Boost and pressure ratio is the same thing.
Pressure ratio = 'Pressure out' / 'Pressure in'.
'Pressure in' is atmospheric pressure = 14.7 psi.
At 10 psi boost you get a pressure ratio of ( 10 + 14.7 ) / 14.7 = 1.68.

What this graph means is that with a centifugal supercharger the boost is a function of rpm. By changing the restriction after the supercharger you will change the CFM a lot, but the boost will not change much.

Yes, According to the compressor map a 350ci and a 400ci engine will show similar boost at the same rpm.

The boost will determine how much volume that gets into a specific engine.
If the air is colder, then more air will fit into this fixed volume.

A displacement supercharger ( Roots or Eaton ) will have a very different compressor map. Changing the restriction after the supercharger on one of those will have a large effect on the boost level, but the CFM will not change much.

[Modified by JoBy, 12:37 AM 5/22/2003]


[Modified by JoBy, 12:45 AM 5/22/2003]

 

WE'VE FOUND THE MISSING LINK!!! :D

Ok, so engine rpm determines supercharger rpm which determines boost, not CFM! The disconnect is with #3. The supercharger will actually flow more air with a properly sized intercooler.

So if I want more boost I spin at a different RPM and it will have a different pressure ratio to feed. Very Interesting.....

 

mn vette Remember me telling you several post back that it was better to use a larger blower and turn it slower instead of spining a small one at warp speeds. If you look at where the blower is most efficient it is in the 30 to 35000 rpm range not the 50,000 rpms

 

Nope :nono:
All the engine cares about is air mass. Air mass is air density * air volume. Air density is increased by compressing it and/or reducing it's temperature. Air volume is increased by spinning the blower faster (but this can also increase temp.)

When the piston goes down there is 14.7psi NA pressure differential pushing air into the cylinder. It will accomodate a fixed volume of air, and therefore a certain air mass at this density. A blower raises this pressure (say 24.7psi with 10psi boost), increasing the density and allowing a greater air mass to fit in the same volume. But remember it does ths by forcing more CFM into a fixed space thus increasing the pressure (density). As the engine spins faster its volume requirements increase linearly, while the blower's go up exponentially. So there's your extra air mass and the pressure differential to push it in there.

So there are three components: airflow in CFM, the pressure behind it, and it's temperature. Of course all of this assumes an ideal shot straight into the cynlinder; the intake may be an airflow restriction. Unfortunately backpressure from the intake also appears as manifold 'boost' which has NO relation whatsoever to how much air mass actually went into the cylinder.

Like I always say, if you want to see high boost on your gauge, heat up your intake air and put on the most restrictive manifold you can find! If you want air mass and power, set the blower for need CFM, cool down the air, and give it a free flowing path into the engine. You could achieve the same air density at a much higher temperature without intercooling, but practically this causes detonation and mangled engines.





[Modified by gcrouse, 2:25 AM 5/24/2003]

 

[QUOTE]Ok, so engine rpm determines supercharger rpm which determines boost, not CFM! The disconnect is with #3. The supercharger will actually flow more air with a properly sized intercooler.

Nope :nono:
All the engine cares about is air mass. Air mass is air density * air volume. Air density is increased by compressing it and/or reducing it's temperature. Air volume is increased by spinning the blower faster (but this can also increase temp.)

When the piston goes down there is 14.7psi NA pressure differential pushing air into the cylinder. It will accomodate a fixed volume of air, and therefore a certain air mass at this density. A blower raises this pressure (say 24.7psi with 10psi boost), increasing the density and allowing a greater air mass to fit in the same volume. But remember it does ths by forcing more CFM into a fixed space thus increasing the pressure (density). As the engine spins faster its volume requirements increase linearly, while the blower's go up exponentially. So there's your extra air mass and the pressure differential to push it in there.

So there are three components: airflow in CFM, the pressure behind it, and it's temperature. Of course all of this assumes an ideal shot straight into the cynlinder; the intake may be an airflow restriction. Unfortunately backpressure from the intake also appears as manifold 'boost' which has NO relation whatsoever to how much air mass actually went into the cylinder.

Like I always say, if you want to see high boost on your gauge, heat up your intake air and put on the most restrictive manifold you can find! If you want air mass and power, set the blower for need CFM, cool down the air, and give it a free flowing path into the engine. You could achieve the same air density at a much higher temperature without intercooling, but practically this causes detonation and mangled engines.


/QUOTE] :yesnod: :yesnod: :yesnod: :yesnod: :yesnod: :yesnod:
:iagree: :iagree: :iagree: :iagree: :iagree: :iagree: :iagree:

 

Yes and No

Facts:

(1) The possible engine power is a function of 'air mass flow'.

(2) The 'air mass flow' is a funtion of 'air volume flow' and 'air density'.

(3) The 'air density' is a function of 'boost' and 'air temperature'.

(4) The 'air volume flow' is a function of 'boost' and how well the engine flows at that RPM.


Look at the graph again.

The corrected CFM is before the supercharger, at atmospheric pressure. The graph is plotted with air mass.

The engine rpm DOES determine supercharger rpm which DOES determine boost, not CFM!

To make more power using the same supercharger pulleys you have a few options.
From the graph you see that boost will be about the same with the same pulleys.

To ger more 'air mass flow' you have two options

(A) Increase 'air volume flow'. To do this with fixed 'boost' you have to port the intake or install a larger cam.

(B) Increase 'air density'. To do this with fixed 'boost' you have to lower the 'air temperature'. INTERCOOLER !!!

The original quote is correct.



[Modified by JoBy, 4:30 PM 5/24/2003]

 

Boost is the result of the backpressure generated by inflow vs. outflow rates. Spin your blower up so it's pushing 1000cfm into open air; 1000cfm in / 1000cfm out- no backpressure, no boost. Now stick a jar over the blower outlet; 1000cfm in / 0cfm out, infinite backpressure / boost (theoretically.) Yes the blower rpm and impeller/compressor scroll tolerances, seals, etc will determine the maximum pressure it is capable of. Now have it blow thru a small tube that flows 500cfm; 1000cfm in / 500cfm out- the resulting backpressure from trying to push 2x the air is 2 atmospheres. Since you have 1 atmosphere at the outlet, the extra pressure you're creating- your "boost" is 1 atmosphere (14.7 psi).

Blower rpm determines airflow AND the max pressure it can create. If you want to make 2x the power you need to stuff 2x the air into the cylinder. So you need the blower to move 2x the NA cfm requirement of the engine; AND enough pressure behind it to stuff it in there.

 

True but ít does not matter.

Using the supercharger 'open air' will put you off the chart to the right.

With a jar over the outlet you are off the chart to the left ( surge ).

Within the centrifugal superchargers operating range my previous post is true.

Within 'operating range' the centrifugal supercharger will change boost very
little with a change in restriction after the supercharger. The mass air flow will change, not the boost.

A displacement supercharger ( Roots, Eaton ) is VERY different. A change in restriction after a displacement supercharger will change the boost a lot, but the mass air flow will be almost the same.

Your resoning works much better with a displacement supercharger than a with a centrifugal supercharger.



[Modified by JoBy, 9:45 PM 5/24/2003]