eniac

This is true and depends on the type of injector you get and also the type of injection(batch or sequential). 95# injectors would work fine with sequential injection, 86# are fine with batch. If using 126# and using batch injection on a stock DME then what you state would definately be an issue.

Different brands of injectors spray differently so some will run better at a low duty cycle then others. I had 83# Python injectors for a couple years until one started leaking. I then switched to Siemen 86# injectors. I have to say the poorly reviewed Python injectors gave me a smoother idle however the Seimens have a better throttle response. I assume this is a result of the different spray patterns. The Seimens still idle great with no issues at all temperatures just the Pythons were a little smoother.

Wormhole

Well I guess I'll see what happens tomorrow at the dyno. Last time I made 350hp and 380ft lbs at 15lbs on horrible software. I went back to my old setup and with the expected power increase I am concerned my 55lb injectors will be at the limit.

Just trying to get through this season, but I agree with you guys. Over the winter I am going standalone and some big injectors. Size is undetermined, but I'll want something that can support 500hp. I'll most likely be moderately tuned in the low 400s, with toggle for a high boost map setting to pull away for a lap if needed.

Jeremy Himsel

I'm not sure what you are asking me but I think we're agreeing here. Injector flow rates change based upon pressure. Most injector flow ratings are rated @ 3.0 bar of rail pressure (at the injector) and a true 55lb injector will flow like a 44 lb injector @ 2.5 bar and a 73 lb injector @ 4 bar. These are just simple guestimates because I don't have the injector spec sheet handy. Under boost @15psi and assuming a rising rate regulator of 1:1 (never actually checked the stock or 3.0 bar regulators), you are flowing more fuel through the injector even though injector duration stays the same. Under vacuum you flow less fuel. Unless of course your don't have a rising rate FPR or one that is adjustable (3:1) or fixed at a higher rate. Agree?

blown 944

Joshua,

I have to agree with Jeremy here. when the boost pressure rises there is more delivery through the same oriface open at the same amount of time.

When looking at the RC chart it should be noted that the line pressure should be whatever your base (atmo) plus boost pressure is IE: if you start at 40 line and run 20 psi (boost) the test pressure for sizing should be 60 psi. This is of course if you are using manifold vacuum/pressure source.

Someone on pelican had done some testing to verify the 1:1 rising rate and it is correct with both 2.5 bar and 3 bar FPR's.

I did the test to figure it out(what the FPR is) when I first got my car by using a tire pump (bicycle) and adding pressure and watching the gauge as pressure was added. Even my adjustable FPR is 1:1.

Rogue_Ant

We are agreeing that injectors flow changes depending on what you set the base pressure to...
However once the base FP is set, assuming a 1:1 ratio FPR, then actual fuel delivery volume/mass doesn't change (assuming same injector duration).

Lets say you set the base FP to 43.5psi (3BAR). Now if you're boosting 10psi, the fuel pressure is 53.5psi. But the pressure differential between the two sides of the injector is still 43.5psi (53.5-10). It takes 10psi of pressure before the injector can inject anything at all into the pressurized manifold.
The same holds true for vacuum. Because it is easier to inject into a vacuum, the FPR lowers the fuel pressure to keep injector flow constant.

BTW calculating injector flow for different base pressures is:
sqrt(newpressure / oldpressure) * injector flow rate @ old pressure

55lb injectors @ 2.5Bar = ~50lbs
55lb injectors @ 4bar = ~63.5lbs


-Rogue

Rogue_Ant

Sid,
Think about it for a minute. You guys are forgetting the other side of the injector. The fuel pressure must overcome manifold pressure first. You are injecting into a pressurized manifold. It takes equal pressure to first overcome this, then what pressure is left is what is actually driving the fuel flow.


-Rogue

blown 944

I understand what you are getting at but injecting into a much larger area vs the same size area is what makes the difference.

If you were injecting against the same size oriface that would hold true it would deadhead, but having a larger plenum vs small pressurized oriface allows the fluid to overcome the difference.

I would have to get out my old books for tuning blown alcohol to get right at the point. I can't quite put my finger on it right now but it was something about the specific gravity of the fuel and or any fluid.

Jeremy Himsel

Nope, I agree with you and stand corrected. If you look at it like blowing water through a straw, if the outlet side has pressure @ the tip of the straw (key being at the tip) essentially mimicking the inlet side you'll essentially have a stalemate. While there may be differences because of the inlet and outlet orifice sizes, the pressure imposed that would limit flow will still be at the tip. I do believe that the changes in overall chamber (small injector orifice to large runner) volume and fuel vs air specific gravity have an effect I'm guessing that they're most likely negligible. BTW, my injector flow calculation were just approximate examples provided without any calculation at all.

Interesting subject, Sid I'd be interested to see what you come up with.

theedge

Youre thinking about surface area I think? As in pounds per sq inch... You still need pressure on the fuel side to overcome the pressure in the manifold. That holds true even though the tip of the fuel injector is tiny. If the manifold pressure is 20PSI, and your fuel pressure is 20, you wont get flow. If the fuel pressure is 30PSI and manifold is 20, youll get flow but it will be a little dribble. Doesnt matter that the injector tip is less than a square inch.

blown 944

IIRC it has more to do with the mass of the fuel against the mass of the air hence the specific gravity reference. AS well as the pressure being spread vs the velocity from a small oriface.

there are a couple of other factors as well ie: kinetic energy of the fuel, the effect of the greater pressure of a quick stoppage of liquid flow. IIRC it actually creates a much high pressure than just the line pressure shown like 3 times more at the head (injector tip)


Man you guys are making me think here

just to give a quick example though

when tuning an alcohol engine at 35-50 psi there was minimal account for the boost pressure change in the equation it was much more relative to the amount of air and the oriface (total area) and line pressure.

IE: we could run the same Nozzle at a higher line pressure and run more boost with the same outcome on the AFR. We did this by changing the pump loop jet.

I did notice that there was a difference when we decided to go with soleniods vs poppets in regards to the size of jet needed to relieve (lean) line pressure we had to use larger jets when using the solenoids for the returns. I believe this has top do with the abrupt halt and the additional pressure generated from the kinetic energy at the valve.

I don't know what the scientific terms are but something is different between air and fuel a(and the types of fuel)

Good discussion non the less. Now we just need a fluid dynamics engineer to chime in

Rogue_Ant

Nothing wrong with that
I agree, good discussion, the more understanding I have can only be a good thing.


-Rogue

blown 944

One other quick thing I just remembered.

I have tuned blower engines that have the nozzles in the hat (atmos) and the same engine with the nozzles in the manifold (pressurized up to 25 psi)

The same nozzle size and line pressure was used. If the above thoughts were true that would not be possible

Jeremy Himsel

Working on that. Outside my office are about 25 Automotive Engineers who are trapped in cubes all day without the sight of daylight. Since there are only five offices in my building and I'm the only car freak that has one big enough, the hardcore car guys tend to congregate in my office during lunch and talk about current projects, racing and restorations. I'm lucky in the sense that they've worked for Bosch, GM, TRW, Delphi, Takata, ect....... So If I'm unsure about something I bring up the question and they fight like pit bulls for the rest of the afternoon. Every now and then I'll throw a wrench in and offer a different scenerio or an alternate perspective Usually it's the guy with the best drawing on my white board that wins.

Every now and then my boss will stroll by and ask "WTF is this". I just respond with ......... Don't bother asking...... but it's funny as hell.

Rogue_Ant

Awesome! Let us know what you find out


-Rogue

dand86951

If you use the TEC3 setup wizard it says you can get 390crank HP, and at 6600rpm and 19psi boost you will run a duty cycle of 100% using 3 bar fuel pressure. Anywhere below either 19psi and 6600rpm the duty cycle would be less than 100%.