To determine what this would do for you, we'll have to make some assumptions regarding your engine, since you didn't provide all of the critical specs. Let's say your engine does currently have 58cc chambers, as you suggest, and that it is at 8.8:1. We'll have to assume your Ford composite gasket is about 0.040-inch compressed, and that you have a dished piston that has a deck height clearance of negative 0.025-inch (typical of production V-8s). We used a Performance Trends compression ratio calculator to figure out where you are and where you would be with thinner gaskets. To make 8.8:1 with the specs we've already listed as well as the ones we're taking educated guesses at, you'd have a 21cc dish in your piston. Installing the 64cc heads with another 0.040-inch gasket would drop compression to 8.33:1.
You will have lost 6 cc of chamber volume with the new heads, but according to SCE's loose rule of thumb, every 0.010 inch of gasket thickness is worth 2.5 cc's of chamber volume. Again, this is a very general guideline, since chamber shape, bore diameter, and other variables play a role. But, using that rule, going from a 0.040-inch gasket to a 0.021-inch gasket is just shy of a 0.020-inch reduction, which would gain you about 5 cc of chamber volume, putting you a little below the volume you had with the stock 58cc heads. The Performance Trends calculator is more specific, showing that the thinner gasket will bump compression back up to 8.63:1.
It's important to point out that while the effect of compression ratio on power output is significant, another general rule says that each point of compression is worth about a 4 percent increase in power output (we tested this back in the Jan. '02 issue). This means that even if your engine made 400 hp with 8.8:1, losing 0.15:1 would cost you about 211/42 hp. There is another issue to consider, however. You're probably going from an iron head to an aluminum head, and generally, when making that switch compression ratio should be increased to compensate for the increased heat dissipation that will occur with aluminum, which will cost power.
So, without knowing all of the variables, it would seem that using race-oriented copper gaskets to only gain 31/410 of a point of compression is somewhat futile. If you have already committed to the 64cc heads, reconsider the milling issue-shaving a little off would probably be beneficial in your case. Ideally, your street-oriented 351 could run around 9.6-9.8:1 on pump gas with no problem, but even milling and the thin gaskets may not get you there. If you're willing to mill the heads but decking the block and/or swapping pistons is out of the question, shoot for as close to 9.5:1 as possible with a composite gasket for regular street use.
Poncho PonderingsI have a '77 Pontiac Trans Am that I've owned for less than a year. It's mostly original, though it is in dire need of restoration. I don't have any of the original paperwork on the car, but as I've been researching the '77 model T/A, I'm finding that there was more than one engine available. Two were different versions of the Pontiac 400, one with 180hp and the other with 220hp, and then there was a 403 Olds. I'm pretty sure my car has a Pontiac 400, but I don't know which one, or how to figure it out. My VIN is 2W87Z7NXXXXX. I don't know if it matters, but this car is also a four-speed manual trans.Jim GordonLima, OH