Technical > Spud Miller's Cave

Volumetric Eff. % of an Engine

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captndiet:
Spud

How do you decide the V.E. % of an engine?

The injected fuel engine I am putting together is 358 c.i. ex-Nascar engine. 410 cfm on intake, 300 on exhaust, titanium intakes, inconol exhaust, roller cam & bearings, Jesel valvetrain & belt drive, so I assume it will be higher than 105 VE.

Computing the the fuel flow numbers can change quite a bit when I change the VE % and I am not sure what value to input.

Thanks again

Jeff

Spud Miller:

 That's a tough one sometimes! Backing into the numbers on some proven tuneups over the years has given me some points on the graph to help extrapolate other setups. It's much harder to do a naturally aspirated system than a supercharged setup and VE is why. The questionnaire I give to naturally aspirated customers is aimed at getting enough info to make a good call on this. The supercharged questionnaire is all about the blower. NA has much more to consider.

 Factors that lower VE:
  - Fuel in the runners
  - Poor flowing heads
  - Low valve lift
  - Flat tappet cam

 Things that don't affect cylinder filling, but can make your VE number go to xxxx:
  - Low compression ratio
  - Weak spark
  - Not enough ignition timing
  - Low load on the motor

 Down nozzles keep your runners dry and flowing max air right to the valve and they help VE by getting fuel OUT of the runner.

 High dollar heads, a big aggressive roller cam, big intake valves, down nozzles, 15:1 CR, good mag...a guy better use 112% for a VE or be lean.

 2.02 "Camel hump" heads, 10.5:1 CR, flat tappet cam, box ignition...95% is optimistic and 90% is more common. While you might be "right on" for the amount of air getting in the cylinder, motors with these types of issues just will NOT want to burn any methanol or behave in a way that the math will make sense unless the VE is pitifully low. These motors are better off on gas...or nitro!  ;D

 I've done Flatheads that come out to 83% and that's a "Stout" build! A more or less stock Flathead running methanol is more likely to be 75% or so.
 
 Having said all that, VE really is only an important number to split hairs on if you're running methanol or percentages up to 75% and trying to tune to an AFR. If you're going to run a high percentage and big volume of nitro...it doesn't matter too much because the AFR is less than 1.00. In that case, changing the VE 1% is like peeing in the ocean! For "volume" tuning you're better off using a "gallons per cubic inch" approach to get started and tune for the load on the motor.

 Spud

captndiet:
Thanks Spud

As usual your experience is appreciated and causes me to take a step back and re-evaluate my though processes.

Thank you for sharing your knowledge.

Jeff

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