CFM question

2rangers · Feb 4, 2009

Need some double-checking on the math for this question: How much air does a stock NA 2.3 intake at full throttle? I came up with @ 100cfm. Here's what I figured:

2300cc's / 4 (strokes/pistons) = 575cc's
575cc's = .02cf (cubic feet)
.02cf * 5000 rpm (full throttle?) = 100cfm

This calculation assumes 1 piston on a stock NA 2.3 intakes a maximum of 575cc's of air per each revolution (1 piston, 1 stroke, 1 revolution) with red-line @ 5000rpm.

Right - or wrong? Thanks for the help.

dieselboy740 · Feb 5, 2009

Somethings not right remember if your engine ran at 100% volumetric efficency. More than likely it does not.

Nipplechops · Feb 5, 2009

Assuming 100% volumetric efficiency?

2300cc = 140 cubic inches = 0.081 cubic feet

That is the amount if intake air used per complete cycle of the crank, which takes 2 revolutions.

Total cfm = 0.081cubic feet/cycle * 1 cycle/2 revolutions * 5000rpm

Total cfm = 202.546

:beer:

rurouni20xx · Feb 5, 2009

still wrong, how long is your intake valve open? fig the pull/force there. most engines only run 80 percent ve.

Nipplechops · Feb 5, 2009

rurouni20xx said:
still wrong, how long is your intake valve open? fig the pull/force there. most engines only run 80 percent ve.

No. Not wrong. I said that is assuming 100% V.E. Let's see your math. If your assumption that most engines only run at 80% V.E., then simply multiplying the number I came up with by the normalized V.E. will yield the desired answer. Same goes for RPM.

CFM @ 5000 rpm and 80% V.E. = 162.048

So the actual calculation would look like this:

Total cfm = V.E. * 0.081cubic feet/cycle * 1 cycle/2 revolutions * RPM

2rangers · Feb 7, 2009

Nipplechops said:
Assuming 100% volumetric efficiency?

2300cc = 140 cubic inches = 0.081 cubic feet

That is the amount if intake air used per complete cycle of the crank, which takes 2 revolutions.

Total cfm = 0.081cubic feet/cycle * 1 cycle/2 revolutions * 5000rpm

Total cfm = 202.546

Ok, I see. On a four cyclinder, four stroke engine, 2 cylinders (not one) are drawing air per a single rpm. That explains the doubling (200 vs 100cfm) of your calculation.
Thanks.

rurouni20xx · Feb 7, 2009

no hes saying it only draws in 100 cfm per 2 revolutions (rpm), the only time you would pull 2 cylinders together would be cuz of crank layout, i wish i knew how your engine was built so i could explain it better w/o confusing you. basically i need to know the crank layout and see if the firing order is sequential or doubled. then i could tell you if its pulling two cylinders at a time or not. most inline fours have the inner cylinders upward (tdc) and the outers downward (bdc). one cylinder pulls in fuel and then the next one does, and they fire sequentially, in this case only one cylinder is pulling at a time hence 100 cfm per rpm. if they fire doubled then two cylinders (2 and 3) and the other two (1 and 4) pull fuel together and then it pulls 200 cfm per rpm. i dont have my manual w/ me so i cant tell you if it fires sequentially or doubled.

Nipplechops · Feb 8, 2009

rurouni20xx said:
no hes saying it only draws in 100 cfm per 2 revolutions (rpm), the only time you would pull 2 cylinders together would be cuz of crank layout, i wish i knew how your engine was built so i could explain it better w/o confusing you. basically i need to know the crank layout and see if the firing order is sequential or doubled. then i could tell you if its pulling two cylinders at a time or not. most inline fours have the inner cylinders upward (tdc) and the outers downward (bdc). one cylinder pulls in fuel and then the next one does, and they fire sequentially, in this case only one cylinder is pulling at a time hence 100 cfm per rpm. if they fire doubled then two cylinders (2 and 3) and the other two (1 and 4) pull fuel together and then it pulls 200 cfm per rpm. i dont have my manual w/ me so i cant tell you if it fires sequentially or doubled.

It makes no difference. In any four stroke engine every cylinder cycles once every two crank revolutions regardless of cylinder phase or the number of cylinders. A 1-3-4-2 firing order will flow the same as a 1-2-3-4 firing order at the same V.E. and same RPM.

rurouni20xx · Feb 8, 2009

is the crank sequential or double? ive seen double on thing b4, i cant remember what it was tho...i dont think its the ford i just want to make sure...

V8RangerBoy · Feb 10, 2009

200 cfm is roughly correct, but slightly high according to my calc sheet for turbo sizing. I show 192cfm at 140 cid, 5000rpm, and 90% VE.

CFM question

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