Air Fuel Ratios

[Klasik-69]

I just ran my 2003 R300 on the dyno to see where I was on power. The engine is the 2.0L Zetec with a claimed 202 crank HP. I tried to attach the dyno sheet on this but the site keeps claiming a database error. However, the dyno sheet shows an AFR reading of around 10.0 at 2500 rpm then climbing up to 14.0 at 3000 rpm, dropping off to around 13.4 with slight ups and downs all the way to 7000 rpm. Torque started at 105 @ 2500 rpm then climbs to around 139.2 @ 5500 rpm for a max. Horsepower at the rear wheels starts at 45 @ 2250 rpm and climbs to 170.1 @ 7000 rpm with a pretty linear climb.

 

What do you guys think of the AFR ratios ? (sorry for the graph being sideways.........I tried to rotate it but wasn't successful)

Edited June 7, 2012 by Klasik-69

[JohnK]

From what i remember, max torque is found at 11:1. But a nice flat A/F curve means you've got good cooling. Power comes from spark. Diddling that delivers max torque is what you're after here. Once spark is done, you go back to do fuel. A load-bearing dyno and lots of patience will give you optimum in both across your power band (and wonderful drivability).

[midgetracr]

AFR for maximum brake torque (MBT) at any particular RPM for a given engine depends mostly on the efficiency of the combustion chamber but will also vary depending on camshaft timing and inlet and exhaust porting and lengths. A gross approximation would be in the area of 10.5 to 13 AFR for 100% gasoline. A lower AFR number would be appropriate for E10 fuel.

 

It is common for well tuned race engines to lean them out slightly at revs above peak torque. This is because an engine's ability to achieve maximum volumetric efficiency falls off above peak torque, combustion pressures fall off and the engine is able to burn less fuel per combustion cycle.

 

The graph you showed seemed to be well over 12.5. I suspect that more power could be achieved by going richer. I suggest going richer in 0.5% AFR increments and comparing results. Once you achieve max power I suggest leaving it slightly on the rich side for safety, then try retarding ignition advance 2*. Hopefully this will reduce power and prove that you have been overly conservative with spark advance. If this turns out to be true, add advance a degree or two at a time until power stops increasing. Then back off one or two degrees for safety.

 

Tuning to the razors edge of AFR and ignition advance is dangerous. Minor differences in ambient temps, engine temp or fuel octane could get you into severe detonation and be disasterous for your engine.

 

Just my somewhat informed opinion.

 

John Meyers

[Klasik-69]

You bring up a good point about detonation. The engine and ECU does not have a spark knock sensor so the safety zone isn't there. I've been concerned about that. I burn 93 octane but that is still E10 fuel and thus a few points shy of true 93. I just took some fuel pump pressure readings and its rock steady at 60 psi, +- 2 psi up and down the rpm range. Also, I just got my synchrometer to syncronize the ITB's as required prior to further tuning. They're now dead even. My next move will be to hook up the computer to the Pectel T2 and hope that the system isn't locked. I understand about intellectural property and the need to protect one's tuning skills but it really pisses me off to have a system you purchased be beyond your ability to manipulate. I would never buy another ECU that doesn't allow me the flexibility to adjust as needed. If I have to ditch this one, I believe I'll go to the MSPNP.

 

How is the AFR adjusted by the ECU ? Is it the duration of fuel injection or the timing of the fuel injection, or both ? If anyone knows who its accomplished, clue me in. I'm old school, carbs and distributors, hammer, screwdriver and channel locks, maybe a coat hanger and a hose clamp will make anything run enough to get you home. Now you need a computer science degree or have a 12 year old in the house :ack:

[Kitcat]

Sign up on the Focus Fanatics forum and post there. You will be swamped w/info. One of their tuning guru's (1turbofocus, aka TurboTom) will likely comment. He is an occasional visitor here, but at something like 20,000 posts there, much more likely to offer his 2 cents.

[midgetracr]

Change AFR by reducing injector duration (commonly called pulse width) to go leaner or increasing duration for richer. I am not familiar with Pectel so cannot comment on their terminology.

 

Be aware that changing pulse width by I.E 10% will not change AFR by 10%. This is because the electric field for the solenoid connected to the injector pintle and the pintle itself have lags that delay the start and stop of fuel flow. These lags are constant and do not change when the pulse width is changed.

 

I suggest you make small changes until you get a feel for what you are doing.

[slomove]

Keep in mind, the AFR for most wideband systems is just calculated from the lambda values as the oxygen sensor sees it. That means, the different values for alcohol mixes are kind of compensated for. Alternatively you need to tell the wideband meter what the mix is and interpret the values accordingly. But that makes it only unnecessarily complicated. I leave mine on 100% gasoline and no matter what the alcohol mix is the AFR 14.7 should indicate a lambda of 1.0.

 

Anyway, I assume the AFR of 10.0 at 2500 rpm means WOT? That may be O.K. and nobody does that for long time, anyway. But if that would be also true for cruising with minimal throttle you would foul the plugs in no time.

[Klasik-69]

Yep, I was thinking just like you so I pulled the plugs to see. They show lean running, no fouling or deposits. They were very slightly pale white but darn near what I call perfect in color. However, keep in mind that this car spends a lot more time on the track than on the street, so the appearance of the plugs will reflect more WOT at higher revs. Since I heard about the high rpm failures of the stock oil pumps, I've been keeping it below 7000 rpm just to be safe until I get the new pump. And of course now with a lean AFR, even more reason to err on the side of caution.