Quote:
Originally Posted by Dick Boneske
Neither one of those tables lists cc's correctly. The '64 GTO heads were specified to cc at 72. The actual chamber size on '64 GTO's from the factory was 74-76 cc. The 421 HO heads were identical in valve size and casting number, but were bigger chambers to achieve 10.75:1 compression. None of the '64 421HO or GTO heads were 68 cc stock.
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I'm in the middle of my 421 HO build and have had the heads cc'd.
Chamber ccs of the 9770716 421 HO heads before starting any work, with the original valves popped back in. The factory lists the spec for these chambers at 72ccs. Pretty close in size with only a .6cc volume variance, attesting to the excellent cylinder to cylinder compression ratio control provided by the fully machined chambers that were a quality feature built into all Pontiac V8s:
1. 74.2
3. 74.0
5. 73.8
7. 73.8
2. 74.0
4. 74.0
6. 73.6
8. 73.8
So rest assured both the 389 and 421 chamber sizes were identical. Heads that were factory spec'd as being 72ccs coming out to near 74ccs actual is to be expected, factory heads from no matter what year quite often measure larger than the factory numbers on chamber volume.
The compression ratio difference was made up using much larger and deeper valve reliefs in the 421 pistons, much like the 428 used a 12cc dished piston compared to the 400 with 6cc valve reliefs.
421 piston is shown on the left and 389 on the right, both examples are 1964 factory original standard-bore pistons. I haven't measured both pistons, but it's pretty easy to see a considerable amount of added volume provided by the 421 pistons. The 421 pistons having 10ccs compared to the 389 pistons having 4ccs wouldn't be too far-feteched.
