CIS tuning for low end power?? SC 3.0
 

I was at a local porsche shop today picking up some Brad Penn. I asked the owner if he knew of any mild hp increases that could be done to my SC. He explained that the stock fuel delivery is very lean up until wide open throttle, where it just starts dumping fuel in. He went on to tell me that he had a way of "rewiring" (he used this term, dont now if he meant electrical or not) the CIS system so that the fuel mix starts sooner. He also says that he could set it up in a way that would let me go back to stock very easily and quickly.

He explained that this was only to give you more power on the lower to mid rev range. Not to increase HP over all.

Has anybody heard of this??

ssi's

You can get more low end performance by re timing the cams, but top end you compromise.

But these cars are supposed to be raced at Hi revs. Mine isn't awake until 3700 to redline.

No harm in hi revs under red line. I timed my car for hi revs.

CIS tricks, my antennae are up. Let us know what you find out.

My car is just not a dragster. Don't think it was designed to be though.

search these:

John Walker

3.5% CO

Unhook oxygen sensor

timing



And stuff like that.

I just want to say that I have replaced the cams for 964 profile, along with higher compression pistons. (9,8:1)
This has altered the "pull curve", if you like... It now has more "ooumph" all the way from low RPMs, and the high RPM output isn't that much greater than at 4000 RPM, like it used to be.
I run this engine at 3,5% CO, and there is no O2 sensor. Timing is spot on.

I don't think there is any easy way out on this, but I've been proven wrong before..!

Hello JW,

Yes, there are the "book" settings and then there is an optimum tune based on the individual engine. The wording of your question is a bit confusing, however, "performance" is all about WOT and any talk about "improved performance" at anything but WOT, is nonsense. The book settings are all about passing the existing emissions tests when the car was new. The CIS lambda system goes open loop after 35% throttle and below that it will try to bring the AFR back to stoich for the converter. You want to adjust the mixture so that the open loop WOT AFR is rich enough to support the optimum timing curve for the fuel you are using. These engines are knock limited and 25 BTDC is far from the optimum advance for making power in a hemi head engine with domed pistons. The later SC CIS lambda engines were detuned for 87 CLC octane fuel. If you have better fuel, and do not have issues with oil temps or blowby, you can advance the ignition and richen up the open loop mixture to get better performance from the engine. CIS lambda will speak to you through the test port, the first step is to start listening.

This last sentence sounded esoteric to me: What test port? How do you listen to it?

Aurel

Psalt,

I'm not following what you mean.

What about the various items others have mentioned, incl removing the lambda system, 964 cams, higher compression, larger pistons (this of course would increase displacement), cam timing changes, distributor recurving and freer flowing exhausts? Are you saying they don't qualify as performance enhancers or that they only enhance @ WOT or...?

My diz advances more quickly than stock. I was under the impression that helped w/ low end torque delivery. Is that wrong?

It's the lambda test port in the engine bay and you listen to it with a dwell meter. Paul (psalt) has frequently posted about it here. The Bentley manual also describes the procedure.

CIS w/lambda owners would do well to learn how to use the capabilities of that test port. My experience is here.

Brian

psalt
 

well said psalt, I dont think most guys know that once you go past the 35 deg switch, lamda goes into open loop (02 sensor quits controling mix) so why dicontect the sensor? dont you like to get 25+ mpg when your just cruising?

+1. This made my SC come alive. This is often referred to as "running open loop".
Unhook the O2 sensor first before setting CO. Leave it unhooked.
A side benefit of this set-up is that your car will loose the "loping" part throttle behavior at low speed (I would feel it driving through my sub at 25 mph).

Hello Don,

My guess is that people think they are disconnecting a smog device and that must be good for performance. CIS lambda was only used for a few years and few people bother to figure out what all of the switches, sensors and control boxes are doing. Unfortunately, most manuals, including the Bentley and Haynes, do not explain it correctly. The BOSCH text and Probst book are still the best sources of accurate info. The system allows you to make open loop WOT mixture adjustments to optimize performance and still have good cruise economy. But only with the sensor connected.

The Lambda computer will always pull the engine back to 14.7 afr, which is not where optimal performance is.
Granted there is enrichment at WOT but no one is jumping on their car every time they push the accelerator. Part throttle performance is improved by running open loop and "re-mapping" via %CO.

The Lambda computer will always pull the engine back to 14.7 afr, which is not where optimal performance is.


This is not true (always), the system goes open loop after 35% throttle and the O2 sensor is irrelevent. "Performance" by definition (i.e. output, torque, 0-60, 1/4 mile, top speed, ) is always measured at WOT, not below 35% load. If by "performance" you mean smooth part throttle cruise, or steady idle, disconnecting the O2 sensor can mask defects like poor compression, vacuum leaks or a worn out distributor. It is better to corrects these problems than load up the converter at part throttle. It is a misnomer to suggest "performance' improvements at anything but full load. In other words, if performance is inadequate at 27% throttle. you open up the throttle.

Don is correct, leaving the O2 sensor disconnected will only low mileage, it has zero effect on performance.

peanut
 

yes, you are correct in saying that 14.7 is not good for performance, its good for economey and emmisions, but once you go past the 35 degree switch,(which isnt much throttle) you are not running at 14.7. the ecm has put the system in open loop and the freq. valve goes to a 90 percent duty cycle which lowers the press in the lower differential chambers in the fuel dist thus enriching the mix. if you are discontecing the 02 sensor by pulling the relay, you are losing your only source of enrichment at wot. adjusting the mix to compensate will work, but at the great expence of fuel ecconomy and drivability. if the system is working you can have both.