4.0 cfm?

[MogifiedXJ]

How many cfm does a stockish 4.0 flow?

[streetpirate]

i cant find it anywhere, but id guess 350.

[5-90]

Take the cubic inch displacement of the engine (should be 242, unless you stroked it.)

Multiply by the expected "redline" RPM of the engine (5000-5500RPM, unless you do some balancing...)

Divide by two (each cylinder moves air every OTHER time, rather than every time.)

Divide by 1,728 (for cubic inches to cubic feet.)

So...

242 * 5000 = 1,210,000 / 2 = 605,000 / 1728 = 350.11 (ish)

So, figuring a stock AMC 242 running a 5Krpm redline, you're looking at about 350CFM. Take that one step farther, and think that the typical VE peak of the AMC 242 is 80% (maybe) and you're looking at 280-ish CFM at peak...

5-90

[badron]

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Take that one step farther, and think that the typical VE peak of the AMC 242 is 80% (maybe) and you're looking at 280-ish CFM at peak...
====================================

VE peak does not happen at 5k. Typically, Max VE comes to be at about torque peak. By the time your up to 5k VE on many stock engines it is about 60-70 percentage typically that is.

[5-90]

=====================================
Take that one step farther, and think that the typical VE peak of the AMC 242 is 80% (maybe) and you're looking at 280-ish CFM at peak...
====================================

VE peak does not happen at 5k. Typically, Max VE comes to be at about torque peak. By the time your up to 5k VE on many stock engines it is about 60-70 percentage typically that is.

Correct - but I have found it easier to just take the peak VE, and apply it to the peak airflow as calcualted. I think VE at 5kRPM is something like 65-70%, but I haven't had a real-world test of that, and I have only had simulation results (which are "optomistic," to put it nicely...)

5-90

[Dr. Dyno]

According to my dyno simulation, the stock 4.0 HO (241.5ci) has a VE of 74% at the 5250rpm rev limiter (peak VE 77%) so the airflow at 5250rpm should be ~270cfm.
My 4.6 stroker (278.5ci) has a VE of 85% at 5250rpm (dyno simulation, peak VE 90%) so the airflow at 5250rpm should be ~360cfm or 1/3 more than stock.

[MogifiedXJ]

Thanks for the replies guys I just wanted to make sure my calulations where close reguarding peak VE

[JoJo97XJ]

Dr. Dyno, where did you get your Jeep PhD at?
haha

[Dr. Dyno]

Stroker University. ;)