Joe Engineer

Why CIS?

After rebuilding my entire engine from the ground up, why on Earth would I stick the stock Bosch CIS K-Jetronic fuel system back on?

I can’t tell you how many times air-cooled Porsche people have asked me that question. CIS sucks. CIS is hard. It will never run right. Just go Webers. Just go EFI.

The reason is mostly curiosity and a little bit of good old Jeremy Clarkson “How hard could it be?” hardheadedness.

What is CIS?

CIS is an electro-mechanical fuel injection system created by Bosch and put into production cars from 1973 to 1994. Interestingly enough the cars to bookend CIS application were both 911s. Im calling it electro-mechanical to distinguish it from the performance-oriented Bosch mechanical pump injection that preceded CIS in high performance and racing 911s. The full name is CIS K-Jetronic, where CIS means Continuous Injection System. It is so named because the fuel injectors are not pulsed on and off, rather they keep spraying fuel during the entire combustion cycle.

CIS consists mainly of a mixture control unit, which is a combination of 2 sub-components: an air flow sensor to measure the air entering the engine and a fuel distributor to deliver a corresponding quantity of fuel to the injectors. Once the throttle plate opens up, the engine inhales and creates vacuum, which causes a big flapper valve upstream of the throttle body to rise by a specific amount as air rushes past it. This is the air sensor. The air sensor flapper valve has a lever arm which is connected to the fuel distributor piston in the center. As the air sensor flapper valve rises or falls proportionally to throttle opening angle, the lever on the air sensor adjusts the position of the piston proportionally and changes the fuel pressure that is sent to the fuel lines and injectors. That is my grossly oversimplified explanation (bordering on inaccurate) of the basic system. There are numerous additional CIS components that we will discuss at a later time.

The early versions of CIS were open loop and had no feedback system to let it measure emissions and adjust fuel mixture accordingly. It was simply a system of air and fuel valves, fuel distribution components, temperature sensors and switches that simply allowed tighter fuel and emissions control than carburetors or mechanical pump injection. Later versions, like the one in my 1983 911SC, added an additional Lambda closed-loop circuit, on top of the existing CIS components. This circuit used an oxygen sensor in the exhaust stream to measure emissions and fine tune the fuel mixture even further. This version has a ton of parts. However it still is not true electronic fuel injection (EFI). CIS is a strange middle child between performance mechanical pump injection and electronic fuel injection.

If you want all the details, here is an extremely dry (but quite good) video showing how CIS works:

Why does CIS exist?

As stated before, primarily for emissions and fuel economy compliance. Porsche introduced this system on the 1973 911T because it could no longer meet US emissions standards with prior systems. It was used on naturally aspirated 911s through 1983 and on most 911 Turbos through 1994. CIS was predominantly used in European cars and was actually one of the world’s better performing fuel systems at the time, while in the US, cars like the 1977 Lincoln Mark V had 400 cubic inch (6.6L) V8s that could barely turn out 179hp thanks to very primitive emissions technology. This era of cars with anemic engine performance is lovingly known in many automotive circles as the “Malaise Era” although the performance of the Porsche 911 was still quite good when measured against its competition at the time. The near-supercar Porsche 930 Turbos were all factory CIS-equipped cars.

Why does everyone hate it?

It didn’t matter that CIS was the world’s premier fuel system at the time, it is the air-cooled Porsche world’s stepchild fuel system. It is difficult to learn, can be finicky if not well-maintained, you cant put big cams in it, and there is no power upside to using it compared to everything else the factory used on 911s. Carburetors are simple, sound awesome, and make great power. Mechanical fuel injection is also challenging, but it looks super cool, sounds awesome and makes even bigger power. Fast-forward past CIS to Motronic fuel injection and you had the best of both worlds, great power and fantastic emissions compliance. Fast forward to now and modern aftermarket EFI systems are super cheap, make excellent power, are great with emissions, and with current computer technology, are infinitely tunable. If you master CIS, your car will run very well, but at a known power-deficit to other systems. Not only is it difficult to learn, but the car wont make as much power as your friends’ cars running Webers, mechanical or Motronic injection, or turbos. So if a big part of your driving satisfaction is having a car that is as fast or faster than everyone else (which sadly, is most car people) then CIS is probably not for you.

Well maybe there are some upsides. If tuned correctly, it is rock solid reliable. Also, with the right exhaust, it makes all the right deceleration pops, cracks, and burbles that all modern performance cars have pre-programmed into their ECMs. More on that later.

So wait, why am I running CIS again?

Primarily to comply with California emissions. If I could take the easy route, rip it out, put in another fuel delivery system that is easier to work with, makes more power, and still gets me my CA registration sticker in the mail, you bet I would. This is my first 911 and my goal was to get it on the road as soon as possible, while making it as reliable as possible, and to keep it for as long as possible, and the easiest way to do that is with factory parts. The quicker I simply follow the manuals and rebuild the fuel system to factory specs the faster I can start driving.

Another reason is I simply want to know how it works. Since I have to literally rebuild every other part of the car anyway, I might as well immerse myself in it, learn as much as I can for the benefit of the car, and if someday I happen to pick up more childhood dream cars that also run CIS, Ill be set! Not to say that I want them because they have CIS, rather if I get any of them I might have a chance at keeping them running.

Cars I want to own someday that run CIS:

• Mercedes 190E Cosworth (KE-Jetronic)
• Volvo 240 Series
• VW Golf MK1
• Porsche 930 Turbo
• Ferrari Testarossa
• Lamborghini Countach (LP5000 QV)
• Porsche 964 Turbo (3.3L)

Will I run CIS forever?

That is difficult to say since this is my first Porsche 911. I’m sure I will take the engine apart more than once and maybe even switch fuel systems down the line if emissions regulations change. This car has already achieved “classic” status and with hybrid vehicles already mainstream, electric vehicles near mainstream, and autonomous vehicles not far behind, who knows what will happen with older gas-powered classic cars.

Ah who am I kidding, Ill probably just end up picking up a project smog-exempt 911 and build a turbo monster to be faster than everyone else. 😉

Enough explanation. Next time Ill discuss some of the known weak points of CIS and my countermeasures to try and make it as robust as possible.