Inliners International Forums General Discussion Hi Performance SERIOUSLY porting a GMC

I've just rescued two 270 GMC's, unfortunatly they have the small port heads. Are these the same inside casting as the BIG port heads? How far can they be enlarged Is there iron above the ports so they can be seriously raised?
Thanks;
Roar

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Greasy Thumbs

The small port heads don't have much meat especially around the exhaust ports. I'd be REAL careful. Might not be a bad idea to take one of those 270 heads and slice it up to see just how much you have to work with. The 302 and 270H large port heads don't have a lot to work with either. Actually the small port head performs well on a street engine, just don't stuff a big cam in the motor.

Test for BB

Another test

and another test!

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Ignorance can be fixed, Stupidity is FOREVER!

and yet another

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Ignorance can be fixed, Stupidity is FOREVER!

SORRY ALL, I was having problems logging in.

Anyone know what is a reliable RPM for a prepared bottom end: smoothed, shot peened, balanced for a 248/270?
Today I noted that V8 1.94" and 1.5" valves fit nicely leaving room for unshrouding and allowing the cheap v8 stuff to be used! Am I right that the cast alloy rocker stands are better than the stamped tin ones?

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Ignorance can be fixed, Stupidity is FOREVER!

The stock Jimmy bottom end is pretty stout but these are long stroke engines and aren't happy at much over 5000 RPM. Old time racers would gain a few hundred RPM by using a 228/248 crank in a 270/302 block, and there are still GMCs at Bonneville allegedly running 7000 RPM, but with major work like reinforced main caps and lightened cranks.