The rest of Geoff's points were right on. Another advantage of the Pontiac is that the distributor does not go through the intake, like it does on the Chevy engines. The connecting rods are the weak link in the Pontiac engines from a high performance standpoint. Because the important parts for performance (heads, intake, cam, distributor and rods) are all interchangeable, they are reasonably priced and readily available. Also, because they are basically the same size, with the same connections, switching from a small engine like a 326 or 350 to a

455 requires no modification. If you are looking for a Pontiac engine, remember that they really dropped the compression ratio about 1972 for smog and gas mileage reasons. The good news is that they dropped the compression by opening up the head chamber size, so changing to the older heads or after markets can gain that back. Hope this helps, Roy

"Geoff Welsh" wrote in message news:KRfhh.69110$ snipped-for-privacy@tornado.socal.rr.com...

A couple of thing that come in different sizes are blocks and crank. The

400 block and crank are what a will call small main journal size and the 455 are large ma>

Another useful point is that the Pontiacs use varying head chamber volumes to control compression ratio for the different size engines. You can increase your compression ratio by swapping on a set of heads from a smaller displacement. You don't want to get small valves in the process, of course, but there are a lot of Pontiac heads with big valves. The late 70's heads all have big valves, and very low compression, so it's a typical fix for late 70's Pontiacs.

When you buy a Pontiac engine, just assume the guy selling it doesn't know if it's a 350 or a 455. They look alike. Be sure to decide for yourself.

Also, the motor mount hole arrangement varies with time. Depending on what you're using it for, that could matter. The 69 engines won't bolt into a 70 and later F-body. After 1972, when the 60's A-body was phased out, some blocks didn't have the old-style motor mount holes. As always, you can build your own, if you have to.

Pontiac engines ran on a 2 year development cycle. 68-69 are alike, 71-72's are alike. The 73-74's are alike, 75-76 are alike, and then 77-78 are alike with 79 composed of leftover 78's. I don't know what happened with 70. There was a hiccup there.

P.S. GM dropped its compression ratio with the 71 models. You probably already know that. It just seems one of those things that everybody gets wrong.

Where did that come from? 87 Trans Ams are Chevy-powered, there, James. NOTHING will fit.

Thank you for correctly pointing out the fact that 1970 was the last high compression and high performance year for GM vehicles, that has been a pet pieve of mine as well. I decided to pull out my old Motors Manual from 1972 to see where the confusion on this comes from.

Oh the memories! The smell of the book, 1972 the year I bought my very first car, and what a car it was, a 1965 Pontiac GTO with headers and a Muncie 4 speed, long time no see old friend.

Anyway, now I understand the confusion, American Motors and GM both dropped their compression in 71, Chrysler and Ford dropped in 1972, so there you have it.