A\F ratios on a 72 mfi

[tobluforu]

I just bought a wideband a\f meter. I bought it for my other cars, but I might as well use it on the 72. My first question is where should I weld a bung onto the stock heat exchangers? Has anyone done this? If so do you have a pic?
For a 72, what is a good a\f ratio for WOT and idle? This car has RS pistons and Ecams.
Thanks all.
P.S. Will I be wasting my time trying to set this up on this car?

[tobluforu]

Bump.

[jluetjen]

Tobluforu;
I think that this sounds like a great project that apparently hasn't been done before. I'm not expert, but I believe that you want the O2 meter right at or immediately after the collector. That way you will pick up data from the whole bank as opposed to one cylinder. Aside from that I think you want the meter as far upstream as possible (as opposed to down by the muffler for example) so that it gets up to operating temperature as quickly as possible.

Since no one answered your question about good A/F ratios, I can only suppose that they don't know, but I doubt that they are different then for any other engine. Why not weld up the bung and get some data and then let people chew on that for a while.

Great project!

[304065]

Well, we know for starters the range of AFR from best power through stoch all the way to lean.

And we know what the factory recommended at a variety of temperatures for check measure adjust. AND we know what we really use, 5-6% CO at Idle, 7-8% at part-throttle.

Now all we need is a table correlating AFR against CO level.

http://www.perfectpower.com/technical_info/afr.asp

Voilla, you want an AFR of: 12.2.-12.6, 11.6-11.8 to 1.

Now that sounds AWFULLY rich to me, but that's what the numbers would suggest.

I would be interested to see your empirical data: when the car is running right, wonder what the numbers are.

I've got half a mind to buy the wideband AFR box and hook the output up to one of the variable voltage inputs of my DL-90 data logger.

Then again, I would probably LOSE a second a lap carrying all that data acquisition equipment around! But as the Germans say, "Zere is no performance vissout measurement!"

[Lorenfb]

The torque curve vs the A/F is fairly flat from 11 to 14, where max torque occurs
at 12.6. The corresponding CO should be around 3.0 to 4.5% max. Much more
than that becomes wasted fuel. Given the imprecise nature of MFI, it'll be difficult
to maintain an optimum A/F over the total torque curve without some feedbeck
type of control system, e.g. closed loop electronic fuel injection. So the A/F for
the MFI system needs to be set at the RPM where the engine will be operating
the majority of the time.

The MFI type of fuel injection system is very simple and as such is referred to
as an alpha-N (throttle position angle, & RPM) system. This type of system
(alpha-N) is used on race type electronic fuel injection systems where a TPS
(throttle position sensor) determines the throttle angle, but also uses a wide
band O2 sensor for feedback to optimize the A/R over the total torque curve.

[Grady Clay]

I recieved this PM and think it is most appropriate here.
Sorry tubluforu, I don't mean to hijack but this is a good combined thread.

QUOTE
MFI Air Fuel Ratio
Hello Grady,

I've read your posts with much interest as you seem to have a wealth of knowledge regarding Mechanical Fuel Injection. I've learned quite a bit since I installed MFI on my 69 911.

First let me give you some specifics on my motor.....72 2.4 T motor that has had the cylinders bored to 86mm and JE 9.5:1 pistons installed. Original MFI pump has been rebuilt and recalibrated to "S" spec by Hans at H & R. Original T cams were reground by Elgin to their "Modified S" profile. Motor was completely rebuilt. Heads, ports, throttle bodies, and air stacks are all stock "T" spec. Throttle bodies while not new, appear to be in decent shape.

Several months ago, I purchased a Gunsen CO analyzer to help me adjust the mixture. I can get the car to run well, but it is very rich....according to the anaylyzer CO at 3200 is 8.9-9.1. CO at idle seems to like 4.5-5.0. Heeding your warnings about oil dilution with mixtures this rich, I began to suspect the accuracy of the Gunsen analyzer. I just purchased an LM-1 air fuel analyzer and I'm confused by what I'm finding with the readings. AFR at idle is running about 11.9(pretty rich). WOT throttle AFR is running about 13.2 from 4000 RPM to 5500 RPM. Cruising AFR at 3000 to 3500 is in the range of 15.5 -16.0(pretty lean).

Do you have any reference for what AFR should be on a well tuned MFI? Secondly, should I be concerned about the lean condition at part throttle. I've just moved to Phoenix and oil temps are getting high so I don't want to compound the problem by running too lean. Any help you can provide would be appreciated.

Thanks much,

P.S. I've been thru CMA numerous times, and I believe everything is set correctly. I just rechecked the throttle plates last weekend again, and have them all reading the same at idle.

Pelican '69 911T
(name changed)


REPLY
Pelican '69 911T,

Can you invent a way to test drive with both meters? The CO meter will need to have the gasses cooled with a long (copper?) tube. Since the CO meter probably uses 115VAC, you will need a suitable 12VDC-to-115VAC inverter.
Remember the LM-1 AFR meter will react quickly and the CO meter very slowly, depending on the length and diameter of the tube.

Be careful to vent the CO meter gasses to the outside. Carbon Monoxide is an insidious toxin. It attaches to the hemoglobin in your blood, displacing the oxygen, and won’t easily let go. You don’t know there is a problem until too late.

Where did you mount the LM-1 sensor(s)?

What muffler do you have? Is it internally clean?
Stock intake air filter assembly?

Do you have a front cooler? What kind?
What is your engine cooling fan & ratio?


OK, on to the engine specifics.
Your increase in displacement is only 2.4% - negligible as far as the MFI is concerned. The change in CR won’t affect the MFI – you just need to use high octane gasoline.
The difficult thing to predict is the combination of the ModS cam and S-space cam pump with the “T” intake runners (stacks, throttle bodies, and intake ports.) I would think the disparity between what the pump supplies and what the actual air flow is would be greatest in the WOT 4500-to-redline and would be rich. If this is the case, and you set this mixture for satisfactory (safe) running, then the spec part-throttle mixture would be lean. Of course you can make the idle mixture anything you want, semi-independently.

Clearly it will help to have both CO and AFR measurements simultaneously. Can you borrow a data logger?

I know you have CMA several times.
Are you absolutely certain that the thermostat turns off fully each time?
What are the cylinder leak numbers?
What are the cranking compression numbers?
What are the cam timing specs? Where are they actually timed?
Are you absolutely sure the pump is in time and not 360 out?
What distributor curve?
What is the ignition timing at idle and 6000?

I don’t think you have a unique situation given the high cost of S stacks, throttle bodies and head modification. It would be easy for me to recommend you “just” replace the stacks, heads and throttle bodies.



Best,
Grady
END QUOTE


It is very desirable to post on Pelican 911 Technical Forum. There are several others who can help shed light on this and many who will benefit.
If you need, please e-mail me at gradyclay@hotmail.com. PM is cumbursome for me.


G.

[tobluforu]

Wow! great info from all. Thank you very much. No prob on the hi-jack, great info there as well. Although those ratios sound rich on most cars, for MFI that is probably right on. About everyone you talk to that has been working on mfi cars state to run the car "Fat". Placement might be a little tricky because there is not much room at the collector after the Heat exchanger and before the the muffler. I'm not sure if the wide band has enough wire to reach the front cockpit from there. Might have to have the girlfriend in the back holding on to the roll bar and working the meter.

[Lorenfb]

The key is that the A/F ratios MUST be checked under load conditions, most likely
using a dyno. Checking the A/F at the various RPMs under no-load is only a rough
first check. The use of a dyno provides the acid test of meaningful A/F ratios.

[rs911t]

Well, I bought the bungs for just this kind of project. Of couse, they've been sitting on the bench two months now. My plan is/was to drill and
weld one to each heat exchanger before it goes into the muffler.

John, I have wired a LambdaBoy wide band AFR to the DL-90. Easy to
do and works just fine. The problem was the AFR reading from sticking
the probe into the muffler outlet.

[tobluforu]

Loren, much is stated on how good dyno's are and how bad they are as well when they come to a\f. I plan on taking readings while under load on the street. Dyno's have a tendencie to show somewhat leaner conditons a\f wise. I also want to see what a\f is while idling(does not mean much, but what the hay). Don't get me wrong, dyno's work very well, but it's under load on the road that works the best. And, that is what i plan on doing. Rs-do you have a wide band that collects data from both banks at the same time? I hear that is the way to go, but they are also expensive as hell.
P.S. I bought this for my very modified turbo saab and blown viper. I figure that since I have it, I might as well check out the porsche as well.

[Lorenfb]

Monitoring the A/F ratios while driving will work. This requires a lot more data and
analysis, though. With the dyno you can quickly check all the data points and not
leave any "holes" in your torque curve.

Check out this web site ( www.systemsc.com ) on the Graphs page for info on
A/F ratios and CO.