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Guest Message by DevFuse

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How to increase fuel pressure?

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36 replies to this topic

#26 Skip


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Posted 14 November 2003 - 10:48 AM

Just some empirical observations please.
Please post me a print out of a wide band 02 setup showing a need for more enrichment of the mixture when blowing say 14 psi.
I would also settle for some certified G-TECH numbers showing an increase in HP or reduction of 0-60 times.
The stock fuel system on an EA82T is overengineered as are most systems on a Subaru.
The mixture stays very "fat" up to the limits of the factory IHI unit.
If you replace it with one having a bigger A/R then maybe.
BUT as I said, show me the need.
Now in a racing application you may need it as the engine is in need of all the cooling it can get, but for us street rodders - ah??
I believe this increase in fuel will not yield higher HP or torque, just more sheckles spent at the pump and for fouled spark plugs.
I ran an 0-scope connected to an injector signal. Our ECUs are not state of the art by any means and only vary the pulse width according to a limitied number of preprogramed "maps". When the 02 sensor "flips" to rich, the ECU shortens the duty cycle of the injector, in turn the mixture goes (flips) lean, this "tick tock" of the mixture is how the ECU controls the ratio adding more fuel per degree of pulse width duration may or may not increase this tick tock timing. I'm sorry empirical studies please as verbage is not conclusive evidence.

I have been running up to 15 psi for about a 5000 miles now.
I have the elementry air fuel metering using the factory 02 sensor (a poor gauge of the true mixture I know) and when on boost it has always been full rich. It has never gone anywhere near stoik.

#27 Do It Sidewayz

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Posted 14 November 2003 - 12:50 PM

well....for us, when we are racing, the motor spends very little time below 4,000 rpms for about a 10-15 minute race.

The problem we are having is that #3 cylinder is melting the piston, as it runs leaner than the rest. This year we are going to try and create new fuel rails. Right now, the fuel rails run in series, and this makes #3 receive less pressure than the rest. Were working on a set-up to make the fuel rails work in Parallel so every cylinder receives the same about of fuel pressure.

the other reason why we run higher fuel pressure, is for cooling. the fuel does a good job of cooling the tops of the pistons at high RPMS which ofcourse saves them from melting.

We've been through about 8 motors (3 inthe last year) trying to figure out what is right. i think we finally got it

#28 Skip


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Posted 14 November 2003 - 02:32 PM

DIS, let me say I agree in a racing app, more fuel can help for the reason any boosted car needs more fuel when on boost. The surplus fuel, when it evaporates in the cyl combustion chamber, absorbs heat and thus cools the chamber.
This is the reasoning behind the ECU going to it's full rich map and ignoring the 02 sensor output when it believes the car is in a positive intake manifold state.

What I do not agree with is your statement that the injectors are plumbed in series, they are in parallel. Study the injector rail system you will see "T"s above the injectors and a 'T" where the main fuel output from the fuel pump (via the fuel filter) is brought in. The regulator is on the output side and dumps pressure back into the return line. Please if you do not believe me connect a pressure gauge to the small flex line above the injectors.
Also please verify my statement by looking at a fuel rail close up, as I just did.

I am not surprized #3 is melting it's piston but not for the reason stated. Although more fuel may help. Why not just put a bigger injector (more cc per percent duty cycle) in the number three location?
I believe the reason #3 has problems is assoiated with
a) it's proximity to the turbo unit (ambiant and radiated heat) and B) the coolant that is dumped from the turbo into the H20 jacket at this location.
Both of these factors increase the temperature of the entire cylinder head in this location and as a result we see more head cracks in the area.
I encourage your comments as my signature states, the more I learn.......

#29 rallyruss


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Posted 14 November 2003 - 04:36 PM

all this fuel pressure business is interesting.

I have to agree with skip for the most part. I have had no trouble leaning out under boost. but I dont have excessive mods or excessive increased boost levels.
I hit about 15lbs at 6000 rpm and then I shift.

#30 Do It Sidewayz

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Posted 14 November 2003 - 04:47 PM

Alright.. i'll believe you on the Fuel rails...as i haven't really looked at them that closely. However are all the lines the same length?? Because that also makes a big difference.

As for our mods...we are runnning full 2.5 inch OPEN exhaust, K&N Filter with opened stock air box, Cam timing adjusted, as well as ignition timing.

We run about 15 PSI of boost consistantly. From 3,500 to redline.

I'd have to agree with you though..You could probably drive an EA82Turbo motor on the street for years with the mods we have an never have a problem.

The problem we have, is that we are running at full-throttle for about 90% of the time for around 15 mins straight.

You name it, and we've done it to these motors. Zillions of blown head gaskets. Thrown Rods so badly the shrapnell has made dents in the hood. Melted pistons. Cracked pistons (took the ring off the piston and it fell apart in 3 pieces). We've melted a hole in the heads.

The only way we can keep them together is to run all the fuel pressure, and run 94 Octane fuel in our rubber cars (not full out all the time) and 110 Octane in our Studded cars (flat out all the time). Yes..it does make it hard to start in the winter, especially with race gas. and running rich at idle really isn't a problem for us, since we only really idle when it's warming up, or sitting on the grid.

#31 Dennis ex24

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Posted 14 November 2003 - 05:49 PM


what car is this?

#32 Do It Sidewayz

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Posted 14 November 2003 - 06:01 PM

umm...there are about 5 of them

We've got a 1987 XT Turbo, with Spider Intake

a 1986 XT Turbo

1988 RX Coupe

1985 GL-10 Turbo Sedan

1987 3-dr coupe with Turbo motor.

#33 Myxalplyx


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Posted 20 November 2004 - 01:05 PM

Bump for a really really good thread. I was/am looking for fuel pressure regulator (and Rising Rate Fuel Pressure Regulator) info and came across this. Really informative!

*Back from da dead*

Here's something interesting I just read at NASIOC about the use of RRFPR.
"Stock fuel pumps can produce about 55 psi max in most cases. Since the WRX and STI both use a Rising rate fuel pressure regulator, fuel pressure increases with boost pressure. Base fuel pressure on both cars is 43 psi static at idle. The FPR raises fuel pressure in a 1 to 1 ratio. This means for every lb of boost you add you gain 1 lb in fuel pressure. The reason fuel pressure increases is not so much to provide more fuel on boost but rather to combat the pressure the injector tip sees in the intake manifold. let me explain:
The injector is being fed with 43 psi of fuel pressure and fuel sprays out into the intake tract at 43 psi. If you add boost of say 10 psi you now have 10 psi of pressure forcing fuel back into the injector. The injector is now fighting the boost pressure to inject fuel. This is differential pressure across the tip of the injector. The overall actual pressure is really 33 psi now. Since we have a rising rate fuel pressure regulator we gained 10 psi in fuel pressure as we created that 10 psi of boost pressure. The injector sees 53 psi of fuel pressure now due to the rise. Since the injector tip sees the 10 psi of boost we have an actual pressure of 43 psi.. In other words the pressure stays the same due to the increase in fuel pressure by the FPR.

From http://forums.nasioc...ad.php?t=668778

#34 Myxalplyx


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Posted 20 November 2004 - 01:28 PM

The stock fuel pump is good for about 80psi, (least the one in the Brat is, it's off an EA82T). So thats good there. As for putting in an adjustable FPR, that isn't a problem either. Just grab a JEGG's, J.C. Whitney, Summit or racing supply mag of your choice and look for an FI adjustable FPR, they can be had for as little as 10 buck. Then your take your shiney new FPR and plumb it into your existing lines on the RETURN hose. Remove the vacume line from the stock unit (which is built into the fuel rail anyways, not much you can do with it without any work), and plug in the new FPR. The other one will over ride the stock one. Since the restriction is downstream of the old FPR the new one will work the way you want it. I've done this for many years on many different rigs, this is how I'm pushing 12psi into an EA81T without any problems. And if you spend the cash and get a Rising Rate one, you won't have to worry about the rich idle syndrome that I've got right now. The stock injectors will handle 80psi without any problems, least on mine none of the hoses burst or anything. If your going to go much higher than that though, I would build a new fuel rail and use injectors that don't have the rubber hose connection. Also make sure you don't have one of those el' cheapo plastic fuel filters (like from autozone), get a good quality all metal one (I got mine from napa). I had a plastic one, and at 45psi the entire end of it burst, really really bad thing.

I was thinking of taking this approach. Putting one right in the return line seems to be the ticket. I'm posting this to remind myself when I look again (I'm at work as always).

#35 archemitis


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Posted 20 November 2004 - 04:06 PM

i run a 90 lb pump in my car, with the stock regulator, and it works just like normal. the fpr is inline, and should be very easy to switch to something else.

#36 Tachometer


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Posted 21 November 2004 - 03:51 AM

One thing I am curious about is the flow rate of the stock injectors...

Sounds like the stock fuel pump is up to snuff for some minor increases in boost pressure. The fuel delivery system though...hmmm...

The pressure is there but can the injectors handle it?

So does anyone know our injector specs? Perhaps there is a car out there with same size (fit wise) and impedance that our ECUs could handle yet would allow a higher flow rate.

#37 archemitis


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Posted 21 November 2004 - 10:07 AM

apparently you have to build a fuel rail to fit bigger injectors in there. afaik

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