Here is something I thought I'd do in my spare time, because I know that many are wondering about injector sizes according to their boost/HP. I don't know how to calculate injector sizes from boost levels, and I also think it is redundant, since different engines will have different requirements at different boost levels. So here it is, a HP vs. inj. size chart for a typical turbo charged 4 cylinder engine, using formulas from Corky Bell's book.

CC/min = lb/h x 10.5

lb/h per injector = (expected bhp x .55) / number of injectors

Calculating for a 4 cylinder engine (rounding up to next integer):

250BHP - 35 lb/h or 370cc injectors (maxed out)300BHP - 42 lb/h " 440cc " " 350BHP - 49 lb/h " 515cc " "400BHP - 55 lb/h " 578cc " "450BHP - 62 lb/h " 650cc " "500BHP - 69 lb/h " 725cc " "550BHP - 76 lb/h " 799cc " "600BHP - 83 lb/h " 872cc " "

Approx. fuel increase due to 50HP increase is 7 lb/h or 75cc.

WHP = BHP x .8 due to estimated 20% drive train loss -> 300HP = 240WHP

Quote from Corky:"The .55 figure is actually the maximum load brake specific fuel consumption (bsfc) of a typical turbo charged engine. In general the number of injectors is the same as the amount of cylinders. Clearly one should choose the next larger size than the calculated value, to offer some margin for improvements."

I'll add to that; improvements AND safety.

I know that those injectors sizes are not quite the same as those used by most injector manufacturers, but I assume you will know to round up a bit when you are selecting your own injectors. For example when you know that you aim for 240WHP, which is 300BHP, and you choose the 515cc injectors (remember one size up), there will be very few if any manufacturers of that specific injector size, so you should go with the more common 550cc injector, which will offer even more of a safety/improvement margin than the one calculated/chosen.

This is also a method/chart erring somewhat on the safe side, but I think that most of us will agree that it's the best way to go Hope this helps,

sil80

P.S.: These formulas can be used for a 6 or 8 or however many cylinder engine you'd like, by substituting those numbers instead of 4, on the division part of the second formula.

Excellent!

I remember how much trouble i had trying to pick what size injectors i was going to need. This will be a big help to alot of people. Deserves a sticky. :ylsuper

What is the lb/hr to cc formulae?

Rick

Multiply by 10.5

it's all up there

sil80

I talked to holley performance. They say that 50lb injectors can support up to 400bhp on a 4cyl engine if fuel pressure is raised. Not really that much of a difference from the chart up there, but just thought i'd add it.

Well yes, this chart assumes stock fuel pressures, some kind of fuel management, either piggy back or standalone, and of course slightly different numbers will result depending on application. It is just a very accurate ballpark figure reference, so that people can't really make mistakes towards the detonation side of things.

sil80

In detail: These figures assume STOCK FUEL PRESSURES. This is a very important aspect in all of these calculations. The fact that our stock fuel pressure is about 40psi, is why these numbers may seem a little TOO much on the safe side to some. YES, people have put down close to 300whp on 370cc injectors. With 100+psi fuel line pressures. That's asking for trouble and REALLY maxing out the injectors. When using an RRFPR, you increase fuel pressure with boost. An 8:1 RRFPR will add 8psi of fuel pressure for every additional pound of boost. But boost is a variable that does not guarantee the same airflow with different turbos. A T4 will flow a lot more at 7psi then a T3. That means more air at same boost level, and if it's not accounted for, it means detonation. So if you DON'T get an aftermarket FPR and stick with stock fuel pressures, the numbers above should be very good ballpark figures for you.

Also, take a look at this site, they have cheap Delphi injectors from small to large: http://www.racetronix.com/

sil80