Quote:
Originally Posted by moore_rb
This is an accurate point when applied to the older style belt-driven hair dryers (which "feel" a lot like turbos as they spool up); but the latest crank drive blowers out there now are able to start adding boost as low as 1500RPM...
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They can angle the blades and make improvements, but a true centrifugal blower, the pressure output is proportional to the tip speed of the blades. The relationship is typically pretty close to: pressure equals tip speed squared. Since it is difficult to fit a 3 foot diameter blower in a car, they end up with a small diameter and a planetary gear set spinning the wheel up to 80,000 rpm to make pressure. At low rpm the boost is very low, with little increase in boost for a 1000 rpm motor increase. At high rpm, boost is high, and a large increase in boost for a 1000 rpm motor increase. Torque is proportional to boost, and boost increases with the rpm squared. This gives a very steep increase in torque over the rpm range, until the engine pumping efficiency starts dropping off rapidly. Once that point is reached, the centrifugal blower and engine combine to flatten the torque out for a while.
Multi stage compressors wheels exist in industry, but I have never seen them in an auto application. The pressure is still proportional to rpm, but not as steep. These need a good deal of application design and cannot handle a lot of variable changes, as they can get into vibration harmonics and destroy themselves. I don't see these going into auto applications, but I get surprised all the time. I just doubt that there is any centrifugal blower that doesn't have a steep rpm to pressure curve. Physics doesn't change much.
Now change to the roots bower, or better, a screw compressor and they are positive displacement blowers. You pretty much get the same boost pressure (thus torque) at all rpms.
So let's assume your engine NA has 300 ft-lb torque from 2500 - 4500 rpm and then it starts dropping off to say 250 ft-lb at 6000 rpm. With a centrifugal blower you may get 350 ft-lb at 2500 rpm at 2.5 psi boost. At 4500 rpm you are likely at 8.4 psi and 470 ft-lb. At 6000 rpm you are likely at 15 psi and 500 ft-lb. Some of that torque would be consumed by the blower belt, but that's more work than I wanted to put into this, so I am ignoring those losses.
In comparison if you used a positive displacement compressor at 15 psi boost (again ignore power robbed to drive the blower) you would double the torque everywhere. So cruising down the road and pulling up a mountain at 2500 rpm the positive displacement would would have about 600 ft-lb of torque available, where the centrifugal would only have 350 ft-lb available (only 50 more than the engine had normally aspirated). Sure drop two gears and rev it up to 6000 rpm, they both have the same torque, but every place below that, the positive displacement wins hands down.
I hate a different torque at every rpm. So you come out of the first turn at 2500 rpm smash it to the floor and get a modest 350 ft-lb of torque. Yea this is fun. The next corner you are at 4500 rpm and smash the throttle, expecting the same fun, only the engine pumps out 470 ft-lb of torque. The tires break loose on a corner, in a 90 inch wheel base car, and you are dead.
With the centrifugal smash it in first gear, and your tires will hold to the ground until the rpms come up. If you do not back off the throttle before the tires break loose they will rapidly go up in smoke. To keep the tires from breaking loose, you have to continually back off the throttle, as the rpms increase to keep the tires hooked. Same thing in every gear. Not for me.