Porsche 911 ST - Racecar - AFR Measurement

[Olsen911]

Gents.

New MFI pump and trying to dial in the AFR on the Porsche 911 2.3 ST racecar.

The car is pulling and running strong, but as always running a tad to "fat", so this time i am trying to connect 2 AFR gauges - 1 for cylinder 1-3 and 1 for cylinder 4-6.

Looking for 12.9 - 13.1 AFR at Peak Torque and Peak HP.

HP: 200+
Max peak torque at: 6100 RPM.
Shift at: 7100 RPM:

Here is my video- Press link:

https://youtu.be/Cd1SNQGQI6g

Please make sure that the quality is set at 720 P (HD) and full screen in order to see the figures on the gauges.

Sorry for the poor sound.

I am taking 3 examples in the video, measuring the AFR:

1: Left: 10.9 - Right: 10.7
2: Left: 10.8 - Right: 10.5
3: Left: 10.7 - Right: 10.4

Was looking for 12.9 - 13.1 AFR, so it´s running to "fat", but how "fat" is this.?

There is a small difference between cylinder 1-3 and 4-6 about 0,3 - Is this something to worry about.?

Let me know.

Thanks - Cheeers - Olsen911 - Tommy Olsen

[turbobrat930]

How fat?? Well, I have always been told ( on a 930 turbo engine that is highly modified with aftermarket EFI) that in high boost settings, 11.0 to 10.6 is good)... If your is normally aspirated, I would say you are quite rich. Is it rich enough to wash the cyl walls? I do not know. But, I would think on a N/A engine, you would be hurting your power output with a AFR setting that rich.

More qualified people are sure to chime in....


Also, do you have a knock sensor (knock box) on this engine? Could help as well, as far as tuning

[jpnovak]

way too rich. lean it out to your specs.

Start with adjusting the entire rack. Though, depending on your cam spec you may need some custom tuning.

Here are some links that may be helpful .

MFI Open Heart Surgery III - Reworking the Space Cam

MFI Pump - Open Heart Surgery

[Olsen911]

Gents.

So went on the dyno today, trying to lean out to my specs.

Before i had perform all the steps in the CMA, so the only thing that was missing was a nice curve to WOT at 12.9-13.2 and then idle.

First run on the dyno shows baseline at 221 HK at 6300 rpm. And AFR showing 10.6 at WOT and 9.6 at idle. About 950 rpm.

Then we lean out on the main rack - 4 clicks lean.

Second run on the dyno shows 225 HK at 6500 rpm. And AFR showing 11.6 at WOT and 10.6 at idle. - all good but now the idle went from 950 rpm to about 1200 rpm. What.???

Now the dyno guy wanted to adjust the pushrods on the trottle bodies to get the idle down, but i told him no, let's go on with the AFR on WOT and then look at the idle, like the CMA says.

Then we did a serie af runs where we adjust the main rack as close to specs. as we could. But every time we came close to the specs. The idle lean out and went up and up.

So now the dyno shows 229 HK at 6900 rpm. And the AFR showing 12.6 at WOT and 17.6 at idle and idle at 2000 RPM. I didn't want to lean out more, before i had idle under control.

So now we did start to rich up the idle with the idle low rack. And this helped with AFR that went to 15 in idle. but still about 1200 rpm.

So before we did any more with idle i did a run to see if this had chance anything in the WOT. And just as i thought the AFR had gone from 12.6 to 11.6. At WOT.

So everytime we lean out the at WOT. The AFR went up in idle and the RPM in idle.

What a nightmare and the dyno guy had alot of suggestions to how to chance this, but he wasn't a P guy but all Mopar. So i didn't wan't to go that way.

I just love my MFI system, but this thing is so complicated and nothing like understanding and adjust Carbs.

Last run we made 228 HK at 6900 rpm. And the AFR showing 12.6 at WOT and about 15 at idle and idle at 1200 RPM.

We did stop there, because i belive that was the Best from both worlds.

Is this how it is.? Or is i doing something wrong.?

The pump is 2.2S all new Exchange pump with ST cam.

Let me know what you Think.?

Thanks - Cheers - Olsen911

[dicklague]

I have been fiddling with the MFI in my 1973 for years and have it working well.

A couple of comments. Data logging is the best way to do this. Does your AFR have this or do you need to video things?

I have data logging on my permanently installed AFR. It records AFR, Vacuum, and RPM. I can chart it and even do excel spreadsheets and see what kind of RPM, load and AFR I have. I use a Daytona-Sensors WEGO IV. It reads both banks.

I think you need this level of detail to know what is really going on.

One trick that really helped me was replacing the warm up system with an adjustment screw. This allows richening and leaning across the whole RPM spectrum.

You are lean, but not too bad. for best power and cooling, you want 12.5 to 13.5.

[Flieger]

I'd say your setup sounds good. Mine idles high but it's not a daily driver. Usually about 1500rpm and I don't mind. Runs smoother there than at <1000 anyway.

[tobluforu]

Its a race car, not sure I would want to be that lean, but I don't race. What I mean is that when you are racing that car its going to get way hotter and that will probably lean it out a tad.
Idle, since its a race car idle doesn't mean much, but you can always synch the stacks and drop it down with the idle adjustment screws.
Many more experienced users on this board that race with mfi cars and will have more to say, might want to also post in the race board.

[kent olsen]

Contact Mark Jung at 356rs@peak.org. He is the west coast expert when it comes to MFI. He rebuilds MFI with attention to idle, mid range and WOT.

Kent