Injector Calcs

[35R]

I've been doing some research into injector size requirements, and came across a lot of info (some conflicting). I thought i would share my thoughts and conclusions with TrackChatters.. From what i can gather this is how you work out required injector size..

First an important term:

Brake Specific Fuel Consumption (or BSFC):

This is how much fuel the engine will use for each horsepower, per hour. Apparently the constant number for N/A cars is 0.50 - meaning the N/A car will use half a pound of fuel - per hour - for each horsepower it produces.

Turbocharged engines are apparently between 0.60 and 0.65 BSFC.

Anyway the calculation to work out your required injector size is:

(DesiredHP / NumofInjectors) * (BrakeFuelConsumption / MaxInjDuty) / Sqrt(SysFuelPressure / 43.5)

This calculation takes into consideration the fact that stocker LS1 fuel system pressure is 58psi whereas most injectors are quoted for 43.5 psi (see last part of equation). You need to know what your fuel system pressure is and adjust accordingly (if not a stocker LSx fuel system).

So for my N/A munro application this is the calculation, assuming say 600hp:

(600 / 8 ) * (0.50 / 0.80) / Sqrt(58/43.50)

= 75 * 0.625 / 1.154

= 40.61 (lbs/hr)

So this calculation is saying that 40# injectors are ok for 600hp @ 80%. And that is just about exactly what i have found works fine believe it or not (i thought it would need bigger injectors).

Now suppose you are running a turbo, let's assume 750hp and a higher Brake Fuel Consumption of 0.60..

(DesiredHP / NumofInjectors) * (BrakeFuelConsumption / MaxInjDuty) / Sqrt(SysFuelPressure / 43.5);

(750 / 8 ) * (0.60 / 0.80) / Sqrt(58/43.50)

= 60.8lbs/hour

I thought perhaps we could implement a "shorthand" version of these calcs thus:

For N/A cars:

- Divide HP by 14

So.. 600hp / 14 = 42# injectors required

For F/I cars:

- Divide HP by 12

So 750hp / 12 = 62# injectors required

These are just approx calcs for a desired 80% duty cycle. And remember these "constant" divisors (HP/14 for N/A and HP/12 for turbo) are for LSx motors running at 58psi. Of course, talk to your mechanic and/or tuner for a sanity check, but i think these calcs are good for a ballpark estimate minimum injector size requirement.

I realise there are spreadsheets around that calculate this for you, but i just wanted to break it down to understand the components.

Hopefully this helps a few others too!

[NickS]

Interesting stuff OzDunc ... :D

[lautray]

Interesting stuff OzDunc ... :D

:lol: :lol:. It does work though :cool: .

[NickS]

:lol: :lol:. It does work though :cool: .

Makes sense to me (sort of ...), I have always been sceptical about these 350+rwkw turbo kits that don't need fuel systems (of any kind ???).

This makes putting that sort of power (especially with F/I) through a factory fuel system even scarier.

:shock:

[seedyrom]

Now, just to make matters more confusing.
What happens with F/I cars using a rising rate regulator?
As boost increases, so does the rate of fuel flow (well above the 60psi at idle).

Interesting, as I run 42lb's in my F/I car, yet I have a set of 55lb's here, but till now haven't really felt the need to fit them (and get tuned obviously), as I believe my duty cycle is only 80% at max power.



Obviously i'm not pumping out 750hp, but if I was looking at 550hp, would I be best putting them in?
Lets do the calcs.


Injector size = (DesiredHP / NumofInjectors) * (BrakeFuelConsumption / MaxInjDuty) / Sqrt(SysFuelPressure / 43.5)
=(550 / 8 ) * (0.60 / 0.80) / Sqrt(58/43.50)
= 68.75 * 0.75 / 1.154
= 44.68 (lbs/hr)

So if I was wanting to step up to 550HP (I'm currently about 350rwkw - engine flywheel HP conversion anyone?), it might be worth upgrading my injectors.
Especially since my fuel system is a rising rate one.

Hmmm ... thoughts?

[Delft Maloo]

Well im running 60lb'ers in my ute atm and the other day on russo's dyno when it pulled 398.7rwkw/535.4rwhp the max injector d/c on the data was recordered at 73%.
All that comes to be 73% d/c at about 715hp at the engine.

Yours seedy comes to about 470rwhp or about 630hp at the engine. According to those rough figures that would mean your 55lb injectors would be more suited to your application.

I was told that you only want your injector d/c to be around 85% at max load that way you allways have some headroom left in them just incase.

[35R]

Now, just to make matters more confusing.
What happens with F/I cars using a rising rate regulator?
As boost increases, so does the rate of fuel flow (well above the 60psi at idle).

Interesting, as I run 42lb's in my F/I car, yet I have a set of 55lb's here, but till now haven't really felt the need to fit them (and get tuned obviously), as I believe my duty cycle is only 80% at max power.



Obviously i'm not pumping out 750hp, but if I was looking at 550hp, would I be best putting them in?
Lets do the calcs.


Injector size = (DesiredHP / NumofInjectors) * (BrakeFuelConsumption / MaxInjDuty) / Sqrt(SysFuelPressure / 43.5)
=(550 / 8 ) * (0.60 / 0.80) / Sqrt(58/43.50)
= 68.75 * 0.75 / 1.154
= 44.68 (lbs/hr)

So if I was wanting to step up to 550HP (I'm currently about 350rwkw - engine flywheel HP conversion anyone?), it might be worth upgrading my injectors.
Especially since my fuel system is a rising rate one.

Hmmm ... thoughts?

Seedy maybe we can have a go at constructing a chart to show curves based on increasing fuel pressure. Say at 300rwkw, and 25rwkw+ increments

[MrKeen]

Interesting stuff OzDunc ... :D

lol
Ive wondered about this myself. As far as I know ive got 42 lb injectors in mine. Running 550hp. I no thats what sam used with the TTkit running 470hp but I dont no if he increase injector size when the bigger turbo's went in.

[35R]

Btw guys those estimates are based on FWHP not RWHP. And of course are only a guide for 80% duty cycle.

[35R]

Seedy here is a little chart I whipped up for variable fuel pressure.

I guess as fuel pressure rises, the injector flow rating requirement diminishes.

First pick a power rating row (say 550hp) and look up the relevant fuel pressure column (psi). The intersection tells you the required injector (in lbs/hr). So for a 550hp example, at 58psi you need 48.3lb/hr injectors. At 60psi you need 47.5lb/hr and so on.. I guess at the end of the day, you would probably take the injector as dictated by the lower pressure end of the scale, for safety.

These numbers are derived using 0.65 BSFC (turbo applications).

http://farm3.static.flickr.com/2417/2293440884_84a9853d9a_o.jpg

[RedVYIISS]

Seedy,

Your rising rate reg simply compensates for boost pressure. Injectors are rated to flow X lb/hour at (normally) 3 bar fuel pressure.

The 42 lb injectors will therefore flow 42lb of fuel at 3 bar fuel pressure when delivering at atmospheric pressure. We run 4 bar fuel pressure [58 psi(g)] so we get about 48 lb out of 42 lb injectors in any case.

What your rising rate reg does is compensates for the fact that (in a boosted application) the injector has to deliver to a pressure elevated above atmospheric, so at say 7 PSI boost, they'll still deliver 48 lb. (the rising rate reg adds the 7 psi to the set pressure of 58 psi to give you 65 psi fuel pressure).

I hope that makes sense!