Cheap horsepower for CIS engines

Tuning the 911 CIS engine for cheap horsepower

Porsche 911s produced 1973 to 1984 are fed by a Bosch K-Jetronic fuel injection system also known as CIS for Continuous Injection System. It was a huge leap forward compared to the old mechanical injection. And in a great many ways CIS is superior to carburetted engines with their wild mood swings. The mileage is way better, and your car will actually start and be driveable in cold conditions. But CIS falls short when it comes to one vital area: Power!

Unfortunately, there’s not a lot you can do about it. Or so common wisdom has it in the form of Bruce Anderson and his indispensable ”Porsche 911 Performance Handbook”. The advice here is quite clear: Get rid of the CIS and install a pair of PMO or Weber carburettors instead. Or, even better, splash out for a modern electronic fuel injection. Whatever you choose, the bill will make you weep for years and ensure a life of eternal poverty for your children, their children and their children’s children.

A set for PMO carburettors will cost you close to 4,000 EUR, and their very nice EFI system is an equal amount. Bitzracing has recently produced an affordable CIS to EFI kit, but it still comes in at 1,500 EUR. Eye-watering, right? Fortunately, an article at Pelican Parts shows a glimmer of hope. Because it seems there is an alternative to a life in debt prison. And even better: These are all parts you should get anyway, before you even start to dream of getting rid of your Bosch K-Jetronic CIS.

Get yourself some SSI heat exchangers

SSI heat exchangers are pretty much the standard for Porsche stainless exhaust heat exchangers. Replacing your 1975 and later exhaust system with these stainless steel beauties is often good for 10 or more horsepower.
The reason is straight forward; from 1975 and onwards, new US legislation brought about necessary changes in the exhaust that severely limited the flow capacities of the engine. This was done to pass stricter smog standards. But the design of SSI heat exchangers is based on 1974 and earlier Porsche 911 exhaust designs, which were highly optimized for the engine.

Replacing the old heat exchangers with SSIs brings a whole range of benefits. The engine will run cooler, it will produce more horsepower, and it will produce more torque. With an extra 35 Nm at 3,000 RPM this will make a huge change for everyday driveability.

All SSI heat exchangers are a bolt-on replacement. In recent years production has moved from the US to the Danish company JP Group. According to them, they produce the heat exchangers using the same specs and tools as the US manufacturer. They can be bought here in Europe, and here in the US. Expect to pay close to 1,400 EUR.


Put a grinder to your cams

The next step is a bit more radical and frightening. Remove your camshafts and regrind them to the more aggressive Porsche 964 profile.

911 CIS tuning

I actually was a bit surprised when I read that particular tip. Because the Bosch K-Jetronic CIS system is notorious for being unable to cope with more aggressive cams. But some inquisitive soul had been experimenting and struck gold. It seems that CIS engines can handle the camshafts of a 911 C2 and C4 from 1989-1994, also known as the 964. With those installed you gain approximately 10 extra HP’s and a steeper power curve that lets you continue on to the fun side of 5,000 RPM.

So what does it take? Well, you can go two ways. Either buy a pair of 964 cams, which should set you back approximately 2,200 EUR. Before you install them, make sure to grind off the drive for power steering. You could also regrind your existing cams. This is not a thing you would want to do yourself. So Google for a professional with Porsche experience.


Get a set of high compression pistons

Next step is to increase compression. Especially if you are driving a US model with notoriously low compression. For the 2.7 liter engine it is in the neighbourhood of 8:1 or 8.5:1. That is what it takes to make your 911 run on the bilge water Americans call gas.

911 CIS tuning

With proper petrol available you should be able to increase compression to at least 9.5:1. All it takes is to replace your existing pistons with a nice new set from JE Pistons. They will set you back 800 Euro or so.


Enjoy more horsepower in your 911 CIS engine

And what’s the end result? My guess would be an extra 20-30 horsepower and a car that’s more rev happy, but doesn’t lose low-end torque. The total bill tallies up to 2,500 EUR with your existing camshafts, or 4,400 EUR if you get a set of new 964 cams.

That is still a lot of money. But remember: If you dream about upgrading to an EFI system for even more horsepower, you will need all of these upgrades anyway to get the most out of it. You need to start here and not with a new fuel injection system.

Read all about it in Pelican Parts article.


What is this Bosch K-Jetronic sorcery anyway?

The K-Jetronic fuel injection system is still a mechanical fuel injection system. But it is a very different beast than the earlier systems used by Porsche.

Bosch K-Jetronic CIS

Commonly called ‘Continuous Injection System (CIS) in the USA. K-Jetronic differs from pulsed injection systems in that the fuel flows continuously from all injectors. The volume of air taken in by the engine is then measured to determine the amount of fuel to inject.

Fuel gets pumped from the tank to the fuel distributor; essentially a large control valve. As the name says, it distributes the fuel to the individual six injectors. The fuel distributor is mounted atop a control vane through which all intake air passes.

This is where it gets complicated. Because the fuel volume supplied to the injectors is based on the angle of the air vane in the air flow meter, which in turn is determined by the volume of air passing the vane, and by the control pressure.

The control pressure itself is then again regulated with a mechanical device called the control pressure regulator (CPR) or the warm-up regulator (WUR). The injectors are simple spring-loaded check valves with nozzles; once fuel system pressure becomes high enough to overcome the counterspring, the injectors begin spraying.

Sounds complex, right? And it is. There’s plenty that can go wrong, but if you have a well-calibrated CIS system it can be an absolutely joy to drive. So think hard before you decide to ditch it.

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