The Stovebolt Site Forums The Shop Area The HiPo Shop SEROIUSLY porting the gmc head

I've just rescued two 270 motors and they both have the worst head. It seems a waste to not make the best of what I have but EVERYONE KNOWS that these heads are junk--OR ARE THEY?
I'd guess the floor of the intake ports is higher than the 'good' heads so if the roof can be raised LOTS along with some widening to get enough volume-- the head might be able to produce decent Hp. The higher floor and roof allow the air/fuel mixture to not have to bend so much --which looses much Hp. while still providing good velocity.
Has anyone really spent some time determining how far these ports can be enlarged?
i'M amazed at how little information is around about these motors! As Fisher said back in the 50's it costs less per Hp with a six!
Holiday cheers
Roar

Jack Halton and others on Inliners have lots of good GMC info, Jack post here by the name of Sixpack.Jim Proffit knows something too, but he's vacationing in Europe at the moment I think.
What Bill Fisher said was before 100 million Chevy V-8's were produced,today a GMC cost per HP is 2 to 3 times that of a smallblock V-8 if you can find the GMC parts in the first place.

I still question that it's cheaper to do a V8, presuming that you are capable of doing more than just bolting stuff on.
The big things you need to buy for a good street motor are manifolds which are simple to build for a 6. True you can buy cams cheaply for SB's but there are also many grinders who will grind your cam for the same price.
I have no problem with your enthusiasm for v8's that's where most rodders are.
I'm restoring a 40 track roadster which came with a 270 GMC. These were the hot setup in the 40's and 50's. Finding parts is a standard problem for restoration/rodding of olde stuff.

What Bill Fisher said was before 100 million Chevy V-8's were produced,today a GMC cost per HP is 2 to 3 times that of a smallblock V-8 if you can find the GMC parts in the first place.

I've been pondering the same question myself. I've been piecing together parts for a trad style track roadster for a few years now, a '26 T roadster on A rails that will run a 261 Chev or 270 GMC. I hauled a reasonable 270 out of a deuce-and-a-half a few months ago but it too has the small port head. I beleive the 270H heads are scarce because they were pulled from production after a few years due to a problem with cracking, so it is likely that if you did find one it would need a ton of work anyway. You could fit a 302 head, but then you'll need custom pistons as the 270 pistons from Patrick's don't work with a 302 head. The speed manuals of the day stated that the ports on the big port heads were considered too large for a street application due to a loss of air speed at low rpm, and recommended porting out a small port head instead. I have a manual by Frank McGurk that recommends enlarging the intake side ports to 1 7/16", enlarging intake valve ports to accomodate a 1 15/16" intake valve, and enlarging exhaust valve ports to take a 1.6" valve, but it probably wouldn't flow much better than the stock head on my 261. I guess it depends on how fast you want to go. In a light car that should be enough to beat a few flatheads. It would be good to hear from someone who has actually used a small port head like this. I hope this helps.

Roar, I have a few running and driving old Chevy 6's ,one a nice 261 modified beyond bolt on stuff.You can build or buy a 300 hp small block V-8 for less than 1000 bucks, cost more than that to build a stock GMC

After dinking with a relatively newer L6 (1970 292) . . . I can tell you that the $/HP ratio is over twice as high with a 6 than a V8. But - that is why everybody and their sister has a V8.

RE: port size: There's a very good book for chevys: CHEVROLET INLINE SIX CYLINDER POWER MANUAL
It gives very good detail on how to get lots of HP from them. They find that they get the BIG HP by chopping out the devider between the paired intakes leaving a giant hole! I'd guess that they have found more techniques in the decades since Fisher wrote the book. I used to have it and built a 270 back in 54. It worked well but hopefully we've learned much since then.
Since I'm "restoring" a 40 roadster for vintage racing, I need to use the appropriate motor even though it is more obsolete than the chevy six.
I'll learn about the cost of building a six and a 327 hopefully this year in that I plan to build one of each.

They remove the divider(boss), but generally place a lump port in either by epoxying or drilling holes and tapping for small bolts.Drilling is easy to mess up, as you are close to water.