How much fuel pressure

[FastRthenU]

I have a set of 550cc injectors, that are pintle style injectors. But they dont cut it for my 50 trim at all. I remember someone saying that by raising the fuel pressure a certain amount, that they will flow like 660's. If this is true, then what fuel pressure should I raise it to?? Or just what is a safe fuel presure for a 1g? My mods are in my list. thanks

 

[kpt4321]

The formula for fuel flow rate with relation to fuel pressure is:

new flow = old flow * sqrt(new fp / old fp)

That's the old flow rate, times the square root of the new fuel pressure over the old fuel pressure. Keep in mind that this is obviously base fuel pressure.

Here are some examples, using 550 cc/min injectors:

36 psi (1g stock) = 503 cc/min
43 psi (2g stock) = 550 cc/min
47 psi (1g NT) = 575 cc/min
55 psi = 622 cc/min

In order to flow 660 cc/min, you would need 62 psi of fuel pressure, which would get you well over 80 psi of pressure behind the injectors under boost. That's getting a litle iffy, I personally would not go much over 55 psi.

Turning the base pressure up to 55 psi will increase your flow by about 24%.

...Kyle T.
93 TSi AWD "TURBO4"

 

[FastRthenU]

Thanks so much.

 

[wira4]

To complicate the issue, as the fuel gets heated up from the heat in the engine compartment, you will lose fuel pressure. You will need a gauge to monitor.

Originally posted by kpt4321
The formula for fuel flow rate with relation to fuel pressure is:

new flow = old flow * sqrt(new fp / old fp)

That's the old flow rate, times the square root of the new fuel pressure over the old fuel pressure. Keep in mind that this is obviously base fuel pressure.

Here are some examples, using 550 cc/min injectors:

36 psi (1g stock) = 503 cc/min
43 psi (2g stock) = 550 cc/min
47 psi (1g NT) = 575 cc/min
55 psi = 622 cc/min

In order to flow 660 cc/min, you would need 62 psi of fuel pressure, which would get you well over 80 psi of pressure behind the injectors under boost. That's getting a litle iffy, I personally would not go much over 55 psi.

Turning the base pressure up to 55 psi will increase your flow by about 24%.

...Kyle T.
93 TSi AWD "TURBO4"

 

[kpt4321]

Originally posted by wira4
To complicate the issue, as the fuel gets heated up from the heat in the engine compartment, you will lose fuel pressure. You will need a gauge to monitor.

I would like to hear the reasoning behind this.

....Kyle T.

 

[hostile]

Most people set their fpr when the car is at normal conditions. So the gas should be normal and you might get more fuel when it is colder outside but that shouldnt hurt performance enough to discuss.

 

[kpt4321]

So wrong, so wrong.

First of all, fuel density and pressure change VERY little when temperature changes, this is true for all fluids.

Second of all, if you were able to record any change in pressure anyway, increases in temperature would result in INCREASED pressure. That's how it works.

But, since liquids are largely not affected by changes in temperature, especially within the normal realm, you would never see this happen.

When it is colder out, you also get more air, and the density of air is effected by temperature changes MANY more times than the densiy of fluid. That means that you'll end up running leaner, if there is no means of compensation (which there is, so yet again the point is moot.)


....Kyle T.
93 TSi AWD "TURBO4"

 

[Turboniam]

Off of which side do you set the base fuel pressure??

Pre-fuel rail or post fuel rail??

Meaning, obviously there is going to be a difference in fuel pressure that the in cabin gauge vs the one on the FPR... so which one do you set the base FP to??