400cui with 455cui heads? - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 05-31-2012, 02:58 PM Thread Starter
 
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400cui with 455cui heads?

Hi!

I'd like to know which heads would be the best fit for a 400cui from 1975.
As it is not my car, I cannot tell exactly everything about it, but he has some heads of a 455cui from 1971 that he wants to use.
Is that possible? And if so, is it a good idea?
Only thing I know is that the 455cui heads have more cc.

Thank you!
Chris
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post #2 of 5 (permalink) Old 05-31-2012, 07:04 PM
 
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Everyone is going to ask for more details about the engine, like cam, pistons, etc. So if you can get any of those details it will help. And ask your friend what are the heads he wants to use-6X or whatever. Unless only one type head was used in 71 on a 455, but even then the heads he has might not be the originals to that engine.

Oh, yeah. Using them is possible, they just might make the engine run really really badly if they aren't the right ones.

Russ
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post #3 of 5 (permalink) Old 06-01-2012, 05:00 AM Thread Starter
 
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He checked the numbers and now he says he wants to use #62 heads from 1969 with 72cc on his 1975 400cui engine (which came with about 110cc heads)
More details are not available.. but can you say if this is a good idea if everything in the engine is original?
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post #4 of 5 (permalink) Old 06-01-2012, 07:53 AM
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Quote:
Originally Posted by Chris-Austria View Post
He checked the numbers and now he says he wants to use #62 heads from 1969 with 72cc on his 1975 400cui engine (which came with about 110cc heads)
More details are not available.. but can you say if this is a good idea if everything in the engine is original?
There are other things that come into play, like has the engine been overbored any or is it still the stock bore size? Has it been "zero decked"?

However, if I assume everything is "factory stock" then 72cc #62's with flat top pistons, stock head gaskets, and "usual" piston deck clearance of .020" yields a compression ratio of 9.978:1 With "good" fuel and/or "enough cam" that's doable but it's getting a little close to the limit in my opinion.

Personally, I probably wouldn't risk it with the readily available fuel we've got here in the USA. There are people who run that much compression successfully here but in general it's not "the norm" because you really have to stay on top of the tune-up, plus the difference in torque/power output from just a few tenths in compression is so small that the risk usually outweighs the benefit.

Bear

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post #5 of 5 (permalink) Old 06-01-2012, 09:03 AM Thread Starter
 
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Thank you Bear, that's what I thought as well.
Too bad that he doesn't know what has been done to the engine before. Reminds me of my own car with the 72cc heads (670) on my 428cui. Until now I had no problems, maybe the engine has been rebuild that it can sucessfully run on the best fuel here in Austria without any problems. (have been driving for about 4000 miles with this setup)
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion

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