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Author Topic: GMC 270H Head  (Read 13403 times)

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Offline 38flattie

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GMC 270H Head
« on: March 03, 2012, 04:54:57 PM »
I need 2 GMC 270H heads,

Anyone have 1 or 2 they want to part with? I promise to show you a really cool build, if you do! :-D

Thanks in advance!
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Offline RichFox

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Re: GMC 270H Head
« Reply #1 on: March 03, 2012, 05:24:48 PM »
What is the advantage of a 270H head over a 302 head?
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Offline 38flattie

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Re: GMC 270H Head
« Reply #2 on: March 03, 2012, 05:27:27 PM »
Rich, both have the big ports, but the 270H have an 'open' chamber, vs a smaller one on the 302.
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With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925

You can't make a race horse out of a pig. But if you work hard enough at it you can make a mighty fast pig. - Bob Akin

http://www.flatcadracing.org/
http://youtu.be/89rVb497_4c

Offline panic

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Re: GMC 270H Head
« Reply #3 on: March 03, 2012, 07:12:18 PM »
The "H" (1950-52 casting #417) has a large volume bowel-shaped chamber. High compression requires a domed piston. Unless the bore is large, there is no quench area to speak of.
The other big-port head (#983) has a (roughly) "D" shaped chamber, which allows a flat-topped piston. The has an effective quench ledge opposite the spark plug, but the chamber outline masks the valves. Here's a picture.

Depends on what kind of mods you're considering. No personal experience, but I'd rather have the closed chamber to start with, but I have no idea how close the water is to the chamber edges which would limit reliefs.
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Offline 38flattie

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Re: GMC 270H Head
« Reply #4 on: March 03, 2012, 07:41:10 PM »
Panic, You're correct , to my knowledge.

Trust me, though, when I say I need the 2193417, 270H casting.
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With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925

You can't make a race horse out of a pig. But if you work hard enough at it you can make a mighty fast pig. - Bob Akin

http://www.flatcadracing.org/
http://youtu.be/89rVb497_4c

Offline Seldom Seen Slim

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Re: GMC 270H Head
« Reply #5 on: March 03, 2012, 08:24:26 PM »
Panic:

Are you sure you've spelled the word correctly?

"...a large volume bowel-shaped chamber..."

That'd be pretty funky for a combustion chamber - might sound less like an engine than a fart-maker. :evil:
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Offline Bob Drury

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Re: GMC 270H Head
« Reply #6 on: March 03, 2012, 09:35:49 PM »
  Flatee, Once again I bring up the name of Gary Cope in Auburn,Ca.
  Gary is a long time Land Speed Racer and set at least one record with his GMC powered 41 Chevy sedan called "Gangreen".   
  Give him a buzz @ (530) 8898GMC.                   Bob
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Offline 38flattie

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Re: GMC 270H Head
« Reply #7 on: March 04, 2012, 08:03:29 AM »
Thanks Bob! I left a message for Gary.
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With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925

You can't make a race horse out of a pig. But if you work hard enough at it you can make a mighty fast pig. - Bob Akin

http://www.flatcadracing.org/
http://youtu.be/89rVb497_4c

Offline RichFox

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Re: GMC 270H Head
« Reply #8 on: March 04, 2012, 11:04:22 AM »
I always use the 302 head. I know the differance between the two. I just never saw the advantage of the 270H head. Unless you already had been running a 270 when the heads came out. And didn't want to buy new pistons for the 302 head so went with the 270H. I sure don't see people running around looking for open chamber heads for Clevland and 385 type Fords. Why GMC?
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Offline 38flattie

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Re: GMC 270H Head
« Reply #9 on: March 04, 2012, 11:40:26 AM »
Well, Rich, I've learned over the last few years, that when you ask me a question more than once, you think I'm on the wrong track!  Luckily for me, you've asked me a lot of question! Lol!

Here's my rational-The valves on the 302 heads are masked by the chamber walls, more so than the 270H heads, and will require more work for good flow. Looks to me that the 270H heads will require less work for good flow.

I'll be running forged blower pistons,so I can get some of the compression back from them if needed, although I only want to be in the 7-7.5:1 range anyway.

So, now I welcome Rich, and anyone else with real experience running these things, to point out the error of my thinking. I'd much rather find out I'm wrong now, than farther into the build!
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With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925

You can't make a race horse out of a pig. But if you work hard enough at it you can make a mighty fast pig. - Bob Akin

http://www.flatcadracing.org/
http://youtu.be/89rVb497_4c

Offline Peter Jack

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Re: GMC 270H Head
« Reply #10 on: March 04, 2012, 11:59:31 AM »
I haven't had one of those apart since the early '70s so I'm flying rather blind but it occurs to me that for the 302 head maybe you could relieve the top of the cylinder in the area of the valve similar to what's often done with a BBC.

Pete
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Offline RichFox

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Re: GMC 270H Head
« Reply #11 on: March 04, 2012, 01:23:22 PM »
As I remember there were a lot of cracked GMC heads out there. It would be nice to cut one apart and see how much the combustion chamber could be laied back to unshroud the valves. Somebody must have already done this. But I understand your point about the shrouding on the "Bath tub" chamber. I also ran a 7.5 to 1 CR with the 302 head.
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Offline 38flattie

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Re: GMC 270H Head
« Reply #12 on: March 04, 2012, 04:29:18 PM »
Rich, I'll probably cut a 302 head up, even if I find a 270H head, just to see what's possible. I'll post pics on the build thread when I start it, after El Mirage.

I have a total of 3 engines coming, so I'll have a head to play with. I learned on the Flatcad, that when you're dealing with cars several states away, you need one block in the machine shop, one in the garage for mockup, then one on the other end, several states away, for more mockup!

I'm waiting for a response from Sissells, about porting the head, and porting the Fontana head. I'm working on figuring out a mechanical injection and Procharger for one setup, and a turbo and EFI for the other.

The Fontana head pushes us to XXO, so I need the 270H for XO class.

Above is just FYI on what this is about, and a teaser for the engine build thread! Lol!
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With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925

You can't make a race horse out of a pig. But if you work hard enough at it you can make a mighty fast pig. - Bob Akin

http://www.flatcadracing.org/
http://youtu.be/89rVb497_4c

Offline SPARKY

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Re: GMC 270H Head
« Reply #13 on: March 04, 2012, 05:11:51 PM »
 :-o  maybe some one has  a ruined 302 head   :cry:
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Offline dw230

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Re: GMC 270H Head
« Reply #14 on: March 04, 2012, 05:55:02 PM »
The use of the turbo and EFI on the GMC in your car will push you into the Modified Category.

DW
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