Z-Dreamer Posted November 21, 2001 Share Posted November 21, 2001 Ok, can someone explain to me what is the formula conversion to convert millimeters to CFM in regards to throttle bodies and carbs? Example, what would be the CFM rating on a TB that is say 60mm? Only correct answer will be appreciated. Mark Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted November 21, 2001 Share Posted November 21, 2001 quote: Originally posted by Z-Dreamer: Ok, can someone explain to me what is the formula conversion to convert millimeters to CFM; Example, what would be the CFM rating on a TB that is say 60mm? Only correct answer will be appreciated. Mark Ahhh, Grasshopper; In order for one to obtain a correct answer one must ask a correct question! CFM is not converted into mm's. CFM is the flow of air thru a passage while the mm's is the measurement of the opening that the air passes thru. Just because you're looking at lets say a 60mm metering device doesnt mean that this particular 60mm metering body will flow the same airflow of another 60mm metering body from another brand. The mm's measurement of the metering body in question would be similar to the measurement in inches of a venturi from a carburetor. As we all know not all carb's of equal CFM will perform equally. Each Mfg of both carb's & flow meters will build their product w/their own special mod's, these mod's enable a tuner to customize their engine's performance. The trick is knowing what those mod's are & when to apply them. I'm not a FI guy; but I've got charts (From a SBF book that gives flow ratings) on 60mm metering bodies flowing from 530cfm to 600cfm; while the 70mm bodies may flow from 730cfm to 820cfm, the 75cfm to 77cfm bodies flow from the 850cfm to the 1550cm, while some 80mm bodies flow in the 1350cfm range the 90mm bodies will flow from 1200 to the 1250cfm range. Grasshopper; when you can take this injector from my hand, then & only then will you be ready to leave. I believe this info to be the best of my knowledge at this moment...and if I'm wrong-well then: Never Mind! Kevin, (Yea,Still an Inliner) Quote Link to comment Share on other sites More sharing options...
blueovalz Posted November 21, 2001 Share Posted November 21, 2001 The Master has spoken. And airflow vs orifice size is also determined by the pressure difference across the orifice (Just thought I'd muddy the waters even more). I'm sure there is a standard vacuum in inches of water used, but I have no idea what it would be. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 22, 2001 Share Posted November 22, 2001 According to Vizard, and I'm sure this can be verified elsewhere, a pipe flows ~115cfm per square inch of cross sectional area at a 20.5" of water, equivalent to 1.5" of Hg, which is how they rate 4-barrels and Throttle bodies. Based on that bit of information, and after a couple of simple conversions and such, we find that a pipe will flow 0.14 * its diameter in mm squared in cfm, which gives results slightly lower than the ones Kevin gave (if you plug them into the equation, and that is probably due to a couple reasons. A throttle body is a lot shorter than a "pipe", reducing the pressure drop across it, furthermore, a TB is usually smooth and polished, whereas an exhaust pipe has a rougher surface, reducing flow somewhat. Also, TB manufacturers no doubt have slightly inflated numbers due to different testing conditions (higher temp, lower humidity, etc...) This isn't very important anyway, the equation I provided is helpful in giving you a ballpark figure to estimate how big a TB you'd need. Hope you found this helpful Quote Link to comment Share on other sites More sharing options...
Z-Dreamer Posted November 22, 2001 Author Share Posted November 22, 2001 Thanks Kevin, I am humbled by your vast knowledge and wisdom. Many thanks, GrassHopper Quote Link to comment Share on other sites More sharing options...
Guest Fast Frog Posted November 22, 2001 Share Posted November 22, 2001 Kevin: You're a hoot!!! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.