# Thread: Who's the mathematician that can compute a CFM figure for me?

1. ## Who's the mathematician that can compute a CFM figure for me?

trying to find out the CFM consumption of the 155 HP 4-tec motor at 7500 rpms

naturally aspirated motor, same 1500cc motor, rated 155 HP from seadoo... how can i figure out the CFM demand of the motor at approximately 7500 rpms?

doesn't need to be exact, but semi-close

Thank youuu

STEVE

2. Originally Posted by Shibby1485
trying to find out the CFM consumption of the 155 HP 4-tec motor at 7500 rpms

naturally aspirated motor, same 1500cc motor, rated 155 HP from seadoo... how can i figure out the CFM demand of the motor at approximately 7500 rpms?

doesn't need to be exact, but semi-close

Thank youuu

STEVE
You're buying a Demon carb for that ski aren't you?

3. LMAO...

no no... i haven't totally lost my mind yet, haha

i think someone once calculated that if u install a bilge blower fan on the RXP air intake that is pressurized the air intake at idle and at something like 3000 or 3500 rpms the supercharger / motor "out sucks" the bilge blower

so what i'm trying to figure is what about the NA 155 HP ski with no supercharger? for arguments sake if i slapped a 4" attwood bilge blower in the air intake, at what point would the CFM demand of the motor over-take the fan... or maybe it wouldn't with this ski?

4. 200-203

5. I can tell you just by looking at the conceot that the motor will out pull what the blowr can push VERY EARLY in the RPM range...

6. cfm is calculated using (rpm*displacement) / 3456.

7500*91.5 Cu In /3456 = 198cfm required

that is theoretical

7. Originally Posted by mxl16
cfm is calculated using (rpm*displacement) / 3456.

7500*91.5 Cu In /3456 = 198cfm required

that is theoretical

per cylinder?

8. Originally Posted by GTXX
200-203
Nevermind Danny posted it while I was posting

Out of curiosity how did you arrive at this number?
What's the formula used?

9. i came up with like 198 myself... wanted to verify

is that per cylinder or entire motor?

i believe a high enough powered large enough fan can out push the motor for a very large part of the rpm band

obviously not a SC motor tho

4" detmar is what i'm using

10. the one mxl16 used is the simplest. RPM x Displacement/ 3456