GT944 wrote:


Not quite.

What happens is there is a simple look up table in open loop mode. It figures AFM position and RPM and then determines the car needs xx of fuel to run properly. In closed loop it reads the O2 sensor to fine tune things. All cars do this. Its called closed loop because it uses and air flow or mass flow meter to predict air flow into the engine and meters fuel. Then it on the back end it measures mixture. Then it adjusts fuel flow to get the right ratio. Ie closed loop. In open loop mode it just dumps in as much fuel as it thinks it needs given AFM position, RPM and value in the look up table. O2 sensor reading is not used.

If you chip a car you are in effect changing the look up table values. Of course this can be done at a wide range of RPM and AFM positions. Most tuners chip just pick one table and everyone gets that. The right way, which is also quite expensive is to custom make a chip for that motor. This can gain you a little power everywhere, but must be done be a real pro with the car strapped to the dyno and so is costly. We do not allow chips for this reason.

Tuning the AFM is really a coarse adjustment. All you can do is trick the computer into believeing there is is a little more or a little les air flow. The computer will then add a little more or less fuel to based on the stock look up table. It is coarse because you cannot add fuel at one RPM nor take it way from another. So you just compromise on something that works for power and keeps the motor happy.

It not an issue of maxing out any part of the system as we are only talking small adjustments.

BJ is alos corrrect in that beyond cat removal are host of other issues that can tweak A/F ratio. It just makes sense to allow drivers to run a safe ratio as often times the AFM's wear too causing a shift. In the end all we can do by tuning the AFM is resetting the open loop setting to provide max power as Porsche intended.

Great thread guys! Lots of great info here. From my experiments over the last few events, I can tell you clearly that our cars are capable of delivering more than enough fuel. I was chasing down some erratic A/F ratios that turned out to be due to wear on the AFM wiper track, I cranked the AFM over 3 or 4 clicks. I then fixed the AFM, and found my A/F ratios barely 11:1. This made the car shoot flames out the back in an impressive fashion , but made it lose a ton of power. In the end, my car ended up at one click richer than stock, and it now runs A/F ratios in the low 13's.

The best method to follow your A/F ratios is to put it as an input on your Traqmate, which can plot out the values for you. This helps tune out some of the noise you get in the gauge, and keeps you from having to try and follow the A/F ratios through the rev range on the track. BJ's method is a great budget alternative, though.

I am bringing this thread back from the dead, since recent testing has shown that AFM testing is valuable, and maybe important to keep you engine running. I also got a question on this today, so I figured I would post my reply here for all to see.

First, some back ground the car tested. It finished 3rd at Nationals after heroic week of driving by Dan Pina, making 133.5 HP on the dyno. That was after fixing a vacuum leak and loose cam wheel (it made it less power before that). It is a low compression motor with a shaved head, late cam, and with about 3 seasons of racing on it. Early ECU & AFM. Seemed like a fairly average motor.

We took it to the dyno yesterday. This dyno showed air fuel ratios in the 15's (!). That's frighteningly lean. Still, it made 136 HP on this dyno. The 2.5 extra HP may be dyno variation, or the switch from 20W50 to 10W40 oil for our last event (temps in 30's!).

So, we cracked the previously unopened AFM and started moving the wheel counterclockwise. It took 8 clicks to get it right (that's huge, don't try that w/o a dyno). This resulted in a gain of an even 5HP. This is with a stock airbox, and short straight exhaust, and a modified collector. We tested a lot of things on the dyno, and this tuning made the biggest difference, by far. More on the rest soon, but this was the most interesting finding.

Tune tune the AFM (on a dyno, with air fuel monitoring, please!), lift up the spring on the black geared wheel (pictured below). Move it CCW to make the car run more rich, and CW to make it run more lean. Estimate 1/3 to 1/2 point of change per click (it's not always consistant).



Here is the before and after dyno:




These results may not be typical, but they illustrate what can be gained. They also illustrate why it's important to get your car checked out - you don't want to run that lean!!!

Great results, Eric! I've started using a Innovate LM-2. It is a wideband that does multichannel data logging. As we have started doing tuning, it has been an excellent tool for getting AFR numbers in the right ranges before going to the dyno. At under $500, it has quickly paid for itself by saving lots of time on the dyno. Now for a Spec car where you can only adjust the FQS and the AFM spring, a $500 wideband might be overkill. Then again, if a few people went in together on it and shared it might be worth it since we should be able to determine the AFR that works best for these motors and just tune to that AFR.

Speaking of AFRs, did you try running richer than your pull that yielded 141 hp? I ask because an AFR of 14.28 is still pretty lean. I should also ask if this was this an early or late car? I've been told, but haven't had a chance to verify that the late ECU ignores the AFM at WOT. And one last question, did you do any part throttle pulls? I'd be extremely interested to see how the AFM changes impacted the results outside of WOT.

-bj

Just to add to this discussion... One factor here is the great variety in pump gas we use. If you tune on strait gas (non-ethanol blend) you will be lean on blended gas. Ethanol requires nearly twice the volume of gas to produce the same net mixtures. You can do the math but in a nut shell if you have 10% Ethanol blend then you will be leaner by roughly 5%. This is often missed by tuners. I would argue that for safety you should tune on the highest blend you will likely run and that will add in some safety.

What A/F ratio makes the most power? That is entirely up to the individual car. My 993 based 3.6ltr Race engine liked 14.8:1, my 944 Turbo liked 11.9:1, my 2.5ltr BMW Touring Car liked 13.9:1. Each could require a change of several points due to fuel changes, temperatures, altitudes etc... I suspect that the low compression, poor combustion chamber design, and such of our engines would not benefit from super lean mixtures like a full race engine. I would bet that 13.5:1 to 14.2:1 would be the magic number.

Another note, my recollection is that the N/A 944 does use the AFM to redline, however if it reached the max table point (full swing of the air flapper) it is in essence only using a 'WOT' map. 944 Turbos do have a WOT map that is activated by the TPS as are the 944 S2 and 968. Tuning once a car is at the limit of the AFM or in WOT mode is now only for chip programming or other non-legal tuning methods like fuel pressure, injector sizing, temperature sensing etc....

BJ, I have a similar wide band O2 monitor in my car, and interestingly, it was showing a good 1 point richer than than the dyno was. After discussing with the dyno operator, we felt that the exhaust pipe configuration (same on both cars) may have been leading to some mixing of ambient air in, and to go by the O2 meter up by the header. If the dyno is to be believed, Dan's been racing all year with 16:1 A/F ratios, which seems hard to believe it would have survived intact. We did further richen the mixture with no benefit in power, in any case.

The ethanol issue Joe brings up is an interesting one. Unlike newer cars, our DME's are not sophisticated enough to adjust for that (especially under WOT), so this may well be an issue.