horrible mpg's when can i lean out?

[DonTZ125]

Your cyl is more completely filled at WFO, with higher compressed pressure, thus greater likelyhood of detonation if you try and spark too soon. The half-filled cylinder of a loafing engine both wants and can withstand more spark advance.

[cygnus x-1]

funny, i always justified lots of ignition timing for cruising around for really crappy, poor combustion chambers!!!

Correct! And in fact my old Honda does indeed have really crappy combustion chambers. It's a 3 valve/cyl design with a hemispherical chamber, and practically no squish.


As for timing advance, it's a multi-variable problem. The two main variables are RPM and mixture density. As RPM increases the pistons are moving faster, meaning the mixture has to be ignited sooner so that the peak cylinder pressure will occur at the optimum crank angle. More dense mixtures burn faster, so as mixture density increases (with increasing manifold pressure, or decreasing vacuum), the timing must be retarded so that the peak cylinder pressure doesn't occur too soon.

Air/fuel ratio and fuel octane also have an effect as well. Leaner mixtures burn slower than richer mixtures. Higher octane fuel also burns slower than lower octane fuel. And then there's combustion chamber shape, mixture turbulence, squish, tumble, and all that good stuff.

So this is why in the end, the best way to tune ignition timing is on a steady state dyno, where you can control the variables better.


C|

[drmiller100]

there are lots of tools which would be nice. a wind tunnel, accoustics to listen to combustion, combustion pressure, knock sensors, dynos, etc.

lots of cars get tuned without that stuff though.

[FOTIS84]

with 100% load in high rpmswith 27 advance i had 12,5-13 afr.when i increased the advance 2 more (29) the afr was 11-11,5.what does it means? must i lean out my mixture or the 27 advance was closer to correct ?

(sorry for my english)

[drmiller100]

my opinion and GUESS is you do not have enough total advance, and you were getting an incomplete burn. When you advanced the timing, burn got better, and now you are into the lean zone.

If it were me, I would consider richening it a BUNCH, then throw a bunch of advance at it.

If you start rich, then lean it down, with "enough" advance, the engine will gurgle and make lots of power, but get not great mileage.

If you start lean, and babystep the richness and advance the timing, you are flirting with disaster. You are running too lean, you are going to be bumping against knock and detonation, and it is going to be confusing to tune.

I REALLY encourage other opinions - I am by no means an expert and maybe I am missing something, but I think you have tuned yourself into a hole. You don't have enough timing, and you don't have enough fuel. I would TOTALLY believe you have no power, and crappy mileage if I am right.

[drmiller100]

CRUD.

I went the wrong way.

When you advanced the timing, it got RICHER, and I read it wrong. My advice above is wrong wrong wrong.

Very sorry.

[FOTIS84]

yes when i advance it get richer but i fill incomplete burning fuel before the rev limit.and now with 29 advance is more than 27.no cuts but incomplete burns from the exhaust.i do;nt know if this uncomplete burn is this gurgle.