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Porsche MFI

Bosch mechanical Fuel Injection is a performance induction system.
It was made famous by the 1973 Carrera RS, (providing 210bhp from a 2.7L engine) but also used on earlier 911S and 911E engines from 1969.
Porsche used various types of MFI on factory racing cars from 1967 to 1984.

It is sought-after, as it provides excellent power and superb throttle response.

I am not near knowledgable enough to provide a detailed description of how it works, but a basic description is that there is a mechanically driven fuel distribution pump (belt driven from the end of one camshaft) which, through a combination of inputs from engine speed, throttle position, ambient temperature, and atmospheric pressure, gives a precisely metered fuel amount to the individual fuel injectors.

There is a famous Porsche procedure for adjusting the MFI system, known as the CMA Check, Measure, Adjust

Basically, it states :

Check, Measure, Adjust

CHECK IN SEQUENCE

Air Cleaner Cartridge
Compression Loss
Spark Plugs (Spark Plug Connectors)
Dwell Angle
Ignition Timing
Fuel Pressure and Flow
Injection Nozzles
Injection Timing
Correlation
Exhaust Emission Test
- at part-load
- at idling speed

NEVER DEVIATE FROM THIS SEQUENCE

Remember, the injection system is not a separate component, as, for example, the generator/alternator. It should be thought of as part of the engine.
No matter how well the injection system is adjusted it cannot make up for problems in the operating condition of the engine.
Always begin injection system work by checking the engine’s basic tune.

Taking each of these items in sequence, a brief description (taken from the CMA manual) is:

Compression Loss

The Cylinder Leak Test provides information on how tight the engine is.

Leakage at the valves
Leakage at the piston rings
Leakage at the cylinder head gasket

As a max. cylinder leakage of 10% per cylinder should not be exceeded. A cylinder leak test is more conclusive than a compression test.

Leaky Valves = loss of compression thru intake or exhaust
Leaky Piston Rings = loss of compression thru crankcase - air vent
A leaky cylinder-head gasket is generally already noticeable at idling speed (engine warm) through a hissing noise.

If a compression test is used, observe the following:

Engine Oil Temp. 70 - 80°C
Throttle wide open
Measure each cylinder with the same number of compression strokes.

Spark Plugs

Burned spark plugs, adversely affect combustion. Therefore, the spark plugs must be thoroughly checked. The exhaust-gas composition can be improved by up to 1% just by replacing bad spark plugs.

Checking DWELL ANGLE and IGNITION TIMING

Dwell angle and ignition timing influence combustion and exhaust-gas composition.

DWELL ANGLE

Mechanical fuel injection engines are equipped with:

Up to 1972 Bosch distributor,
from 1972 (2.4 litre.) Bosch or Marelli distributors

Note: Marelli distributor rotors are fastened with a set-screw.

2.0 and 2.2 litre engines
Bosch-distributor - dwell angle 38° +/- 3°

2.4 and 2.7 litre engines Bosch-distributor - dwell angle 38° +/- 3° or Marelli-distributor - dwell angle 37° +/- 3°

Ignition timing must always be checked whenever dwell angle is adjusted.

Adjusting IGNITION TIMING

Ignition timing is always adjusted at normal operating temperature, oil temperature 70 to 80°C
2.4 and 2.7 ltr. ignition timing differs from that of 2.0 and 2.2 ltr. engines.

2.0 and 2.2 Litre Engines

Note:
2.0 and 2.2 litre engines
Ignition timing 30 BDTC at 6,000 rpm
On 2.0 and 2.2 litre engines, always check the ignition timing art idling speed. (0 - 2 ATDC at idle speed)
If the setting is too late surging may result. Note:
If ignition timing is too late at idle, do not re-adjust until it is checked at 6,000 rpm. Timing that is earlier than 30 BTBC at 6,000 rpm will burn pistons.
If timing cannot be adjusted properly at both ends, remove the distributor and check the advance curve.

2.4 litre engine

The vacuum controlled distributor of the 2.4 litre engines retards the ignition timing at idle (up to 5 ATDC).

When the throttle is opened and the engine accelerated, the ignition timing must shift from retard to advance.
Check by disconnecting the vacuum line while the engine is idling. The ignition timing must be between 4° and 6° BTDC.
This approx. 100 timing variation (from 5 ATDC to 4 - 6 BTDC) is a result of the precise throttle-valve synchronization.
If the air-correction screws are opened too wide, the vacuum needed for ignition retard cannot build up.
The timing may retard, for example, to only 3 ATDC. If the timing is now corrected to 5 ATDC, it will only change by 2 to 3 BTDC on acceleration. The engine might have hesitation and bad transition performance.

2. Let the engine continue to run at idling speed, disconnect the vacuum line and watch the ignition timing mark change from late to early. When the vacuum line is disconnected, the ignition timing should be at 4 to 6 BTDC.
3. With the vacuum line disconnected, check the ignition timing at 6,000 rpm. It must be between 32 and 38 BTDC.

2.7 litre engine

Set ignition timing at idling speed (900 ± 50 rpm) to TDC.
The vacuum hose must be attached during this operation.
At 6000 rpm, with the vacuum hose removed, the ignition timing must be between 32° and 38° before TDC.

Checking Injection Timing End of Delivery Stroke

The end of delivery stroke is adjusted to 40° after overlap TDC/Cyl 1.
Check as follows:
Set engine in direction of rotation at (TDC/Cyl 1).
From this position, turn 360° further (one revolution) until the Z 1 (Cyl 1) marking again lines up with the notch in the blower housing.
Turn further to the FE marking, the engine is now in the correct position.
On the cover of the injection pump and on the hub of the drive wheel there is a notch mark. When the pump is correctly adjusted the marks line up. If the marks do not line up, adjust by moving the toothed belt, or, for fine adjustment, after loosening the three screws on the drive gear

Checking and Adjusting CORRELATION

Correlation is the relation of the movement of the throttle valves to the movement of the pump regulator lever.
The throttle valves have to move in precise relation to the pump regulator lever in order to maintain the most favorable fuel-air ratio. Skipped the text from this section, as it requires special tools (Protractors) to measure the angles of throttle butterflies and pump lever See the CMA manual for detailed explanation of this procedure

SYNCHRONIZING THROTTLE VALVES

Synchronizing throttle valves is done at 3,000 rpm (engine at operating temperature).

Execution:

1. Fully close the air-correction screws (without applying excessive force). Then open :

5 half turns for 2.0 or 2.2 litre engines
3 half turns for 2.4 and 2.7 litre engines.

2. Start engine and run at 3,000 rpm. Use the hand throttle.
3. Using the synchrometer, measure the air flow of each intake pipe and note the values (see example below). Add the values for each cylinder and divide by 6. Adjust to the average value with the air control screws.
Example:
Cylinder 1 - 12.5
Cylinder 2 - 11.0
Cylinder 3 - 10.5
Cylinder 4 - 13.5
Cylinder 5 - 11.5
Cylinder 6 - 10.0
Total 69.0: 6 = 11.5

So, the average value 11.5. This means that cylinder 1 is too high, the aircorrection screw has to be closed.
Cylinder 2 is too low, the air-correction screw must be opened, etc.

Note:
When adjusting be sure that the air-correction screws are not opened more than 8 half turns.
If the air control screws are opened more than 8 half turns with little change on the synchrometer, the air channels are carboned up. The channels must be cleaned.

MEASURING EXHAUST EMISSION

Prerequisites: Make sure the test instrument is operating correctly.

The emission test is in two phases:

the part load measurement
the idling-speed measurement.

Always observe the following sequence:

first part load
then idling speed

Reversing the sequence results in incorrect values.

PART LOAD MEASUREMENT (road test measurement)

snipped, as it means measuring while car is driven, or on a rolling road ! See CMA manual for details

IDLING-SPEED MEASUREMENT

Check the idling-speed CO only after the part load measurement.
The idling-speed CO should never be measured without previously measuring the part load CO and, if necessary, correcting it.
Reason: An incorrect part load CO value influences the exhaust-gas composition so strongly that it can no longer be corrected with the idling-speed adjusting screw. Therefore, never regulate the idling-speed CO without first checking the medium-speed CO and, if necessary, correcting it.