How to determine needed injector size

[mhubble91]

Ok, I thought this would seriously help some fellows out there who are really confused with injector sizes. Bigger is not better. Nor will it improve performance at all, unless you require more fuel... More Fuel = Need for more air... Turn that boost up

*To determine the correct flow rate for forced induction applications (turbo'd):

- Find Boost/Pressure Ratio (PR) -> (boost to atmospheric pressure ratio)

PR = 14.7 + Manifold Pressure (how much boost you're pushing)

________diveded by___________

14.7 (atmospheric pressure)


- Now calculate CFM ( VE = volumetric efficiency, use 0.9 for 90% - for most boosted motors)

CFM = Engine Size (C.I.D.) x RPM (optimal power RPM - use redline) x VE x PR

______________________ diveded by_______________________

3456


*C.I.D. can be calculated by multiplying the engine Liters by 61. (ex. 2.0L = 122 CID ... whereas 5.7L = 350 CID)

Now that both Pressure Ratio (PR) and CFM have been calculated, the correct injector size can be estimated. Fine tuning the injector flow with fuel pressure adjustments will still have to be accomplished when the fuel system installation is complete.

Injector Size lbs/hr. = CFM x 0.44298

_____divided by_____

# of Cylinders

Now take this number and multuply by 10.5 and there you have it... you're correct CC injector size

Also, All i can say is be very realistic when doing this... do not say you're redline is 8k when its a stock SR

Cheers

[Croustibat]

This still is a theoretical approch, which will give ballpark figure, but it does not always work well.

It also does not account for fuel used (e85 usage goes stoeich with a near 9:1 ratio, whereas standard fuel is 14.7:1).

The engine we use in our S cars are not brand new, most of them already have some numbers throught trial and error. Maybe we could do with a list of injector size needed for a ballpark power figure ?

For the CA18DET (remember this one use a 2.5bar fuel reg, not 3bar):
so far this is what i got on my own
regular fuel :

stock 370cc = maxed at around 240hp ( T25, 1bar, remap, intercooler, exhaust, air filter). a T25 in these condition does not last long mind you ...
~440cc : good for 240hp with a safe margin, can go up to near 280 (T28, 1bar)
~ 550cc, 300HP (T28, 1.2bar), with a little margin can go up to 320-330
700-800c for 400+HP (gt2871r , 1.6 -1.8bar)

e85fuel:
510cc (with 3bar regulator) : maxed at ~ 280HP
600cc (with 3bar regulator) :for 300HP gives a little room, but i recommend 650cc for peace of mind.
1200cc for 500+ HP (low latency injectors needed)
1600cc for 600/700+ HP (low latency injectors needed)

[Walperstyle]

Glad to see someone young seeing the math behind the madness!

two good books with the same math, and more
http://t1.gstatic.com/images?q=tbn:ANd9GcR3bvAhZY_FdNBI5OyG5Bh9-2XftVIY-1AqAfu7ITCctPMz5Lvn6ySzSH3q
http://www.automotobookshop.com.au/images/b13864_turbocharging_performance_handbook.jpg

[Darren]

I ran your calcs through a spreadsheet and it seemed right for 10 - 12 psi (.8 - .9 bar) and then it didn't really make sense to me..

according to the calcs, i could push my stock sr20det 370 injectors up to 40 ish psi?? or are we not taking duty cycle into account with the calculations?

just wondering cause this is pretty important (and powerful) stuff to know.

Thanks btw for posting this

[mhubble91]

Wow, So sorry for taking forever to even come back to look at this, lol.

I thought it would just help a few people out there. But you need to be very, very careful when you do the calculation because one wrong miscalculation would give you strange results.

And thanks, Walperstyle. I like to do stuff correctly and not just go off of hear-say. I know a lot of younger folks like to talk crap but it's nice to figure things out the right way :)