Pontiac Block Size (vs SBC/BBC)?

[James Drinkwater]

Hi all,
Got a quick question. How do Pontiac V-8 engine blocks compare to small
block/big block Chevrolets?
The question points out that I'm familiar with Chevy and have never worked
with Pontiacs (pleeeennnty of experience with performance SBC's over the
past 20+ yrs).
Also, any insight to the peculiarities of the Poncho engines or a simple
link that'll wrap it up would be greatly appreciated.
Thanks for any words,
Drink

[Geoff Welsh]

James Drinkwater wrote:
> Hi all,
> Got a quick question. How do Pontiac V-8 engine blocks compare to small
> block/big block Chevrolets?

It's a medium block.

> Also, any insight to the peculiarities of the Poncho engines or a simple
> link that'll wrap it up would be greatly appreciated.

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

[ROY DARNELL]

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" <geoff...@some.rr.com> wrote in message
news:KRfhh.69110$si3....@tornado.socal.rr.com...

[Stan Weiss]

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 main journal size.
Stan

[Joe]

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.

"Stan Weiss" <srw...@erols.com> wrote in message
news:45861B80...@erols.com...

[dmtaurus]

Here's a bit of info. Pontiac was using nylon timing chain gears in the 69's.
Because these were good for only 20 - 25k the teeth would strip away and the
pistons would hit the valves. They would still run! those good ol' engines.
Anyway, for those engines that did get replaced the factory would stamp the
replacement engine on the with an SR xxxx number alongside the timing cover,
written vertically on the machined pad. Hence, the usual spot, on the
machined angled pad on the block, would be ground away. It was a way to track
replacements. Unfortunately, I never found a way to trace the number back to
displacement, so I didn't know for sure until I opened the engine up. So be
careful someone doesn't give you a line that their engine is an experimental
and rare unit.

[James Drinkwater]

Cool. That explains some of the visual differences. So, a Pontiac V-8
should be a bolt-in for an '87 TransAm? Bolt patterns? Motor mounts? And,
there's no difference, externally, between a 350, 400 or 455?! If that's
true, it's all a question of bore and stroke, correct?
Cylinder bores are siamesed and the heads use steam holes, and all the
blocks are two bolts if I'm not mistaken. Or, is it that they also come in
four bolts but the two bolts are actually stronger because of additional
bolts installed in the weakest point of main caps? Last, how does the
cylinder wall thickness measure up when a block goes to 455cid?
Thanks for your reply over Sunday coffee, BTW:)
Jim

"Geoff Welsh" <geoff...@some.rr.com> wrote in message
news:KRfhh.69110$si3....@tornado.socal.rr.com...

[Geoff Welsh]

whoaaaa. Stop.
Pontiac stopped making V8 engines in 1978. The engine in 80's and 90's
Firebirds is a Chevy smallblock.
Nothing interchanges, or bolts right in.
GW

[Joe]

"James Drinkwater" <jdrin...@satx.rr.com> wrote in message
news:45942d90$0$18127$4c36...@roadrunner.com...

> Cool. That explains some of the visual differences. So, a Pontiac V-8
> should be a bolt-in for an '87 TransAm?

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

[GTORon]

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.
https://www.motorsforum.com/img/3s6

--
For full context, visit https://www.motorsforum.com/pontiac/pontiac-block-size-vs-sbc-bbc-7915-.htm