Quote:
Originally Posted by comptiger5000
That sounds like a pretty good explanation. However, wouldn't accessory losses count more at high rpm, where they're spinning faster, and frictional losses, as well as pumping losses (in the power steering pump) are greater?
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That is a common belief because it is more intuitive and easier to measure the changes at WOT & high RPM - (racing or dyno'ing). Even though a % gain in power is similar, people tend to bias their judgment based on the difference in raw numbers.
The situation with judging the effectiveness of under-drive pulleys is further confused because they tend to show such poor gains in power during steady-state testing. Where the reduction of accessory drag really shines is during positive engine speed transitions. An under-drive pulley set that shows only 10 horsepower gain in a chassis-dyno test can show a solid performance gain equivalent to 20+ horsepower during a quarter mile pass.
A large chunk of this extra performance gain is during the first 60 feet of the run where average engine power output is the lowest, and the engine speed transition rate is the highest. Hard for some to imagine, but the vehicle acceleration rate is also the highest during the first 60 feet of a quarter mile pass.
Though the % power gain isn't exactly linear in relation to RPM, it is close enough for practical theory. It may help the intuitive understanding to realize that at lower RPM the engine speed can transition at a faster rate. Going from 1000 RPM up to 2000 RPM is doubling the engine speed, the same as 3000 RPM up to 6000 RPM. The same % of engine speed transition is concentrated into a visually smaller range of engine operation.
At the drag strip an under-powered vehicle may experience some drive-ability problems when launching at a low engine RPM. The load on the engine may be so much that tuning changes aren't enough to resolve the drive-ability problem without also lowering the vehicle performance. In a case like this, the reduction of accessory drag can make an unusually large improvement in vehicle performance (acceleration) by unloading the engine to the point that the low RPM drive-ability problem is either resolved, or then can be resolved with a change to the tune. Unloading the engine by reducing accessory drag can change the engine speed transition rate to the point that a given spark advance curve can be used without experiencing lug or knock.
This effect would not be so pronounced in a situation where the engine was already over-powered, or the launch RPM was significantly increased.
Under-drive pulleys perform in this way because it takes much more energy to accelerate the rotating mass of the accessories than it takes to spin them at a steady speed. The relative % energy required to accelerate this mass is proportionally larger when compared to the net engine power output when at very low RPM (especially at part-throttle). During normal driving the rate of RPM transition is also proportionally larger for a lower RPM range of operation, than it is for a higher RPM range.