injector size Options

[xian 1g]

Hey,
I apologize for the plaque of questions cluttering the board, but its relatively hard to do a search while I'm driving and read an extensive write up... something about keeping the eyes on the road is atleast better than driving with no hands.


anyways...

what size injectors should i run with a fp3065? rewired supra fuel pump. fmic, etc....

eric, you mentioned something about the supra being maxed out with 750's at 43psi? Looking to hit tens.

thanks in advance...

[JMoushon]

780s. Might have a bit of trouble tuning for idle, but if properly tuned and with the right gas these should have no problems getting where you need to be. Notice my opinion is based on the fact that you want to run in th 10s, and for that I figure you would run no less than 112-116 octane at the track. If you want to run pump gas and 26+psi, well, I have no idea.

[awd4kicks]

I don't know if idle will even be a concern to you Xian but since it was mentioned, I was able to get my 780cc injectors with the BR500 and with the 14b to idle just fine with the MAFT. I did not have enough time to get them tuned in properly for performance so I can't vouch for that. BTW - I am running the Walboro 255HP with an FPR.

[xian 1g]

i've actually got most of the makings of a afpr... we're going to come out with a kit. idling's not too much of a concern. car is mainly for track purposes, though, like i said before occasionally romping on the streets is fun. especially when 'daddy's mustang' sitting next to me gets a kick because the car can't idle.

imo with enough time and tuning the car should be able to idle fine... and yeah, i've got the maft set up as well.


so, 780's would be large enough? and anthing larger would just be over kill? would it be wise to go larger to make sure that the injectors would never be a limiting factor? or is that just going to cuase problems or be pointless?

[zach]

First what is your desired flywheel hp?
and at what psi of boost will this be attained at.
are you batch firing of sequential.
take total hp multiply by the brake specific fuel comsumption of your motor. ( most claim o.62 on 2 liter turbo. an divide that by the number of injectors to be used times the max duty cycle, that will get you lbs/hr for injector. multply this number to by 10.5 to convert to cc's

example: 500hp x .62 =310. divide by 4x.80 =96.875lbs/hr
and maximum 80% injector duty multiply by 10.5 to get cc's wich would equate to 1017 cc injector at 43.5 psi
this injector is way to big, because you will not be making max hp at only 43.5 psi instead...

the more boost the more fuel
example 25 psi of boost, then fpr 1psi boost = 1 more psi of fuel.
you will now be running 68.5 psi fuel pressure

I love math (remember this 80% duty cycle, it should not be breached or possible failure of injector could result)

since you will be using 68.5 psi instead or the 43.5 psi, you take 68.5 and divide by 43.5 to get 1.5747
you then take the square root of this number, which would be 1.25487 and multiply this by the original injector fuel flow of 1017cc of fuel and you would be pushing 1276 cc's of fuel into this beast.
which is way to much fuel.. so lets go backwards and take away 25 psi by dividing our 1017 cc by 1.25487 and we would then get 810 cc's


so size 810cc's @43.5 psi is what is required to support our 500hp motor runing 25lbls of boost and 68.5 psi of fuel pressure.

now... batch or sequential?
by the numbers 810 will work for both sequential an batch
sequential...810 at the minimum, throw in a set of 1000cc injectors, tune your aem ems, call it a day.
batch... you can probably get a way with a little smaller ie. 750s with out a hint of a lean condition problem. but i would get the 800 + injectors just to be safe

next question.

do you have enough fuel pump.
in our example you need a pump that will support 310lbs/hr of fuel at almost 70psi
make sure its there.

[wortdog]

Or, you could just go here and play around to your heart's content:

http://www.rceng.com/technical.htm#WORKSHEET

0.62 is WAAAY conservative for race gas IMHO. According to all of the worksheets, running my 6 38lb/hr injectors at 46 psi and 100% duty cycle with an assumed crank HP of around 450 (from a 112 mph trap speed in a 3800lb car) results in a BSFC of .52 or so. Its actually probably more around 0.51, as I'm not running my injectors at static quite yet. The wideband shows between 12.5:1 and 12.8:1 AFR on the run.

Also, the part about taking the square root of the fuel pressure doesn't make any sense. The whole point of the fuel pressure rising with boost is to maintain a set pressure differential (in this case, 43.5 psi, which is what most injectors are rated at) across the injector, such that opening the injector for a given pulsewidth will always result in the same amount of fuel flowing across the injector. If you want to change your base fuel pressure, you can divide the square roots of the pressures and multiply that times the injector flow to get your flow at the new pressure, like so:

Sqr(45psi) * 450cc / Sqr(43.5psi) = 457.69 cc

[xian 1g]

zach and eric,
thanks for the incredible write ups. your math skills blow my english major's mind away.

How would i find out the pressure at the rail for the link you sent eric?

[wortdog]

The pressure at the rail is whatever you adjust it to with the adjustable regulator. Stock is around 37psi I believe.

[xian 1g]

thanks again eric,

with a fpr, what should I set the pressure too?

[wortdog]

Fuel pressure is a thing you can tune, you don't just set it up at a specific number and call it good.

I'd start with stock, its what the translator and the ECU are going to expect.

[xian 1g]

thanks again eric


thanks for the offer, but right now i'm leaning towards 850 or 950.