How much can I mill down a 6X-4 head? I’m building a stock 455 that’s 0.030-over with flat-top pistons and an “068” cam. I’m trying to get my compression up, but still run on pump gas. The engine is going into a ’76 Firebird backed with an automatic trans and 2.73 rear gears. This is my first Pontiac build so I’m a bit green, and I’m looking for as much power as I can get. Can you help?
Steve, congratulations on your first Pontiac engine build! Hopefully, you will find it a rewarding experience and you’ll be able to appreciate the generous amount of low-end torque that Pontiacs are known for and hobbyists have come to love.
The 6X-4 cylinder head was introduced during the mid-’70s smog era and used on the 350-inch V-8 and the ’77-’79 T/A 6.6 (400). It has a combustion-chamber volume that measures between 89 and 91cc’s and is desirable because it can be used to increase an engine’s compression ratio without spending big bucks for early small-chamber castings (72 cc) and has large intake valves (2.11-inch diameter) and threaded rocker studs.
Cylinder heads from the ’70s can typically be milled as much as 0.060-inch without any issue. Each 0.005-inch removed from the deck surface reduces the chamber volume by 1 cc, effectively increasing compression. And because the Pontiac V-8 is a 90-degree design, an equal amount of material must be removed from the intake flange to maintain proper intake-manifold alignment.
There are no negative attributes associated with adding compression to an engine as long as suitable fuel octane is available. Each point of compression can increase horsepower by 2 to 3 percent through the rev range. A compression ratio that’s too great for the fuel octane can induce detonation, which can seriously damage the engine’s internals, ultimately ending in complete engine failure.
When assembling a Pontiac V-8 using original cast-iron cylinder heads that will be primarily driven on the street, I generally shoot for a compression ratio between 9.1:1 and 9.5:1. It ensures that the engine will operate reliably on commonly available 91- to 93-octane pump gas without any issue—assuming it’s properly tuned. If you’re looking for maximum power output, compression can be pushed toward 10:1 or slightly more, but it requires precise control over the fuel and timing curves, and a highly efficient cooling system.
When combining your 6X-4s with your 0.030-over 455 with flat-top pistons, the compression ratio will be very close to 10:1 before the cylinder heads are milled at all, and any amount removed during the machining process will simply increase the compression ratio. In my opinion, that’s a bit too much, especially if you’re just learning about Pontiacs and how they operate.
You might be better off shelving the 6X-4s for a future project or subsequent rebuild and finding a set of 6X-8s to use for this project. The 6X-8 is functionally identical to the 6X-4, but its combustion chamber measures closer to 100 cc’s, and that will provide you with a much more manageable compression ratio for street operation on pump gas. Good luck!
I’ve been an HPP subscriber since 1990. I’m restoring a ’79 10th Anniversary Trans Am. The first body shop separated the Shaker-hood metal base from the fiberglass/plastic top. Can you tell me what kind and size rivets attach the Shaker base to the top?
Steve, thanks for being a loyal subscriber. I’m glad to assist you with getting your Shaker reassembled.
The sheetmetal frame that fastens the Shaker assembly to the air cleaner is a separate piece that’s retained to the fiberglass housing using truss-head rivets with a body diameter of approximately 3⁄16-inch. Since the rivets were installed prior to the Shaker being painted, body color would be the correct finish.