SC parasitic loss when NOT in boost?
04-23-2011, 12:16 AM
#22
Supporting Vendor
Thread Starter
And while rotational mass is a factor when changing speed, maintaining speed on a rotating object requires VERY little power assuming the object has minimal resistance....which is the key point behind my original question: When not in boost, The Centri and Whipple blowers have additional resistance because they are always compressing air (even if it is vented away) while the Roots style blower does not actually compress air at cruise.
04-23-2011, 08:17 AM
#23
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Radioflyer, here's my guess on all this during a low throttle opening, cruise situation:
Roots blower without a bypass:
Significant power consumption, because the blower is always compressing air, and has a pressure differential between the inlet and outlet. Delivered air will be below atmospheric pressure, but air on the intake side is even lower because of the restriction of the throttle plate. So you're still boosting, taking source air at maybe 14" vacuum, and boosting it to 7" vacuum, which takes some power.
Roots blower with a bypass:
Much lower power consumption, because the bypass equalizes pressure on both sides of the blower, so it isn't doing the work of compressing air. There's still some power used, and heat produced as the rotors re-mesh, and squeeze out the air to keep it from returning to the top of the blower. It's a pretty high-turbulence situation.
Twin screw:
I'll guess this is pretty low on efficiency at cruise, because it's always compressing air internally, even when the pressure is the same on both sides of the blower. Think of it maybe like a shop compressor with a blow gun attached. The air goes back to atmospheric after leaving the blow gun, so there's no pressure differential on each side of the system, but in the meantime, it's done the work of compressing the air in the tank.
Centri with a blowoff or recirculation valve:
This might be the most efficient, as long as the blowoff valve is large enough to keep pressure roughly the same on both sides of the blower. There's still some pressure difference (if there wasn't, there wouldn't be the pressure differential required to make air come out the blowoff valve). I'm not sure it's correct to think of it as an internal compression blower, in the same sense as a twin screw, particularly when not in boost. There wouldn't be much internal compression produced at steady state, low rpm highway cruising.
Maybe the tech wizards can clean these ideas up a bit.
Bulldogger, Radioflyer is correct about rotating mass (not reciprocating mass, because there isn't any) being an insignificant factor. Like a flywheel, it can eat some power when engine rpms are rising, but then much of it can be recovered when the engine is decelerated when shifting to the next higher gear. This recovered energy is the "jerk" you feel when an auto trans upshifts (if there aren't schemes in place to remove it).
With steady-state cruising, it doesn't really come into play.
Roots blower without a bypass:
Significant power consumption, because the blower is always compressing air, and has a pressure differential between the inlet and outlet. Delivered air will be below atmospheric pressure, but air on the intake side is even lower because of the restriction of the throttle plate. So you're still boosting, taking source air at maybe 14" vacuum, and boosting it to 7" vacuum, which takes some power.
Roots blower with a bypass:
Much lower power consumption, because the bypass equalizes pressure on both sides of the blower, so it isn't doing the work of compressing air. There's still some power used, and heat produced as the rotors re-mesh, and squeeze out the air to keep it from returning to the top of the blower. It's a pretty high-turbulence situation.
Twin screw:
I'll guess this is pretty low on efficiency at cruise, because it's always compressing air internally, even when the pressure is the same on both sides of the blower. Think of it maybe like a shop compressor with a blow gun attached. The air goes back to atmospheric after leaving the blow gun, so there's no pressure differential on each side of the system, but in the meantime, it's done the work of compressing the air in the tank.
Centri with a blowoff or recirculation valve:
This might be the most efficient, as long as the blowoff valve is large enough to keep pressure roughly the same on both sides of the blower. There's still some pressure difference (if there wasn't, there wouldn't be the pressure differential required to make air come out the blowoff valve). I'm not sure it's correct to think of it as an internal compression blower, in the same sense as a twin screw, particularly when not in boost. There wouldn't be much internal compression produced at steady state, low rpm highway cruising.
Maybe the tech wizards can clean these ideas up a bit.
Bulldogger, Radioflyer is correct about rotating mass (not reciprocating mass, because there isn't any) being an insignificant factor. Like a flywheel, it can eat some power when engine rpms are rising, but then much of it can be recovered when the engine is decelerated when shifting to the next higher gear. This recovered energy is the "jerk" you feel when an auto trans upshifts (if there aren't schemes in place to remove it).
With steady-state cruising, it doesn't really come into play.
Last edited by Warp Factor; 04-23-2011 at 08:20 AM.
04-23-2011, 09:38 PM
#24
My take on OP's original question was that he wanted to know how much less mpg you would get from a s/c compared to a n.a. car. In our case, we were getting 29.8 mpg at 70 mph, and now after the A&A s/c install, we get 26 mpg at 70 mph.
I really don't care about the mpg loss... I am happy with the hp increase.
I really don't care about the mpg loss... I am happy with the hp increase.
