How do you size injectors before the engine is built?

[ciper]

Id like to do this right the first time, I know others probably have the same question.

What information can you use to correctly size injectors.
A related question would be what is available at a decent price?

From the different calculators available online I find that I will need 550-650cc injectors?! Seems very high to me.

I also read on autospeed's site that a fairly reliable way to get the size is to take the CC and divide by 5 to get Horsepower (multiply by the number of injector) Using this method Im guessing I need 410cc injectors.

Those are two very different numbers. Can I easily calculate BSFC?

I know for fine tuning I can play with fuel pressure but at the moment Id like to stick with 45 or so PSI

[vrg3]

I don't know of any way to calculate BSFC... it changes all the time anyway, since VE does too, and it also depends on your tuning (how rich you want to run).

But a rule of thumb is that turbocharged gasoline engines generally have BSFCs of around 0.55 or 0.60 lb/hp/hr when producing maximum power.

So...

injector flow = power * BSFC / (duty cycle * number of injectors) * 10.16

The 10.16 converts from lb/hr to cc/min because BSFC is usually given with pounds of fuel per hour.

Well, you have 4 injectors, and you probably want to stick to duty cycles below about 85%, so

injector flow = power * 0.60 / (0.85 * 4 ) * 10.16
injector flow = power * 1.81

Was the Autospeed article talking about turbos? Unless they were allowing for higher duty cycles and/or lower BSFC numbers, dividing by 5 would give you smaller results than I'd think would be ideal.

Sounds like you're shooting for around 330hp... 550cc injectors are actually pretty appropriate for that power level. Some people say 90% duty cycle is okay, and some people say it's okay to fuel a lot less than a 0.6 BSFC, and plenty of people do get away with using injectors significantly smaller, but it can be a risk.

Consider that the stock MkIV Supra TT had 550cc injectors for power levels similar to your goal, and it had 6 injectors. Of course, that car was ridiculously overengineered in many ways, but still.

[Matt Monson]

I don't know how I missed this thread until now, Must have been while I was on my Holiday Hiatus for a week. Anyways, I ran the numbers based on similar BSFC and similar HP goals and came to the same conclusion. 500-550cc injectors. Seeing as the ones I've got are 480cc, I am gonna start there, and if I find I have to have more fuel for more power, I will go up from there, after all I can always have them b;ueprinted and modified to flow a bit more...

[Yukonart]

Well. . . I believe the stock injectors in my ride are 550cc . . . . if that helps.

[JasonGrahn]

I posted this on another forum where people were being stupid on picking their injectors...

Okay, for those who are new or need a refresher course, picking your injector size out of a hat is not a good idea. It's costly, inefficient, and downright stupid. While there there is plenty of information specific to most cars available these days, some applications make it difficult to track down information. In these cases, you can find the injector size you need by following some simple math.

Please see the following websites for more information:
http://www.holley.com/HiOctn/TechServ/T ... tcl07.html
http://www.mindspring.com/~steveflyer/f ... r_size.htm
http://www.rceng.com/technical.htm

To use the calculators in the above websites, it is good to know the BSFC of your car, or Brake Specific Fuel Consumption. Thank Al Gore for the internet! Here is some more information:
http://www.westechperformance.com/pages ... /bsfc.html
http://www.webcalc.net/calc/0756.php

So good luck in your endevors for power. As usual, please use the 3 R's for buying car parts: -R-esearch your options, -R-ead all the information, and -R-eevaluate your needs.

[MY92]

Just tell me what hp you want at the crank and I'll give you a figure

[ciper]

Crank will be 300-400. It looks as if 500+ cc units are my target. Ill reply with more information once the engine is actually installed (still sitting on a stand).

[MY92]

Hmm, could get away with the STi yellow top 440's and a malpassi.

The problem with bloody Subaru's is the lack of available inj, unlike most other cars you can just raid them out of a rx-7.

Other options are, run another set parallel or run additional inj's (although this is not my fav method).

What ECU are you looking at or have?

[Matt Monson]

Just tell me what hp you want at the crank and I'll give you a figure

I am going with the inverse of that, which is generally easier to tune. I am using an STI v6 manifold with it's injectors, fuel rails, ignition coil and VF-28 turbo. Given that was all factory equipment on the car, I think I am playing it pretty safe. Once it is all installed on the engine, I will then get it tuned properly, and whatever HP it ends up at will be what I run.

Of course, too much is never enough, and that is where my comments about bigger injectors down the road will come in. How long will that keep me happy? After all, why build an engine that is capable of 450hp if you are only gonna run it at 300-325hp or so...