25THRSS

When running a draw through blower, how is the cfm requirement for the engine effected? Any cfm calculators out there that will compensate for boost?

Kingtal0n

Treat them just like motors.

that is, a 150 cubic inch draw through blower will move just as much airflow as a 300 cubic inch motor (PER REVOLUTION) just think of the blower as a 2 stroke instead, and so long as the throttle body pressure drop is @ Atmospheric pressure (Wide open throttle, no restrictions), and assume 100%VE per revolution.

for instance, the formula CID X Blower RPM / 3456 = CFM @ 100% VE

so a B&M 144CID blower would move:
144 * 2 (because its not a 4 stroke blower) = 288CID

288 X 6000 (blower RPM) / 3456 = 500CFM Airflow moved into motor.

If you do the equation for a 355CID motor,
355 @ 6000 / 3456 = 616CFM

You will realize that that blower if driven 1:1 on a 355 would actually Hurt performance (161CFM worth of performance) @ that 6000RPM.

But then you realized B&M(weiand) is not THAT dumb and supplied you with a 1.8:1 overdrive pulley setup.

25THRSS

So I will actually need less cfm?

BMmonteSS

No, the example he used was with the blower speed at 6000 rpm. If you calculate that with the 1.8-1 pulleys that turns out to be 10800 blower RPM at 6000 engine RPM. That turns out to be 900 CFM.

Now for my question, because blowers are mechanically driven do you really need to give it that big of a carb? I'm workin on a setup for my GF. We're plannin on a 383 with decent heads along with a holley 177 mini blower. We're aiming for 500-550 hp and runnin pump gas. I was kinda hoping that we wouldn't have to use a huge *** 1100 cfm carb, I can't imagine ever seeing a carb that big on top of one of these little blowers.

Damon

800-850 CFM will do fine. I run an 800 CFM QJet on top of my 383 with a Weiand 142 roots blower on top. I'm making a REAL 470HP at the flywheel. With your larger 177 blower you should be able to achieve your goal more easily than I would with my smaller 142 (which is just about maxed out). The carb is plenty big, I'm just pushing my smaller blower further into an RPM range where it gets inefficient. Your larger blower won't be pushed as hard as mine since it's bigger and won't need to be spun as fast.

BMmonteSS

Thanks Damon, I e-mailed you about this setup but I figured out most of my questions by searching your name. Only thing I didn't find was exactly what kind of milage you got.

Kingtal0n

And now that we have that down, we can move in for the kill on why we dont need 1100CFM carbs to feed our "1100 CFM" of drawn through blower airflow. The answer falls in two categories... Adiabatic efficiency being the most important. A roots blower in its efficient range will be almost 68% Adiabatic efficient, from what I've read in weiand's book. that means that for 68% of the Airflow moving in (say, 500CFM to keep it easy) once you hit boost, for every extra CFM you get from the blower, 32% of that airflow's density goes up in heat produced by the blower. the more air pressure you generate, the more heat you pile on top of the already heated air going into the motor. So just for example, if your motor originally took in 500 CFM, and you pile on another 500CFM, if the AIR DENSITY remained the same, your engines output would double itself. but thats not real world physics. if your motor receives 500CFM and makes 400 horsepower, adding another 500CFM of airflow in the 70% Adiabatic effiency range will only add roughly 70% of that 500CFM worth of horsepower, so you will wind up with more like 650Horsepower instead of the 800 you would have had if you could keep the air temp the same as you started with.

check this out first: http://www.hotrod.com/techarticles/e...bo/index1.html

Cfm boosted = Cfm unboosted x pressure ratio
if you have 600CFM unboosted, then you would do 600 X pressure ratio. Pressure ratio at 10PSI of boost is 1.68, so
600 X 1.68 = CFM boosted or 1008CFM.
Now convert to Lb/min (assuming 80*F outside temps) :
1008 * .07 = 70.56 Lb/Min of airflow. Now we can figure out a general horsepower number:
Every 1Lb/Min is worth about 10 horsepower on a fine tuned EFI motor, so

70.56 = 705 Horsepower. but now you need to factor in A/E, so just lose 30% of that to get you in the right ballpark:

700 * .3 = 200
so you wind up with a 500 Horsepower Small block chevy. Typical numbers from a small roots blower on a small block, typically. correct me if im wrong though

and theres way more to it than that, thats very oversimplified, Crossfire TA can shed some more light on the subject.