How much airflow?

[EShaw]

Here is the formula for figuring out how many C. F. M. our CTD flows at specific RPM, boost.

Example: 360 CID, 36 PSI (max boost), 3200 RPM

(some of your figures may be different)



I calculated the CFM requirement of this motor. It will need 1133 CFM. Part# RE-0880 flows enough CFM for this motor. RE-0880 will flow 1400 CFM



CID x Max. RPM divided by 3456 = CFM

Pounds of Boost divided by 14. 7 (add 1. 0 to this answer)= A



Take A x CFM = CFM requirement of turbo or supercharged motor.



360 x 3200 divided by 3456 = 333. 3

36 divided by 14. 7 = 2. 4 + 1. 0 = 3. 4



333. 3 x 3. 4 = 1133 CFM



Hope you have fun with this.

Eric

 

[Strick-9]

I just love numbers.



My turbocharger book varies slightly in the formula used to find CFM.



Instead of multiplying by the pressure ratio (3. 4 in your case), it uses a density ratio instead.



This number is simply the pressure ratio times the ratio of the temperatures across the turbo. (i. e. inlet over outlet in absolute temperature)



Using your numbers, and assuming a 65% compressor efficiency yields a density ratio of merely 2. 09 which leaves the intake CFM at 693 CFM.



You also must consider the volumetric efficiency is much less than 100%, so the actual value is much lower. More like 520 CFM with a generous 75% VE.



I'm too tired to look up the details on why a density ratio is used instead of just the pressure ratio, and the book is on loan to a friend. It's interesting how much difference there is in the two answers though. Maybe someone can enlighten me?



Anyone know what the stock filter/box is rated for? With the mods listed in my signature, I've yet to see a substantial gain (EGT or throttle response) going to a more free flowing filter.



-Chris

 

[EShaw]

Chris,

I would like to point out that K&N technical support supplied that formula and it is probably on the high side as a safety factor. Your formula seems more accurate, because we know it is not 100% effecient.

Eric

 

[Strick-9]

I did some searching on the net today about supercharging gassers (actually briggs and stratton engines) and it listed your formula as the way to size airflow.



I understand needing to be on the high side of estimates as far as filtration is concerned, but I would like to see a better explanation as to when to use the density stuff versus just pressure stuff.



-Chris

 

[Bigsaint]

Midgets

Eric, very interesting, but I hate working with numbers. I have a much simpler method to measure CFM's... .



1) Find a large pack of midgets congregating along the roadside.

2) Remove your airfilter.

3) Drive past the group with max. boost, one of the midgets will be sucked onto the hood.

4) Stop the truck and weigh the midget, then multiply his/her weight by 10. This will give you your CFM's.



Example: If the midget weighs 100 lbs. you need a filter that can flow 1000 CFM's at a minimum.



Sorry in advance if I offended anyone.



Scott W.

 

[EShaw]

Hey Scott,

Ouch... ... ..... Just because you're taller than me... ..... I will not subject myself to such a test and if you think I'm short..... you should see my fiance'... ... . LOL

Eric

 

[Bigsaint]

Sorry Eric, wasn't meant for either you or your fiance, just thought I'd lighten up the topic a bit... DOH !!!! there I go again, better stop before I get banned.



Scott W.

 

[EShaw]

Airflow/midgets

Scott,

A little fun (pun intended) goes a long way. I know you were just kidding... ... . Now about those shredded service manuals you loaned me... ...