Q: : I have a 1967 Pontiac GTO with a 400-cubic-inch engine that originally came from a 1968 Grand Prix. The 10.75:1 compression engine is rated at 340 horsepower. The car has an M-20 four-speed transmission and 3.55 rear-end gears, and curb weight is about 3,500 pounds.

What I really want is a strong, daily-driven street car that can run on pump gas and take an occasional trip down the drag strip. I’m on a limited budget, so I want to keep modifications to a minimum. I have been told to put a 455 engine with 6X heads in the car, as this will give me plenty of torque. I know keeping my 400 would be a lot cheaper. Could I put the 6X heads on the 400, or would they cost me horsepower? Would a cam change make up for any loss of power if I do switch the heads? If I stay with the 10.75 compression the engine has now, will 92-93 octane do any damage?

I planned on using an Edelbrock Performer RPM or an old Pontiac aluminum intake. If I swap cams, should I run a Pontiac Ramair IV grind or a Performer RPM? I would like to keep my Quadrajet carburetor if it will work OK.

Finally, can I swap the timing cover to a 1970s version so I can use that later model water pump?

A: Your 400 seems to be about right for what you want to do. We do suggest backing off on the compression if you want to run pump gas. At 10.75:1 compression, pump gas will definitely cause detonation, which may lead to engine damage if left uncontrolled.

The 6X heads have a 101cc combustion chamber, which would give your 400 around 7.75:1 compression — that’s too low. Heads with a 72cc chamber would give you 9.7:1, which is a better deal. If you have access to the heads from the original 1967 engine that came in the Goat, they would be ideal.

If you want to change camshafts, we’d recommend Crane’s CRN-968781. This is a blueprinted version of the 068 Pontiac grind. The Q-Jet carburetor will be fine once you rebuild and upgrade it.

As for the timing cover, you can use a ’70s style cover as long as you get ALL of the later model pulleys. Pontiac went to a bigger and longer water pump in the early 1970s, which forced them to reposition the pulleys.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all

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