How do I calculate airflow of a supercharged engine?

[Handlebars]

I'm helping a friend figure out an air cleaner solution for his JK with a supercharger and need to know how much airflow is necessary. The Donaldson catalog specs airflow at 6", 8" and 10" of H20 of restriction. The engine is a 3.8 running 8 psi boost with a maximum RPM of 5500.

Also, which restriction rating should we use? Thanks!

 

[Boostwerks.com]

simple answer:

Fit the largest filter possible. The more surface area, the better it will breath and the better it will filter.

 

[winterbeater]

The air still has to pass through the engine whether it is pushed or sucked. It may make some difference, but the basics should be close:

3.8 liters per 2 revolutions, so 1.9 x 5500 = liters of airflow per minute.

 

[Handlebars]

The problem is that there isn’t even room for a normal air filter. His current air filter is about the size of a coffee mug, it is stashed underneath the tube at the upper left of the engine bay, behind the supercharger:

The wadded up dirty blue rag is a vacuum cleaner bag that he was using as a prefilter.

I found the equation to calculate 4 stroke engine airflow: ((CIDxRPM)/3456)volumetric efficiency = CFM airflow.

To calculate supercharged volumetric efficiency : VE= 14.7/pounds boost

We came up with an airflow requirement of 550 CFM. To cope with the dust of off-road use this is going to require a LARGE air filter. Right now the most likely solution will be an external air cleaner.

 

[Boostwerks.com]

I love it how people try and make a simple problem as complicated as possible. There is no need to solve an equation when you already know the answer...

Fit what you can.

 

[5-90]

You want a "quick and dirty" answer that is close to correct?

(Supercharged Airflow) ~= (NA Airflow) * (14.7 + boost psig)/14.7

8spig boost? Then your ratio is (14.7 + 8)/14.7 = 22.7/14.7 = ~=1.544

3.8L displacement = 1.9L displacement/revolution = (1.9x61)cid ~=116cid/rev.

What's redline for you, 5500-6000rpm or so? Let's use 6K.

6,000rpm x 116cid/rev = 696,000ci/min (assuming 100%VE for NA - it's usually considerably less.)

(696,000ci/min) / 1,728 ci/cf ~= 403cfm.

While "fit the largest filter you practically can" is sound advice, it does help to know what your minimum filter size would be - so here you go! This is still going to be an oversized filter (as I said, I didn't account for VE,) and going larger will allow the filter to get clogged up (restricted) gradually without your losing airflow capability, but now you've got a number to start working with.

Surpercharged and turbocharged engines typcally have a VE well in excess of 100%, but using a 100%VE assumption for NA allows you to keep things well in hand.

 

[Handlebars]

Fit what we can is what we'll do. The oiled foam coffee mug sized air cleaner is staying. The Donaldson PSD10 is the smallest air cleaner that we could find that is rated for sufficient airflow and medium to high dust environments. It measures 17"x15"x10" and weighs 11lbs. It would most likely have to go on the roof and he does not want that.