Porsche
911s produced 1973 to 1984 are feed 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.
But an
article at Pelican Parts shows a glimmer of hope. Because it seems there is an
alternative to a life in debt prison.
First of
all you need to install a proper sports muffler and a pair of SSI heat
exchangers. Actually, you should do that anyway, even if you decide you do not
need more horsepower. This is where you need to know that SSI has been bought
by JP Group who also manufacture the DANSK muffler.
The next
step is a bit more radical and frightening. Remove your camshafts, regrind them
to a 964 profile and install a pair of high compression pistons.
I got a bit
surprised when I read that article. Because the 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, for starters get hold of a
pair of 964 cams and grind off the drive for the power steering. Or regrind
your existing cams. It can probably be done quite cheaply.
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.
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.
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. And it will cost you a lot less than changing the CIS.
Read all about it in Pelican Parts article.
Labels: 911, Bruce Anderson, CIS, Porsche, tuning
