dumb question

[GreyGoose006]

hey, i have a dumb question about superchargers

i was always under the impression that superchargers made boost in proportion to RPM.
ie, at 4500 rpm, you get 8 psi, at 5500 rpm, you get 12 psi, ect...
however, after actually driving a supercharged car for the first time, and watching the boost guage, i realized this isnt true.
at 3K, the car begins to make noticable boost on the guage, but this is at mid throttle.
when i close the throttle some, boost goes away.

now i know this is to be expected, but it got me thinking...
how does a turbocharged or especially supercharged car deal with low throttle situations.

if the blower is only spinning so fast, but the engine is sucking more air than it is blowing, the you have a massive vacuum, and a huge loss in efficiency.

is there some kind of releif valve built in to let extra air in so that there isnt a vacuum BEFORE the throttle?


also, how do power brakes continue to work in a turbo'd or blown car?
i thought the booster in brakes worked off engine vacuum, but if the car is making boost, then you have no vacuum, and no brakes...



i realize this is a dumb set of questions, but i just started thinking about it and dont have a answer.

[UncleBob]

paxton/vortec type SC's (turbo-like compressor wheel) are very linear just like your previous assumptions, don't peak until redline. Roots (twin screw, etc) are close to flat-line from near idle up (aka, 10psi at 2000 RPM's, 12psi at redline). Thats because twin screw is a true positive displacement pump, where as the paxton isn't. True positive displacement pumps, since they are turning the same speed as the engine, will make similar boost regardless to the RPM

Most SC cars have bypass valves for partial throttle operation

booster brakes work the same way on NA cars. its assumed that you'll be decelerating soon after applying the brakes, which gives you a vacuum supply. There is enough vacuum stored in the booster for braking while on boost/WOT, but after that, if you don't let off the gas, you will lose the booster, regardless to the engine having forced induction

You can see this by pumping the brakes while holding the gas WOT on any car. You will lose the booster until you let off the gas.

[GreyGoose006]

i didnt think about that.
i guess it makes sense that you wouldnt be wot and braking at the same time.

the whole throttling issue still doesnt make sense to me, especially on a roots blower where you have pressure at low rpms

[Iliess]

You have pressure at low rpms because a supercharger is belt driven and running off of the engine. The faster the engine spins the more boost you will get because the supercharger spins faster. If you want more boost you go to a smaller pulley.

[beef_bourito]

as engine speed increases, the volume of air sucked into the engine per unit time increases, as does the volume of air produced by the blower. they increase at a constant rate so the boost stays constant. in a centrifugal blower (turbocharger, paxton, etc) as the engine speed increases the cfm produced by the blower will increase at an accelerated rate whereas the engine cfm will increase at a constant rate so the boost increases.

if i didn't explain that clearly let me know, i'm pretty tired so i'll fix it tomorrow if need be.

[UncleBob]

Originally Posted by GreyGoose006

the whole throttling issue still doesnt make sense to me, especially on a roots blower where you have pressure at low rpms

its the bypass valve. I'm assuming you drove a Grand Prix GT? (most common SC car that I see anyway)

The bypass valve stays open until 1/2 throttle or so, then closes. No boost until you get over that

[GreyGoose006]

actaully it was a 2003 mustang shelby 10th anniversary edition (with a supercharger... dur)
it must be a bypass valve issue then for me.
either that, or the scenario i am envisioning doesnt really happen.