AFM ADJUSTMENT w/o gas analyzer

vwlover77 · Post by **vwlover77** » Sun Sep 21, 2008 6:12 pm

Not sure what difference it makes, but you've got the Camper Special setup, correct?

Westy78 · Post by **Westy78** » Mon Sep 22, 2008 6:42 am

Yes, which actually may want to run a little more lean than a stock engine.

vwlover77 · Post by **vwlover77** » Mon Sep 22, 2008 9:16 am

Geek that I am, I pulled out my copy of "Bosch Fuel Injection and Engine Management" last night and was reading about air-fuel ratios.

I was reminded that the "stochiometric ratio" of 14.7 to 1 is not the "best" ratio, but is in fact a compromise between the best engine power (delivered at 12.6 to 1 ratio), and best fuel economy (delivered at 15.4 to 1 ratio) to end up with a good running engine that can meet emission requirements.

I'm wondering if the "engine speed change" method of adjusting the AFM is in fact leading us to set it to the maximum power ratio of 12.6 to 1? It would make sense that the engine would be running fastest at its power peak for a given throttle setting and that a change in ratio either richer or leaner would result in a power output reduction and therefore a speed reduction.

Post by **Amskeptic** » Mon Sep 22, 2008 9:55 am

vwlover77 wrote:I'm wondering if the "engine speed change" method of adjusting the AFM is in fact leading us to set it to the maximum power ratio of 12.6 to 1?

Yes, and I then suggest that we put the "window" of wiper movement such that the merest movement clockwise causes an rpm drop. This puts it at 13.3749:1 and makes the exhaust valves happy. And as we say here on the Itinerant Air-Cooled forum,
"What Makes Your Exhaust Valves Happy. . . Makes YOU Happy."

vwlover77 · Post by **vwlover77** » Mon Sep 22, 2008 11:25 am

Do we also want the "window" of wiper movement such that pushing the wiper further CCW results in no increase in engine speed?

And isn't the desired final ratio really 13.3649 : 1 ?

Gypsie · Post by **Gypsie** » Mon Sep 22, 2008 12:03 pm

I have an LM1 hooked up right now (Thanks Westy78 and IFBWAX) and am finding that with the 'beer fueled bull in the china shop adjustment' (8 cogs to the lean on the 'dynamic' adj wheel) and a corrected timing adjustment (from 12BTC to 7.5BTC) I am running at 13.5-14.5 AFR on the city streets, and 14.5-15.5 at idle with shifter in D (automatic). W/Shifter in N 14-15.

Haven't had a good freeway run yet.

Pretty close IMO.

So, the idea here is to be a 'touch' rich? (i.e.-if optimal burn is 14.7 then a touch rich would be 12.6-14ish?)

Does anyone know how I should account for the Cat in these equations?

I have a pre-cat bung but have been told the LM will not attach here as they will not match threads er sump'n.

Not even sure the Cat is working. I have heard that there should be a HUGE diff in temps pre and post cat (an indicator of function).

Even without the sniffer telling me where mix is I got 16 mpg freeway speeds with a few 70+ mph stretches.

(Y'all have a good time with the 'pre-cat bung' line......

.....John)

markd89 · Post by **markd89** » Mon Sep 22, 2008 12:20 pm

Paraphrasing John @ Aircooled.net posted on Samba that for economy you want 16.1-17:1 for 1/3 throttle crusiing and 13:1 for > 1/3 throttle.

To me this makes sense as less load we can get away with being leaner. When we need the power it has to richen up...

Mark

Gypsie · Post by **Gypsie** » Mon Sep 22, 2008 3:37 pm

Just had a lunch time run on the freeway.

Cruisin Speed 14.5-16 AFR, Mild accelerator low 14's, big accelerator-drops down to 12.5-13ish.

deceleration brings the mix to the 20+AFR range for a quick blip then back to the 14.5-16 range.

???

Seems grand to me. Feels right.

Comments?

markd89 · Post by **markd89** » Mon Sep 22, 2008 3:53 pm

I think he was saying mild acceleration 16 or 17:1 is ok so you may be able to lean a little...

Westy78 · Post by **Westy78** » Mon Sep 22, 2008 5:14 pm

For more confusion here's what's been recommended by Jake for my engine. 13.1:1 at light throttle cruise. 12.5:1 at WOT. This is almost exactly where I'm at right now and power seems very good. With the wiper bump at 3000rpm adjustment I was way rich at around 10.8:1.

markd89 · Post by **markd89** » Mon Sep 22, 2008 5:16 pm

Westy78 wrote:For more confusion here's what's been recommended by Jake for my engine. 13.1:1 at light throttle cruise. 12.5:1 at WOT. This is almost exactly where I'm at right now and power seems very good. With the wiper bump at 3000rpm adjustment I was way rich at around 10.8:1.

What MPG are you getting on the highway?

Thanks,

Mark

Westy78 · Post by **Westy78** » Mon Sep 22, 2008 5:20 pm

This past weekend I got around 17mpg but that was going camping and climbing 3000ft. This engine is still tight too. Only around 3000 miles on it. I expect once everything loosens up and gets dialed in I should see mid 20's on the highway.

Post by **Amskeptic** » Mon Sep 22, 2008 6:50 pm

Gypsie wrote:Does anyone know how I should account for the Cat in these equations?

I think catalytic converters have to remain "lit" by having enough hydrocarbons to stay heated up. Since they are doing a nice job cleaning, a little richer upstream still comes out clean and it keeps the cat toasty for best catalysis. IIRC
Colin

airkooledchris · Post by **airkooledchris** » Thu Jan 01, 2009 8:53 pm

bringing this old thread back once more....

I tried adjusting the way this was laid out but I think im just too far off my starting point to rely on these changes.

I an get the results were looking for, but only when I run the idle adjustment at full rich. I have to keep cranking the idle way up when I lower the idle fuel mixture - which makes sense, you lower the fuel the motor isn't as happy, but I mean WAY up. Probably 6 or 7 full turns to keep it running when the slider and cog are about where we want them. - where the little flicker in either direction nearly does nothing at all, bigger push CCW and the rpm's jump up, bigger push CW and it goes down, but both snap right back to where they were.

I do have an LM1, and just wondering where to start from. With the above changes made, I was seeing a pretty rich mixture, which might be good? About 22:1 at idle, 13.5-15 at 2500 RPM, pushing 12's when I rev higher than that (my handheld tach doesn't go over 2500 rpm's, duh.)

What do you want at just idle, which I know isn't as important as at higher RPM's and especially under load, but what do you want to see usually around 1000 RPM's?

Im just not sure how to get it set where I can have the idle adjustment screw anywhere other than at full rich in order to get these numbers, which makes me think im missing something someplace else. )or just chasing my tail)

also, can running too lean cause bad fuel mileage as well?

markd89 · Post by **markd89** » Thu Jan 01, 2009 9:24 pm

Some will disagree but, John @ aircooled.net posted that for economy you want 16-17:1 at 1/3 throttle and 13:1 above 1/3 throttle. This assumes you are taking measurements when actually driving...

Mark