MFI Simplification Project - Help wanted!

[356RS]

David and Milt got me thinking about the Shut Off Solenoid. Is there a way the RPM transducer/speed switch can be set up for a pointless/CDI type distributor like the 3.0 SC dist? My MFI SS engine is twin plugged and using an Andial converted SC distributor and I want to be able to use the shut off solenoid.

[David E. Clark]

Originally Posted by 356RS

David and Milt got me thinking about the Shut Off Solenoid. Is there a way the RPM transducer/speed switch can be set up for a pointless/CDI type distributor like the 3.0 SC dist? My MFI SS engine is twin plugged and using an Andial converted SC distributor and I want to be able to use the shut off solenoid.

Mark,

I haven't read about anyone doing this but I don't see why it wouldn't work. The RPM transducer gets it's signal in the early cars from pulses emitted from the coil so you should be able to integrate a speed sensing switch to any coil or other electronic RPM pickup device. I'd study the early wiring diagrams to replicate the way the RPM Transducer was set up to work with the shut-off solenoid on those cars.

[David E. Clark]

Mark,

Take a look at this schematic. It should give you a good idea of how the RPM transducer, Speed Switch and Microswitch are wired in the early cars.


© Intereurope Workshop Manual, Porsche 912, 911T, 911E, 911S, page 62 (October 1972). Posted here for educational purposes only.

[David E. Clark]

# 5:

FUEL PUMP – is an electric roller-cell type with a delivery capacity of up to 33 gallons per hour. The pump is mounted on a bracket under the fuel tank. The fuel is delivered from the tank to the fuel filter. The fuel filter contains an overflow valve which regulates the fuel flow at approximately 12psi to the injection pump. The fuel pump is specifically designed to provide excess fuel capacity so that the excess fuel can be used to cool the injection pump. After flowing through the injection pump, the excess capacity is routed back to the fuel tank via a return fuel line. If fuel pressure increases above 14psi, an independent bypass valve installed in the fuel pump routes excess fuel to the fuel tank.

The Exact Fuel Pump Specifications are:
Regulated pressure = 14.2 PSI, 0.8±0.2 Bar
Flow rate = 125 liters/hour
Porsche P/N = 901.608.105.00
Bosch P/N = 058097 0001
Current draw = 3.5A @ 12V
RPM = 2800

The fuel pump is # 1 in the Fuel Injection System Diagram and it's location is shown in the diagram below.

['71 911E Coupe]

About the microswitch, RPM trandsducer, and speed switch; I ran my car today and pulled the wires off of the microswitch with the car at idle, and I could smell it got much richer, since the system is designed to not deliever as much fuel when activated. I hooked it back up and it smelled better and right when I hooked it up I heard a little bang, which I think was the speed switch since the sytem was then live. Just my experience, and plus like you guys said, I don't have any backfiring. I'm pulling my injectors again and sending them to Supertec Performance to get them tested and cleaned. I just had them out and tried to clean them but I want them tested. I may need new ones.


David,

I agree with you on the thermostat discs arragement. I don't understand it either, and when I first took mine off and cleaned them, I think I put them back in the correct order, but I may have mixed one or two up. I don't think I did though, but I dont' know why it would effect it either.

Also, is that fuel pump one that they used on earlier fuel injected cars? If these pumps go bad, instead of replacing with the $600+ original one, has anyone put another type of fuel pump in it's place and has not had any problems with it (as far as fuel flow, etc.)????

[David E. Clark]

Anthony,

I removed the Bendix pump that I had pictured under "fuel pump" because I don't think that particular pump was ever used on the MFI system. I couldn't find a picture of the roller-cell type pump and I didn't think about the Bendix picture being confusing. As far as using a different pump, I imagine any electric pump that matches the appropriate specifications would work. I think the tricky part will be finding a substitute pump with a relatively low current draw of 3.5 amps and a 14psi bypass valve since there are lots of modern pumps that can put out in excess of 28 gallons per hour. Keep us posted if you find an acceptable substitute. Suppliers like Summit Racing or JEGS might be a good place to start your search since they supply loads of racing pumps which, by design, have to pump fuel like crazy!

David

['71 911E Coupe]

David,

Yeah that diagram is more familiar to me. I am going to look for another pump because mine is old and I think I can find one that will be more reliable. Also, when I got the car, the pump was stuck, so I'm sure putting a new one on will be a good idea, cause it's probably only a matter of time with the original one. I'll let you know what I find.

David, do you think I need to have a return from the pump if I was to get one with just an in and an out, if one is coming from the injection pump and then into the fuel filter housing. Could I just leave that line in place and then get rid of the T in my line that splits it from the fuel coming from the engine, and the return from the pump, both which go into the T and to the fuel tank. So in other words, I would just have the return from the filter housing flowing into the tank. Or do I need to find a pump with all 3 directions of flow?????

Thanks! I hope this will help others, maybe if I get this to work, when other people's pumps die they can do this instead of spending over $600.

[David E. Clark]

Anthony,

I don't see how you could get by with a two directional pump since you need to (1) flow excess amounts of fuel into the filter then to the injection pump and back to the filter, (2) overflow out excess fuel from the injection pump through the filter and, (3) bypass fuel (different from overflow) back to the fuel tank in case of excess pressure. If you look closely at the above fuel pump diagram you will see that the lines are directional arrows that show all three areas of fuel flow. I think a two directional pump would probably cause all kinds of problems, not the least of which would be constant flooding.

['71 911E Coupe]

Well I have a brand new fuel pump now, that has only an inlet and an outlet. David, what do you think if I put a pressure relief valve, or a check valve of some-type that is built into a T after the outlet? This way fuel would still flow up to the filter and if there was too much pressure, some of the fuel would go back to the tank which along the way I would just hook the normal T into from the filter. Only things here is that I have to have enough flow to keep the right amount of pressure going to the filter.

I'm gonna keep looking online for a pump with a return though.....

David, So the fuel coming from the injection pump and the filter, need to meet with a return under pressure? (Isn't the fuel coming from the injection pump controled by a valve above the filter, that lets the fuel flow back to the tank. If so how is this affected by a return from a pump?) Or are you just saying it may cause flooding because the pump pressure may be too high going to the filter. So Maybe then a relief valve would do the trick? We both on the same page? Thanks!

[David E. Clark]

Anthony,

I'm not promising that this will work but here's a very simple schematic of what I've come up with for a standard fuel pump:



The bypass valve is in the factory pump but I don't see why it couldn't be on the outside like you suggest. It shouldn't be hard to find a relief valve rated at 14psi.

Originally Posted by '71 911E Coupe ... Or are you just saying it may cause flooding because the pump pressure may be too high going to the filter.

I think the filter is designed to operate at a fuel flow of 12psi. If it gets above 14psi without "relief" I think the filter will be unable to continue flowing causing flooding in the filter or (just guessing here) the "overflow" will go down the only available route which is back to the injection pump again flooding or, worse, it will just leak out creating a serious fire risk.

Hopefully, someone who has tried this will chime in -- maybe this deserves it's own thread?

['71 911E Coupe]

David,

It's worth a shot...and yeah I'll start a thread on this subject alone. Let's get back onto the main subject of this thread...sorry for getting off of it.

I'll do the Fuel Injectors next...let me just get a picture of them...

['71 911E Coupe]

Fuel Injectors- The fuel injectors that are part of the MFI system are also mechanical. They operate by opening up when fuel is pushed through the line of that injector at around 213-256 psi. A Very Very high pressure! The fuel comes down around the bottom of the injector and pushs it upwards against a spring, and then the fuel forms a cone and is sprayed out of the injector. These are located in the head, behind the intake valve. There is always fuel sitting in the line, even if it is not under pressure, because of this the injectors need not to leak. They should be checked if your injectors are old. Supertech Performance, and Pacific Fuel Injection will both test your injectors. If you need new ones, these are still available for around $180. These injectors were made by Bosch and can not be repaired. There are also screens in the injectors that can get blocked if you don't have clean fuel.

Below is a picture of a set of original fuel injectors for the MFI system

[pjh69911]

David,
I was rereading your thread and thought a revised text might further clarify the control unit's function. The picture is somewhat confusing as the governor control regulator does not directly connect to the fuel rack. It only acts to rotate the space cam. Anyway, here's what I propose for the text:

The heart of the control unit is the 'contoured cam'. It is a 3-dimensional cam whose radial dimension varies both with rotation and with axial displacement. The 'governor control lever' (connected to the throttle) only acts to rotate the contoured cam. The 'centrifugal governor''s weights fly outwards with increasing speed, causing the contoured cam to move axially. The 'sensor on the contoured cam' moves radially in response to the rotational and axial contour variations, and thereby provides a specific output of radial displacement for any combination of throttle position and engine speed. This radial displacement of the sensor is fed to the flat metal plate with rectangular slots via the upper pin in the rectangular slot. The lower pin in the lower slot acts on the fuel rack. The ratio of the input upper pin to the output lower pin is influenced by the thermostat and barometric cell, that move the pivot center of the flat slotted metal plate, further or closer to the rack lower pin. Refer to earlier control compensating unit illustration.

Hope this helps. Paul.

['71 911E Coupe]

Thanks Paul, I know that helped me some!

['71 911E Coupe]

There are still a lot more parts to be identified!

[David E. Clark]

OK, here's my attempt at describing the Injection Pump. Once all these parts are successfully described, I will edit this post into one comprehensive thread describing the entire MFI system. I'll post that to a new thread so it's more readable and to solicit corrections before making it final. Hopefully, once we've finalized the thread, a moderator can delete the prior threads to avoid confusion. We'll see ...

INJECTION PUMP: The MFI Injection Pump consists of two major parts: (1) the pump assembly which contains the camshaft (#19 below), the six plunger units (#s 11-14 below) and the toothed control rack (aligned horizontally behind #27 below); and (2) the control and compensating unit (described above). Each of the six plunger units consists of a plunger and a cylinder. Each plunger has teeth which mesh with the control rack on top and a roller tappet (#20 below) which rides up and down on one of the camshaft lobes on the bottom. As the camshaft rotates, the movement of the lobes forces the tappet into the plunger. At the same time, the various components of the compensating unit (thermostat, barometer, enrichment solenoid, etc) are sending instructions via connecting levers to the toothed control rack causing it to slide back and forth in the teeth of the plunger. This movement of the control rack causes the plunger to rotate exposing a slanted "metering land" in each plunger. This metering land is designed to close or open the inlet port to the fuel injector (see diagram below). The amount that this inlet port is open determines the quantity of fuel to be delivered with each stroke. As a result of these complex movements, the fuel contained in the plunger chamber is forced out in appropriate quantities through the hard fuel pressure lines then through the injectors and into the intake port.



pre-1971 MFI Injection Pump
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.


Pump Plunger and Cylinder showing fuel delivery.
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

[Jim727]

David, this is a great idea. I'm a bit too new to the MFI cult to be able to speak with any depth of knowledge, but I'm learning and will chip in knowledge permitting.

Jim

[David E. Clark]

OK, here is the final major definition. When I get home tonight, I'll try to edit this thread into one comprehensive post and resubmit. Please take a look and get back to me with any additions, corrections or deletions. Thanks!

SHUT-OFF SOLENOID, MICRO-SWITCH AND RPM TRANSDUCER CIRCUIT: The shut-off solenoid is part of the control and compensation unit (on the lower right in the diagram below). Its function is to move the control rack to the fuel off position when the vehicle is decelerating in gear. The shut-off solenoid is controlled by a micro-switch in the accelerator linkage and an RPM Transducer on the relay panel which receives its RPM signal from the ignition coil. The RPM transducer senses engine speed. It is supposed to activate the shut-off solenoid cutting off the fuel supply when the throttle is closed (for example when the car is decelerating) and engine speed is above 1,500 RPM. If everything is working correctly, the solenoid is supposed to release when the RPM transducer senses engine speed dropping below 1,300 RPM regardless of throttle position. This allows the fuel supply to resume and the car to idle correctly. If engine speed is again increased, the solenoid circuit is interrupted by the micro-switch allowing the RPM transducer to again become activated at 1,500 RPM.


© Glenn’s Diesel and Gasoline Fuel Injection Manual. Posted here for educational purposes only.

The RPM Transducer is #5 in the diagram below and wired through #30b on the terminal.

© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Here's a schematic of the RPM transducer thanks to Warren Hall - "Early S. Man" (may he rest in peace) and Mike Gillies of Brisbane, Australia.


Finally, here is a simplified wiring diagram for the micro-switch and RPM transducer.

© Intereurope Workshop Manual, Porsche 912, 911T, 911E, 911S, page 62 (October 1972). Posted here for educational purposes only.

[Jim727]

David -

I have recently had to troubleshoot and replace my RPM Transducer, and I think there is a change that needs to be made in the text.

Pin 1: Output to shutoff solenoid;
Pin 2: Power from engine fusebox;
Pin 3: Ground and pin 31 of time limit switch;
Pin 4: CD Ignition and distributor (not coil).

How about also including a pic of the time limit switch for reference?

Jim

[David E. Clark]

Originally Posted by Jim727

David -

I have recently had to troubleshoot and replace my RPM Transducer, and I think there is a change that needs to be made in the text.

Pin 1: Output to shutoff solenoid;
Pin 2: Power from engine fusebox;
Pin 3: Ground and pin 31 of time limit switch;
Pin 4: CD Ignition and distributor (not coil).

How about also including a pic of the time limit switch for reference?

Jim

Good catch Jim,

The RPM Transducer gets its RPM signal from the coil but to say it is "connected to it" is confusing. I modified the sentence so that it is less confusing. I'm at work and I can't locate a picture of the micro-switch. That is what you are referring to as a time-limit switch, isn't it? (We all use so many diffferent names)! I'll see what I can find when I get home.