The Bosch pump setup in the early 911 and many of the Porsche race cars is derived from a diesel fuel pump setup. The pumps are set up like little engines, with one piston per cylinder of the engine. The piston pushes the fuel at high pressure into the fuel lines, sending it to the injector.

The pistons in the pump are asymmetrical, and push more fuel when turned one way and less when turned the other. Mixture control is done by turning the pistons in the pump. They are all turned together in response to throttle position and engine temperature. The temperature is provided by a heat riser from the exhaust, or an extra "leg" from the heat exchanger. (The race car versions did not have temperature compensation; a crew-member had to physically hold a lever in the "cold engine" position during engine warm-up.)

Here's an article about the diesel pumps that the Bosch setup is based on: http://en.wikipedia.org/wiki/Injection_pump

--DD

Can you dumb it down a little for me

Older BMW's also used it.

Think of it as a 4 cylinder fuel pump. Fuel goes in and there are 4 outlets - one for each cylinder.

It is mechanical because the "injection" is not based on any electronics. I believe on the BMW's they were cam driven.

no offense, but if you need that explanation dumbed down, you really shouldn't be messing with MFI. It's quite tricky to get right. If EVERY other aspect of the engine isn't in spec, you're wasting your time trying to set the injection up

You should get a copy of Mechanical Fuel Injection ... Check, Measure & Adjust, to see what you are getting into. Without the proper tools and equipment necessary to set a pump up, you're pissing into the wind (and it will blow back on you).

Actually you missed a step there buddy.....

The Bosch mechanical fuel injection on a 911 engine is an evolution of the fuel injection used on the Messerschmidt Bf 109 and a lot of other German aircraft during World War II. That was derived from the diesel fuel injection.



Now..on to the explanation of function.

There are 3 basic types of fuel injection....

1. Mass Air flow systems.
Think L-Jet.. Load is determined by amount of air coming into the engine.
2. Speed Density Systems
Think D-Jet..Load is determined by throttle position and and manifold vacuum.
3. Alpha-N systems.
This is MFI. Load is determined by engine speed and throttle position.


The Alpha-N system is not as efficent as a MAF or Speed Density system. It cannot accurately determine the load on the engine. An Alpha-N system is using a preprogrammed fuel curve based on throttle position and engine speed. Because it does not use engine vacuum, it can be used with very high overlap camshafts that would mess up the Speed density or MAF systems.


What the MFI system does is to use a "space cam" (A 3 dimentional cam) to adjust the mixture in response to some basic, mechanical inputs. There is a pointer that follows the countor of the space cam. That pointer moves the mixture control rack. (I won't go into detail about how that part of the MFI works).

The first thing it uses is throttle postiton. As the throttle is opened, the space cam is twisted on it's shaft to change the location of the pointer on the space cam's sorta egg shaped profile. That richens the mixture to account for the additional air getting into the engine.

The second thing is the govenor style centrifegual weights. When the engine accellerates, the weights move outward. This causes the space cam to move forward (in a 914) or backward (in a 911). This also richens or leans out the mixture.

With those two items, the MFP pump can determine the correct mixture for the engine speed and load. It knows you have your foot all the way to the floor, but the engine is only at 1500 RPM because the cam is rotated to max, but the flywheel weights are still in.


Now there are added things to make it more user friendly on a day-to-day basis. There is a barometric cell that leans the mixture out when the car climbs in the mountains by moving the relationship between the pointer and the mixture rack. There is also a warmup thermostat that leans the mixture out as it heats up from air in the heat exchanger.



The MFI's control system can be duplicaed with modern electronic fuel injection. Most aftermarket systems can be programmed to use the MAP sensor for BARO input only, and to use just the TPS and RPM sensors to adjust the base fuel curve.


I have studied the MFI system extensively, and have lived with it on a 914/6 for years now. It is a very intricate system with lots of adjustments required to make it run right. When it is right, there is nothing else like it for pure performance. The throttle response is phenomenal, and the sound it makes a full throttle will give any true gear head chills.

The downside to it is that it gets really lousy fuel mileage, and it does not like the ethanol in the gas. The gaskets and seals in the pump were never designed with ethanol in mind. So I have to limit my use of the car to keep the pump seals from dissolving. It is hard to start cold, and requires some patience waiting for it to warm up.

If I were building a new six, I would go with Motronic. It is the most modern injection you can get for a 914/6, and it truly works with no cantankerous quirks.

I really just trying to understand the very basics

Or theory

Before I start tuneing or messing with my great running

D jet

But I appreciate that last tip





If EVERY other aspect of the engine isn't in spec, you're wasting your time trying to set the injection

D-jet is not MFI. D-jet is an old, funky, analog electronic fuel injection system. It uses a variety of electronic sensors to determine how much fuel it thinks the engine needs, and opens the injectors for long enough for that amount of fuel to get sprayed into the intake. The injectors are opened for longer times for more fuel, or shorter times for less fuel.

CIS (Bosch K-jetronic) is sometimes called a mechanical fuel injection system. It isn't the same as MFI, though. It is a fuel-pressure-controlled form of injection. The bits and pieces vary the fuel pressure going to the injectors, which continuously spray fuel into the intake. Higher pressures mean more fuel goes in, lower mean less goes in.

MFI (yes, Clay, it went through aero engines in the 30s and 40s) is a purely mechanical system. The MFI pump is a piston pump, much like an engine is. There is one pump piston per engine cylinder, and when the pump piston goes up in its bore, it pushes fuel out of the injector in the intake. The pump is hooked up to, and runs at the same speed as, the camshaft. The pump has a means that lets it vary how much fuel is pushed by each piston for one stroke. Rotate the pump piston one way, it pushes more fuel. Rotate it the other way, it pushes less fuel. That rotation is hooked up to several different things, mostly the throttle position and a mechanical RPM sensor.

Is that simple enough?

Also check the Wikipedia article.

--DD

Google it man!

D-jet, or jetronic , Bosch fuel injection

Pages and pages of info, more info available than anyone wants to type up.

Books available also.

I have googled it but was unclean about many terms used in the descriptions of various systems


So d jet is analog electric system and modern systems are full digital I would assume

So what are the fundamental differences other then the cpu

All of the electronics are different. The D-jet uses all analog signals, while many of the inputs to and outputs from the modern EFI are digital.

Most modern systems physically measure the amount of air coming into the intake to determine how much fuel the engine needs. There will be a MAF (Mass Air-Flow) sensor of one kind or another.

D-jet uses manifold air pressure (MAP) to determine how much of a load the engine is under and therefore how much fuel it needs. There are modern MAP-based systems (Honda used them for years and years) but the D-jet sensor is an interesting analog device that varies the inductive coupling between two coils of wire, instead of a digital device that provides a changing voltage with pressure changes.

--DD