Tweaking CIS

[911nut]

I was reading last night in a book about Bosch fuel injection that a K-Jetronic Lamda CIS can be enhanced with a larger Weber throttle body and a Micro Dynamics rising rate fuel pressure regulator. I wasn't able to find sources for these parts. Does anyone have experience with doing this or something similar? Thanks.

[emcon5]

I saw the same thing in my Bosch book, so I dug a little deeper, researching in other books, and on this and another forum. Keep in mind, that the Bosch K-Jetronic FI system is found on a bunch of different cars, with different performance/economy requirements. A Volkswagen Rabbit may benefit from a larger throttle body, but a 911 probably won't. The main limitation to performance increases with CIS engines (after 1974) is the exhaust system.
On 911's the hurdle on the intake side is the CIS sensor plate. This limits how aggressive a cam you can use, and makes a rather twisted path the intake air must follow to get to the heads. I am no expert, but I would think that as long as the throttle body can flow as much air as the sensor plate when both are fully open, there would be no benefit from a larger TB.

Most people who have tried to modify a 911 will find that the engineers at Porsche did their homework. With the exception of backdating the exhaust system to the 1974 and previous type, there is very little you can just bolt on for more horsepower.

You can "bolt-on" higher compression 3.2L pistons and cylinders and a slightly more aggressive cam. Combined with the early style exhaust this should get you in the neighborhood of 220 HP, with the stock FI system, or so I hear.

Tom

[911nut]

Ok, let's try this again. Does anyone out there have any FIRST HAND EXPERIENCE with modifying their CIS for improved performance?

[rstoll]

I tend to agree with Tom. The restriction is on the output side.

[emcon5]

More discussion on the topic here:

http://www.pelicanparts.com/cgi-bin/vb/showthread.php?s=&threadid=8559

As to "FIRST HAND EXPERIENCE" go buy Bruce Anderson's book, and read the chapter on Performance Modifications. I'll give you a teaser: Early exhaust system, displacement/compression increase, or if you junk CIS and go to Webers, displacement/compression increase and more agressive cam.

Think of it this way, take a look in your book at all the different cars that used the Bosch Jetronic system in all it's forms. Not a lot of performance cars in there. In fact, the only one's I see are Porsches (and the Saab Turbo, but turbo's are a different story). For a car that is 183 cubic inches, they get up to 200HP with a fuel injection system designed for economy. Porsche did their homework.

There are no shortage of folks out there whose business is to seperate the automotive public from their money. If simply bolting on a larger throttle body was any improvement (or in many cases no improvement, see K&N) everyone would sell them.

Tom

[911nut]

Well, I read the thread and I re-read an old edition of Porsche 911 Story. Freiere states that the engineers at Porsche saw the sensor plate as the big restriction in the intake path as well as the component which prevented the use of the more radical cams Porsche was fond of using back then. So, at the least , this has been informative and I understand my K-Jetronic better than I did a few days ago. I declare this thread officially dead.
Thanks to all who posted to it.