PMO Carb's using MFI Fuel Pump - Issues?

[Tom C.]

In general does anyone have pros/cons or good/bad experience using a (factory installed) MFI fuel pump with PMO Carb's running through the PMO suppiled fuel-block with the return-line still active?

I'm running this setup and have not had any problems . . . but have recently heard from sources on this board . . . the MFI pump may create too much pressure.

The only points of failure would be the PMO fuel block . . . or the fuel filters.

So what do you think . . . any reason for me to consider replacing the fuel pump?

Thanks,

[Zeke]

I'm not an expert by any means but I've owned a few of these cars with various fuel systems. I would say that the MFI fuel pump is putting out too much pressure for all but the best fuel pressure regulators ever made. It can't last forever like that. Most carbs have around roughly 3-5 lbs. pressure supplying them. The MFI pump is putting out many times that. Your regulator must be pretty good. Tell us, what brand is it? Or it the regulator in the fuel block (I don't know what PMO supplies)?

[e p slick]

Have had PMO lash up and ran Holley blue pump + pressure regulator. 5 to 6 lbs as I recall. No return line needed.

EP Slick - Tucson

[Tom C.]

Milt . . . Thanks for jump in on this one.

PMO supplied a 'fuel block' that contains:

1) main input line (from tank)
2) return line (to tank)
3) two carb output lines
4) fuel pressure guage (on the output lines)
5) fuel pressure valve (with pressure controlled by restricting the return line)

It's all in a one-piece machined unit with brass barb connectors and brass fittings for the guage and valve. It really looks like a solid piece of hardware and I have no reason to question Richard Parr. He told me this would work with my MFI pump . . . no problem.

In theory if the pressure valve is just 'opening' up the output circut to the carbs . . . then it really isn't under any more pressure than what is displayed on the fuel pressure guage. Then the remaining fuel is just getting pumped back to the fuel tank. Seems this PMO fuel block should be pretty bullet proof.

What do you think?

I'll try and post a picture of the fuel block tonight.

Thanks,

[JP911]

The PMO regulator uses a brass needle valve instead of a diaphragm. Pretty bullet-proof if you ask me. I used it with a stock CIS pump, which is high pressure, and it worked fine.

[RWebb]

mfi >>> cis pressure

is it ok to use? i dunno

mfi pumps are worth $$$ - why not sell it?

[Flieger]

The MFI pump only is rated at about 1 bar, I believe. Maybe less.

I think the PMO fuel pressure regulator can handle the step down to 5 psi.

Your engine seems to run strong from what I see.

[Tom C.]

Hey RWebb,

Are you saying MFI runs at a different fuel pressure than CIS?

I'm not sure what you mean . . . and I'm interested to know.

After all I think it was your comment in another thread that got me thinking about this.

Don't want to sell the MFI pump.

Thanks,

[Tom C.]

Hey Max . . . hope all is well. We need to catch up on lots of progress with the car. And hear more about your projects and school.

Let me know when your back in town.

[Tom C.]

JP911,

Thanks for the feedback. I've wanted to remove the valve and have a look. You save me some work. Also glad to know you've been running the fuel block without problems.

Cheers,

[RWebb]

yes, lots higher afaik

[YTNUKLR]

Carb fuel pump is usually 4-5psi

The MFI (911E & 911S, ’69 and later) is 901.608.105.00 (Bosch 0 580 970 001) and is specified with operating pressure 1 bar (14.2 psi). The safety pressure relief valve opens at approximately 2 bar (28 psi), delivers 125 liters per hour, uses 3.5 Amperes at 12 Volts and spins at approximately 2800 rpm.

The extremely detailed response is courtesy of Grady Clay, of course.

[Grady Clay]

Woah guys. Lots of misconception here.

Carbs (Weber, Zenith, PMO, etc.) all like 3-5 psi. The Bosch rotary pump 0 58096 0009 (Porsche 911.608.107.00) is regulated by an internal relief valve to 2.8 to 4.3 psi (0.2-0.3 bar). The excess fuel is returned to the fuel tank. This pump appears the same as the MFI pump but has the Bosch part number stamped into the metal. Specs on this pump are; flow of 80 liters/hour, current draw 2.5 A @ 12 V and 2000 rpm. This pump is designed for a 'dead end' systen where the outlet of the pump goes to the carb inlets and there isn't any return flow past the carburators.

The MFI Bosch rotary pump 0 58097 0001 (Porsche 901.608.105.00) is internally limited with a pressure relief valve to 28 psi (2 bar). The excess fuel is returned to the tank as above. This relief valve does not operate during normal operating conditions. It is a safety only. The MFI fuel pressure is regulated to 11.8 ±3.0 psi (0.8 ±0.2 bar) by the center fitting on the fuel filter console. This fitting constantly returns the excess fuel to the tank after having passed through the MFI pump to keep it cool. Specs on this pump are; fuel flow of 125 liters/hour, 3.5 A @ 12 V and 2800 rpm. The reason for the larger capacity is that it is designed for a circulating fuel system.

The CIS pumps for normally aspirated engines do not have an integral fuel bypass regulator – safety or control. The pressure is controlled by the CIS Fuel Pressure Regulator in the range of 15-52 psi (1.0-3.5 bar) depending on engine operating conditions. The system fuel pressure is regulated 69.6- 72.5 psi (4.7-4.9 bar). Fuel flow is 1.7 to 2.0 liters/minute (102 to 120 liters/hour) depending on model year and current draw 8.5 A @ 11.5 V. The CIS pump is a powerful, high volume pump and flows this quantity all the time to keep the CIS system cool(also necessary to keep the 100+ Watt pump cool).

Turbocharged CIS engines use two CIS pumps in series. The system pressure is regulated at 89-99 psi (6.0-6.7 bar).


Obviously a pump designed for carburetors is best (and safest). The MFI pump, filter and filter console without the regulating fitting can be very successful with the bypass regulator from PMO that Tom has. In fact that is better than any Factory original carburetor set-up.

In fact, with a slight change it can be better yet if you allow the return fuel to flow past the four carb inlet fittings. This keeps the coldest fuel at the needle & seats. This prevents vapor bubbles from forming in the fuel as the pressure drops from 3 psi to atmosphere.

I also use two phenolic spacers (matched to the port and manifold) to insulate the carb & manifold from cylinder head heat. (Remember to put appropriately longer studs in the heads.)


I would only use a CIS pump with a capable bypass regulator and all screw-together hoses capable of 150 psi for safety.

Best,
Grady

PS: Scott, I started this last night and didn't see your post. Thanks.

[JP911]

FYI, the PMO fuel block being discussed is a bypass regulator.

[Tom C.]

Damn Grady . . . now that's what I call a 'reply'

Thank you for taking the time to set me straight on this. Now that I know the PMO supplied 'Fuel Block' will work with the MFI pump . . . I just have one more question to your point.

How do I . . . "In fact, with a slight change it can be better yet if you allow the return fuel to flow past the four carb inlet fittings. This keeps the coldest fuel at the needle & seats. This prevents vapor bubbles from forming in the fuel as the pressure drops from 3 psi to atmosphere."

I'm trying to imagine how this gets routed, but I just don't understand it.

And yes, I've got the heat insulators (spacers) between the cyl head and the intake manifold with long studs.

Thanks again for all your help.

[Grady Clay]

EDIT - I confused the location of the supply and return. Corrected.
I shouldn't write so early in the morning from memory.

Tom,

You should confirm this with Richard Parr @ PMO.

I would put the ‘fuel block’ and carbs in the supply and return circuit normally used by the MFI pump. On the fuel filter console the supply is the fitting pointing down out of the bottom of the filter and the return is the banjo pointing toward the rear. The banjo fitting pointing toward the front is the inlet from the electric pump.

The center fitting (pointing toward the engine) on the fuel filter console is the MFI pressure regulator. That straight fitting should be replaced with a regular banjo fitting and banjo bolt. This allows the fuel to return to the tank without restriction.

I would plumb the supply from the fuel filter console to a ‘Tee” fitting, then through the two carb inlet manifolds bolted to the carb tops and then to the two common ports on the ‘fuel block’. This will also be common to the pressure gauge. The ‘return’ port should then go to the return banjo (the one toward the rear of the car) on the fuel filter console.

This way all the circulating fuel goes through the carb inlet manifolds, keeping them cool.
You should remove the MFI cold start solenoid from the top of the fuel filter console and replace it with a suitable metric plug and crush washer. It has straight threads – not tapered.

You didn’t say which 911 – ’69 or ’73. The ’69 electric pump at the front cross member is better than the ’73 pump at the rear torsion bar tube. The waste heat from the heaters exits close to the rear location.

On SOME ’72 & ’73 MFI there is a 2 mm restrictor inside the “Tee” hose assembly that returns fuel from the fuel filter console and the rear mounted electric pump to the pipe in the chassis and eventually the fuel tank. You need to remove that restrictor. This involves disassembling the “Tee” hose assembly as the restrictor is inside the hose just upstream of the tee on the fuel filter console side.

Use this opportunity to replace ALL the flex hoses in the fuel system. Even though this normally operates below 5 psi, I would only use the OE Norma-Schellen hose clamps and not the ‘worm screw” type. This is also the appropriate time to clean the fuel tank and the screen filter in the fuel tank outlet fitting.

The point of continuing to use the MFI filter is that it is a far better filter than most carburetors ever see (or need). Its size gives it the capacity to absorb a lot of debris and still function properly. I would measure the return fuel flow (engine off) at the fuel tank on your new system. Future measurements will allow you to gauge the condition of the filter. They are relatively expensive so you don’t want to replace them needlessly.

Several manufacturers sell a product called a ‘fire sleeve’, Aeroquip for example. These are insulating and fire resistant sleeves for hoses. It is appropriate to use this over the hoses where they pass above the transmission and are exposed to the heater outlets. Some continue them to the engine.

The point of this entire exercise is to keep cool fuel at the carburetor inlets in almost every situation. While running with a full fuel tank, the ‘thermal inertia’ of the large amount of fuel keeps the fuel from getting too hot. ‘Heat soak’ can allow fuel to boil. Until the circulation has purged the vapor back to the tank and cooled all the components, the carbs get hot fuel and vapors. They don’t like to run properly in this situation. Insulating the hoses and components is the only solution.

Many racers go to extreme measures to insure cold fuel at the engine, including refrigerated fuel. Many (most?) EFI systems circulate the fuel through the electric injection nozzle to keep it cool also.

If I were doing a system from scratch, I would have the fuel pump, in-line filter and return bypass-type regulator mounted on the front cross member. Only insulated supply and return hoses to the engine. I would use only screw-together fittings and a minimum number. I would have the pair of fittings for engine removal as male-female pairs so they could be coupled to maintain fuel system integrity when the engine is removed (sorta like pre-2.4 MFI). I have an old post on EarlyS Forum about fuel cell considerations.

Best,
Grady