Whats the ideal Air Fuel Ratio

[cmcfaul]

73 911 E with MFI
DC 30 cam grind on E cams
JE 9.5 / 1 CR

Following are my readings

Idle - 1000 RPM's - 17
65 MPH - 3200 RPM's - 12.5
WOT - 2500 RPM's to 7000 RPM's -15

What's the consensus. Too lean at WOT?

50 degree day
car fully warmed up to 180 degrees (as hot as it would get)

Looks like I am lean at idle
a bit rich at cruising speed and around town
Right on at WOT

My thoughts are, as the outside temps increase it should run a bit richer as the air thins out.

The dc 30 cam grinds lower compression a bit but improve VE thus reducing a detonation condition.

I already added two click of enrichment to the idle circuit. Am I too lean elsewhere?

98% daily driver. Maybe two DE days per year

Thanks for any advice,

Chris
73 911 E

[Joe Bob]

14.7 has always been the optimal read......your variances can be attributed to vacuum leaks, timing and/or fuel delivery.

[KTL]

Too lean for HP production, however 15 is not "bad" for MPG consideration since it's right on the button for 14.7 stoich ideal like Joe Bob Z said. Lean idle is not a big deal, as there's not a lot of danger there since you're not under load. If you do a lot of cruising around, you'll not like your gas mileage.

Just out of curiosity, what'd you use to measure the A/F ratio?

For those of us stupid enough to race these things, the A/F in the mid-high 12's is where the power is at, as well as safety from detonation. 15 all day at a DE is not what i'd be shooting for.

[aigel]

I would not want to see anything over 14, especially at WOT. Best power is not made at 14.7, it is only ideal for fuel economy. Also, at WOT you want to be away from any possible lean condition to help cool the engine ...

I think your FI needs serious adjustment to get optimal power and safe WOT operation.

George

[tobluforu]

Fatten up the top end first and shoot for around 12, then fool with the bottom end to get it around 14.5. MFI's love it a tad rich. Better safe than sorry.

[cmcfaul]

OK, I added two click of richmment to the idle circuit last night. Will that have any effect on the other readings?

I will add on click of richment for the upper end. Will that be enough or should I do more?

I could enrich everything by adding a turn to the linkage that pushes on the MFI pump. Is one metheod better than the other? Since I think I am close with now just a bit too lean at WOT I am leaning toward a click or two on the main enrichment screw.

I borrowed an LM2 for a couple hours to get these readings.

I just completed a rebuild so there should be no air leaks except through the throttle linkage.

Thanks,

Chris
73 911 E

[David]

[cmcfaul]

From the chart, I am leaner then I thought. Will I see HP increase by enriching or just a safer, less detonation prone environment?

I always thought the leanest environment, without detonation was the best (faster fuel burn).

thaks again.

Chris

[MBruns]

Chris, for 911,s MFI and carbs for safe performance we shoot for 12.9 to 13.1 at peak torque and peak hp because the 911 is cooled by fuel as well, you may make a little better hp at a touch leaner # but on the dyno it may be for only 1 or 2 pulls, the above AFR is safe and proven, as with mfi and carbs it is a bit of shuffle to get the curve decent and have safe #'s at the peak demand. Did that piston work out for you ?

Mike Bruns JBRacing.com

[tobluforu]

Originally Posted by cmcfaul

OK, I added two click of richmment to the idle circuit last night. Will that have any effect on the other readings?

I will add on click of richment for the upper end. Will that be enough or should I do more?

I could enrich everything by adding a turn to the linkage that pushes on the MFI pump. Is one metheod better than the other? Since I think I am close with now just a bit too lean at WOT I am leaning toward a click or two on the main enrichment screw.

I borrowed an LM2 for a couple hours to get these readings.

I just completed a rebuild so there should be no air leaks except through the throttle linkage.

Thanks,

Chris
73 911 E

Richen up the the idle mixture last as the mid/top end adjustment will affect the a\f throughout. If the rods are setup right, do not touch them. Is the rod from the rack to the pump the correct length?

[cmcfaul]

Hey Mike,

Good to hear from you. The piston is installed. I sent it to a machine shop in NY and they were able to cut it down to size and balance everything. So far so good (my first rebuild).

The engine runs really strong. Seat of the pants HP curve says it builds HP all the way to redline. When doing the LM2 test, I ran it from 2500 to 7000 rpms in 4th and it was scary how fast it got there. Especially while glancing at the LM2.

I definitely owe you a bottle of single malt or what every your preference is.

Chris

[Flieger]

Pump rod should be 119mm center to center, IIRC.

[cmcfaul]

The problem with following the CMA to the letter is that I modified the cam slightly. So unusual ajdustmetns might be needed, like a slightly longer pump rod.

Chris

[911st]

My two cents.

I agree standard set up procedure might go out the window but would be a good place to start.

And that it is probably best to try to get the WOT AFR right first. Shoot for the high 12's using the rack adjustment.

Then see if you can get your idle to somthing in the 14's or even 13's if you have a flat spot off idle. You can try using the pump's idle adjustment, air ballance screews, and even the throttle stops to effect this. Just do not go to far with the stops.

You might even be able to fine tune idle speed some by playing with timing a bit. Just do not let your WOT timing get to advanced.

Playing with the rod length can be done if all else fails. This may have the most effect on idle fueling and how fast fuel comes in off idle. Would not expect it to effect WOT fuel much unless you really change the length a bunch.

You could try a couple of turns on the shaft and see what effect it has. I think it bassically gets you further down the acceleration ramp side of the space cam faster. It also can have an effect on cruse AFR.

I am rusty but cruse AFR I think is at somthing like 2500rpm. Do not expect to get into the 14's at a higher RPM. It would probably have more like an acceleration AFR at at higher rpm's under cruse load(13's to 12's).


If you want it any better than what you can get doing this, call Gus and see what he can do.

[Joe Bob]

Ahhh, you didn't mention the fuel pump cam mod......I was going ask if it had been checked for spec.

MFI has never been known for fuel economy and as a previous poster noted, the system like being a bit rich. You might want to data log your exhaust while on a dyno or out driving. Makes changes based on your results.

MFI ran afoul with the US EPA due to high emissions at the tail pipe. Great system for kick in the butt fun....

[cmcfaul]

I am going to add a click of main rack enrichment. Anyone know what the change per click is supposed to be.

Chris

[911st]

MFI has no air flow sensing compensation at all and only limited environmental sensing ability.

At least a carb has some ability to vary fuel delivery with changes in air flow.

It is a full reference system that only inputs throttle angle and rpm once it is up to temp.

Any changes, including gearing, muffler, flairs, and spoilers can have an effect on load. Change load and the injection is still going to send the same quantity of fuel.

Same with changes in air density to a degree. Tune it on a 75 deg day to perfect and on a 100 deg track day it is probably going be richer. Go for a run on a 50 deg day and it is probably going to go leaner.

Change things like motor capacity, port size, or cam and this just adds to the fun.

The easy part is getting the right fuel at TQ or HP peak on a given day. The hard part is getting it right at both and everywhere else. Even if all systems are as originally intended.

DIY on cruse AFR adjustment seems to be a weakness with MFI. It is probably going to be very hard to adjust it without having the pump's internal springs re-calibrated. And even then, if you want to cruse at say 3200rpm because one is running 23" tall 225/50's it is probably not going to be as hoped.

One other level of adjustment could be added by modifying the lever arm connection to the pump by length and angle. This could allow changing the rate the space cam advances with throttle angle. Would only consider if building something really out of the ordinary.

From what I remember, even with the adjustments we can make, it did not seem like the correlation between AFR and the adjustment was as direct as one would think. On top of that, one adjustment usually effected more than the area one intended.

MFI is a blast but more designed for performance than highway.

Not an MFI expert, just what I belive so far.

[911st]

David,

Love the AFR chart!

WOT AFR's in the 11.8 to 12.5/1 seem proper for and air cooled N/A motor if I read it right.

We keep hearing 13/1 make more power but for how long...

I know on my 3.2 we changed the AFR's with a custom chip from about 12/1 to 13.1 and it did not seem to make that much differance on the dyno.

[cmcfaul]

Anyone know how many clicks I need to go to get from 15 to 13 to 12.5 ish?

Each click on the main enrichment rack should = some percentage change to AFR according to Bosh. Anyone know what it is or supposed to be?

Now it seems like I am a long way away.

"so close and yet so far",

Chris

[MBruns]

911st, I have been tuning 911 race engines to 12.9- 13.1 for years, and have alot of race hours at that tune up. Our 3.8 w/ PMO's runs 30 to 40 hrs. between refresh at that AFR, HSR runs 3hr and 4hr enduros at sebring with only fuel stops so properly built its not an issue.
Mike Bruns JBRacing.com