DME, Dyno and Air/Fuel ratio

[ewave]

Hi,

I spent the day playing with the air fuel ratio, and doing dyno runs. Here are the results:

Engine: 1989 3.2L Carrera Engine Euro type 930/20, with stock 10.3:1 ratio compression. Euro pre muffler (No catalytic converter) and I am running code from the "Low quality fuel" DME part number 911.618.111.19. The weather in Dallas today was about 60 deg F, dry and sunny. All dyno runs were done in forth gear. Everything is stock for a Euro car.

I did experiments with the fuel quality switch in position 0 and position 1. Position 1 is listed as 3% rich and standard advance.

In the + 3% rich setting, there was a fuel increase at idle, at part throttle, and at WOT during the dyno runs.

I did a total of 4 runs: Two in the 0 position, and two in the + %3 percent position. I'll post the graph later- I have to borrow someone's scanner.

Remember this is the DME for "low quality fuel". I was running US 93 octane (R+M)/2 gas. We also monitored the Air Fuel ratio during the dyno pulls:

The peak HP was with the fuel quality switch in 0 position was 196.2 HP at 5900 rpm.
With the switch in the + %3 the max hp was 191.0.

This shows that maximum HP was achieved in the standard position. Running richer did NOT increase HP.

Also interestingly enough, if you accept the 15% drive line loss then 196.2/0.85 = 230.8 (Amazingly close the the published 231 hp number!)

The Air Fuel graph starts at about 14.5 at 2700 RPM and drops to about 11.5 at 6200 RPM for the 0 position.

For the + 3% position it starts at 14.1 at 2700 RPM and drops to 10.8 at 6200 RPM.

I'm guessing because this is the DME for low quality fuel that it intentionally runs way to rich at high RPM and max throttle to help avoid detonation.

I wanted to do a set of dyno results BEFORE I put the cat on to see how much HP is lost. (Unfortunately with the new rolling dyno testing in Dallas I can not get my car to pass with out the Cat.) The Nox number was about 20% too high. All the other numbers were in spec.

I'll put the Cat on in the next few days, and also try to get a normal DME 911.618.111.18 (I'm still looking for this EPROM hex dump if anyone can PLEASE send it to me?) for the 10.3:1 ratio engines to do a second set of dyno runs. I'll post the results, and then we will know for sure how much HP the cat really robs, and also what the HP differences are between the 111.18 and 111.19 DME's.

[chuckr]

I have good friend that has an 89 911, and he has a K&N cone filter, a Autoauthority Chip and a B&B exhaust and no cats, he gets about 210 on a Dynojet Dyno, for a Euro car, you are a bit on the low side

[Jim Smolka]

It makes sense that as the Fuel to air ratio decreased at WOT. The 'most' HP is generated when the ratio is about 12.5 to 1.

The O2 sensor sets up the idle ratio for about 14.7 air to 1 part fuel.

I am not sure why your system is running so rich at WOT???

[Bill Verburg]

Great post!!

That makes pefect sense. According to all of the Bosch tech lit I have ever read, max hp will always be achieved at Excess-Air factor

l = 0.95 &#133 0.85 which corresponds to AFR 13.965 &#133 12.495

• l = 1 Stoichiometeric or theoretical optimum
• l < 1 Air deficiency or rich mixture l = 0.95 &#133 0.85
• l > 1 Excess air or lean mixture l = 1.05 &#133 1.3
• l > 1.3 The mixture ceases to be ignitable

Bosch further recommends l = 0.9 &#133 1.1 (AFR 13.23 &#133 16.17)as providing the best results in actual practice

[ChrisBennet]

I recently spoke a Porsche engine builder and he said no, these cars do not make the most power at 12.5:1 - they like it leaner than that. He also said that different types of motors behave differently. 12.5:1 may work fine for some motors but it isn't ideal for a Porsche motor.
-Chris

[Bernd Andritzky]

Hi all,

i do agree with Chris, Bill and Jim, best hp output of that engine is at about Lambda=0,87-0,9 (AFR ca. 13). So your engine even runs a bit too rich with the stock setup (maybe thatīs necessary for the inner cooling) and so a further enrichment of 3% will only make You loose more power.
Please send me a email to my home adress info@an-tec.com then i will send You the DME 911.618.111.18 EPROM hex dump.

Regards, Bernd

[AES]

Doesn't the excess fuel also contribute to premature valve guide wear?

[Wil Ferch]

We should get "SteveW" to chime-in on this post...specially as he collects hex dumps of various chips...which he has used to good effect over the years to provide probably the best 3.2 chip on the market..IMHO. I had him burn me a custom chip for my slightly modded 3.2...and he offered me two chips to try...the first well matched to my minor mods....the second running yet richer at 4500+rpm...close to his SSI chip. I have a USA spec car...and prior to installing his chip I got 200 Rear wheel HP on a DynoJet 248C, suitably correctred to read SAE hp ( removes *that day's* ambient temp affects and normalizes the run for later comparisons to other runs). Once I get the car out of hybernation. I intend to dyno the car again, with both chips. Since that original dyno run, I've also added a pre-muffler in lieu of cat, so I am hopeful for good numbers ( which I think are good already running stock manifolds, with a 9.5 CR USA spec car, running a cat to get 200 rwhp !!).
--Wil Ferch

[Rick Lee]

Excess fuel can mean big problems. This week there was a long post on this subject, albeit on an MFI car. Rich mixture is what killed my last 3.2. More specifically, gas will dilute the oil and kill your rings and maybe even cylinder walls. My new 3.2 is a lot more powerful with the Webcams, but the gas mileage sucks. I have rich-mixture-phobia now and would love to be able to toy with leaning it out a bit. Tomorrow I plan to go for a long hard drive and then try to ignite my dipstick with a cigarette lighter. If it lights, then I'm gonna be really upset.

[930fan]

Would heat soak affect the second run's results? I thought I read somewhere that 911 engines don't cool properly on the dyno so several runs in succession w/o a cool down period will adversely affect the numbers even though you don't change anything.

[Peter Kelly]

Severl builders of race motors (911 air cooled) have told me to shoot for 12.8 A/F.

I have autothority MAF plus other mods on my car, and mine goes real rich (11 on a hot day) above 5700 or so.

Soo..., rather than going though all of the burn chip/change chip/run car cycles I am modifying my motronic so that I can re-program it on the fly (just like a real ECU ).

If anyone else is interested in this mod email me though our web site www.competitiondata.com

-peter