36 lb/hr Injectors Maxed Out

[KirkEvil]

When getting the car tuned, the 30 lb/hr SVO red top injectors, (36.5 lb/hr @ 58 psi) were at 110% duty cycle at 6400 rpm. The car has full bolt ons and a 228/232 cam, stock 241 heads. Flowtech Inductions claims 390 rwhp with supporting mods for this cam with stock heads.

I really dont think Im making enough power to max these out now. I guess my next step is to run some injector cleaner through them. Input?

[Whiplash Performance]

through my findings tonight i was looking through my advanced tuning book
for calculating flow rate:
( Crank HP*.5)/(8*0.80)=lbs/hr flow rate
(460*.5)/(8*0.80)=35lbs/hr injectors recommends

so you should be good on injector but yet we were getting 110% duty cycle.

[BarneyMobile]

Sounds like a weak fuel pump. Is your fuel pump still stock? Put a fuel gauge on it and see if your fuel pressure drops off in the upper rpm range.

[KirkEvil]

fuel pump is a walbro 255 with a hotwire kit, only a few months old. fuel pressure was verified when I fixed this problem

http://www.njfboa.org/forums/showthread.php?t=47253

[Zero4488]

What are you using to log it? If it's HPTuners post up the log and I'll take a look at it....

-Dave

[Pampered-Z]

You are probably not getting a true 36 out of the injectors. You need more fuel which is why you are so far over the duty cycles. FP isn't really going to overcome that.

Below is the formulal I use for my car. This is from my Electromitive DFI, I shortened some of the text! Plugging in you numbers I come up 34 lb injectors.

1) Knowledge of the break specific fuel consumption of your engine at maximum output, values can be from .39 (lean and efficient) to .69 (inefficient cumbustion chambers and lots of boost). Example of how to determine the fuel requirement of an engine with the a BSFC of aproximately .53.

500 HP x .53 BSFC = Fuel requirement of 318 lb/h

2. How many fuel injectors are being used? This is pretty obvious, we need to know the per injector fuel requirement.

318 lb/h divided by 8 injectors = 39.75 lb/h per injector

3. At what RPM are we making this power? This is probably where most people get stuck. Not unless you are being so crude as to run your injectors at 100 % duty cycle, can you make do with injectors sized exactly by the lb/h per injector number. A fuel injector needs a certain amount of time to recover from being engaged between spraying events. It is important to know when your injector fires for this step, but let us
assume the injector fires once per engine revolution, just like ELECTROMOTIVE's phased sequential injection for multi-point systems. The engine speed for our ongoing example will be 7,600 RPM, and we need to know how much time there is between injector events.

60,000 ms (one minute) divided by 7,600 RPM= 7.8947 ms between Top Dead
Centers

4. What is the % of duty cycle that can be used at this point? This is determined by the time it takes the injector to close after it has been opened, which is usually somewhere around 1 ms. Here we need to know the ratio between the total amount of time available and what we can actualy use.

7.8947 ms - 1ms = 6.8947 ms 6.8947 ms / 7.8947 ms = 0.8733 or 87.33%

5. Injector Size Requirement! Now we have all the info required to select an injector.
Fuel Requirement of 39.75 lb/h divided by the maximum duty cycle of 87.33% = 45.5 lb/h
With this information we can now go get an injector.


So plugging in your information:

460hp x .53 = 243.8

243.8 / 8 = 30.475 (Fuel required)

RPM = 6400

60,000 ms / 6400 RPM = 9.375 Ms - BTDC

9.375 - 1ms = 8.375 ms | 8.375 / 9.375 = 0.8933

30.475 (fuel) / 0.8933 (Duty cycle) = 34.11 Fuel Injector size.

From this your 30s are too small, you need 36s to stay in the 80% duty cycles.

[KirkEvil]

You are probably not getting a true 36 out of the injectors. You need more fuel which is why you are so far over the duty cycles. FP isn't really going to overcome that.

Below is the formulal I use for my car. This is from my Electromitive DFI, I shortened some of the text! Plugging in you numbers I come up 34 lb injectors.

From this your 30s are too small, you need 36s to stay in the 80% duty cycles.

That is basically the calculation I first went through when I was selecting injectors, and I came up with 32 lb/hr injectors using slightly different values. The 30 lb/hr SVO ford injectors are rated at 39.5 psi, and LS1 FP is 58 psi... so at 58 psi they should flow ~36.4 lb/hr, and accommodate both of our calculations.

If you back solve those equations with the 110% duty cycle, and 36 lb/hr for horsepower it comes to 598 bhp, which is definitely not what the engine is making.

Also, Im just trying to understand steps 3 & 4 of those equations...Is that supposed to calculate acceptable duty cycle based on max rpm? Ex. the duty cycle calculated with the steps increases with decreasing max rpm and vice versa. The calculations I did when selecting injectors always used 80% for IDC.

[LTb1ow]

Kirk, any chance that you are not supplying enough voltage to the pump?

[KirkEvil]

Doubt it, the hotwire kit comes right off the alternator.

[LTb1ow]

And you have such stellar charging qualities.... :lol:

[Whiplash Performance]

I'll get the HP tuners log up in a little bit.

[Pampered-Z]

That is basically the calculation I first went through when I was selecting injectors, and I came up with 32 lb/hr injectors using slightly different values. The 30 lb/hr SVO ford injectors are rated at 39.5 psi, and LS1 FP is 58 psi... so at 58 psi they should flow ~36.4 lb/hr, and accommodate both of our calculations.

If you back solve those equations with the 110% duty cycle, and 36 lb/hr for horsepower it comes to 598 bhp, which is definitely not what the engine is making.

Also, Im just trying to understand steps 3 & 4 of those equations...Is that supposed to calculate acceptable duty cycle based on max rpm? Ex. the duty cycle calculated with the steps increases with decreasing max rpm and vice versa. The calculations I did when selecting injectors always used 80% for IDC.


Yes you want to target no more then 80% duty. likewise, with a super charger we used FMUs to push FP well up and still didn't get the flow we were expecting and needed larger Tis was even with factoring in 100 extra HP to "feed the blower".