I just purchased a 1970 LeMans\ GTO conversion with a 400 from a dealer across the country. I'm currently waiting to receive the car. The dealer indicated to me that he thought it needed a bigger carb. It has an Edelbrock 450 cfm. When I received the title in the mail, it had the previous owner's name on it so I thought I'd give him a call and ask him about the car and restoration process. I had a great conversation with him and learned a lot about the care he put into the restoration.
When I told him about what the dealer said about the carb size he disagreed, but did say that the previous owner he bought the car from had fitted the engine with 670 heads and a mild cam and once he discovered he couldn't use Regular pump gas, he installed low compression pistons. He (the man I was talking to) felt that there was significant power loss as a result.
So, my question is, what is my solution to get this engine running as intended or better? Put the original heads and pistons back on? Keep the heads and put higher compression pistons on and take a beating at the pump? I don't know much about mechanics so any your thoughts would be very welcome.
Well, I doubt it has a 450CFM carb. Edelbrock makes both the Q-jet and the AFB. The Q-jet is 750CFM. AFB comes in 500, 600, & 750CFM. Many dealers have no clue about cars they sell unless they are also mechanics and/or Pontiac enthusiasts.
You can figure out what carb is when you get it. Factory is the Q-jet along with a stock cast iron manifold which when set-up correctly cannot be beat.
The 670 heads are 1967 closed chamber. The '67 engine was rated at 10.75, but most likely 10.5 and required premium gas. To use them today, could not use pump gas and would then have to go the racing gas route or an octane booster.
The solution when using any high compression head, whether closed chamber or 1968 and up open chamber, is to use a dished piston or one having more cc's volume for the valve reliefs in the top of the piston. A pump gas friendly iron headed engine should be around 9.0-9.3 compression if set-up correctly.
You will lose some power by using a lower compression, but not a whole lot. Often times the timing and distributor curve can be changed and can wake up and engine. Many do not understand the function of timing and the advance curve with regards to a Pontiac - it is not a Chevy.
So at this point I would not be too discourage yet, but wait until you receive the car and then go from there. You may be able to make a few changes and fine tune the set-up and be pleased with how it runs. All to often people can mismatch parts when building a Pontiac engine and then it runs poorly, so a few changes may be in order.