Camshaft/Compression Ratio relationships
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| − | + | The final word in cam selection should always be determined by the grinder for your specific application. It's a free service offered by all cam manufacturers. | |
| − | + | OK, now with that disclaimer in place, I will attempt to help you see how the SCR (static compression ratio) plays out with the cam timing. There may be a mathematical formula, but I'm not aware of it, so I just use what information I have available to me and try to figure it out from there. We will not get into DCR (dynamic compression ratio) here. That's a whole other subject for another time. | |
| − | + | The main point to consider is the intake closing point after bottom dead center. That's what controls the amount of cylinder pressure the *motor will make. You don't make any compression at all until the intake valve closes. | |
| − | + | First is the SCR range of the motor, then the intake and exhaust duration @ 0.050" tappet lift, then the lobe separation angle, then the intake closing point and finally the operating range. Now please, don't anyone take this as the final word on camshaft choice. I only posted it to help the OP understand the relationships and this is a very short list of the thousands and thousands of cam grinds that are available to you. This chart was taken from the catalog of small block Chevy grinds with emphasis on the 350 cubic inch motor. Be aware that larger displacement motors will tolerate more camshaft and smaller displacement motors will be less tolerant of more camshaft. In other words, with the proper choice of cam for a given set of conditions in a 350, the same cam will be milder in a 406 and wilder in a 302. | |
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*7.50-8.50........184/194....104....16....500-4000 | *7.50-8.50........184/194....104....16....500-4000 | ||
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*A rule of thumb that I use is that you need to begin considering a looser torque converter when the operating range of the camshaft hits 1400 rpm on the low part of the range. A looser converter will make any cam more driveable, even a stock cam. You won't have to stand on the brake at stop lights and the motor gets up into a useful rpm range more quickly. For instance, I'd be thinking of using a 2000 rpm stall converter with a stock cam just to make the car more driveable. | *A rule of thumb that I use is that you need to begin considering a looser torque converter when the operating range of the camshaft hits 1400 rpm on the low part of the range. A looser converter will make any cam more driveable, even a stock cam. You won't have to stand on the brake at stop lights and the motor gets up into a useful rpm range more quickly. For instance, I'd be thinking of using a 2000 rpm stall converter with a stock cam just to make the car more driveable. | ||
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