Question on the effects of throttle position on boost level for PD vs Centri Blowers

ScortLX · Jan 8, 2009

Hello. I am thinking about putting a KB on my 2v in the next couple of years and I have a question about PD superchargers vs Centri when it comes to boost levels. I know the KB will make peak boost (lets say 9 lbs) at around 2500 through to redline while the Centri would have a more progressive boost curve peaking at 9 lbs (for the sake of discussion) at around 4000 through redline. I’m assuming these numbers would occur under full throttle correct? What if the car is being driven around town at half throttle? What is the effect on boost levels for both tyoes of blowers? My car will be driven around town mostly, and I’m afraid that boost will only be felt if my foot is to the floor. I can’t be doing that everywhere, so I’m trying to get some help understanding how the two types of blowers would act under half-throttle situations and the level of boost they would provide. Is the boost just cut in half? Will you get any noticeable boost out of either at throttle positions less than full throttle? Let me know if the question isn’t worded correctly and I will try to clarify what I am asking. Thanks everyone! I’m just not very knowledgeable in FI yet.

BlownStangGT · Jan 8, 2009

Well I will say this is my first boosted car. I like having the vortech because when I just pop around the city I am not getting into boost at all. It drives like any other car. I can get into it slightly and it still really will not get into boost. Now once I get into the loud pedal and hit around 3k it starts making boost and it makes it quick And keeps building to redline. It happens quick also. It all depends what you are using it for I guess. It might be nice having full boost at 2500 with a kb, but then agian, most of the time I am driving I am just popping around under 2000 rpms so it doesnt affect me at all. But when I want it, its there. Hope that helps some

trinity_gt · Jan 8, 2009

ScortLX said:
Hello. I am thinking about putting a KB on my 2v in the next couple of years and I have a question about PD superchargers vs Centri when it comes to boost levels. I know the KB will make peak boost (lets say 9 lbs) at around 2500 through to redline while the Centri would have a more progressive boost curve peaking at 9 lbs (for the sake of discussion) at around 4000 through redline. I’m assuming these numbers would occur under full throttle correct? What if the car is being driven around town at half throttle? What is the effect on boost levels for both tyoes of blowers?

The Kenne Bell has a bypass valve that allows air to flow around, instead of through, the screws. This allows "normal" driving to occur with no boost at all. The presence of manifold vacuum actuates the bypass valve. When you get into the throttle heavily and manifold pressure increases naturally toward atmospheric, the bypass valve will close and air will start flowing through the screws allowing boost to occur.

In this image, the bypass valve actuator can be seen as the black bit hanging off the right side:

Is the boost just cut in half? Will you get any noticeable boost out of either at throttle positions less than full throttle?

Absolutely. The KB displaces a fixed volume with each rotation. The one on my car, for instance, is their smaller, earlier 1.7L unit meaning it displaces 1.7L for every rotation. It is overdriven to something like, say, 3 times crankshaft speed. The engine displaces, ideally, 4.6L for every two rotations of the crankshaft. In those two rotations, at 3x crankshaft speed, the KB would've spun 6 times and would have displaced 10.2L or just over twice the engine's displacement. No matter what the engine speed is, the KB, in this example, will always be displacing 2.2 times the volume the engine itself is displacing. When the bypass valve is closed, boost is really controlled by how much air you allow into the supercharger inlet (i.e. the throttle.) If the inlet side of the screws is operating in a low-pressure environment (i.e. throttled) then the output pressure will be a fraction of the maximum boost theoretically available at ambient pressure (i.e. if the throttle was wide open to the atmosphere.)

So for the KB, the amount of boost is a function of both the bypass valve and your foot.

blkhorse · Jan 8, 2009

you can drive around without getting into boost real easy with a KB. You can can shift at 3000 to 3500 with no boost pretty easy. if your cruising around at 2000 to 2500 and quickly stab the throttle to half you will see ~ 5lbs instantly(what I see with 9si kit)

DBMSTNG · Jan 8, 2009

with my S trim, if i give it a moderate amount of throttle, it will have 0 vacuum/boost. light throttle will have vacuum. heavy throttle will get about 4psi boost. full throttle 10psi.

ScortLX · Jan 8, 2009

BlownStangGT said:
Well I will say this is my first boosted car. I like having the vortech because when I just pop around the city I am not getting into boost at all. It drives like any other car. I can get into it slightly and it still really will not get into boost. Now once I get into the loud pedal and hit around 3k it starts making boost and it makes it quick And keeps building to redline. It happens quick also. It all depends what you are using it for I guess. It might be nice having full boost at 2500 with a kb, but then agian, most of the time I am driving I am just popping around under 2000 rpms so it doesnt affect me at all. But when I want it, its there. Hope that helps some

Thanks for the info man! I'm still keeping the door open for a Mongoose kit, but with an I/C like I want, it will be close to the same price. Good stuff though

trinity_gt said:
The Kenne Bell has a bypass valve that allows air to flow around, instead of through, the screws. This allows "normal" driving to occur with no boost at all. The presence of manifold vacuum actuates the bypass valve. When you get into the throttle heavily and manifold pressure increases naturally toward atmospheric, the bypass valve will close and air will start flowing through the screws allowing boost to occur.

In this image, the bypass valve actuator can be seen as the black bit hanging off the right side:

Absolutely. The KB displaces a fixed volume with each rotation. The one on my car, for instance, is their smaller, earlier 1.7L unit meaning it displaces 1.7L for every rotation. It is overdriven to something like, say, 3 times crankshaft speed. The engine displaces, ideally, 4.6L for every two rotations of the crankshaft. In those two rotations, at 3x crankshaft speed, the KB would've spun 6 times and would have displaced 10.2L or just over twice the engine's displacement. No matter what the engine speed is, the KB, in this example, will always be displacing 2.2 times the volume the engine itself is displacing. When the bypass valve is closed, boost is really controlled by how much air you allow into the supercharger inlet (i.e. the throttle.) If the inlet side of the screws is operating in a low-pressure environment (i.e. throttled) then the output pressure will be a fraction of the maximum boost theoretically available at ambient pressure (i.e. if the throttle was wide open to the atmosphere.)

So for the KB, the amount of boost is a function of both the bypass valve and your foot.

WOW! Thats some good KB stuff. That really helps. I knew I would get some great info on :SNSign:

blkhorse said:
you can drive around without getting into boost real easy with a KB. You can can shift at 3000 to 3500 with no boost pretty easy. if your cruising around at 2000 to 2500 and quickly stab the throttle to half you will see ~ 5lbs instantly(what I see with 9si kit)

Perfect!

DBMSTNG said:
with my S trim, if i give it a moderate amount of throttle, it will have 0 vacuum/boost. light throttle will have vacuum. heavy throttle will get about 4psi boost. full throttle 10psi.

Thanks also. This is great to know if I go the Mongoose route instead. All I know is that boost=fun so I'm not going to really lose either way I go. Thanks again everyone!

Question on the effects of throttle position on boost level for PD vs Centri Blowers

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Question on the effects of throttle position on boost level for PD vs Centri Blowers

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