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Old 08-15-2012, 10:55 AM
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Originally Posted by 64Joker View Post
What does "Not under diving the pump" mean?
Under-driving in place of under-diving, seems the spell checker looks at incorrect spellings but doesn't fix the wrong word I used for the one I was thinking. I hate that in an otherwise smart machine.

Under-drive pulleys are used with competition engines to reduce the power extraction the pump takes from the crankshaft. The power curve on the pump is such that power used is a geometric function of RPM (goes up in powers like doubling the RPM squares, or quads the power extracted) so at 6500 RPM you can be looking at 15 to 20 horses just to spin the pump. Additionally the pump output, while not growing quite so much as the power extraction, does start to output more coolant through the system than is needed at high RPMs to keep the engine temp stable. So without a thermostat or suitably sized restrictor it over cools the engine in addition to taking a lot of power from the crankshaft.

Taking these two events, it then becomes advantageous to slow the pump down which reduces the power extraction from the crank and does not force a situation where more coolant is being pumped than is necessary to maintain proper temperature. A win-win, how often do you see that?

This, however, does not work on the street in-so-far as moderate highway cruise speed and RPMs in the range of 2000-3000 are concerned. Under-drive pulleys will lead to insufficient cooling in these street settings, and like I said above, except at very high RPMs the power extraction of the pump is almost incidental. Of course one alternate solution is to use a high flow pump along with under-drive pulleys, but since very few aftermarket makers and sellers of these pumps provide numerical data compared to the OEM pump output and drive speeds, buying this stuff can lead you down a merry path of spending a lot of money to get unknown to incorrect results. Which is why for a street engine; I always recommend staying with what the factory puts on the engine unless you have numeric data, not making engineering decisions on advertising superlatives.


P.S. Back in the good old days of NASCAR level stockcar racing when much of the equipment was in fact stock and modified for the peculiar needs of racing. It was common to control power extraction and coolant delivery of the pump at high RPMs by not only slowing it down with under-drive pulleys but, also, grinding the impeller blades down. Needless to say when a caution yellow or stop red flag came out these engine immediately puked coolant and overheated.
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