Porsche Mechanical Fuel Injection - Bosch MFI

Porsche developed a mechanical fuel injection system, used versions of it on numerous race cars, and also used it on several production models in the early 1970s, most famously the early 911, including the iconic 1973 Carrera RS

The big advantage of the MFI system over the other fuels systems of the time was it's instantaneous throttle response, and it is still favoured today for classic 911 racing.
Being an open-loop system, the quantity of fuel supplied to each injector was regulated simply by engine speed, throttle position, and had some other inputs, such as barometric pressure, and engine operating temperature.

With todays technology, it is possible to use modern Electronic Fuel Injection systems (EFI), but still it is argued that nothing beats the MFI, both for simplicity, and performance.

The drawback with MFI is finding a good mechanic who can work with this system, and get it tweaked/adjusted to its optimum performance.
But all is not lost. At first glance, the system seems complex, and a lot of people simple dump it in favour of either the later CIS injection system, or convert to carburettors, which were also used on the early 911 engines.

In July 2005, after many months/years of searching and researching, I finally took the plunge and bought a 1975 911 Coupe. I was not knowledgable enought at the time to realise what I had just bought! The original 2.7 engine from 1975 would have had the CIS fuel system. However, this car had undergone a detailed restoration around 1995. During that restoration, I was told it had undergone lots of changes.

Upgrade to oil-fed cam chain tensioners (the previous style were prone to failure)
Turbo style lower cam covers
Front mounted 1984 spec copper pipe oil cooler
High Flow oil pump
New Valves
Gearbox oil cooler
Upgraded brakes to later 3.2 carrera spec all round
RS Bilstein suspension
Later 915 gearbox from 3.2 carrera with factory short-shift kit
Stainless brake hoses
Brake servo
Complete Burgundy leather interior from a 930 (911 Turbo)
Stainless Steel Heat exchangers

So, I drove it for a while, figured it was running a bit rich (black smoke, and rough idle) and decided I would read up on the MFI system and find out how to adjust it. Little did I know what lay ahead !

I found lots of useful information about MFI on the Pelican Parts forums, and I needed some special tools to adjust the MFI pump, so I bought those from Pelican Parts in the USA.

I tweaked the idle mixture setting a bit, without much success.

I drove it for another bit, just every now and again, and although it was a fantastic drive, and performing really well, I knew it just wasn't right. It was a bit embarassing sitting in traffic with black smoke bellowing out the exhaust.

I read through the famous Porsche document Check, Measure, Adjust known as the CMA a few times, and decided I'd better get an expert to sort it out for me. Trouble was, I was in the Northwest corner of Ireland, and the closest "god MFI guy" was in the UK. I though about it for a while, and put it on the back burner.

Aside:
In the meantime, I changed my daily driver a few times, ending up with a 1983 MK1 VW Golf GTI 1.8 Campaign. I drove this for a while and found it to be a fantastic little car. Very quick and very good handling. As the suspension was a bit soft, I decided to replace the shock absorbers & springs, but I got carried away, and replaced all the suspension bushes, topmounts, ARB bushes, etc etc. In the process of that, I had bought some more tools.

Since I was in motor-mechanic mode, I decided it was time to sort out the 911.
I again looked around for a professional who could do it for me, and at the same time I read up a lot more of the MFI system, and the CMA procedure, and eventually decided I'd tackle it myself.

I posted a thread on the Pelican parts forums, about what I intended to do, asking for advice, and got some very good feedback, encouraging me that I actually could tackle it myself, together with suggestions on what I should do, and what I shoudl measure, etc, etc.

I started work. I discovered the MFI pump was the correct pump for the 2.7 engine, so that was a good start
I was advised to measure the valve lift, to gauge what camshafts were in my engine. For this I needed a dial gauge which I didn't have, so I ordered one from pelican together with a z-block (P207) which makes it easy to mount the dial gauge in the right place. I also ordered a set of cam cover gaskets which came with new lock nuts and washers. (I would have to remove the cam covers to check valve clearance, and to measure the valve lift)

I was also advised to remove the intake stacks and throttle bodies to measure them also. This would mean new gaskets for refitting, so I ordered them, again from pelican.

I removed the throttle bodies, and found they were very dirty. The butterflies and the inner bores were covered/caked with carbon deposits.

After a lot of scrubbing, and a few cans of brake cleaner, I got them cleaned up, and measured as the correct throttle bodies for an S engine, and went through an advised adjustment procedure, using a vacuum cleaner and a synchrometer, to get equal aiflow through all the butterflies, setting the stop positions as required.

I had bought a leakdown tester, so I measured the cylinder leakage across all 6 cylinders. All but one were very good. Jury still out on the reason for that one. Could be carbon build-up preventing the exhaust valve from seating properly.

I got delivery of the dial gauge, so I remove the inlet (top) cam cover, adjusted the valve clearance, and set about measuring the valve lift.
I had a figure of 5.4mm in my head, so I assumed my 10mm dial gauge was more than adequate. I was wrong. I started to measure the inlet valve lift, and got 10.7mm. This confused me, and I wondered about the relationship between 10.7mm and 5.4mm.

Then I realised my gauge only read to 10.7mm maximum, so it was possible the valve lift was more than that. I was annoyed that pelican would sell a dial gauge which was too small for the task.

After a bit more reading up, I found that I should have been measuring the valve lift at TDC overlap - This is 1 turn after TDC compression, where the exhaust valve is closing, and the inlet valve is opening in preparation for the intake stroke. Specified at 5.0mm to 5.4mm

Silly me.

I found the specs for valve lift for an S cam, and decided I may as well measure total valve lift, as well as the TDC overlap measurement, so I searched and found a Draper 25mm dial gauge on ebay.

While waiting for this gauge to be delivered, I went ahead and refitted the throttle bodies, and stacks.

While under the car to adjust valve clearances & measure the exhaust valve lift, I noticed I had SSI's and I had a 7R case.

Measured valve lift at TDC overlap - Perfect for an early 911 S cam. Measured total valve lift - close to early 911S cam - still trying to verify this. So far so good.
Bought some new oil, Silkolene Pro S, 10w50 and put 10L of that in to start with.
Bought a compression gauge and did compression tests. This matched the results of the leakage test, showing #5 as being 18% down on the highest compression figure of the other cylinders.

Fitted new rev-limiting rotor arm, distributor cap, and new magnecor kv-85 plug leads.

Hopefully I'll get the engine started next and continue with some more adjustments to ignition timing, measure ignition advance, etc.... basically continue step by step through the CMA procedure to be continued.......