'93 VR4 State of the Union

[AdamVR4]

I'm surprised you didn't go with a dual in --> dual out setup. I've never been fond of the loops... too nervous about starving the 6th cylinder in the loop... cylinder #6.

I almost made a thread about this.

Dual parallel walbro 255lph's will flow anywhere from ~550 L/hr @ 30 psi to ~350 L/hr @ 85 psi. They have internal pressure relief valves @ 85 psi and begin dumping volume there. If you're running 35 psi boost on top of 38 psi rail pressure, you're talking ~400 L/hr @ 73 psi.

400 L/hr or 6.67 L/min... in more useful units, that's .00011 m^3/sec. It's really not much flow and that's why people have been able to conduct reasonable fuel pump flow rate tests in their garage with a large bucket.

Assuming no pressure drop through the -8 lines en route to the rail...

Ethanol has a dynamic viscosity of ~.0011 kg/(m*s) and a density of 789 kg/m^3.

We'll assume the stock rails have a -6 AN diameter of .31" ... x-sectional area of .075 in^2... or .000049 m^2

Velocity = volumetric Flow Rate / x-sectional area =.00011 m^3/sec / .000049 m^2 = 2.28 m/s or about 7.5 feet per second.

Reynolds number = 12,900 ... turbulent

I used this calculator to find the friction coefficient because my spreadsheet was for laminar flow

Moody Friction Factor Calculator

It's reynold's number jived with mine and the friction coefficient is .0378

The stock rails and loop are about 30 inches long. We'll call it 3 ft to be conservative.

I will also conservatively neglect fuel sprayed into the cylinders that would reduce pressure drop to cylinder #6 for simplicity. I may explore this further later.

I'm getting 1.3 psi pressure drop across 3 feet at these flow rates. Jives well with available calculators like this one: https://www.lmengines.com/fuel-line-calculators/

Differential pressure across injector #1 is 38 psi

Differential pressure across injector #6 is 36.7 psi ... or 3.4% less... Seems significant, right?

If you assume my 1200cc injectors flow that at 38 psi (a slightly conservative assumption) and plug the differential pressure numbers numbers into RC Fuel Injection

That 1.3 psi pressure difference only amounts to 11 cc/min or less than 1%.

For reference, my current 6x 780cc injectors flowed a maximum of 744cc/min and {*edit* a minimum of} 734cc/min... or a totally acceptable 10 cc/min deviation.

That and... The #6 cylinder gets the least air flow simply due to our throttle body location and intake manifold design. I want less fuel @ cylinder #6.

[futurevr4man]

That and... The #6 cylinder gets the least air flow simply due to our throttle body location and intake manifold design. I want less fuel @ cylinder #6.

I almost made a comment about that in my post as well. But meh. When everything is under pressure, I have a hard time believing there is THAT much of a difference, but I haven't seen any models of it, or the numbers they used in said models.

I will also conservatively neglect fuel sprayed into the cylinders that would reduce pressure drop to cylinder #6 for simplicity. I may explore this further later.

Maybe I am being ignorant here, idk, but I don't think neglecting fuel sprayed is being conservative. There would be an inherent pressure drop at each point, every injector taking it's piece of the pie right? Sure, they fire for just, what? a ms? (don't want to calculate that out right this second), but at 7000rpm, or a rotation ~every 10ms, they are firing QUITE a bit. Am I just totally looking at this wrong?

The last thing I would want to happen is starve an injector for even a few ms, and get a lean spike in your injector firing. Create quite the hotspot pretty dogone quick.

And to be clear - I don't think you have a bad setup going at all. I am simply asking questions out of curiosity. You're smart enough that you know what you're getting into. A nice discussion is a rare thing to have

[AdamVR4]

I almost made a comment about that in my post as well. But meh. When everything is under pressure, I have a hard time believing there is THAT much of a difference, but I haven't seen any models of it, or the numbers they used in said models.

I'd be impressed if there's only a 1-2% difference in air flow from runner 1 to 6 through our asymmetrical intake manifold... How good was the CFD in 1990? How about the CFD of anything aftermarket for a 3S? haha

Maybe I am being ignorant here, idk, but I don't think neglecting fuel sprayed is being conservative. There would be an inherent pressure drop at each point, every injector taking it's piece of the pie right? Sure, they fire for just, what? a ms? (don't want to calculate that out right this second), but at 7000rpm, or a rotation ~every 10ms, they are firing QUITE a bit. Am I just totally looking at this wrong?

8000 rpm's is 133 rev/sec... my 1200cc injectors @ 92.5% IDC will dump 1.11 L/min or .0185 L/s or .000139 L per revolution. There is .0487 L of fuel sitting in a 1 m long fuel rail... Each time an injector opens it draws a negligible 0.2% of the available fuel.

Also, the 6.67L/min parallel walbro flow rate I assumed in my last post is exactly what 6x 1200cc injectors @ 92.5% IDC would need.

The last thing I would want to happen is starve an injector for even a few ms, and get a lean spike in your injector firing. Create quite the hotspot pretty dogone quick.

Thanks to the fuel pressure regulator, the only way that happens is if you run out of fuel pump, right?

And to be clear - I don't think you have a bad setup going at all. I am simply asking questions out of curiosity. You're smart enough that you know what you're getting into. A nice discussion is a rare thing to have

Thanks for asking. This was something I'd started to do a few weeks ago and blew off. Glad the math worked out! haha

[futurevr4man]

8000 rpm's is 133 rev/sec... my 1200cc injectors @ 92.5% IDC will dump 1.11 L/min or .0185 L/s or .000139 L per revolution. There is .0487 L of fuel sitting in a 1 m long fuel rail... Each time an injector opens it draws a negligible 0.2% of the available fuel.

Also, the 6.67L/min parallel walbro flow rate I assumed in my last post is exactly what 6x 1200cc injectors @ 92.5% IDC would need.

I'm surprised it's that little. Man. Seems like it would be so much more.
And this discussion is good because when I sized my fuel setup, I didn't go through the math real strictly, but ours are very similar with the exception of my pumps being a bit bigger. Gives me a little comfort.

Thanks to the fuel pressure regulator, the only way that happens is if you run out of fuel pump, right?

I thought about that myself, just was hard to convince myself that the injectors pulsing so many times per second wouldn't pull a significant amount of pressure out of the system. Your math seems to hold up though.