Fuel Pressure vs. Air Pressure calculations, need input

[Checkered-Member]

I have been doing some calculations of fuel pressure vs. air pressure, at N/A and at different PSI levels, here is what I came up with.

If atmospheric pressure at sea level is 14.7PSI and the stock fuel pressure on a KA is exactly 3 times that, at 43.5PSI. So the ratio is 3:1…every 3 psi or fuel there are 1 psi of air going into the engine under NA conditions. For turbo applications if we want to keep the same A/F ratio we need a 3:1 FMU, but because we need a bit richer conditions a 4:1 will do the job. So why do people run 6:1 8:1 10:1 FMUs…?

We also know that for every 1 psi of boost you will increase the air flow by 7%, and consequently power by the same amount. At 5psi there will be 35% more air flow. To keep the same A/F ratio I need to increase the flow of fuel by 35% one way will be to increase the fuel pressure by 35%, (or 40% to keep it a bit rich)Now is that correct?

If I to double the fuel pressure will the injectors supply twice the fuel?

[DRIFTEADOR]

thats actually not how a stock fpr works. the fpr has a 1:1 ratio with ~43psi static pressure at sea level. like a boost gauge, it starts counting pressure change below/above atmospheric. 43psi is the beggining point from where it increases or decreases fuel pressure depending on vacumm. for example at idle, roughly 10psi vacumm, it decreases fuel pressure by 10psi to ~33psi. same for positive pressure (if it was 3:1, it'd read ~13psi fuel pressure at idle).

and btw, it does this to equalize fuel pressure that enters the cylinder not to add more fuel. when there is vacumm inside the cylinder the fuel is being pulled out of the injectors tip, again using 10psi vacumm as an example, you only need 33psi fuel pressure to level it out to 43psi. at 10psi boost, cylinder pressure is pushing against the fuel coming out of the injector so you need more pressure to push the fuel out of it.

your psi calculations are also off. what decides how much fuel is needed is airflow, which is not the same as pressure. 5 psi doesn't mean the motor is getting 35% percent more airflow. that would assume that 1 bar of boost would double hp of any motor with any turbo. im sure you know thats not true.

and finally injector flow is not proportional or linear to fuel pressure. I don't think theres a generic formula that applies to all injectors

[Import_Ant]

and finally injector flow is not proportional or linear to fuel pressure. I don't think theres a generic formula that applies to all injectors

Thank you, I couldnt think of a good way to put that. Since fuel flow is based on fuel pressure and pulse width AND injector size you have alot of variables to account for.

[Checkered-Member]

thats actually not how a stock fpr works. the fpr has a 1:1 ratio with ~43psi static pressure at sea level. like a boost gauge, it starts counting pressure change below/above atmospheric. 43psi is the beggining point from where it increases or decreases fuel pressure depending on vacumm. for example at idle, roughly 10psi vacumm, it decreases fuel pressure by 10psi to ~33psi. same for positive pressure (if it was 3:1, it'd read ~13psi fuel pressure at idle).

why are you using PSI number for vacuum...isn’t it measured in: in/hg?

your psi calculations are also off. what decides how much fuel is needed is airflow, which is not the same as pressure. 5 psi doesn't mean the motor is getting 35% percent more airflow. that would assume that 1 bar of boost would double hp of any motor with any turbo. im sure you know thats not true.

1 bar of boost by definition means twice the air in the engine, twice the air doesn’t equal twice the power because air temperature charge is higher, there is back pressure, but for low PSI it should be fairly close.

and finally injector flow is not proportional or linear to fuel pressure. I don't think theres a generic formula that applies to all injectors

thats what i need to find out...

[DRIFTEADOR]

because inches of mercury can be translated to pounds per square inch, a term that we're all more familiar with. 10psi=~20in Hg

1 bar relative pressure means twice the pressure, not twice the flow. psi and cfm are two different units measuring different things. cfm is directly proportional to power, psi isn't.

[FliMSiCaL]

atmospheric pressure is not 14.7 psi, its 0 psi. there are 2 different psi's. psia and psig, a meaning atmospheres, and g meaning gauge. atmospheric pressure is 14.7 psia, which = 0psig. Almost every gauge is refrenced in psig, unless it obviously states psia, in which case it will also read 14.7 when not hooked to anything.

[Checkered-Member]

atmospheric pressure is not 14.7 psi, its 0 psi. there are 2 different psi's. psia and psig, a meaning atmospheres, and g meaning gauge. atmospheric pressure is 14.7 psia, which = 0psig. Almost every gauge is refrenced in psig, unless it obviously states psia, in which case it will also read 14.7 when not hooked to anything.

The pressure of the air going to a N/A engine at sea level, is 14.7 psi.

I know what you are saying thou. For calculations purposes I used 14.7

I read some technical articles and it said that to double the fuel flow I need to quadruple the fuel pressure…major ouch …FMU’s are worthless , I’m going with bigger injectors...
Modified by Checkered-Member at 5:25 PM 6/9/2005

[sanioll]

PSI and CFM are totally different things.

you can run smaller turbo at high psi and push same CFM.You can also run bigger turbo at low psi and push same CFM.

Bigger turbo is efficient and doesn't heat up the compressed air as much, So you are compressing more molecules into the same volume, thus more power.

My advise, read a book called "Maximum Boost" It talks about fuel delivery a lot.

[:: orion ::]

I If atmospheric pressure at sea level is 14.7PSI and the stock fuel pressure on a KA is exactly 3 times that, at 43.5PSI. So the ratio is 3:1…every 3 psi or fuel there are 1 psi of air going into the engine under NA conditions. For turbo applications if we want to keep the same A/F ratio we need a 3:1 FMU, but because we need a bit richer conditions a 4:1 will do the job. So why do people run 6:1 8:1 10:1 FMUs…?

It's been pointed out already, but it's buried in another post...

The stock 1:1 regulator is ONLY used to keep the injector flow constant...Nissan injectors are rated at 43.5psi. So a 270cc injector flows 270cc ONLY when kept at a constant 43.5psi of fuel pressure.

Under vacuum, the fuel is being sucked out of the tip of the injector (manifold vacuum), so the fuel pressure is lowered by the reg. to keep the flow static.

Same goes for under boost...with 1psi across the tip of the injector (positive manifold pressure), the fuel pressure needs to increase 1psi to keep the injector flow EXACTLY at 270cc.

Only reason we use an FPR is to keep the flow CONSTANT, no matter what vacuum or boost is in the intake manifold.

...

With that said...let me address one other thing:

I read some technical articles and it said that to double the fuel flow I need to quadruple the fuel pressure…major ouch …FMU’s are worthless , I’m going with bigger injectors...

It's easy to figure out the flow needed to double the injector size...and I'm sure you came across it in your search(s):

[ (sq.root of (new pressure/old pressure)) x (injector size) ]

So if you went from 43.5psi to 60psi...

sq.root of (60/43.5) = ~1.17

1.17 x 270cc = ~316cc is the new size of your injector.

So to double flow (make a 270cc injector a 540cc...)...

(sq.root of (new pressure/old pressure) needs to = 2.

2 squared is 4...like you said. 4 times the pressure to double the flow.

Any fuel pressure above ~100psi should be considered dangerous, so keep to less than 100 NET (90psi base plus 10psi boost, or an 8:1 FMU at 7psi will increase pressure by 56psi (plus 43.5) is just under 100)

Anyway, I'm rambling. Sorry.

- Brian

[Checkered-Member]

Thank you for the post Brian it really clears things up.

now for the million dolor question, how do I calculate the CFM that a turbo flows at a certain psi?

[DRIFTEADOR]

you'll have fun with this one...

http://www.stealth316.com/2-3s-compflowmaps.htm

[Checkered-Member]

you'll have fun with this one...

http://www.stealth316.com/2-3s-compflowmaps.htm

cool, thanks...that should keep me busy for about a day.

[nissanfanatic]

And in Turbochargers by Hugh Macinnes, there is a long formula that will determine how much your engine will flow at a certian PSI. It figures in density ratios too.