The neat thing about GMC's is that you can mix and match blocks, cranks, and
heads for various performance combinations. Everything is the same
externally so whatever you build will fit right back into a 1/2 ton with no
problem.
GMC heads are all virtually interchangeable but some combinations require
different pistons due to combustion chamber shape. The 270 "H" (big port)
head was only made for a couple of years. It has the larger port size of the
302 head but a "hemi" combustion chamber, so needs pistons with a full
pop-up dome for decent compression. 302 heads have a "D" shaped chamber and
302 pistons have a corresponding "3/4" pop-up area.
228 and 248 heads are the same, so no advantage would be gained by swapping.
Milling the head gives limited compression increase and increases chances of
valves hitting pistons. Increasing compression is much better approached by
domed pistons.
Any GMC rebuild with improved performance as a goal should include lighter
pistons with some kind of "dome" for increased compression. 228, 248 and
later 270s all used flat-top pistons. But an otherwise stock 248 would not
really benefit much from a big port head, in fact low-end torque would
probably suffer due to loss of port velocity. Chevy V8 valves (1.94
intake/1.60 exhaust) are easily installed in the big port heads but would be
of little advantage in the small port heads, for the same reason.
If you really want more output from a Jimmy, I'd look for a 270. They can
safely be bored 1/8" for a total of 292" and since you are looking at $300
for new pistons anyway,
why not start with a block that has more potential to begin with. Same with
a 302, but they are getting relatively scarce, and are usually priced
accordingly.
Jack / Winter Park FL
----- Original Message -----
From: <CLLLSLS@aol.com>
To: <oletrucks@autox.team.net>
Sent: Monday, September 13, 1999 11:26 PM
Subject: [oletrucks] 270 big port VS standard gmc head.
> Hi everyone,
>
> I've got a question for the inliners:
>
> What defines and seperates the 270 (302 also?) big port head from the
248-270
> standard head? I think I've been told that the big port head has larger
> valves?
>
> I'm trying to plan out the rebuilding of the 248 in my '50 GMC and I need
to
> know what would be best to do for the head. I have heard the 270 big port
> mentioned before and was curious if it is a feasable route to take for
> performance. I would imagine that even if these heads are great I probably
> won't be able to find one. Can I have large valves installed in a
standard
> head? What would be the better head to install these in: a 228 head with
> higher compression or the stock 248 head?
>
> Please let me know if I'm on the right track here or if I need some
serious
> enlightenment.
>
> Dustin
> 53 GMC 1/2-long W/Hydra-matic
> 50 GMC 1/2 ton shorty
> oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
>
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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