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455CI Pontiac Engine Build - What's Your Problem?

Building Big Bird

Tom 455bird via CarCraft.com: I just bought a Pontiac 455 for my '68 Firebird. The car has its original 350, and I'm hoping to slip in the 455. I read "Junker to Thumper" in the June '03 issue-you know, '76 455, 210 hp, add headers, intake, and cam, and you get 351 hp. I love that article. Anyway, the 455 is a '72 YC code, which I believe has 250 hp, but I discovered the heads on it are '75 vintage with the 5C code and a 7.6:1-compression ratio. The heads on my original 350 are 17 code with 8.6:1 compression. Should I swap the heads? Obviously I'm an amateur, so any help would be great.

Terry McGean: The 124cc heads we used on our '76 455 for the story you referenced were indeed worthless, having the hugest chambers of any Pontiac V-8 heads, but our sources show that your 5C heads are actually from a '75 400 and that the chambers should measure 101 cc. This would make them very similar to the 6X heads used on many late-'70s 400s, and not a bad choice for a modest 455. Most Pontiac street guys look for No. 96 heads-used on '71 400 two-barrel engines-because of their 96cc chambers, which reputedly create a theoretical ideal pump-gas compression ratio of 9.5:1, something Pontiac never offered on a 455. We used a different set of factory iron 96cc heads, the No. 7K3 castings, which are less desirable because they lack the outer exhaust-manifold bolt holes and bosses. They worked for our dyno test ("New Aluminum for Pontiacs," Jan. '06), but with the stock pistons being significantly below the deck at TDC and the thick composite head gaskets we used, our compression was probably closer to 9.0:1. If we had rebuilt our 455 0.030 over and had it slightly decked, we would have easily found that half a point, if not more, particularly if our new pistons had provided a "zero deck" condition.

If you had your 5C heads cleaned up and milled slightly, you could probably get them to provide around 9.0:1 on your exist-ing engine, assuming it hasn't been rebuilt. If you're going to build it, take the time to figure out the deck height before you order pistons to get as close to zero deck as possible. As for the 350 heads, their combustion chambers are probably small enough to make more compression than you'd want on the 455 (such as over 11.0:1), plus they have the smaller 1.96/1.66-inch valves instead of the 2.11/1.77-inch valves of the big-inch Pontiacs.

Ask Anything

Ask Anything is the portion of What's Your Problem where readers send questions for industry bigwigs, then we get the answers. So pick a hero, then come up with good questions and send them to Car Craft@primedia.com.

Curve Confusion

Steve Donnelly, Las Cruces, NM: I have a '67 Mercury Cougar that I recently acquired from a family member. It's all stock and has a 289 with a two-barrel carb and a single exhaust. I'm going to mildly upgrade the car without making major changes. The planned four-barrel, aluminum dual-plane intake and headers, and dual exhaust are no-brainer items, but I need help figuring out the ignition. I wanted to tune up the car for increased performance even before changing the hard parts.

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