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air box design articles
smart51 - 6/9/17 at 07:13 AM

Hi everyone. I'm thinking of making a new airbox for my tin top to free up a few extra horses. Something that will feed cold filtered air into the throttle body. Can you point me towards any articles on what I should be looking out for when designing it? I get that you want smooth air flow and large radius corners. I can design a tuned cavity if needed. But the best volume of airbox and the best diameter of pipe for a certain engine size I don't know. Where should I look?

Pete Jordan - 6/9/17 at 09:05 AM

There are a lot of conflicting views on the net in my experience but I found this forum post quite useful. It is aimed at racing engines but at least has a formula for calculating inlet diameter into which you can enter your own efficiency factor (depends on how well ported/gas flowed your head is) and also your peak revs, which are likely to be lower than for a race engine.

Air box design post - Ten tenths motorsport forum

Regarding air box volume, anywhere between 3-5 times engine displacement seems to be the range suggested online.

I hope that is useful and doesn't further muddy the waters!


smart51 - 6/9/17 at 01:24 PM

Thanks Pete. I'd read that the ideal size was 2 to 4 time the engine cc. 3 to 5 is similar. The OEM air box is 1.56 times the engine cc before the filter and perhaps 2 to 2.5 times if you include the volume after the filter. It is perhaps a bit on the small side.

Accepting the default numbers in the article, you get an ideal inlet diameter of 42mm. I was expeting more as the throttle body ID is about 60mm. Suggesting a smaller inlet diameter than throttle body seems counter intuitive. It doesn't match what I found when making a box for my BEC. I tried bigger and bigger inlet pipes and felt more and more difference until the inlet area matched the sum of the carb areas. After that, more made no difference. The stock inlet is 37.4 which is a bit small even compared to what the artice says.

While I could make a box that is closer to ideal, I'm not sure it is worth it if the ideal inlet really is 42mm. 37.4 to 60 would make more difference.

bikecarbfred - 9/9/17 at 06:59 AM

thanks pete for the article. love reading this kinda stuff