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Discussion Starter #1
I have a 67 GTO 400 that somewhere along the line someone installed heads from a 1973. There marked 4X. First question, whats the compression ratio of these heads? Second question, I'm thinking of putting 670 heads back on to get it back to original HP. I read in a previous post that the gas of today doesn't have high enough octane to run 670s. A little help for a new to GTOs guy. Thanks
 

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64-67 Expert
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Answer to second question: don't install the 670's unless you run 100 octane fuel at all times. Or tear down the engine and install dished pistons. They do not work well on today's pump gas. Believe me on this one.
 

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Be very careful pushing the limit on compression. There are too many variables that go into the mix to be able to say with confidence "you can always run X with Y octane". Climate, altitude, cam shaft (biggie), tune up (fuel mixture), ignition timing, the shape of the combustion chambers, piston deck height (just to name a few) all play a part. Generally, it's not worth trying to push the limit anyway. Consider this: the 461 in my car (the Beast) is making probably somewhere in the neighborhood of 520 - 530 HP right now (it dynoed at 495 before I changed to the aluminum heads). Pushing the compression by a full 5 points (going from 9.5:1 to 10.0:1 for example) would only buy me about 8 hp on this motor. On a street engine, taking that much risk for less than 10 hp gain isn't a good idea, in my opinion.

Also, those 4x heads will yield different compression ratios on different engines. You've got to know some details to calculate what the real CR is, namely bore, stroke, compressed head gasket thickness, head gasket bore size, piston deck clearance, piston dish/valve pocket volume, and actual combustion chamber volume.
For instance, 4x heads came in two chamber sizes (nominal - actual sizes can vary quite a bit - this is why you have to measure them) - 98 cc's or 114 cc's

On a 400 with "nominal" measurements (.020 deck clearance, standard bore/stroke, 6cc's in the valve pockets, .042 compressed gasket, 4.16 gasket bore) 98 cc 4x's will yield a 7.961:1 CR. 114 cc's would be 7.129:1

72 cc 670's would be at 9.935:1

If the engine is bored +0.030 those numbers change to 8.063, 7.218, and 10.065 with the same heads

On a "standard/nominal" 455 with those three chamber sizes you'd be looking at 8.925, 7.978, and 11.170.

Be aware that the "compression ratio" topic is one of those that tends to arouse very high emotion and much debate amongst gear-heads. Everyone has an opinion that they're passionate about, some of it grounded in experience/fact, some of it purely emotional. In the end it comes down to how much risk you're willing to take and how much effort/money you're willing to put into making it work. There are people (although a very precious few) who are getting away with 10:1 "or so" with iron heads on pump gas with the "exactly right" parts on a "meticulously maintained" combination. There are also others with engines that have detonated themselves to death at less than 9.5:1 with the "wrong" combination of parts and sloppy maintenance.

Bear
 

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Discussion Starter #8
Thanks Bear for all the technical info. I'm new to GTOs but I'm learning fast mostly due to you guys. Originally my thoughts were to restore it the way it was from the factory so it would have its original HP. Thoughts have changed due to, modern day octane, the fact that the car runs fine the way it is, the cost of changing it back, I don't race it, it only gets driven for pleasure and car shows and cruises and the old saying "if it ain't broke don't fix it". Joe
 

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Joe, you are a wise man. If and when the engine gets tired, you can build it up to deliver as much or more power than the original rating, using your 670 heads and pretty much all of your original parts except for the cam, pistons, and rods. New technology has made a lot of parts available that work very well building up a lower compression vintage Pontiac that will tear it up on today's low octane gas. Glad you are out there enjoying and driving your car!!
 

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Discussion Starter #10
There's a guy on ebay that rebuilds 400 engines claiming 424HP complete from pan to carb, even including distributor, wires etc just drop it in for 3,900.00. I don't know how he does it for that kind of money. I had over 3K just getting my four cylinder Model A engine redone. And that don't include carb, distributer, starter etc.
 

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Discussion Starter #12
Bear, His listing on ebay ended, but I did save his phone number. He's in Spring Tx.
281-546-6232 His name is Herb, I talked to the guy, he was more than willing to answer any and all questions. Joe
 
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