Diesel World - - Understanding Compression Ignition -


Another handy no­ta­tion to have con­cerns air­flow, which is sim­ply cu­bic inches mul­tim­lied by rpm and di­vided by 3,456. That means a 360 cid en­gine (can you tell we like Cummins?) spin­ning at 3,000 rpm would con­sume about (360 x 3,000 / 3,456) 312 cfm of air when op­er­at­ing at 100-per­cent vol­u­met­ric ef­fi­ciency. How­ever, vol­u­met­ric ef­fi­ciency is usu­ally pretty far from 100 per­cent; it’s usu­ally closer to 80 per­cent. So that means we now get an an­swer of (312 x 0.8) 249.6—let’s just say 250—cfm of air. The great thing is, with these sim­ple for­mu­las you don’t even have to be that good at math. You just plug in a cou­ple num­bers and there’s your an­swer. If you’re won­der­ing what air­flow has to do with any­thing, we’re get­ting to that.

Tur­bos and En­gines

Let’s say you have your Cummins spin­ning at 3,000 rpm and you want to add some boost. Ev­ery at­mos­phere above out­side air adds another bar to the pres­sure ra­tio, so if am­bi­ent is 14.7 psi (it varies by el­e­va­tion) then 14.7 pounds of boost would be a 2:1 pres­sure ra­tio. That means 29.4 psi would be a 3:1 pres­sure ra­tio, and 73.5 psi would be a 6:1 pres­sure ra­tio. If we take our ear­lier the­o­ret­i­cal 250 cfm fig­ure, that means to make 14.7 pounds of boost you’d need a turbo that flows 500 cfm, 750 cfm for 3:1, and a whop­ping 1,500 cfm

 When it comes to se­lect­ing a fuel sys­tem to make horse­power, it’s best to talk to some­one who’s al­ready done it to get some ad­vice. Also keep in mind that one com­pany’s “100% over” in­jec­tors might not be the same size as another man­u­fac­turer’s 100%...

 Fig­ur­ing out ex­actly how much a turbo will flow re­quires a bit of help from the man­u­fac­turer. Com­pa­nies like Gar­rett and Borg­warner pro­vide ac­cess to com­pres­sor maps, which show the max­i­mum pres­sure ra­tio, flow and com­pres­sor speed.

 Com­pound tur­bos can be a bit more dif­fi­cult to size than sin­gle tur­bos and can be geared to­ward re­sponse, all-around per­for­mance or peak power. In gen­eral, the larger turbo should flow roughly twice what the small turbo will flow.

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