Well Dynoed the RB25 yesterday, Fuel Pressure Problem

[SillyRB]

Ok, well yesterday we dynoed the car just on pump gas nothing major. But the car is having a Fuel Pressure problem. I have 2 In tank Walbros (Getting good voltage) 13.5, -8 Feed and Return with 1000cc injectors, with an Aeromotive FPL, i have a stock tank.

The problem is that when we tried dynoing the car with a 1/4 tank of gas the Fuel pressure was dropping a a ridiculous amount. Then put a 1/2 tank in there and it didnt drop as bad. Could it be that my Sump is getting sucked dry ?

Like yesterday on 14 psi, the fuel pressure went from 40 to 58 then back down to about 44 during the pull. What is happening ?

next time i dyno shold i do it with a Full tank of gas ?

What else could it be ?

By the way the car made on a MUSTANG DYNO 376 @ 1 bar ~15 psi Pump gas, no real timing, 10.4-10.9 A/F all the way through, and the window switch for the VTC to the microtech was NOT turned on.

[Joe]

did you mean aeromotive FPR?
Modified by Kamin at 1:06 PM 6/23/2005

[SillyRB]

NO i mean an Aeromotive FPL (fuel Pressure Regulator)

[JonPowell]

You do know that as RPM and manifold pressure change, fuel pressure changes also, right?

The regulator should allow 43PSI @ ambient and as boost increases 1PSI fuel pressure increases 1PSI so at 14PSI you should have about 57PSI of fuel pressure in the rail...

Starting with 44PSI and increasing the 14PSI would make it 58PSI in the rail, absolutely 100% correct on a boosted motor.

[Imissmyturbo]

How are the pumps mounted in the tank? There is a plastic anti-sloush peice in the tank that keeps fuel around the pump when the tank gets low. I bet your pulling too much fuel out of the anti-sloush tray and starving the pumps for fuel.....also make sure the return lines arn't pointed at the pickups for the pumps

Nice #'s BTW. Remeber he is running lower compression. Are the cams still stock?

[JonPowell]

Remember also that at idle the intake mani is pulling a vacuum and that lowers the fuel pressure from 43PSI to 35PSI...

[SillyRB]

Yes i still have stock cams,we had the car really tightly strapped on the dyno because of the small 225's on the back. It was very conservatively tuned. I guess Friday it will go back on the dyno with race gas and some DR. Why would the fuel pressure drop as the RPM's increaseLike i said on 40 psi it went up to 58 psi from 44 held there for a little bit and as RPM's increased it dropped back down to 44 psi...What could be the cause for that ?

[goofynick6]

Sounds like a fuel leak to me that only shows up when the pressure gets above a certain point.....like 48psi or something.

[Joe]

NO i mean an Aeromotive FPL (fuel Pressure Regulator)

fuel pressure regulator

FPR.

and your pressure changes depending on engine load, it sounds like you just dont know what you are looking at, if your AFR's were fine during the pulls i wouldnt be worries.

[JonPowell]

Yes i still have stock cams,we had the car really tightly strapped on the dyno because of the small 225's on the back. It was very conservatively tuned. I guess Friday it will go back on the dyno with race gas and some DR. Why would the fuel pressure drop as the RPM's increaseLike i said on 40 psi it went up to 58 psi from 44 held there for a little bit and as RPM's increased it dropped back down to 44 psi...What could be the cause for that ?

Fuel pressure drops as vacuum increases in the intake mani.Fuel pressure increases as pressure increases in the intake mani.

Creates a more easily tunable system. At idle the duty cycle is really small, sometimes with larger injectors, 370's for example, it is hard to control the injectors fine enough to get that little fuel. Lower the fuel pressure and you can increase the duty cycle creating an easier environment to control.

At boost you are going to quickly max out the stock injectors on 35PSI of fuel, so increase the fuel pressure and you can LOWER the duty cycle again creating an easier to manage environment.

Higher fuel pressure at higher RPM/boost=lower duty cycle to deliver adequate fuel.

Lower fuel pressure at idle/vacuum=higher duty cycle to deliver adequate fuel.

The fuel injection system does not use a static fuel pressure, it fluctuates with vacuum/pressure in the intake manifold. Quite a genius design actually!

His AFR's are actually on the RICH side and he could use a little less pressure or less duty on those monsterous injectors.

[SillyRB]

Fuel pressure drops as vacuum increases in the intake mani.Fuel pressure increases as pressure increases in the intake mani.

Creates a more easily tunable system. At idle the duty cycle is really small, sometimes with larger injectors, 370's for example, it is hard to control the injectors fine enough to get that little fuel. Lower the fuel pressure and you can increase the duty cycle creating an easier environment to control.

At boost you are going to quickly max out the stock injectors on 35PSI of fuel, so increase the fuel pressure and you can LOWER the duty cycle again creating an easier to manage environment.

Higher fuel pressure at higher RPM/boost=lower duty cycle to deliver adequate fuel.

Lower fuel pressure at idle/vacuum=higher duty cycle to deliver adequate fuel.

The fuel injection system does not use a static fuel pressure, it fluctuates with vacuum/pressure in the intake manifold. Quite a genius design actually!

His AFR's are actually on the RICH side and he could use a little less pressure or less duty on those monsterous injectors.

I understand what you are saying. So the way the Fuel pressure is going down during the pull is because of what--------?The Air fuel is good but i suppose its being compensated for with more fuel in the duty cycle for the lower PSI in the rail.I guess on Friday its gonna be a full tank of race gas and another tune

[JonPowell]

fuel pressure is probably going down because of the boost tapering off. Fuel pressure on your car is DIRECTLY related to intake pressure/vacuum. Its a tool used by vehcile manufacturers to make tuning easier. When fuel pressure rises at a 1:1 rate under boost, then fuel pressure becomes a non esistant issue. It's constantly 43PSI above the intake pressure so it becomes a constant relative to boost pressure.

If your fuel pressure is 43PSI at ambient(Engine not running, just pump on) and is 43PSI above your boost pressure(58PSI Fuel pressure @1BAR[14.7PSI] boost pressure) than you are PERFECTLY FINE. EVERYTHING is working as it should.

I would concentrate on getting the AFR's down to the mid 11's under boost and in the 13.8-14 range at idle/off loa rather than worrying about race fuel.

Are your Walbro pumps set up parallel or series? Series will allow for higher pressures, but not necessarily higher flow. Parallel will allow for higher flow, but not necessarily higher pressures. ONE Walbro fuel pump should allow for about 700 crank HP if it is flowing at 255LPH as they are rated which it doesnt at pressure above 40 (below ambient)and voltages below +14.0V...assuming you use pump gas where viscosity and specific density are all basically the same. If you use race gas then you gotta take all that into consideration also when tuning to account for different flow properties. race gas is a little different animal all around than pump gas and if you arent having detonation problems(which your AFR's suggest you arent) I wouldnt waste my money on it.

[Redline240]

^^^^^....what he said, forget about the race fuel, this is a street car...get those A/F numbers leaner, JonPowell's numbers are a good guide since there's still conservitive so you won't have detonation from being too lean...

Redline

[Imissmyturbo]

Sorry to bring the topic back up but how do you like the T04R turbo? When do you see boost? What hotside wheel + housing are you using on the turbo?

[SillyRB]

well i found out why i was loosing fuel pressuer. I took the pumps out that were sitting parrallel to each other the -8 hose i was using inside the tank from the Y to the fitting to the -8 was split down the middle. SO it could not hold pressure thus dropping at higher RPM and higher boost when more fuel was needed. I am going with a different approach. I sold the 2 walbros. Say hello to the A1000. The turbo was bad from PTE, so waiting on the turbo and then gonna hook up the new fuel system. Say BYE BYE To the blue -8 line and say hello to stainless steal. If anyone is intrested in -8 fuel line, with a bunch of aluminum fittings let me know. Will let go for dirt cheap. Got about 20-25 feet of it.

[JonPowell]

Email me with your price on that stuff!

A1000 is an AWESOME pump, but you may wanna go -10 for feed on that baby and -8 for return, thus facilitating the need for your -8 stuff you have now!

[goofynick6]

Sounds like a fuel leak to me that only shows up when the pressure gets above a certain point.....like 48psi or something.

[SillyRB]

Email me with your price on that stuff!

A1000 is an AWESOME pump, but you may wanna go -10 for feed on that baby and -8 for return, thus facilitating the need for your -8 stuff you have now!

I dropped you an e-mail.Thanks man i must sell ASAP, i appreciate the help.

& GoofyNick, i guess you were right Fuel leak it was.

I guess the turbo going bad was a gift in disguise otherwise we probably would not have taken the car of the dyno. Nevertheless, before we had taken it off it made 45x hp. @ like a 11.2~11.6 A/F this was like 2 weeks ago so i dont remember the tq. Still the VTC was not activating (no window switch hooked up on the auxillary for the microtech). Still just babying the motor. The turbo was bad, blue smoke.
Modified by SillyRB at 8:47 PM 7/12/2005