Posted: August 18, 2011 in Superchargers

Positive Displacement vs. Centrifugal

Centrifugal blowers almost always have a greater adiabatic efficiency than a positive displacement blower. This results in less heating of the charge and more power per pound of boost. The problem with them is that they are high speed devices and need rpm to make boost, they simply cannot make boost at low rpm like a positive displacement blower. You are also limited to how much you can overdrive them because you don’t want to over speed them at high rpm. For a race car that does not need low end power, the centrifugal blower’s better efficiency can make more power. If you want low-end boost, the positive displacement blower may be better for  you. There are screw type blowers that offer the best of both, but there is a healthy price tag to go with them.

Camshaft Selection

As with everything else, it’s a compromise. You must decide where you want the power to be. If you want good power at low-rpm with low boost, you will have to trade off some top-end power. The same goes for the reverse, if you’re looking for maximum output up top, you will trade off low-end and throttle response. At low rpm with no or low-boost levels, the best camshaft selection would be the same as a normally aspirated engine, meaning that it would have a tighter lobe separation than a camshaft used with a positive displacement blower, and installed a few degrees more advanced. For a street motor, this approach is a good compromise. As the cam starts to fall off on power, the blower will be making up for it, which will result in a flatter torque curve. It can be a hard choice, low-rpm no boost conditions will need a completely different camshaft than higher rpm boost conditions.

To Sum It Up

If you’re looking to get the most power per pound of boost and do not care about low-end drivability, then you can the same rules apply as a positive displacement blower. Extending the exhaust duration, widening the lobe separation, thus decreasing the valve overlap period, and installing with 2-4 degrees less advance usually works well. A wider lobe separation helps cut pumping losses by opening the exhaust valve earlier, and also helps stop fresh intake charge from being blown out the exhaust (over scavenging) during the overlap period. Adding duration to the end of the exhaust lobe helps by giving the cylinder more time to scavenge, small losses of low-end power from this can make large gains in top-end performance, so it’s usually worth the sacrifice.  Keeping the air volume and pressure trapped in the cylinder instead of wasting it in the exhaust is paramount to improving the overall volumetric efficiency of the engine.  This is the reason for desired overlap to be at its minimum.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s