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Old 09-08-2010, 02:08 PM
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benwantland benwantland is offline
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All the theoretical discussion going on here is fun, but I think this gets to the meat of the original question:

Quote:
Originally Posted by Adam65
Straight up timing is defined as when the lobe separation angle and the intake centerline angle are the same. Most racing cams are ground with no advance in them, so if they are installed "dot to dot", that is straight up: a 110 lobe separation cam will be installed at 110 intake centerline. Most aftermarket street cams are ground with 4 degrees advance. If you have a aftermarket street cam with 110 degrees lobe separation angle and put it in dot to dot, your intake centerline will be 106 degrees. You would have to put in a 4 degree bushing and retard it back to 110 degrees intake centerline to be straight up, even though dot to dot is what everyone thinks is automatically straight up timing. It is dependant on how the cam is ground.
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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