L-EATER

iTrader: (2)

With all else equal, what effect should adding say 6 degrees of timing have on the air fuel ratio? Leaner, richer or no change? Going to the dyno tomorrow and I am trying to "pre-write" some programs to try. Thanks in advance for any advice !

gtodoug

iTrader: (1)

great question!! I'd like to know as well.

DynoDR

iTrader: (1)

Generally I would say that if you are short from ideal timing, adding timing will lean it out, cleaner and more complete burn, etc. Just as if you are way low on ignition timing, sometimes going richer, like from 12.8 to 12.0, will show more power.

jimmyblue

iTrader: (11)

Timing is independent of AFR. Though, there appears to
be some sort of spark advance adder table indexed by fuel
air multiplier, it's all zeroes in my .bin.

However for a given motor, a given spark advance will
want a slightly different fueling to maximize output and
vice versa. But spark does not change the chemistry,
only its timing relative to mechanicals, and the product
of burn and leverage is what determines the power
delivered and the power wasted.

What I've always wanted to see, but never had the time
or resources to map out, is a "response surface" of HP
vs fuel and spark. Too bad nobody has the urge to flog
the car through 25 or so separate tunes and pulls, to
satisfy my curiosity.

L-EATER

iTrader: (2)

Let me be more specific about what I am doing. Have been running 11 degrees in non-intercooled form. Any more and it would detonate (IAT's up to 230 at the end of a 1/4 mi pass) Now I am intercooled and the IAT's are in the 140's max. I have the A/F dialed in perfect for max power (maybe a little lean for safe daily driving) but am still only running 11 degrees. I want to sneak up on timing on the dyno, but If the addtl timing is going to lean it out then I need to add some fuel in the programs with the extra timing, right?

Another_User

I have seen increased timing push my trims further negative. I would imagine that with a lot of overlap that would make sense...I think. It makes my head hurt to think about it while I have MAF tables on my mind.

greentahoe

iTrader: (2)

I have been told that adding timing does lean out the AF. You could always write up the new tune and have them hookup the wibedand and dial it back in over a couple of runs.

gametech

iTrader: (1)

I just finished putting a Holley 950 mpi setup on a 400 smallblock in a '52 chevy pickup. I had never even thought of the timing affecting the A/F ratio until I started tuning that truck. The o2 sensor readings were affected by over 5% in some areas while playing with different timing curves.

MNR-0

Timing does alter AFR. However, at WOT, with typical AFRs in the mid to low 12s with CATS, it will be negligible. It will be most noticable at the very top end. Extra timing provides for a greater burn time post ignition and so exhausting less unburnt fuel. Too much timing and you invoke detonation. Poor plugs and superheated floaties in the combustion chamber and you invoke pre-detonation. You won't last a minutes when that happens.

L-EATER

iTrader: (2)

Well, I went to the dyno today and ended up making about 9 pulls all together. Each time I would add a couple degrees of timing and a little fuel and the A/F ratio stayed right in there around 11.5 to 11.7 right where I wanted it. Each 2 degrees seemed to take about a .02 increase in the PE fuel to keep the A/F the same.

I went to the dyno with 11 degrees total timing and ended up coming home running 23 or so. The peak hp went from 420 to 489 so I was definitely a happy camper !!

JNorris

iTrader: (4)

This is good info. So you proved that adding timing without adding fuel would lean out your mixture because you are providing more burn time by igniting the mixture earlier.

John

SSpeedracer

iTrader: (8)

Physics: Like stated above, AFR is independant of timing. However, free oxygen is dependent of timing and will change the lambda reading downstream of the combustion chamber.

Shade Tree: For a closed loop EFI system, spark would change the lambda/O2 reading and hence AFR via trims. This will have no effect on a carburator assuming manifold vacuum is constant.