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Airbox Calculations

  #1  
Old 05-02-2013, 05:42 PM
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Question Airbox Calculations

So Im working on making an airbox for my cold air intake. Ill bring the filter up to make a SRI, run piping in front of the radiator and feed it to an otherwise air tight chamber. Im gonna run a few tests to see if this does anything since I suspect under the bumper is actually a low pressure area for the car. Originally this housed the stock air box.

Im wondering about the calculations to see what pressure would be created in the box. I have that at 3000rpm my engine draws about 185 cfm but im not sure about the formula. (engine displacement in ft^3 (.077 ft^3) multiplied by the rpm (3000rpm @70mph 5th gear) and then multiplied by the volumetric efficiency (.8 assuming))

Then finding the flow rate at 70 and 35 mph with a 3" dia piping would be 302 and 151 cfm respectively. This leaves between 34 and 117 cfm not taken in (but assuming this raises the efficiency shifting the amount a little lower).

That would build up pressure to a certain extent, increasing the VE even more to a certain limit.

Assuming Im thinking correctly, help me if Im on the wrong track.
What would the pressure build up be? Resistance from bends and materials is not taken into account yet

Or does is this not work like im thinking it should and my calculations are in vain?
 

Last edited by RobinsonRicer; 05-02-2013 at 05:45 PM.
  #2  
Old 05-20-2013, 06:45 AM
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Looks like you posted this a while back, have you considered a velocity stack either before or after the filter? You could fab one up out of fibreglass or maybe some kind of plastic casting if you're dedicated. Then cut out the underside of the fender liner on the intake side and use some kind of filler material at an angle (say fiberboard or what have you) to create more of a pressure differential under the bumper?
 
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Old 05-20-2013, 07:27 PM
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I have thought about that. Im considering a few different ideas. The biggest issue is that the air flow basically stops and is most dependent on the engine sucking in air at the filter. I need to depend on pressure building in the space outside the filter to increase volumetric efficiency but it wont seem to create a large impact. Its just not the same as if I had a straight pipe from the front of the car going directly into the intake with no filter.

It seems many have tried this topic but haven't tried too many unique ideas, ive been thinking of mixing this with an electric supercharger with a bypass for when the engine cfm exceeds the fan. Dont laugh, it could work in theory :P
 

Last edited by RobinsonRicer; 05-20-2013 at 07:30 PM.
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Old 05-21-2013, 07:50 AM
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Originally Posted by RobinsonRicer View Post
It seems many have tried this topic but haven't tried too many unique ideas, ive been thinking of mixing this with an electric supercharger with a bypass for when the engine cfm exceeds the fan. Dont laugh, it could work in theory :P
I've yet to see a single electric supercharger that wasn't a complete scam. Said another way, every electric supercharger I've looked at would require the bypass you spoke of at all engine speeds above idle.
 
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Old 05-21-2013, 08:02 AM
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Oh shipo

Actually my engine could benefit from a 250 cfm fan past 3000 rpm (3000 rpm at 231cfm at 100%VE). That would only draw about 4 amps. Thats a 4" dia marine bilge. And thats highway cruising speeds for me. I never saw the electric superchargers as something to create actually boost but something to help increase the volumetric efficiency thats lost due to the basic design of an intake. Then design a bypass that would open at a set point where the fan cfm would actually choke the engines intake and have the engine suck air through the unrestricted bypass... Somehow incorporating that into the air box. So you would have the benefit of the high pressure in front of the car building up in the intake as well as the fan mimicking high speeds when driving at low speeds and rpms. You can see above that you only get about 300 cfm with a 3" intake at 70mph.

Thing is people see the electric supercharger as nothing but a scam and a foolish ricer mod. I understand it doesnt work how they advertise, but it does have a good idea behind it. But not for racing purposes. Perhaps theres something to it that people havent done because of the stigma.
 

Last edited by RobinsonRicer; 05-21-2013 at 08:10 AM.
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Old 05-21-2013, 08:16 AM
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You understand how they work (as in benefiting the performance of your engine)? Well that makes one of us, and I'm an engineer with plenty of automotive industry experience on my resume. Long story short, every "electric supercharger" product I've ever seen will reduce intake flow and turbulence and reduce volumetric efficiency at all speeds above idle.

The above said, logic has never been something that deterred you from trying stuff with your car, so mod away and have fun.
 
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Old 05-21-2013, 12:19 PM
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No it has not and ive learned a lot that way. Ive found many things look bad on paper but when you try it out you see results, good or bad, seeing the results is always more satisfying then calculating and stopping there. And its more fun that way. I have the numbers right up there though.

Ive had too much fun experimenting with my car to stop now.

I should add, the idea would be experimented with by adding it to a feed to the air box rather than directly in the intake pipe. The idea of placing a pipe fed to the air box in front of the car is not so foreign. This is just taking that idea and creating highway speed air at city speeds. Not to create boost but to provide less work for the engine to suck up air from a negative pressure zone.
 

Last edited by RobinsonRicer; 05-21-2013 at 04:11 PM.
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