The basics. What AFR makes best power?

[mspiegle]

I got myself into this incredibly retarded argument with someone I thought was fairly intelligent. Essentially, he dyno'd his car and made XXXrwhp and XXXrwtq. He dyno'd again and got an additional 30rwhp and 20rwtq. He didn't specify, but I think in between those runs, he put straight-pipes in. When he re-dyno'd, he noticed his AFR went from 13.x to 14.x and he also had more power. He's saying that he's making more power because his car was leaned out into its proper efficiency range. I made the mistake of not calling bullshit on an unexplained 30rwhp gain in power, but that's besides the point. We're both going to goto our local dyno shop and ask the master tuner who's right. I know he's wrong, but I just want to make sure my argument isn't flawed. Here it is:

https://raptor.nauticaltech.com/~mspiegle/afr_argument.html

Thanks for reading it... I know its somewhat painful to follow

[Hick]

14:1

[mspiegle]

14:1

wrong

[Hick]

1800:1 ????

[mspiegle]

1800:1 ????

dude, did you read the conversation?

[Hick]

dude, did you read the conversation?

huh? what converstaion? what is AFR???

[92 mtechnic cabrio]

dude, I'M RIGHT.

[mspiegle]

sigh @ you people

[gstuning]

In you conversation you completly forget the main ingredient to power.
Ignition timing.

The more advance timing is the more hotter the gases become and more power is made, the point just before pinging is ideal for max power for a set AFR,
more fuel and you should theoratically be able to advance the timing again ,

this is best viewed with a exhaust temp monitoring device

There is no magic AFR value, and they change with rpm´s too,
You should have had that conversation with me ;)

[mspiegle]

In you conversation you completly forget the main ingredient to power.
Ignition timing.

The more advance timing is the more hotter the gases become and more power is made, the point just before pinging is ideal for max power for a set AFR,
more fuel and you should theoratically be able to advance the timing again ,

this is best viewed with a exhaust temp monitoring device

There is no magic AFR value, and they change with rpm´s too,
You should have had that conversation with me ;)

Thanks for your input Gunni - you're just the person I wanted to hear from. The argument was in the context of holding all variables constant except for AFR. I know timing plays a big role in it, but I wanted to just talk about AFR with the guy since he clearly had no clue.

As for your comment, I may sound crazy, but assuming you aren't advancing ignition past the point of being knock-limited - the EGTs should actually be cooler. This is because advancing the ignition should allow your pistons to extract the most work from the combustion and apply it to the point of best mechanical advantage on the crank.

If you retard the ignition, you're making the combustion process less efficient which creates more heat and pushes the combustion closer to the time that the exhaust valve opens. While running retarded timing, you would actually need to add fuel to cool down the EGTs to a sensible value.

Have you heard of the old tip --- advance your ignition if you failed smog because of NOX? That's because NOX is created during high combustion temps and advancing the ignition should reduce this.

Lets get a discussion going since we have a difference of opinion :)

[gstuning]

You just passed me,,,,

I didn´t know temps go cooler, but I guess advancing until you reach max temp should be clever, before going cooler, this is why a dyno is nice to have

[Insanity]

For maximum power, you want your car to run really lean. The mixture will burn hotter, and as heat is the source of the power in your engine, you will get more power. The one time this isn't true is with turbocharged engines, because running rich (around 13:1) will keep the EGTs down which greatly improves the efficiency of the turbo.

And of course running really lean is a bad idea anyway, because it makes your engine more prone to exploding, and the gains are fairly minimal anyway.

[gstuning]

Turbos run on hot gases as well, so running more heat per psi of air in the turbine is better as more heat will turn the turbine more.

The problem with turbos is charge temp,
when you compress 1liter of air wich was under 0psi pressure before the charge will heat up,
but if you compress 2liters of air(1bar pressure in the same space) the heating will be even more as the air is already hotter then before, so that is why turbo engines need to run cooler gases.
If you had fuel wich would provide better prevention to knocking you could run hotter gases as wich lower compression,
This is how I understand it,


to have a larger safe zone

[mspiegle]

For maximum power, you want your car to run really lean. The mixture will burn hotter, and as heat is the source of the power in your engine, you will get more power. The one time this isn't true is with turbocharged engines, because running rich (around 13:1) will keep the EGTs down which greatly improves the efficiency of the turbo.

And of course running really lean is a bad idea anyway, because it makes your engine more prone to exploding, and the gains are fairly minimal anyway.

I agree with this up to a certain point. When you start getting closer to 14.7, there just simply isn't enough fuel in the combustion chamber to fully make use of the air in there.

If you compare a newer car to an older car, yes they run hotter and are far more efficient - BUT, efficient is the keyword. With respect to maximum power, you can still take these cars that run 14.0 nominal AFR @ peak torque down to 13.5 and make power because you're able to use more of the oxygen in the combustion chamber. As we all know, air alone and fuel alone don't make power. Its all about having both of them in the right quantity (and LARGE quantities :)) I don't think we'll see cars that can make best power on 14.0 for a few years at least.

[mspiegle]

Also, I just wanted to add that i'm only referring to pre-cat AFR measurements. I've seen cars on dynos that did simultaneous pre and post-cat monitoring and there is a huge difference. I also witnessed it on my 928 (and 3 other 928s that I personally tuned with pre-cat measurements) when I did 15 back-to-back tuning runs of my techedge pre-cat 7057 sensor vs the dynapack's post-cat 7057 sensor. There was a difference of about 1-2 AFR points, but it wasn't consistent.


Here's a graph from the dynapack. On my pre-cat TechEdge wideband unit, the entire run was dead-on 12.5, but the dynapack measured very different AFRs post-cat.