I just got finished reading an awesome tech article in Sport Compact. I'm going to paraphrase it but this isn't my info. (Although, I already knew this PDT_Armataz_01_22)


Everyone needs to read this. I know it's kind of advanced stuff but if you plan on running any type of forced induction or if you want to up your fuel injectors, you'll need ot know this stuff. Welcome to the world of high performance tuning, kids.
First off, you need to be sure the ECU knows what size injectors you have. The ECU just tells the injectors to open for a certain amount of time and assumes the injectors will flow the same around during the time that they did back when the ECU was programmed. If you have changed from 260 cc/min injectors to 550 cc/min, the ECU will have to send a shorter pulse to deliver the same fuel.
Once you've handled that task, you need to make sure the injector is big enough for your power goals. This is simple. Just plug your pipe dreams into this formula: Horsepower x B.S.F.C. / (divided) # of injectors x Duty Cycle = Injectors flow (lb/hr). BSFC just means brake specific fuel consumption, or how much fuel your engine takes to make power. Most naturally aspirated engines will consume 0.5 pounds of fuel per horsepower, per hour, so use 0.5 lb/hp-hr. If you're turbocharged or supercharged, try 0.6 or 0.65. BSFC might be lower if you're running on the ragged edge of too lean, or if you have really high octane fuel that lets you advance timing and make more power from the same fuel. None of that matters here, though, since you more than likely want a concervative number that will ensure your injectors aren't too small.
For duty cycle, you want to use 0.80. This means the injectors are open 80% of the time. Not only is this about the highest duty cycle you can run and still have the injectors follow the ECU's commands accurately, but it's the duty cycle at which injectors are flow rated. You have to use 0.80 if you want the number from this formula to mean anything when you go to buy your injectors. So, if you think you can make 200 crank hp, the formula works out like this: 200 x 0.50 / (divided) 4 x 0.80 = 31.25 lb/hr.
Which is great, except for the fact that most sport compact injectors are rated in cc/min. You now have to plug 31.25 into another formula to convert to metric units: Flow (cc/min) = Flow (lb/hr) x 10.5. This would give you 328 cc/min. Time to shop for something atleast that big. Don't forget, of course, to use flywheel horsepower in that formula. If you use wheel horsepower, the result will be too small of an injector.

ya good stuff, if you ever run 100% duty cycle on your injectors you most likely to damage or burn up the motors in them.

Also what it was talking about on pulse cycles and letting the pcm know your injector size ...heres another example: you have 10lb injectors at idle they are open 10 percent (1lb) , if you run 100lb injectors and you don't reprogram it..you'll be running 10% still which will = 10lbs (thats one hell of and idle!!) ..so by letting the pcm know, it can change from 10% idle to 1% idle.. its all a number and ratio game.

Doing small injector upgrades are usaully ok without a pcm upgrade, but you'll get the most from upgrading the pcm to. Going from 16lb injectors to 24 usaully works with existing mods..but going fro 16 to 32 or more usaully doesn't work.

also injectors when they say lb's on them, thats how much weight of fuel it flows per hour, when it says CC's , thats how much it flow per min


-500cc per minute is approximately equal to 49lbs
-lbs/hour = cc per minute / 10.2
-lbs per hour = HP / 2.04
-cc per minute = lbs per hour x 10.2
-cc per minute = HP x 5
-HP = cc per minute / 5
-HP = lbs per hour x 2.04

500cc per minute is approximately equal to 49lbs per hour which is equal to approximately 100hp.

wow good stuff. now i know how people figure when injectors are too big for thier HP goals and when they are too small. i was always curious. i never like taking information "because someone said so". having a formula to figure this stuff out is quite nice 8)

I don't think that math above adds up right, I got 20 as an answer not 31.25... and I'm only on my 2nd beer.

how many injectors you got? :)

The only thing I see that may be a variable that is not mentioned would be fuel pressure

I believe most injectors are rated at 38psi , but I'm not 100% sure..I know there is a standard though

I think you are correct but What if you aren't running 38 psi? A stock neon is 49 psi

Fuel pressure only keeps constant pressure to the injector. As long as there is fuel, the injector will spray. Most factory fuel pressure regulators and fuel rails can handle the added volume for pretty much any basic upgrade of injector as long as you don't go to crazy. This is obviously a car by car situation.

Agreed, I just wanted to throw that out there to let people know they need to consider all the variables

Then you would be right on

Got 4, and the divided 4 is in there for the 4 injectors I would assume. Still doesn't make sense why the answer is 31.25. I did play around with this formula back at UTI last year in the HotRod University class.

Here is a great page I found with pictures of different injectors. http://www.sdsefi.com/injectors.htm