Flow vs. Line Size - From another forum

[attworth]

I picked up this thread from a Corvette Forum while trying to answer a question on zcar.com

 

It holds some very interesting information, some that would be valuable to many people I believe. Have a looksy if you have a few minutes of time.

 

http://forums.corvetteforum.com/c4-forced-induction-nitrous/1758285-fuel-flow-vs-line-size.html

[Bartman]

Good write up and I agree 100%. There are plenty of posts here where people talk about having to increase the size of their fuel lines, and in a lot of cases I really don't think it's necessary. I'm running stock lines on my install and it works fine. As is stated in the post you referenced, if you have the proper pressure at the fuel rail - the fuel lines in conjunction with the fuel pump you're using - are just fine.

[attworth]

I wanted to make sure this information didn't get lost to the hybridz'ers.

 

Here is the information contained in that thread, borrowed 100% from that thread, and not written by myself or anybody I know.

 

I've made this post before' date=' but it was a long time ago and I guess we need to examine this topic again - specifically to address Tony Dee's thread. I thought it better to make it a new separate post so new members a year from now could find it more easily. This post will address the required fuel line size from your fuel tank to the injector rails as well as the injector rails themselves.

Let's start with a random off the shelf injector - say a 60 lb/hr. In order for the flow rating of any injector to be meaningful we must also have a pressure rating. Most injectors' flows are rated at around 42 to 45 psig (pounds per square inch gage) and typically 80% duty cycle.

 

So we take this injector and hook it up to a test rail. The test rail has a regulator on it which we set to say 42 psig. We also attach an accurate fuel pressure gage right at the rear of the injector to verify the pressure setting. Now we run the injector at 80% duty cycle with its business end pointed into a calibrated cylinder so we can measure its' flow over time. Some labs have very accurate in-line flow meters that make the measuring task much easier/quicker. With everything in place and functioning we measure a flow rate of 60 lbs/hr.

 

Now I ask you did it matter whether I fed the fuel rail with a 1/8" id fuel line or a 3/8" id fuel line or a 1/2" id fuel line or a 1" id fuel rail as long as I was able to maintain the 42 psig? No - it does not matter what the supply line size is as long as I can maintain the desired pressure at the injector head !!!

 

I took this round about way of explaining flow because some people have it stuck in their heads that they must increase their line size when making big HP numbers like a thousand or more. A #6 AN line can easily support 2000 HP as long as you have enough pump head !!!

 

That is to say as the flow thru any given line size increases so do the friction losses. These losses are published in engineering manuals such as the Cameron Hydraulic Data Handbook published by Ingersoll-Rand. Any engineer who works with designing pump installations has one of these manuals.

 

Let's take a typical Corvette. I'll say there is 20 feet of steel 3/8" id fuel line from tank to rail (there is actually less but we'll be conservative because bends in the line account for some pressure losses also - each bend adds 'x' amount of equivalent feet).

 

Now you go into the table and look up the pressure drop for say 110 gph (gallons per hour) for 20 feet of 3/8" id steel line (12 HP/ gallon of gasoline is a conservative number): it's going to be around 2 to 3 psig.

 

This is the same way an electrical engineer determines what size wire to run for a given load. If the load is say 40 amps he could use a #14 AWG or he could use a #8AWG. Both size wires can handle the load, i.e., 50 amps (or if gasoline amps = volume in gallons). However the smaller wire has a much higher voltage drop (in gasoline thats pressure). If our voltage is fixed at 120 VAC the engineer will go with the bigger wire to minimize the voltage drop. Your Corvette pump head (i.e., voltage) is not fixed until after you pick your pumps etc.

 

So what this means is that if you need to maintain 42 psig at the fuel injector you must have enough pump head to not only make the 42 psig but an extra 2 or 3 psig for the line losses. In other words if your pump(s) can deliver 110 gph at say 60 psig, you have plenty of pump with some to spare.

 

Where all these line size "wifes' tails" started is back in the days of carbureautors and low pressure electric pumps or low pressure mechanical pumps. These pumps had plenty of flow but at low pressures. These pumps could not afford a 3 psig drop from front to back. So they increased line size - without looking it up perhaps a #8 AN line has only a 1 psig drop at 110 gph for the given 20 feet.

 

The whole point of this discussion is that if you have an accurate fuel pressure gage placed at the end of your fuel rail and you are able to maintain the head i.e., desired fuel pressure PSIG, your fuel line size is fine!!! If the fuel pressure starts to fall as you go up in HP you need more pump - if for some reason you cannot get more pump then line size can be considered - but if you look at the numbers for #6 vs. #8 there is very little difference.

 

Tony Dee's thread also addressed hydraulic shock from the opening and closing of the injectors. I've used for years a small Bosch unit #0 280 161 212. It is 2" in diameter and about 1.5" high. It is suitable for pressures well in excess of 100 psig. It's a little difficult to use because it attaches with a M10 banjo bolt - hence you need a banjo adaptor. A pair of these (one on each rail) virtually eliminates the fuel pressure spikes which can be disastrous. The biggest problem with these pulsation dampers are $. They may cost you in excess of $125 each and then you still have to adapt them to your rail.

 

Foot Notes: Gasoline weighs 5.994 lbs/gal @ 60 degrees F (this number may vary slightly depending on its' octane rating). Divide CC/Min by 10.5 to obtain Lb/Hr. With a little math you'll see that ML/Sec x 5.69722 = Lbs/Hr. At W.O.T. you need 1/2 pound of fuel per hour per HP (that's at a conservative .5 BSFC). Well tuned engines will run at .4 to .45 BSFC at WOT.

 

Hope the above saves some members the cost of replacing their fuel lines without really needing to. Best regards, Greg[/quote']

[ShaggyZ]

Are turbocharged and supercharged engines different somehow? I'm guessing not.

 

That was a good read, though I've already resolved to stick with stock lines to supply the 2JZGTE hopefully upwards of 500whp peak.

[attworth]

The engines themselves are probably different yes. The point of the gentleman's thread is that if you've got proper flow at the end of the line, the size of that line is irrelevant to most extents.

 

I deal with this at work when sizing sewer systems, and I agree with everything he said. There's many different ways to calculate flow in my situation, but it's the same concept.

[WizardBlack]

It's the same thing as saying it doesn't matter how small your exhaust is as long as you actually hit the power number you need/want. Bigger hose (to an extent) will make it easier on the fuel pump to hit the demand you place on it.

[ShaggyZ]

It's the same thing as saying it doesn't matter how small your exhaust is as long as you actually hit the power number you need/want. Bigger hose (to an extent) will make it easier on the fuel pump to hit the demand you place on it.

 

It's all very similar to the standard "how much power does [this] give you." It doesn't really make sense to ask that question for various reasons I have trouble putting into words.

[ktm]

What attworth is not saying, but rather implying, is that your fuel pump must be sized correctly. Fuel pumps are not spec'd the same way as sewer pumps, ground water pumps, etc.

 

Most fuel pumping curves are shown plotted against voltage; what is important to note that in order to use the smaller lines, you need a true pumping curve shown in flow rate versus pumping head. You then need to determine your true pumping head when using the smaller lines. Using your power requirements as a baseline, you then determine an estimate of your fuel demand (flow rate). You then need some idea of what your pumping head will be. Smaller lines have higher losses per foot than larger lines. Fittings, bends, etc. are losses as well. Typically for a given pump, the higher the pumping head, the lower the flow rate.

 

Keep in mind as well that the discussion where attworth pulled the text from was based on using OEM fuel lines that were already 5/16. This is larger than the stock 1/4-in. 240z feed line.

 

You have to treat N/A and forced inducted engines differently. N/A engines are not nearly as sensitive to fuel tuning as a forced inducted engine. If a turbo'd engine leans out under high boost, you can blow an engine. Lean here could be 13:1; 13:1 is right around where N/A engine is tuned (lean best torque). You need a higher factor of safety when dealing with forced inducted engines. This is where larger fuel lines come into play as well as the entire fuel system design.

[ShaggyZ]

2 more cents from me: I'm mostly tired of the guys who think they need 1000cc injectors to make 500HP at the flywheel and have no intent of going higher. Same thing happens with turbochargers.

 

Correct sizing is what it's all about.

[garvice]

Surely this TonyDee isn't our TonyD?

 

If it is, he really is everywhere on the net!!!