Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Need help choosing a cam. SBC 350
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Old 10-10-2012, 12:47 PM
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Quote:
Originally Posted by vinniekq2 View Post
tech,I dont know why you posted that.If the cam is slightly more than he had and cr from 8.5 to 10 :1 a slightly bigger cam will improve the engines breathing.If his engine is 8:1 or 11:1 the cam will make a bigger difference and choice is more important.(The engine is mainly stock) was posted in OP,please tell me a slightly bigger grind that would make this engine perform worse?and
OK Vinnie, let's work through this. Please see the link I posted in post #14 of this thread, in response to the OP's post #10. If his motor is maxed out now for cylinder pressure, then it is at the highest level of efficiency that it's going to be for pump gas. If going with more cam (which will extend the intake valve closing point), the piston will be further up the bore when the intake valve closes, thus pushing some of the mixture back up the intake tract (trapping less mixture than it does now) and making less cylinder pressure. That's the whole big deal with knowing the static compression ratio, the fact that you can figure the dynamic compression ratio from it and maximize the cylinder pressure for the fuel you're using. Otherwise, it's just a guess. In the case of this young man, I would advise him that his combination is maxed and that the only way to go faster now is with more cubic inches, because more cam will lower cylinder pressure and the car will likely slow down. I am aware that there are fellows who have run over 200 psi cylinder pressure on pump gas, but they had to ditch PCV and EGR and de-burr and polish everything in the chamber and on the piston to prevent detonation.

I will agree with you that this thing of building motors is not rocket science, but there are laws of physics that have to be observed if you intend to hit a home run with your build. I certainly don't know all the answers and I'm no mechanical engineer, but I have been doing this stuff since 1958, when I was 16, and I have made many mistakes. When I finally got fed up with guessing, I began reading what the engineers and professional engine builders were writing and learned enough to get a lot closer on my choice of components to reach the goals I had set for the motor I was building.

Choosing components for a build just is not that difficult if you pay attention to what the experts/professionals are telling you. Again, I'm no expert, but I do listen to the experts and read what they have to say. If I say something on these boards, it is the result of either having done it myself or quoting an expert who has done it. No hocus pocus, no guesswork, just physics and reality.

One final thought. If you did not buy this motor in a vehicle brand new and you were the first owner, then you have no idea what components are in the motor or what machining has been done to it. Don't assume.
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