Air fuel ratio

[Christian 3]

Hello everybody,

Last winter I did a partial engine rebuild on my euro Carrera 3.0
I installed SSI heat exchangers and SSI 2-in-1-out end muffler.
Recently I had the car on a dyno. The engine produced exactly 200hp, which it should have stock. (Old dyno test, 212hp stock config) The max torque in mid range was very marginally increased by 8 Nm, but not in a good looking curve. A bit disappointing after all the stories about SSI’s. In mid range and partial load the engine feels a little like there are some misfires.
The dyno expert told me the car was running too lean under partial load and too rich under full load and send me to a CIS expert. The CIS expert told me the SSI’s are the cause. In his opinion the CIS is only working 100% accurate on the original exhaust. He could only adjust the mixture richer or leaner over the whole range. Not partially.
He might have a point, but on the other hand it must be possible to adjust something to let it run better. I never heard other people with this problem.
I checked the CIS for leaks. Injectors were cleaned and tested, seals replaced. Timing is according to specs. Idle mixture is set.

Does anybody have an idea what the problem might be?

Thanks,
Christian

[mradovan]

Hi

Here is a link to an extensive discussion on this topic related my CIS 911SC. I have modified the WUR and added a wideband O2 sensor. Full rebuild with SSI and 964 cams. No Dyno data yet because the gearbox is apart for a complete refresh and upgrade.

I am using the warmup circuit but it starts really rich because I don't want it lean when warm. Next go around I plan to unplug the WUR and adjust it so it starts rich enough not to backfire and it's happy at idle when warm. Full throttle enrichment can be adjusted by modifying the control and supply fuel pressure as discussed in this thread.

I highly recommend adding a wide band O2 so you can log data. There are graphs in the thread if you log in. It's long so I don't want to cut and paste it here.

Rennsport Systems Community • View topic - Optimal AFR for 1980 911SC

FYI my car runs open-loop below 2500 rpm. If you are lean it is because you are too lean at idle.
Over 2500 rpm but not wide open you can see the lambda system working in graphs. Full throttle the system goes back to open-loop. To enrich this you can raise the supply pressure by shimming the full pressure regulation circuit.

[Christian 3]

Hi mradovan,

Thanks for your reply.
As I understand most of your adjustments are done in the closed loop. My engine is a euro ‘76 3.0 that has no oxygen sensor and no frequency valve.
Is there anything else to solve the problem of running too lean art partial load and too rich at WOT?
Spec CO at idle is 1-1.5 but they put it at 2-2.5%.

Best regards!

Christian

[mradovan]

Christan,

Duh! Long day yesterday. You are correct. My SC is the first year if the lambda system. I not sure but I would guess that the full-throttle enrichment adjustment for your car would be adjusted the same way as in my car. If I am correct you only have 2 possible adjustments. The idle enrichment is done by turning the adjustment screw and the system fuel pressure which is done by changing shims under a spring assuming everything else in your system is working correctly. THat could be a big "If" depending on the history of the car

You are probably are aware that this system doesn't like dirty fuel. I still recommend installing a wideband O2 sensor. It comes with a meter so you always know the AFR and it allows data logging. If you are going to try and fix this it is very helpful. You also will need a CIS pressure tester. That topic has been covered on this site by others

Good luck

[Christian 3]

Thanks.
I’ll start checking the CIS pressures and WUR. I hope I’ll find something. As my engine is stock, except for the SSI’s, I guess I should have less difficulties than you had, with the 964 cams.
Dit anybody else had AFR problems or made adjustments to the CIS after only installing SSI’s?

['78 SC]

Originally Posted by mradovan

Here is a link to an extensive discussion on this topic related my CIS 911SC. ... There are graphs in the thread if you log in. It's long so I don't want to cut and paste it here.

Rennsport Systems Community • View topic - Optimal AFR for 1980 911SC

Unfortunately the graphs (and Steve W's comments) are not visible to unregistered users and the registration process is broken. Any chance you could repost the information? Thanks.

[mradovan]

I will look to see if I can dig up the document. Gvie me a day or two and I will see what I can do.

[targa72e]

HI Christian, I think you need to take a step back and look at your different systems. Remember your engines output is a sum of a bunch of parts. Each of these parts can be turned to work together for a desired result or work against each other.
Based on what you posted I will relate some of my experience in tuning SC's with CIS. Most of this information was gathered over 15 years ago. First you mention you did a partial rebuild and added SSI's at the same time. The dyno chart below (dynojet chassis dyno) is from the same car (78 sc big port with later 9.3 to 1 pistons, CIS and SSI's and early muffler modified with two outlets) with the cam timing at far end of advance spec for SC and far end of retard. I originally set the cams for best top end power not knowing what I was giving up everywhere else. Ended up running cams in most advance spec.



I am going to guess you did not measure where your cams were set when you did disassembly and set them at spec when rebuilt. This can significantly change your power output and mid range torque.

In adding the SSI's you also changed the muffler. SSI's are just headers covered for heat. I have no idea how well your new muffler works. I tried many muffler options to find something close to megaphones. The early muffler modified with two outlets was hard to beat. I also ran a Dynomax (lighter) that many on this forum have used. It also worked well but I had issues with it cracking. Below is a dyno of another SC that had headers with two glasspacks straight out the back. You would think these had very little restriction as you could see thru them. The higher dyno number is when the glasspacks were replaced with straight pipes. Mufflers make a big difference




The stock CIS system is pretty adaptable, but not necessarily the best for power. If more air is taken into the engine then the throttle plate will move higher providing more fuel. You can play around with control pressure to tweek the curve. When I went thru my CIS system I did the following. Verified all fuel injectors flowed the same over the output range (they did not). If one injector is providing more or less fuel than the others you will never get things tuned correctly. Next I verified that the flow to each injector from the fuel distributor was the same. I did this by using 6 graduated cylinders to collect fuel output and lifting the air flow plate to simulate air flow. The flow from the fuel distributor was also not equal. Once the fuel was equal to all cylinders I played around with control pressure. Then I worked on ignition timing. The graph below shows my car and another similarly built and tuned local car. Power output was almost identical.



I have some other dynos of nationally competitive PCA stock class club race cars and they look about the same. With a car running stock CIS, Stock heads and headers with a good muffler and correct tune this is about what you can expect for output.

john

[AndrewCologne]

Originally Posted by Christian 3

Hi mradovan,

Thanks for your reply.
As I understand most of your adjustments are done in the closed loop. My engine is a euro ‘76 3.0 that has no oxygen sensor and no frequency valve.
Is there anything else to solve the problem of running too lean art partial load and too rich at WOT?
Spec CO at idle is 1-1.5 but they put it at 2-2.5%.

Best regards!

Christian

Seems your air sensor plate is out of adjustment. A typical bavavior you're describing.
Squeeze the spring a tiny little bit more so the plate rests a very little bit lower. Re adjusting the initial CO is needed after that. This makes the plunger in the FD moving a bit higher at the same amount of air at partial load. At WOT this won't affect the AFR that much as the enrichment isn't linear due to the significant diff. angle of the air plate sensor funnel at WOT.

[Christian 3]

Hello John,

Thanks for your extensive reply! Indeed I didn't check the cam timing before disassembly.
My engine is a little different from the SC's. Compression is 8.5. Ignition timing is more comparable to the 2.7. Cam timing is a bit more retarded than most SC's, causing it very happy to rev and peaky from 4000 rpm. I like to keep it that way. (set cam timing just a little before middle of spec, to allow for chain wear) However, an overall improvement of torque is welcome. (main reason I got the SSI's) I'm surprised that you had such a big difference in power and torque at different cam timing. BTW, is it flywheel hp?
Further my muffler is SSI brand, made by DANSK, comparable to the DANSK muffler, of which I heard good reviews. I like the original look of 1-out.
I guess I'll start with checking the fuel pressures and compare delivery per injector.

@Andrew:
I checked the throttle plate. According to most drawings, the plate is flush with the beginning of the upper cone, on the side of the arm. On the opposite side it's slightly lower.
In my case it's the other way around: About 1mm below the beginning of the upper cone on the side of the arm, and flush on the opposite side. This is the only way I can prevent it from protruding in the upper cone.
However I don't understand why the throttle plate (and plunger) would be in a different position under partial load, when its position at idle is changed. The position on partial load is dependent on the airflow through the cone, where it doesn't feel its idle stop. Or do I miss something? I'm just a beginner.

Best regards, Christian

[AndrewCologne]

However I don't understand why the throttle plate (and plunger) would be in a different position under partial load, when its position at idle is changed. The position on partial load is dependent on the airflow through the cone

Cause the screw depth makes the lever to the plunger lifting more .... or .... less – at that "same" amount of airflow.

You cant measure the corrcet position of the senor plate with a sense of proportion, just 1/8 of turning the screw already affects the partial load significantly.
Prooven by many testing with AFR sensor output.

[Christian 3]

Andrew, now I understand! After adjusting the sensor plate, mixture at partial load is affected because the CO screw is adjusted as well. Thanks!

[AndrewCologne]

Btw. don't expect that much from the SSI "+20HP myth".

Here is a "mas o menos" curve where you can see a comparison.



Look in my signature there is a thread with also nice infos independent from a 930/16 engine

[targa72e]

HI Christian,
The dyno jet is a chassis dyno so all number are at the wheels. I have probably done 100+ runs testing different things, engines cars. Dyno time has been best bang for buck I have ever spent. My experience with cam timing on multiple SC's is that retarding the cam timing only causes you to lose torque. On the dyno I posted you will notice the most retarded and advanced will still make the same HP at around 6300RPM so no HP power loss on stock spec motor. The early US SC's had big ports and runners and 8.5 to 1 compression so similar to your car. The later cars were 9.3 to 1 and had small ports. The later 9.3 to 1 pistons (or Euro 9.8) with big ports and runners and cams at full advanced is the most power across the board configuration from much testing.
I bring the muffler up because I tried many on the dyno. Many were very popular at the time. Most cost quite a bit of power. If you tune to max power with megaphones and then add a muffler and retune you can see how much HP that muffler cost.



Here is a 79SC 8.5 to 1 compression motor with big ports and runners, headers and megaphones. Starting point was factory timing on cams and ignition. Advanced cams and ignition , CIS flow equalized is higher end result.




Comparison same spec SC similar settings 8.5 vs 9.3 compression.

john

[Christian 3]

Thanks for the comparisons. It always helps if you don’t have to figure it out all by yourself by a lot of trial and error. I understand a stock engine will have most gain from cam timing and the right muffler. Anybody experience with the 123 ignition distributor, that lets you fully adjust the ignition advance curve?

[AndrewCologne]

Be aware that Euro/ROW cars already come with curves where premium gazoline is needed.
Without lock limited engine design a too agressive advance curve can cause a huge damage to your pistons.