WHY?????

[04CHASE]

ok i keep thinking ( i do that to much) but i have always wondered why does a forced induction vehicle not make boost when it does not have a load on the engine? for example when you rev up your truck when its not in gear it will not make boost, i dont understand, your engine is still reving and your still spining the blower or spooling up the turbo, so why doesent it make any boost?? i know its probably a simple answer but i cant think of any logical reason that come straight to mind? someone school me please!

[dcairns]

Engine RPM vs throttle position. With no load, the RPMs are higher, for a given throttle position. With the engine flowing so much air, no boost will build. It is the same reason the manifold vaccum stays higher when reving out of gear vs in gear

[SSplaytoy]

Engine RPM vs throttle position.  With no load, the RPMs are higher, for a given throttle position.  With the engine flowing so much air, no boost will build.  It is the same reason the manifold vaccum stays higher when reving out of gear vs in gear

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[tawss04]

Engine RPM vs throttle position.  With no load, the RPMs are higher, for a given throttle position.  With the engine flowing so much air, no boost will build.  It is the same reason the manifold vaccum stays higher when reving out of gear vs in gear

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[PladdPezzPunk]

different way to put it is thinking of boost as work that the blower is doing. the blower has to do no work without load so it dosen't make any boost. when you put a load on the engine, everything has to work harder to acheieve the same rpm so that will show boost.

[04CHASE]

it still does not make sense to me, i just see it as the blower spins a certain rpm it should make boost . how does it determine if it has load on it?

[misterp]

it still does not make sense to me, i just see it as the blower spins a certain rpm it should make boost . how does it determine if it has load on it?

I agree with your question - that's why they call it forced induction, right? If you are spinning the head unit at max RPM it should be producing 800-1000 CFM whether the engine has need for it or not, so you would think the guage would show boost and the blow-off valve would be venting off the excess

 

Mr. P.

 

edit - now that I think about it, the answer may differ depending on the specific type of supercharger...

Edited November 20, 2006 by misterp (see edit history)

[dcairns]

Try this, with the trans in neutral, throttle postion at 50%, engine will spin at some RPM, lets say 5,000 RPM and manifold vaccum will be high. Now try it again, in drive, same throttle postion, engine RPM will be considerably less due to the load of accelerating the truck manifold pressure will be low or in boost. Think of boost as the air that can't get through the cylinders fast enough because of the difference in RPM.

[Black2003SS]

Also, boost is made by resistance (pressure), without resistance (load), boost won't be made.

 

dcairns hit it on the head, it is difficult to understand without seeing. If you are more like me, you dont get it until you have seen it or done it.

[06VHO]

It will make full boost in neutral. You would have to hold it at WOT for a while though, KABOOM!!!!!

 

Centrifugal superchargers are not positive displacement compressors, so there is some lag.

 

With the Whipple Supercharger I put on my truck it makes instant boost in neutral, because it is a positive displacement compressor, no lag.

 

Only turbos need load to make boost, due to the fact that they need exhaust heat to expand the exhaust gasses to spin the turbine fast enough to make boost.

[zippy]

The easiest way to understand this is to consider the blade factor. On a system like a Procharged truck or Vortech the blower will move the air and produce boost either way as you rev it, but the throttle body will stop that much air from getting into the engine and creating boost. There will be alot of pressure in the tube, but not on the other side of the throttle body. On a draw through system such as a Radix (draw through meaning the blower draws air through the throttle body instead of pushing past it) the blower won't produce boost by simply revving the engine because the blower can't get enough air past the throttle body to produce boost in the lower intake.

[misterp]

Thank you 06VHO & zippy; that all makes perfect sense. I guessed that about the turbo but forgot about the relationship between the TB and the head unit.

 

Mr. P.

[Dirt.boy]

The easiest way to understand this is to consider the blade factor. On a system like a Procharged truck or Vortech the blower will move the air and produce boost either way as you rev it, but the throttle body will stop that much air from getting into the engine and creating boost. There will be alot of pressure in the tube, but not on the other side of the throttle body. On a draw through system such as a Radix (draw through meaning the blower draws air through the throttle body instead of pushing past it) the blower won't produce boost by simply revving the engine because the blower can't get enough air past the throttle body to produce boost in the lower intake.

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So does this correspond to gas mileage differences during daily driver duty?

It would seem that the Radix is doing less work when boost is not required therefore using less energy.

[04CHASE]

The easiest way to understand this is to consider the blade factor. On a system like a Procharged truck or Vortech the blower will move the air and produce boost either way as you rev it, but the throttle body will stop that much air from getting into the engine and creating boost. There will be alot of pressure in the tube, but not on the other side of the throttle body. On a draw through system such as a Radix (draw through meaning the blower draws air through the throttle body instead of pushing past it) the blower won't produce boost by simply revving the engine because the blower can't get enough air past the throttle body to produce boost in the lower intake.

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ok that is the first thing anyone has said that really makes sense to me, thank you !!

[zippy]

Sometimes explaining the simple plain way of looking at it just does the job. It's about the easiest way of understanding it from how I see. That kind of reminds me on my explaination of the Tornado product. Why bother getting air spinning in the intake tube if the throttle body is just going to stop any spinning that the air might have actually been doing before it even gets into the intake. Simple thoughts often make a much more simple answer.