wow 12.7 in the 1/4th not to shabby..

 

I'll try to give a quick and simple explaination here. If you want more detail, let me know and I'll try to dig up some resources.

Basically the blower used by BRP is a positive displacement pump. That means that for every revolution, reguardless of RPM, it will move the same amout of air.

The centrifugal blowers are more like a fan. Think of an ordinary desk fan, if it is spinning very slowly, it moves a very small amount of air. However if you bring it up to 2000 rpm, it will begin to move a lot more air. Though the centrifugal s/c is technically different, in principle it is the same effect.

The following numbers are for example only, not at all accurate I'm sure.

So for example at 2000 engine rpm (with bypass valve closed, which depends on throttle position, in this case WOT) the BRP s/c will move 1.2L of air per engine rev, while the centrifugal s/c will move 0.2L of air per engine rev. However at maximum engine rpm, both chargers would move the same amount of air (in this example).

HTH

 

A centrifugal compressor (centrifugal supercharger or compressor stage of a turbocharger) provides boost (and additional flow-rate) exponentially proportional to the speed it is spinning. The faster it spins, the more boost. Note that it is NOT a linear relationship.

A positive displacement supercharger (Roots, Eaton, or lobe type) is less of a compressor than it is an air pump. It does not compress the air, it merely moves it from one location to another. All of the compression takes place after the air exits the supercharger. A centrifugal compressor increases the air pressure inside it's housing, in addition to any compression that will occur downstream due to lots of air being rammed into a small space.

Let's speak in ideal circumstances. The roots-type blower has the ability to move X amount of air at Y RPM in a linear fashion. It will move twice as much air (flow-rate) with twice the rotational speed. Coincidentally, an engine also needs air in a linear fashion. If paired correctly, the roots-type supercharger will provide exactly Z times the amount of air an engine would normally need at all RPM. This corresponds to constant boost throughout the rev range.

Of course, ideal circumstances are just that. No matter what someone selling a roots-type supercharger will tell you, they do lose efficiency at higher rotational velocities. This is why you see the boost drop off at higher RPM in almost all Roots applications. However, there have been many advancements in their design and they are vastly superior to the chargers of old.

 

beat ya

 

Habitforming, you probably beat me by ten seconds

(and one part of yours is incorrect)

 

The tune was done by the best but the construction of the kit he was using was flawed. He has modified the design to get a more linear powerband since his last dyno.

BTW, would it be wrong to say that $1100 for an extra 30whp is too much?

 

[QUOTE=slimjim8201,Feb 7 2006, 08:08 PM] Habitforming, you probably beat me by ten seconds

(and one part of yours is incorrect)

 

The amount of air it moves is linearly proportional (more or less) to RPM. If it moved the same amount of air at all RPM, your boost would start off high and linearly decrease with engine RPM. The anti-turbo

 

You VT guys sure aren't edumacated too well

No error in my statement: fixed amount of air per REV. So yes at higher RPM it will push more air, per TIME, which is what you are saying.

I think UK College of Engineering still has some openings, and it's not too far of a drive

 

Thanks guys for all the advice, after reading your posts and doing some more research on Wikipedia I finally have it figured out. Here is my laymens recap please correct me if I am wrong:

The roots type supercharger moves a constant amount of air which is higher than the engine's (because it is geared higher than the engine). It moves more air the higher the RPM but the engine needs more air the higher RPM, so the engine still gets the same (higher) concentration of air from the supercharger throughout the RPM range. Hopes this makes sense. Again, thanks in advance.