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Old 09-08-2010, 01:11 PM
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Originally Posted by benwantland
All the theoretical discussion going on here is fun, but I think this gets to the meat of the original question:

They're just words, and a lot of this phraseology isn't exactly textbook, but the widely-accepted meaning of installing a cam "straight up" is that the Lobe seperation and Intake centerline are the same, or, in other words, at TDC at the end of the exhaust stroke, the cam is perfectly centered between its intake and exhaust events.

The general consensus among folks who seem to know is that installing the cam 4 degrees advanced from straight up is usually optimal for street applications, and - as mentioned - most street performance cams are ground this way. Most. And most timing sets are made so that installing dot-to-dot doesn't give any additional advance/retard. Most.

But then, the cam grind could be off a bit, or the timing set could have an advance or retard built in that you're unaware of. Or you could have an adjustable timing set and not be sure how to set it. This is why we "degree" cams when installing them. To make sure they're installed in the intended phase relative to the crankshaft.

FWIW - I've always heard that some of the blame for low-compression emissions engines being dogs in the 70's and 80's was because of retarded (at least relative to what we consider normal, i.e. 4 degrees advanced) cam timing.

In general - if all other aspects of the engine are supportive of the change - retarding the cam pushes the power curve farther up the rpm range (moving valuable power out of the lower rpms), while advancing the cam slides the power closer to the bottom end (due to closing the intake valve sooner, letting the engine build more cylinder pressure, i.e. torque).
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