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.
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Just my 2 cents-worth; unless you’ve already purchased the aluminum intake, your original non-EGR intake is more than enough through 6500 rpm. It is a slight weight penalty over the aluminum, though. That could be made up by relocating the battery to the right rear of the car.
If you want a little more I would add a set of roller rockers or roller tip rockers. However you will need to change the rocker arm studs to the big block Chevy style and adjust them. The motor will scream and rev faster than the standard stamped steel rockers. I would also go with a little more cam, crane part number 283941. Great low end torque and power all the way through 5000 RPM’s. Vacuum quality is fine for power brakes. Idle Quality is also non noticeable. Would also have nice headers.
If you plan on using the car mostly on the street, and want decent performance from a 400 ci, the 4X heads (96 cc) from about the 73 – 74 years, are a very good choice. Remember to purchase a complete kit for the cam. Dont forget about the Spring pockets that are milled into the heads. If you go with a RAM AIR IV heads, you will need matching springs. If you want to do it right, those springs are too big for the spring pockets in a set of 4X or 6X heads. Yes you can run them like that, but it is not doing it rights. Put you a Comp Cam XTreme 262H Kit, complete with springs etc, with Roller Tip 1.52″ rockers, an Eddy performer or Performer RPM which the latter would be my choice. A holley 750 HP with mechanical secondary’s or a Rochester stock spread bore carb if you want to save a little cash. Go with a Holley 110 gph fuel pump, and definitely invest in a set of 1 3/4″ headers. Don’t buy the 1 5/8″. You will basically get the same HP, but you will lower your rpm power band where the peak of the torque kicks in a lot sooner with the 1 3/4″, and you will definitely feel it. That cam will give you the HP at the top end. This combo is not a pro-stock combo, but a sensible combo.
Don’t believe the 6500 rpm range on a poncho motor. Stock Pontiac engines don’t have the lower end for that, unless you want to spend $1000s. Stay focused on your goal and don’t let someone talk you into something that you will not get on a budget. Keep it fun. You are not going to get an 11 second street car on a budget if you are talking about a limited budget as you stated above. If you want to get a trouble free ignition, go with the MSD ready to run distributor and a street blaster coil. Don’t buy junk from China. You will regret it and only throw away money. Stick with the proven US made compenents, Holley, Edelbrock, Hookers, MSD, etc. They have good tech support and will go out of their way to help you.
The cam above will give you a little lope, not a lot but enough to make you feel like you have something under the hood, and will be good for making vacuum for your brakes. The cam is very streetable. Remember, for every modification you do, it can have a detrimental re-action if not done right and if the parts are not compatible. Going with a 068 Cam, would not be my choice. Bigger is not always better on camshafts. You gotta have the valve train to match, such as matching springs if you want to do it right. A reduction of vacuum due to the higher overlap of the valves with the 068, with the kind of streetable compression. Keep your duration at .050″ around 225° – 230° max if it is a budget car and mostly on the street.
I have had Pontiacs for years and love em. I keep my 74 at 5300 rpms or lower. Any higher and you are asking for trouble if you have stock lower end. Be sensible, not stupid, otherwise it will cost you a lot in the long run and you will become disappointed and lose interest as well as wasting a lot of money. Keep it fun, not a money pit, unless of course you have plenty of cash.