SC parasitic loss when NOT in boost?
 

I know blowers take power to make power when in boost, but in cruise mode, (with bypass valves or whatever open) I'm curious how much drag blowers make (Centri style AND PD)

That saying is such an overstatement. They take like 5 whp to give you 300 whp...in other words an insignificant amount

Edit: To answer your question, they are always "taking" power. But it's really a non-issue.

:iagree: ^

The power to turn it is really minimum at best - not even significant, can't even feel!

Thanks,Matt

This question is aimed more at fuel economy when using cruise control.

On a PD blower, rotors continue to spin bot don't make boost because the bypass valve prevents boost from building. However the lobes don't spin that fast compared to a Centri blower.

In the centri setup, there is no internal bypass valve so the head unit's s design doesn't seem to notice the difference between when it needs to make boost and when it's just cruising. I would think that for centri blowers there's boost still being made but being vented out via a valve.

If I'm talking out of my ass, please let me know.

Interesting, my educated guess would be exactly the opposite
For two systems of the same boost - I would expect PD to see lower mileage.

PD rotors mass and moving parts is far greater than the single impeller wheel mass of a centri design.
As far as method of bypassing - equivalent effectiveness as I see it. venting is venting whether in housing or piping.

Not certain what stock mpg is for a C5Z06. My procharged 550rwhp Z06 does 25mpg on highway with numerous on-ramp blasts and motivated passing. I expect it would do better if driven for mileage. Perhaps some PD owners can chime in the mileage they see.

True the rotors have a larger mass, however the centri is also geared for much higher speed rotation. My theory is that the centri will always make boost (which has a higher parasitic cost) that is vented at cruise while the PD blower can internally regulate boost allowing the rotors to free spin with minimal parasitic drag.

Example: Cruising at say 2000rpm

A PD blower that would normally be making boost isn't because the bypass valve is open. At no point does the blower actually compress the air.

A centri blower however is compressing the intake air about 1-1.5 pounds of boost, but this boost is being vented to the atmosphere. The boost is still being made, but never reaches the intake.

:lurk:

I understand your hypothesis, but think you are dreaming :)

Your premise is venting in the housing is somehow reducing energy required.
Whether Bypassing in the housing or in the plumbing you are still moving/equalizing/venting air.
Consider two blowers on a workbench. The energy it takes to spin the pulley for a PD and a centri has no relation to how it is vented.

PD and centri have static rotation ratio based on pulley/engine rpm

So the question is - does the gearing step in the centri take more energy to rotate than the substantially higher masses and friction in moving parts of the PD.

I tend to think not

On a centri setup it is bleeding "boost" under cruise partial throttle through the blow off valve

yup.

PD blower is equalizing pressure inside. Does not mean you are not moving air.

An internal boost leak vs an external boost leak.

Most pd systems use a bypass valve as well so I dont see how a centri would be any diffrent as radio said...

do not agree,
the blowers with bypasses use the least power when not in boost and the blowers without internal compression (roots) eat up even less power.

without getting in to turbos it goes like this
not in boost
1. roots
2. twin screews
3. centri

in boost
1. centri
2. twins
3. roots

you will notice that auto manufacturers really like the roots (eaton) blowers and this is part of the reason (along with reliability).

you are on the right track just add in the internal compression of the diffrent SC systems to see the rest of the pic. the worst case is the centri that has internal compression and is stacking air aginst a back preasure while the SC systems with a bypass are not loading the blower up with boost preasure (except the Internal compression).

the weight of the internals are all low inertia (light and small radius) loads mopstly driven at constant rpm (somewhat) so they are not 1st order power losses. keep in mind that the SC's are using 40+ hp when under boost vs a fraction of a HP when not under boost so any boost backpreasure will be significant.

Not really that simple. A PD blower can be roots style or twin screw. My TVS on my GT500 roots style has the by-pass valve located where it opens prior to compressing the air so parasitic loss at cruise is less then a whipple style. A Whipple twin screw compresses air in the supercharger which is before the relief valve, so parasitic loss at cruise is greater. On a Centri like on my vet it is compressing but dumping right off about 12-14 inches down stream, also being so small in size with alot less reciprocating mass the a PD blower, loss is real minimal. Prior to install vet averaged 25-26 on a 65mph cruise. After install mileage was unnaffected. Now I don't have exact number like say 5-7 hp but I can tell you it is not enough to notice in any way shape or form

Is this in order of efficiency? Do you have any specific numbers on parasitic drag?

yes in order of efficiency.

the number of variables are :crazy2: things like what RPM are you running and the amount of presure ahead of the TB, the layout of the bypass etc. unless you can arive at some common specs any talk of specificy numbers is like talking to a blind man about how blue the sky is.

keep in mind that Centri's have internal compression also just like the Whipple and internal compression is good for efficiency under boost but bad when not. of course the argument could be made for a centri that is sized such that at a given RPM it is not making any boost, just suppling the exact amount of air the engine needs but then below that RPM the centri would be a drag on the intake and its boost would be delayed at higher RPM.

for example sized such that the Centri is not making any boost at 1800 rpm and 900 rpm it would making less air than the engine needed and would be like a turbonator that you see on Ebay that resticts flow. above 1800 rpm the air flow would ramp up in its typical rpm squared curve but the boost would lagg behind.

the centri blower (and turbo) flows air with the square of the rpm where the PD blowers flow perportionaly with the RPM.

That' pretty much my thoughts. The Internal compression is what causes the extra drag on the pulley of a centri at cruise.

Every supercharger is going create some drag, nature of the beast.
My order of efficiency would be
1 Roots most efficient
2. Centri lower resiprocating mass while making boost.
3. Whipple least efficient

Where is the reciprocating mass in any of these blowers?