Quote:
Originally Posted by serialk11r
I know that...do you see why pressure drop * cfm moved is a close number?
Say the manifold vacuum were 7psi (excuse me, I'll use 50kPa for easier calculations). Say we have a 2L engine, 1000rpm idle. 2L engine draws 1L every revolution. So you have 1 cubic meter of air each minute, drawn against 50kPa pressure. That's 50000J per minute, or 833W, slightly over 1 horsepower.
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1.1 horsepower.
what kind of mileage does that get you at idle stopped in traffic?
Lets look at cruising down the road at 2000 rpm. that means 2.2 horsepower is used in throttling losses.
That means in an hour, we are burning about .20 gallons of gasoline to maintain the vacuum inside the intake manifold, assuming 30 percent efficiency or so.
That is nothing.
Heck, if we are getting 50 mpg at 50 mph, the difference is only 20 percent in our mileage.
If we could go to aerocivic guy, and ask him if he would like another 20 percent mileage, think he'd be up for it?????
FUNDAMENTALLY, this is the EXACT reason smaller displacement engines get better mileage then large engines.
Large engines have much higher throttling losses BECAUSE THEY ARE BIGGER.
This also TOTALLY explains why taller gears get better mileage then shorter gears - because the RPM is lower. And the RELATIONSHIP IS LINEAR.
Plus, a smaller engine can run at higher absolute manifold pressure, for even less throttling losses.