The Physics of Boost?
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Instructor
Joined: Mar 2002
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From: Brentwood Essex
The Physics of Boost?
Someone explain this to me, because it's driving me crazy!
I have a Vortech V1 on my 85, and I know that she only developes boost at WOT. This makes my head hurt, because the blower spins at a rate constantly proportional to rpm, yet the engine at WOT is demanding more air as it's being fed more fuel. ...isn't it?
So the blower can only shift a given volume of air at a given rpm, but the greater demands of an engine under WOT should mean that pressure between impellor and manifold will reduce, not increase? (Which clearly it doesn't!)
I'm guessing the answer is related to rate of change of rpm (as this is simply another way of expressing acceleration, but I just can't see it.
Help meeeeeeeee. (Insert emoticon for voice dwindling as I fall into the black abyss of despair :confused: )
Steve
I have a Vortech V1 on my 85, and I know that she only developes boost at WOT. This makes my head hurt, because the blower spins at a rate constantly proportional to rpm, yet the engine at WOT is demanding more air as it's being fed more fuel. ...isn't it?
So the blower can only shift a given volume of air at a given rpm, but the greater demands of an engine under WOT should mean that pressure between impellor and manifold will reduce, not increase? (Which clearly it doesn't!)
I'm guessing the answer is related to rate of change of rpm (as this is simply another way of expressing acceleration, but I just can't see it.
Help meeeeeeeee. (Insert emoticon for voice dwindling as I fall into the black abyss of despair :confused: )
Steve
Melting Slicks
Joined: May 2000
Posts: 2,148
Likes: 1
From: Chandler AZ
Re: The Physics of Boost? (steveelsbury)
All supercharger systems should have a 'compressor surge valve' somewhere that dumps excess intake pressure between the SC and TB. This keeps the throttle plates from bending and the seals blowing out on the SC when the throttle is closed. Mostly these valves are held open (dumping all boost) until manifold vacuum falls to near zero at about 2/3 throttle; so you won't feel it until then
A centrifugal blower like Vortech/Procharger moves a volume of air proportional to the square of impeller rpm. For example my p600b is rated for 1200cfm flow at 60,000rpm (6000 engine); but at 1/2 that speed 30,000rpm (3000 engine) at only moves 1/4 the air: 300cfm.
With zero restrictions a normally aspirated 350 will only suck in 600cfm of it's own free will at 6000rpm; but the blower will be pushing 1200cfm at that speed. Boost is a measure of the manifold pressure increase above standard atmospheric (14.7psi). So to put that 1200cfm of air into a container that holds 600cfm requires doubling the manifold pressure (29.4psi); and that extra 14.7psi above normal atmospheric would measure as boost on your gauge.
A centrifugal blower like Vortech/Procharger moves a volume of air proportional to the square of impeller rpm. For example my p600b is rated for 1200cfm flow at 60,000rpm (6000 engine); but at 1/2 that speed 30,000rpm (3000 engine) at only moves 1/4 the air: 300cfm.
With zero restrictions a normally aspirated 350 will only suck in 600cfm of it's own free will at 6000rpm; but the blower will be pushing 1200cfm at that speed. Boost is a measure of the manifold pressure increase above standard atmospheric (14.7psi). So to put that 1200cfm of air into a container that holds 600cfm requires doubling the manifold pressure (29.4psi); and that extra 14.7psi above normal atmospheric would measure as boost on your gauge.
