I don't understand what this means?

It's a low tech way of checking for near stoichiometric ratio. If all the air is burning all the gasoline, there will be no Oxygen left over to combust the propane. If there is a bit extra air(or very lean FA), then the RPM will increase slightly when fuel (propane, in this case) is added.

It's also known as running Lean Of Peak EGT, and can be very, very dangerous for engines if not done right. What you're trying to accomplish is to become almost like a diesel where all the fuel is combusted spontaniously. Except in a spark discharge Otto cycle engine, the spark provides the ignition. Once the fuel is all burned, there is left over O2 in the cylinder to burn the added Propane.

Again, can be very dangerous for the engine. You also need to closely monitor the spark timing to make the successful.

Doc

I don't understand how you can vary things much on a CIS engine. I know from adjusting mine there is a sweet spot. Too much one side or the other and the car runs like crap. Adjustment was done when hot and then back and forth after a couple of runs. Once set in the sweet spot the car runs great at all speeds and all temperatures.

Other than idle and the adjustment on the Fuel Distributor what else is there to adjust on the motor. Unless you are going to tweak the WUR how are you going to adjust the fuel pressure. Am I missing something.

If you are driving 65 MPH to save gas you are wasting your time. The best gas mileage I ever got (18 MPG) was on a trip averaging around 80 in nice weather. That was the best.

As far as the cold start valve - yes it will close when the engine get hot, but it can drip a bit once closed adding a little fuel and causing the car to run rich. If the car blows smoke on a hot start after sitting 20-30 minutes then there is excess gas in the intake and it is probably coming from a leaky cold start valve.

Maybe it is me, but if you are looking for mileage get a Prius. You can drive the hell out of one of them and still get 50 MPG. The last car I would be keeping track of the mileage on is a 78-79 CIS Porsche 928. They were not designed for good mileage. They were designed to jump up and go.

Did anyone notice the EPA sticker on the "last built" GTS posted a few days back. The sticker said 26 MPG highway with a note that 30 MPG was possible. Did Porsche do something to the last car we do not know about or were they making a German joke?

Dan the Pod Guy

Dave, your analysis is spot on. The various mechanical(pressure) type of FI can be made as efficient as an electronic system by adding the closed loop sensing. That is, put some type of combustion sensor in the tail pipe and deteriming the CO% - then use that info to vary the FA ratio on the incoming side. Sadly, by the time Lambda censors came out, there were already better electronic FI systems being designed and deployed in the market(Honda PGM was a good one). The CIS lived on as long as it did because it was cheap to deploy and many cars built in the mid-70s just kept using it cause it was reliable. That all changed around 1977 when the L-jetronic appeared.

Why Porsche and other makers chose to still use the K-jetronic is baffling, all I can think of is the munee needed to change to LH system. Which, of course, they did about seven years later. There is no simple way to modify a K-jetronic car to any type of Lambda sense car. You just have to do it with pressure. I've tried to think of a way to incorporate temperature and tailpipe sensors too, but it's a dead system, and the cost to try and change the WUR pressure based on an output from the Lambda would be pretty hard.

Good luck in your quest, let us know if you make advances in efficiency.

Doc

Agreed. Mileage is not something I pay a lot of attention to. I do track it on long trips, just for grins. I do notice that I get 200-250 miles between a fillup and the low fuel light coming on ~19.5 gallons. But it just is what it is, if you're sweating over a couple mpg the 928 is the wrong car for you.

There is a CIS variant used on many cars, incorporating an oxy sensor with some analog electronics to drive a solenoid in the control pressure line. Vary the duty cycle of the solenoid and the control pressure is modulated. With baseline pressures set, the system is tuned with a voltmeter and a dwell or oscilloscope for 50% valve operation at 0.6v at the sensor. I'm sure that the system could be adapted if someone really wanted to put some thought into it. I made a 'brain' for one using essentially Radio Shack parts. A few 4558 op-amps as comparitors and a 556 dual timer, that kind of technology. Does anybody remember analog?

It was most interesting that the CIS, when balanced correctly, really needed little or no correction from the electronics throughout the load range. Getting the pressures "just right" requires a good analyzer. I used a IR analyzer in the car for a while to test under extreme load. A chassis dyno would be much easier, but it's harder to find anything other than the inertia type (Mustang/DynoJet) these days. Since you need to replicate steady-state conditions, the Dyno-Jet type is almost worthless for this duty. Options today include some of the data recorders that attach to a wideband oxy sensor, so you can do your rolling tests without having to watch the meters on the analyzer. Bottom line is that getting a particular car perfect is not impossble. Getting a 'universal' set of pressures would be.

If you 928 is starting OK when cold, warm and hot and you are getting less than the 13 to 18 mpg, start looking for other problems. Spark plugs, wires, partially plugged cat, low compression, incorrect tire pressure, etc all come to mind.

Concerning the CIS in general. Porsche used the most basic version on the 928. Later versions, with upgrades, were used by MB through 1995 on their V-8's. Besides the Lambda upgrade, later CIS used individual adjustors for each injector which provided much better fuel economy and fine tuning.

Dennis

I always thought about adapting a volvo lamba sond system from a late 70's 240 onto the 928 cis system.Pretty sinple system...throttle switch,coolant temp switch,O2 sensor,couple of relays,simple stand alone computer and harness that could be lifted right.It really only needs to be set up to trim fuel mixture on study state cruise conditions.Just walked past one last saturday when I was at the local pick and pull

When the 83 Euro I have was Federalized they added an O2 sensor, a little box under the dash and an injection in the WUR line. Since I have never driven this car I do not know if this helps mileage, but if you are looking for a feed back point for some modification this might be it.

Dan the Pod Guy

Dan,

The gas mileage improvement was about 20-25% on the 924's when they implemented the lambda system in 1980. It also helped the HP rating by deleting the air pump. Conversion is pretty simple in that most is just plug and play. O2 sensor, ECU and harness are the easy part. Installing the frequency valve in a bypass line between the control pressure line and the fuel return line are the hardest part.

Dennis

Well guys, I know a lot of you guys have pockets full of money and may not notice $38.00 every 3 days. Now its more like almost 45.00. I drive my 928 cause I have never found a car I liked any better. I maintain it myself and enjoy working on it. I also have what Dave calls a rev limiter that i live with who thinks i should get a Prius. I have driven them and like it ok for her:) I would kill myself in one of htose darn things. I cant wait to see what European Performance has to say tomorrow.

Thanks for all your help. I should also point out that folks like Dennis, Brad and Jay K. had a huge hand in helping me get the car as good as it is.

Frank,

Instead of a rev limiter, I believe you have a voice activated automatic speed control (VAASC). Mine is intermittently installed in the passenger seat. :)

Dennis

Results:)

I took the car to European Performance today and they checked it out pretty well. The results, they couldn't believe we had it as close as it was. co was 1.6%. They recommended that i close the spark plug gap to .29 and maybe if anything enrich it a little. They got rid of a nagging vibration between 70 and 80 and now it is smooth as silk. The mileage is 13.5. I am going to try their suggestions and maybe rebuild the fuel dist sometime in the distant future.

Thanks for all your help and input. You guys proved that you are the real experts:)