Fuel tank outlet size.
Turbo Mini Forums

So after an interesting discussion arose in one of the Faceache groups it got me thinking.
20+ years ago when I first turbo'd mine it was the general consensus that the fuel tank needed a larger outlet fitting. My original one was removed and a larger 8mm one duly welded in place.
But now it seems this may not be needed as it appears plenty of folk are running on the standard 6mm outlet from the tank.
So who, if anybody has had any fuelling issues using the standard outlet ? Is this once considered "essential" modification simply not needed or just overkill for a road car and only needed for the most fuel hungry high boost track motors ?

Phil.

WWW.TURBO-MINI.COM

For power outputs typical on this forum, flow rate through a 6mm pipe is probably fine but pump performance becomes the issue.

Any injection type pump, whether used with injection or a carb will have an optimum flow/pressure curve it should be run in on the discharge side and to stay within that curve it will require a nett positive suction head (NPSH or overall inlet pressure) that is high enough top prevent cavitation, which can be very damaging to the pump.

So, if the pipe sizes beyond the pump, through the regulator, and return to the tank are also restrictive - to the overall flowrate through the system, most of which is just un-used fuel returned to the tank, not the fuel amount required - it will tolerate a low NPSH. If all the piping after the pump is sized for high flows - even if high flows are not needed - the pump will cavitate without an adequate sized inlet.

Other key factors for NPSH are basic things like the height of the pump inlet relative to the tank and the fuel level (height again) in the tank.

Standard size inlet on a typical EFI pump is 12 (or is it 14?)mm to try to avoid those issues.

A general rule in any pumped system is the pump suction is always larger than the discharge.

In my industry we bury the condensate extraction pumps 10-15m below ground under the bottom of the condenser to ensure they have a physical static NPSH (condensers on steam turbines run at near perfect vacuum, absolute zero pressure).

Just my opinion, I'm sure others will have different views.

Schrödinger's cat - so which one am I ???

Whilst you may get away with 6mm....you cannot say because power outputs are low it is fine.

Restricting a high flow efi pump...which most will use will reduce its performance, reduce its life and generally make it unhappy.
And the pump doesnt care what power the engine makes, as the pump runs flat out all the time so is always flowing maximum volume all the time.

It's also why you can get away with what some call the tee piece trick, so the tank outlet only then needs to feed the engine, rather than the pump, because the pump will then re-circulate fuel via that small loop you've created. Not an ideal scenario either though and could lead to fuel heating.

On the pressure side however...even though it's still relatively low pressure, 6mm OD there is probably more than most A-series can cope with, but most regs etc will likely have 8mm or larger fittings so makes sense to go with that ( as does the OEM fuel pump in Metro turbo cases )

It's just difference between good practice, and bad practice.

And lets face it...most probably dont do huge miles either way ? Which is another factor why they're probably getting away with it without any apparent problems.
The lower flow of the efi pump itself, the longer it would probably last for too. I'm sure if you stuck an 044 with a 6mm supply...it'd start screaming fairly soon. ( Although I've seen some M18 inlet fittings used on those that really do neck down inside too for some reason )

Edited by stevieturbo on 13th Feb, 2018.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

As long as pump manufacturers insist on the silly unnecessary practice of putting these large bore inlets to the pumps, I’ll keep using supply lines the same size.

I’m was going to say what did the MPI tanks have?

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

i will say that my return line is 8mm the same as feed and the reg cant reduce the pressure correctly i suspect the fitting into the tank/pump hanger flange however if i did it again id just use 10mm ret and 8 feed or a smaller volume pump...

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

I did have a problem running large bore feed and return; couldn’t get the pressure low enough. I ended up putting a sleeve into the rubber feed hose just before the reg; basically reducing the flow potential of the feed, and I was able to reduce down to almost zero psi.

No trouble with anything using 8mm feed and return.