Joined: 05 Mar 2008
Location: Pittsburgh, Pa
|Posted: Mon May 26, 2008 3:57 pm Post subject: Injector/pump sizing info
|Here is some info for sizing your fuel system.
[COLOR="Blue"]1. 60# injectors typically flow between 62-63# at 43psi. This equates to 72-73# at 58psi. This injector will reach 900rwhp, but will be maxed out.
2. A 72# injector at 43psi will do the same, but at lower required fuel pressure. A 72# at 58psi will be roughly an 83# injector.
3. An 83# injector should be good for approx 1045rwhp maxed out.
[/COLOR]NOTE!!!!!!!!!!!!!!!!!!!!This is based on a car, using gasoline, with an approx 15% driveterain loss & a BSFC of .55. This is commonly done on manual trans cars & locked automatics. It is possible, but not recommended to drive the injector this hard. Keep in mind that dyno's vary greatly & these are based on Dynojet numbers.
[COLOR="Red"]If you have a 4L80E or a loose unlocked converter, you will see less rwhp for a given fuel consumption.
Also your air fuel ratio greatly effects these numbers. Example running 10:1 AF requires 10% more fuel than an 11:1 AF. This is a huge difference in flow.
Be aware you should not design your fuel system for a maxed injector at 100% duty cycle.[/COLOR]
[COLOR="SeaGreen"]1. You should strive for 80% duty cycle as a general rule for continuous use, but can go slightly higher for short term drag racing etc. 85% should be OK. I would not design for 90% as at this point, you do not know your power output...... If you are off by 10%, you bought the wrong injector.
2. You should not design your system to run over 70psi or you will need an exceptionally large pump(s) to feed the elevated pressure requirements.
3. You do not design the fuel pump to feed the injector, you size it for the HP requirements at the correct fuel pressure.[/COLOR]
Now with this out of the way....................................................
Start to size injectors with the following info:
Desired HP (Don't fantasize here, be realistic or you will have poor drivability & spend a lot of extra money)
I want 900rwhp.....
I lose approx 18% through my auto & 9" rear.
so... I need 1062 flywheel HP.
1062hp with a .55 BSFC (forced induction/nitrous) is 584#/hr
584#/hr divided by 8 injectors = 73#/hr each at 100% duty cycle.
The theoretical 72# injector is maxed at this power level.
Using the 80% DC theory, you should use a 91# injector.
Using a 85% DC, you come up with an 83# injector.
Can I use a 72# with higher pressure to get my 83# flow?
Take your highest boost pressure & subtract from 70psi.
Example 70-20psi boost = 50psi base pressure... not 58psi.
This would work for a car with 12# boost, but above this you are asking a lot of the fuel pump to run over 70# max pressure. If you notice, the cars running high pressure are the ones with lower power & small injectors. Probably FMU cars with series pumps & under 500rwhp.
Now size the pump.....
The problem here is that most pumps are rated at 43psi. Many do not publish flow curves.
Lets take the common 255 Walbro 340.
255liters is 67.36 gallons.
Gasoline weighs approx 6.2#/gal.
This means a 255 pump should flow 417.6#/hr.
Will it support the calculated 759 flywheel hp?
Maybe in a perfect world. There are flow losses for lines, filters etc.
A safe limit is 80% of this which is 607 flywheel or 515 rwhp.
I recommend 550rwhp as Ive seen this commonly accomplished.... 85% of capacity.
Again I would size the pump for 80% of required flow to be safe.
Unfortunately here is where you have to compromise....
otherwise you will have a huge race only pump in the car.
If you want to run streetable pumps, I would consider lower pressure if you are above 750-800rwhp. Start with a big injector.... like a 96, lower the base pressure to 35 or so. Now it acts like a 86# injector.
You boost reference 20-25psi to it & you still have only 55-60 total pressure. Now the pumps are happy & you get a lot more flow from the pumps. You will take a 10% penalty in pump flow between 60 & 70psi.
My pumps have already supported over 750RWHP on E85 & 1050RWHP on gasoline. I still have not found the limits.
Last but not least......
E85 takes 25 to 30% additional fuel.
Take the above calcs & multiply the HP values by .7 to get the equivalents for E85. Ex. 900rwhp x .7 = 630rwhp
Multiply the injector calcualtions above by 1.3 to get the E5 equivalents.
A 60# injector needs to be a 78# on E85.
Hope this wong winded reply helps.
Remember your results may vary..... be safe & buy things once.