Dangers of raising fuel pressure? [Archive] - DSM Forums: Mitsubishi Eclipse, Plymouth Laser, and Eagle Talon Forum: DSMtalk.com

: Dangers of raising fuel pressure?


oharajo
07-21-2006, 05:36 PM
I think there's a flaw in this, so please set me on the straight and narrow:


I was thinking of a way to get the most out of the stock injectors.

I get a good fuel pump (say Walbro 255, whatever).
I get a fuel pressure regulator (FPR is required as I understand since the stock regulator won't handle all that fuel).
I get something to control fuel delivery (say Apex AFC II).

I use the AFC to turn the fuel delivery down (say to -25%).
I turn the fuel pressure up to compensate for fuel delivery (up 25% to say 55 PSI).

Then I should be able to run 25% higher boost before hitting fuel cut right? With this uhh .... scam ... and this 25% I'd be delivering more fuel than 550's would deliver at stock fuel pressure right?

So I'm sure someone will be kind enough to tell me why this is a terrible idea and how far it would be safe to go with (if its at all safe)?

Thanks

2G6
07-21-2006, 05:52 PM
Do not turn your base fuel pressure up to 55 btw. Maybe to 46 or some shit, but 55 is too much IMO.

BlinkerFluid
07-21-2006, 08:23 PM
Well, turning up your fuel pressure 25% will not get you 25% more flow through the injectors.

I will show how to figure change in flow rate due to pressure change.

Here are our standards.

Stock fuel pressure 43psi
Stock injector size 450cc/min

Our variables.

New fuel pressure 55psi (roughly 28% higher)

What we want to find.

New injector flow rate=X

The formula is.

(square root (new fuel pressure/ old fuel pressure))stock flow rate=X

(square root (55/43))450=X

(square root (1.279))450=X

(1.13)450=X

X=508.5 a 13% increase in flow, not enough to make it worth the effort IMHO. I can understand how you saw 25% more fuel pressure lead to a 560cc/min+ flow rate, but this just isn't the case.

Sorry about my use of mathmatical formulas, the way I presented them is the only way I know how.

Also keep in mind, 25% more boost does not mean 25% more airflow.

I hope this helps you understand pressure and flow change, and make your decision about fuel system upgrades. Start simple, fuel pump, DSMLink, injectors and so on. Take it from someone with experience, supporting mods, then turbo. ;)

JRM

oharajo
07-21-2006, 09:52 PM
Well, turning up your fuel pressure 25% will not get you 25% more flow through the injectors.

I will show how to figure change in flow rate due to pressure change.

Here are our standards.

Stock fuel pressure 43psi
Stock injector size 450cc/min

Our variables.

New fuel pressure 55psi (roughly 28% higher)

What we want to find.

New injector flow rate=X

The formula is.

(square root (new fuel pressure/ old fuel pressure))stock flow rate=X

(square root (55/43))450=X

(square root (1.279))450=X

(1.13)450=X

X=508.5 a 13% increase in flow, not enough to make it worth the effort IMHO. I can understand how you saw 25% more fuel pressure lead to a 560cc/min+ flow rate, but this just isn't the case.

Sorry about my use of mathmatical formulas, the way I presented them is the only way I know how.

Also keep in mind, 25% more boost does not mean 25% more airflow.

I hope this helps you understand pressure and flow change, and make your decision about fuel system upgrades. Start simple, fuel pump, DSMLink, injectors and so on. Take it from someone with experience, supporting mods, then turbo. ;)

JRM


Dude I'm all over the formulas, I wish I knew WAY more about the math of my car.

And funny you picked up about 25% more boost does not mean 25% more airflow, because right before this thread I posted a question to help my understanding of that: http://www.dsmtalk.com/forums/showthread.php?t=158716

If you've got two cents and some math to add on that topic feel free.

I like the formula, where does it come from, some kind of engineering law about injector operation? (them engineers are clever devils...)

And how far can you safely push the fuel pressure? Do you agree with 2G6's 46? Working with your formula, that's like a 3.4% gain, pretty much nothing. I guess that's why I should buy bigger injectors when the time comes eh?

BlinkerFluid
07-22-2006, 04:10 PM
Here's another issue with raising fuel pressure.

One thing to always keep in mind when discussing it is, in our engines that see pressure in the intake manifold (boost) we need to keep our pressure drop across the injector tip consistant. So if we have a 43psi base pressure set with the fuel pressure regulator disconnected (0 psig boost) we have our 43psi difference across the injector tip. The fuel pressure regulator automatically compensates to keep this 43psi difference by adding 1psi of fuel pressure per 1psi of boost it senses. So at 15psi of boost, fuel rail pressure will be 58psi, once again we have our 43psi difference. If pressure wasn't upped 15psi, we would have a differential of 28psi across the injector tip, that takes about 20% of our injector flow capacity away. I'm sure you could see how this could be a problem. :D

Another issue rears its ugly head when operating the fuel pump at higher pressures, we see flow drops because the pump has to work harder. So you can see that it is better to keep the stock fuel pressure and use a higher capacity fuel pump and larger injectors.

Search for "fuel pump re-wire" on this site to learn about the process. Re-wireing the fuel pump helps keep the voltage drop to a minimum and keeps the fuel pump from dropping off drastically in flow as higher pressure is requested. It should be done to any DSM that is to be modified. You can also check out www.vfaq.com for info.

On some setups, with DSMLink for instance, in which base injector settings can be changed. The user may actually go way overkill on injector size and run lower than stock fuel pressure. The injectors make up for the flow needed, and at the lower overall pressure the fuel pump(s) can keep up with the required flow of an extremely high boost number. This is just a "for instance" to further the discussion about fuel pressure, pump, and injector flow.

Hope this helps,
JRM