Stephen Max

iTrader: (59)

Here is an interesting graph I lifted from an aviation website (http://www.avweb.com/news/columns/186619-1.html).

The graph was made from data collected on a particular aircraft engine. Aircraft engines commonly use fixed engine rpm and ignition timing, and throttle mixture (afr) is varied for different operations (take-off, cruise, climb, descent, etc.).

The 50 deg rich line corresponds to approximately 12.5:1 afr, and is about where peak power is made. I'm not sure what 50 deg lean corresponds to. ICP refers to the peak combustion pressure after ignition. CHT is the cylinder head temperature.

It is interesting to note that egt peaks at stoichiometric, and falls off for both richer and leaner air-fuel ratios.

IceY2K1

iTrader: (3)

That's exactly why it's commonly recommended NOT to tune exclusively via EGT temps unless you know your starting point is excessively rich. Also, you will generally make peak power as you lean right up to the point of failure, so don't get too greedy eeking out every last hp.

slimer

iTrader: (13)

Good find. Sticky.

Don't you think once the engine reaches the 12.5 Stoimetric AFR, the Law of Diminishing Returns comes into effect and that is why it peaks like that?

IceY2K1

iTrader: (3)

I wouldn't look too much into the "12.5:1 AFR = peak power", since that is all relative to your timing(where peak pressure occurs) and more importantly your fuel octane.

I didn't read the whole article, but these guys run AV gas and have a pretty constant load vs. our 91-93 craptane and pretty much constant varying load. Just stating the obvious, but I don't want people to think 12.5:1 is the golden AFR for EVERYone.

Boosted Maxima

I've played with my AFR's quite a bit and for my VG low 11's to high 10'sAFR are where it tends to make the most power. The leaner I make it the less it tends to pull.
Mine is still setup mild with 10lbs of boost Timing set back a little bit and obviously safe with fuel. On my G-Tech I MPH at 106.7-109.9 for 1/4MI.
We checked the G-Tech in Brad92's car friday and it MPH'd the same as what he ran at the track the previous friday at 101mph. So that verifys the readings of G-Techs VS Real track MPH.
Sorry got off the topic of AFR... anyway.. Brad's FMU tends to creep the AFR richer and richer as the RPM's go up. boost seems to be steady. It'll start out at 11.4 then creep down to 9.8 or so in 4th gear..
Anyone with FMU experienced that before?
Ideas?
Thanks, Scott~

I30tMikeD

iTrader: (8)

My AFR is not perfectly stright but it is fairly constant once full boost comes on, doesn't steadily drop. This is with just the FMU and no SAFC corrections

I also made just as much power with a low 10.5:1 - 11:1 as I did with a 11.5:1 - 12:1 AFR. The only time leaning it out made power was when I went to a larger FMU disk and droped below 10:1 for most of the rpm range. When I brought it up back up to 10.5:1 - 11:0 I made a solid 12-14 hp throughout the curve.