Supercharged MPI Fuel pressures
Turbo Mini Forums

Hey All

Had my supercharged MPI back at emerald today for a few more map tweaks after dropping the boost. All seems very very good but i noticed the fuelk pressure seems to start dropping when held consistently at high revs/full throttle.

I installed a fuel pressure gauge in the fuel supply line and the system runs at 2 bar with an FSE regulator. Injectors are in the mouth of the charger so are never exposed to boost.

The pump is the standard MPI as it came from from the factory early in 2001

I was watching the gauge whilst Dave was putting it through its paces at Emerald today and saw that the pressure began to drop off when held at high revs/throttle.

Strangely this didnt register though o the AFR's! which remained perfect.

My worry is that if i finally get round to doing a track day where it may be held at full throttle for a considerable period it will run lean!

Im using a wideband LC1 but dont have an in car AFR gauge.

Injectors have plenty of capacity remaining as are only running at about 50% duration.

What could be the issue?

1) Could this be being caused by the increasing vacuum the Injectors are experiencing (as the afr's didnt seem to alter, but it was a short power run)

2) Fuel pump is on its way out or cant keep up??

3) Fuel lines arent big enough? Power is 130bhp

i think iff the fuel pressure is dropping off as described either the pump cant keep up or its fuel feed lines are too small

8-10mm is recommended for boosted engines feed n return i think the mpi's standard lines are 6mm?

Yes i moved to the darkside

Instagram @jdm_brett

I have standart fuel line and I don't have that kind problem.
I change that fuel pump steight a way when I start my own project.
That was hateful job to take fuel tank of and change that pump inside the fuel tank, well I have some extra stuff inside the trunk.
In my opinion is that you have to change more powerful fuel pump there.

I use bosch 0580 464 070

JAni

1399cc, Eaton M45 supercharger, N2O system, compression ratio 9.23:1, boost pressure 0,8bar/11.6psi.

http://www.garaget.org/?car=176816

You should be closer to 3 bar fuel pressure really for injection - mine (1293 Injection Turbo) runs about 42 PSi pressure at idle and then rises 1:1 with the boost.

I forget mention that fuel pressure, I have that 3bar and it rise when boost go higher.

1399cc, Eaton M45 supercharger, N2O system, compression ratio 9.23:1, boost pressure 0,8bar/11.6psi.

http://www.garaget.org/?car=176816

Change the pump for a Walbro or an FSE job.

I have the 250l/h job in mine. bit overkill, but 've had no issues, other than when I left it in the tank in 2007 and ran the SPi manilfold. It overfueled like abitch

The fuel pressure regulator should be refferenced to the point on the inlet where the injectors are located. This way the fuel pressure will rise with the manifold pressure keeping the pressure differential across the injector as close to constant as practical.

Where is the FPR connected to the manifold at the moment, or is it refferenced to atmosphere and where is the fuel pressure gauge installed, between the pump and regulator, before or after the fuel rail?

If the FPR is refferenced to atmosphere the FP should remain fairly constant. if it is refferenced to the manifold after the throttle before the charger ( where I think it should be) the fuel pressure should rise up to aprox 1 bar up to full throttle. If the FPR is refferenced to the manifold after the charger before the engine, FP should rise aprox 1 bar plus the boost.

If the engine has been tuned with the fuel pressure as it is, the fuel map will be compensating for the lack of fuel pressure.

If the FPR is refferenced to atmosphere or after the throttle body, and the FP does indeed drop to 2 bar at full throttle peak power, i could well suggest that the fuel pump is starting to struggle. those using the MPi tank and pump on a turbo with a carb, wouldn't nescisaraly notice this problem as there is a float bowl full of fuel, and the FP is regulated down to 3 or 4 psi and up to 20psi on boost, therefore the pump flows more anyway

Edited by Sprocket on 2nd Sep, 2009.

Ahh cheers for the info guys.

Sprox the regulator sits on the fuel line after the t-piece take off for the injectors. The gauge is on the feed line before everything, injectors are before the charger so in theory never experience boost.

The regulator is referenced to atmosphere, fuel pressure has been 2bar ever since Stu at vmax first mapped it, the problem is that if i take it up to 3bar i will need a full remap and Dave at emerald reckoned it would only make matters worse, particulary as the idle is hard enough to get right as it is (it swings from rich to lean by adjusting the map by 1 unit)

Im starting to believe its the pump! Where can i get a Walbro or FSE one and are they a direct replacement for the standard?

Ah right, referencing the regulator to atmosphere is fine as the fuel map will compensate, however, if you were to take the FPR reference off the intake runner after the throttle before the charger then set the regulator at idle for 2 bar, this should help a little with the idle fueling as the 'differential' fuel pressure at idle is now 2 bar rather than the 3bar its set at. That however will again require a remap.

I dont know how emerald does it, but Vems and Megasquirt use a required fuel value that determines the ideal pulse width at 100% VE. The fuel map is then just a percentage of that value. I had a similar problem that the numbers were just too coarse. The way this is normaly resolved is to halve the required fuel number and double everything in the fuel map. so a change of 1 is now half what it was before, giving a finer control over the fueling.

It does sound like the pump is at or close to its limits. They were only ever meant to suport 60 - 80 HP

As for a replacement fuel pump, buy the newest cheapest Walbro one you can find on ebay, then you need to modify the in tank pump manifold to accept the new pump, it is fairly straight forward afair. You could by the FSE but im not sure on flow rates makes or models, which is why I went for the Warlbro 255 as there is really only the one

Edited by Sprocket on 3rd Sep, 2009.