# DIESEL MATH

Diesel World - - Understanding Compression Ignition -

for 6:1. This can be very help­ful in siz­ing tur­bos for your de­sired power lev­els.

Tur­bos and Air­flow

We get the ques­tion all the time: How do you size tur­bocharg­ers? In real­ity, the an­swer is sim­ple. Tur­bos should be sized as small as pos­si­ble for your de­sired horse­power. Luck­ily, air­flow is one of those rare ar­eas where the stars align, as roughly one pound per minute (lb/min) of air is equal to about 8 rear­wheel horse­power on a good-run­ning en­gine. That means a turbo that can flow 50 lb/min (we’re talk­ing stock-ish here) would be good to about 400 rwhp. For those look­ing to be right at 1,000 rwhp an S480 is a pop­u­lar choice, which at 120 lb/min (120 x 8 = 960 rwhp) we can see why. Note that it’s also pop­u­lar to make more power than the num­ber the for­mula yields, but it’s usu­ally at the risk of over­speed­ing the turbo.

Fu­el­ing

En­gine air­flow math is fairly sim­ple (with some ex­cep­tions, like in­ter­cool­ing), but fu­el­ing is a tough one to crack. Lots of things come into play, in­clud­ing in­jec­tion pres­sure, tim­ing, du­ra­tion, noz­zle size and so on. One thing we can cal­cu­late pretty eas­ily, though, is the needs of a lift pump. Start­ing with a fac­tory ex­am­ple, let’s take a truck that has a turbo that flows 50 lb/min of air and cal­cu­late the fuel it needs to sup­port that power level. Most fac­tory en­gines run at around a 20:1 air/fuel ra­tio (or even higher) to keep smoke to a min­i­mum, so we can see that at that air­flow level we’d need (50 / 20) 2.5 lb/ min or 0.36 gal/min (diesel is around 6.93 pounds in weight), or (0.36 x 60) 21.6 gph. Not very much. How­ever, just en­rich­ing the air/fuel ra­tio to 14:1 in­creases that need to 30.9 gph. On a big-horse­power me­chan­i­cal truck (let’s say 120 lb/min turbo, and 12:1 air/fuel ra­tio) we can see the need goes all the way up to 86 gal­lons per hour.

Now, you may be won­der­ing why there are 100 and 150 gph pumps out there if there’s only a need for 86 gph, even on a hot en­gine. The an­swer has to do with pres­sure. As pres­sure goes up, flow de­creases, as the pump has a harder and harder time push­ing fuel to the en­gine. A pump that free-flows 150 gph might only flow 120 at 20 psi, 100 at 40 psi and 80 at 60 psi. Some pumps aren’t even made to run at those types of pres­sures, and might not get there at all. Also, the in­stant need for fuel once the pedal is mashed means that most peo­ple go with overkill so there’s no pres­sure drop when the en­gine sud­denly de­mands fuel.

Ni­trous Ox­ide

Ni­trous ox­ide can be great fun on a diesel, or it can lead to a lot of

 Drag rac­ing has be­come more and more pop­u­lar with diesel trucks. If the launch is a de­cent one, you can con­vert eighth-mile time and speed to quar­ter-mile by mul­ti­ply­ing by 1.57 (time) and 1.25 (speed), re­spec­tively.

 Changing tire size can dras­ti­cally af­fect your over­all gear ra­tio. The tire/trans­mis­sion/axle cal­cu­la­tion can be es­pe­cially help­ful dur­ing sled pulling, where you’re try­ing to hit a cer­tain wheel speed down the track.