When changing to a larger injector, do you also need to bump up the fuel pressure? I am running SVO 30lbs on my setup (sig) and stock fuel pressure 43.5 Is this ok?

 

yea except on our fuel pressure they are higher lb injectors, ford rates theirs at 39 psi of fuel pressure i believe.

 

I believe the svo would be about 31.5 on an lt1. I got 38 but could squeeze a max of 42 out of the by increasing fuel pressure.

 

I would assume that setup is approaching at least 500 flywheelHP. I would recommend at least a 36# injector. You could get your 30's to flow 36# by running 4bar (58psi). Why did you pick a 30# injector?

 

yeah...i would say 30# would be too small

 

Fred, what would increasing the psi do for his performance? I understand the 30# may be increasing his duty cycle @ 43.5 psi. But wouldn't increasing the injector just decrease the cycle at the same psi? I've never understood the benefit of raising the psi (if) if everything else is dialed in correctly. I thought if you lean it out you gained more pwr since they run pig rich from the General. I run 30# and if I raise my psi above 40psi the car has fuel just dumping on the cylinders. Even w/ it programmed for 55 psi the car was a dog. It was recommended that I run 30's at 55 psi. Currently going to 36's and going back down to 41psi, just waiting on the chip to return and see what happens.

Sorry to hijack you thread but it's along the same lines.

 

when i purchased the motor (at the time had a hotcam), It had these injectors. So, lets see if I get this. So by raising the fuel pressure, your actually lowering the injector duty cycle? But would this do anything for power, also, does the tune that is in the car need to be played with, or is it just a simple change in lt1 edit? Thanks Fred, and Guys!

 

First, lets make sure we all know what "duty cycle" means.... its the percentage arrived at by dividing the injector pulse width by the time available in 2 revs of the crankshaft. Its a calculated number.. the injectors can't "change" the DC.

Raising the pressure from 43.5psi to 58 psi increases the flow of the injector for any given pulse width by 15%. That means the pulse widths will be reduced by 15% for any given load on the engine. If an injector was running at 100% DC at 43.5psi, it would only be running at 85% DC at 58psi. Running an injector at 100% DC is a recipe for injector failure and engine destruction. Running the same injector at 85% provides a better chance of both the injector and the engine surviving.

With a stock tune in a stock PCM, playing with the fuel pressure is a useless way to "tune". Yes, you can lean it out to try and overcome the overly rich PE mode tuning from the General, but it doesn't work. And that's not what we are talking about here.... this is a 500HP setup that will (I assume) be tuned correctly, for the selected injectors. The tuner needs to know the type of injectors, their flow capacity, what pressure that flow capacity was measured at, and what pressure they will be run at. That's the only way he can program the flow constant into the PCM.

One benefit of running an injector at higher pressure is to reduce the variation in fuel flow that result from minor fluctuations in pressure.... things like the G's acting on the fuel in the lines on a hard launch. A 5psi "blip" would reduce flow through a 43.5# injector by 6.3%. A 5psi loss on a 70psi system will only produce a 4.6psi loss of flow. The higher the system pressure, the less affect on flow from small variations.

How an injector responds to higher pressures depends on the type of injector. The stock ball/seat injectors don't like high pressure. A pintle type injector, on the other hand, responds well to higher pressure.

Not sure why your example failed. I run 64# Bosch low impedance injectors in a 58psi fuel system.... they flow about 78#/HR in that setup. With a 300-shot of dry nitrous (all the fuel through the injectors) on top of a 500HP stroker, and with a measured brake specific fuel comsumption of 0.53#/HR/HP, I'm running just about a 70% DC.

Another advantage of a very large injector.... you can time the pulse width to the intake valve opening. With 100% DC, you are spraying fuel almost continuously, which means a good part of the time, you are spraying the fuel on a closed valve. With a low DC (and an ECU that allows you to alter injector timing) you can get a much larger part of the fuel flow into the moving air stream caused by an open valve.

 

now it all makes sense... The car was tuned with these SVO 30lbs, and stock fuel pressure. Correct me if Im wrong Fred, but doesnt Datamaster display Injector Duty Cycle? I might want to take a log at WOT, and see what the duty cycle is. I just recently purchased lt1edit myself, and can now do any changes I need to do to the car myself. Im guessing the only benefit of playing around with the Fuel pressure, is if the injector is at 80-85%..With that being said, I need to see where on datamaster, it records duty cycle...Thanks Fred!

 

I would also think that by increasing fuel pressure, you would get a better "atomization" of the fuel and air mixture, which would then burn fuel more completely and also produce more power, correct?

 

TTS DataMaster "calculates" duty cycle from base pulse width (BPW) and RPM. I personally think something is screwed up with DataMaster's "BPW", resulting in unreasonable DC number being produced. I've seen mostly stock LT1's show close to 100% DC using the DataMaster calculation, and since you can see how they calculated it using BPW and RPM, the source of the error has to be in the "BPW". I've seen DataMaster report 115% DC's on slightly stronger setups, where the engine is not experiencing any fueling problems. I've suggested to DM owners that they ask TTS to explain this, but as far as I know, no one has ever done that.

I think that how pressure affects atomization will depend on the type of inector.