what are the basics of mechanical injection, trying to learn the basics and what does what Options

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[Dave_Darling]

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

[ClayPerrine]

The Bosch pump setup in the early 911 and many of the Porsche race cars is derived from a diesel fuel pump setup.

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.