Quote:
Originally Posted by lust4speed
Here is a photo showing pistons available in 1967. If your pistons are original then they are flat top. Notice the valve reliefs are different in the piston tops and your pistons would have the eyebrows in the wrong place for the 670 heads.
If you wanted to continue to use regular gas then the current 8.6:1 compression ratio is about perfect.
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Basically what I said earlier, but was met with, "I have 670 heads already". 670 heads aren't going to be the answer to the question, IMO.
Throughout the years the 670 head has been given a lot of ink, but there are better choices for the car in question. This is a cruiser, not a racecar. They surely wouldn't be my choice if I were going to try to wring power from a street car. They likely won't run on pump fuel, even premium fuel. I had a 67 engine for my 67 GTO racecar that I took off the 670 heads in favor of 62 heads before installing in my car, because I always felt the open chamber was superior to the closed chamber bathtub.
The regular fuel 400 basically used the 66 389 heads, hence the reason for one year only pistons with the older holdover valve inclination angle. In 68 the regular fuel engine became the same valve inclination angle as the premium fuel engines. I believe they gave up the bathtub combustion chamber also in all 1968 engines. When I worked at a Pontiac dealer in 1970 I remember a running change during the 1967 model year for broken valve springs, so depending on where in the model year the heads fell, they may have the old springs that were recalled.
If the car runs as poorly as the OP says, there has to be something fundamentally wrong with the engine, but I already have mentioned this in my previous posts. Putting hi compression heads on an engine that runs poorly is just a waste of time. Finding out why this engine runs so poorly would be my first priority, before pouring a bunch of money into a unknown quantity.
If the engine is worn out, or has low compression due to a mechanical defect, the OP might be ahead to find a 428/455 in running condition, and come out with more engine, for less money. Win, win
