FI and wideband O2 guru’s, What is the perfect AFR ? Options

[Mark Henry]

I know that 14.7 (what the hell do they call it?!?) is the best for economy and they say to add 20-30% for power. This would be around a 12.5 to 13.2 AFR.

So what would you say is the best?
Is this what you are running? Do you have real experience Or is this what you read.

I know there’s no perfect setting, I’d just like to know what others are shooting for.

[ArtechnikA]

QUOTE (Mark Henry @ May 29 2005, 09:12 AM)

I know that 14.7 (what the hell do they call it?!?) I know there’s no perfect setting, I’d just like to know what others are shooting for.

"Stoichiometric" or Theoretical Combustion is the ideal combustion process during which a fuel is burned completely.

IIRC DJet tries to control around 13,5:1 - pretty lean.

some of the factors are - do you have an enrichment facility? you can run closer to stoichiometric under constant load but it's too lean to have excess power for accelleration. you can run leaner if you have a way to temporarily richen the mixture during accelleration. D-Jet does, via the TPS. some CIS does, via a WOT switch.

Bosch MFI doesn't - which is one reason it has a reputation for running rich - throttle response sucks if it's adjusted lean enough for optimal economy in cruise...

i ran a realtime AFR on my narrowband-equipped CIS car while trying to diagnose an intermittent loss of power issue; the control box did a very good job of dithering it rich/lean around center at 3-4Hz. i adjust my MFI to around 4% CO at idle. that's about the extent of my actual instrumentation, alas...

[Katmanken]

What he said.....

At stochiometric (14.7:1), you have exactly enough fuel to mix with the air in the cylinder and no unburned hydrocarbons when ignited. You also get best fuel economy.

At 12.7:1, you have maximum power and unburned hydrocarbons coming out the tailpipe. In the 60's you had black exhaust coming out of the muscle cars- lotsa gas and lotsa power. With the catalytic converters, you don't see that black rich exhaust anymore.

Most people stay away from stochiometric as the leaner the mix, the hotter it runs. Go over stoich and you can overheat- even a watercooled.

On the other hand, running rich helps cool the burn and provides more power.- but wastes gas. Too much gas can also wash the oil off the cylinders.

So, for aircooled, I'd run a little rich for cooler running and for more power.

That's the theoretical answer.

For the actual answer, As Jake says, it's all in the combo. Have a lumpy cam and I can see some static/dynamic CR issues to tune for. At some speeds, some of the fuel/air charge may go down the tailpipe. That's where a wideband sensor AND a dyno AND a head temp gage would help.

Hope that helps...

Ken

[Bleyseng]

Factory settings for djet were 13.7 to 1 for partload and 12.5 to 1 for WOT. I would use those as a starting place or settings to aim for with any EFI. Going leaner with a air cooled engine could mean running too warm unless you are running Jakes DTM which has more cooling. (IMG:http://www.914world.com/bbs2/html/emoticons/chowtime.gif)

[ArtechnikA]

QUOTE (kwales @ May 29 2005, 10:06 AM)

Go over stoich and you can overheat- even a watercooled.

yes, but -- lean of peak (LOP) EGT is also a bit less efficient and will run cooler. it's important to know how all the curves (HC, CO, NOx, CHT, EGT ...) interact, and under what circumstances, and how they can be used to your benefit. i've read the articles but i'm not conversant with all the subtleties.

and as much as some people would like to pretend they're exactly the same, there are fundamental differences between aircooled aircraft engines and aircooled vehicle engines. airplanes burn leaded gas, and we don't. and when you have a 2500rpm redline, valves are on their seats a LOT longer.

[Mark Henry]

I knew this would be good (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif)

Maybe this thread should have classic status?

[lapuwali]

Stoich (14.7:1) provides the best EMISSIONS. Leaner than this (16:1 or thereabouts, just this side of a lean misfire, so it depends on the engine) provides the best ECONOMY. Best power is richer than this (just this side of a rich misfire, so it depends on the engine, but figure around 12-13:1) provides the best POWER.

This is documented up, down, and sideways in every book on engine design that's been written in the past 50 years (thought the emissions part often wasn't talked about until 30 years ago). Old news. It works for all gasoline, spark-ignition IC engines. And don't start going on about how lean mixtures make an air-cooled engine run hot. They don't. The hottest CHT is very near stoich, and CHT is cooler on either side of it. You have the tools, Mark, verify this for yourself.

On emissions, HC and CO (and CO2) continue to fall as you go leaner than stoich, until you reach the lean misfire point. NOx, however, are higher on either side of stoich, and bottom out there. Some engines (some Hondas and recent Fords) are deliberately designed to run very lean (17:1), and cope with increased NOx using a better catalyst. These engines give excellent economy and very low HC/CO numbers naturally. The trick is in combustion chamber design encouraging the mixture to be richer near the plug, and leaner everywhere else.

It's very common to tune a road car aftermarket ECU to provide leaner than stoich mixtures where MAP is low (usually where cruising takes place), and richer mixtures where MAP is high, when you're asking for power. Good fuel economy AND good power.

[crash914]

Just to confuse things more...I am running mine at 2.35V. this is somewhere around 13.2.

My crossover point is 2.5v..

[MecGen]

Hi Guys
I agree with lapuwali 99%
but I don't understand the ...

QUOTE

And don't start going on about how lean mixtures make an air-cooled engine run hot. They don't. The hottest CHT is very near stoich, and CHT is cooler on either side of it. You have the tools, Mark, verify this for yourself.

I have the tools and check them all the time...maybe I don't follow excactly...A complete combustion on a leaner side, will cause heat...use forced air for example, I find the temp tells me boost specs pretty good if I don't compensate with added fuel (before ping).
I have to addmit I never tryed it on an air cooled...
I just made a phone call to a serious expert on air cooled planes anyways...he was eating dinner, but I squeaked a question at him..
He says yes of course, leaner is hotter to the point of giving problems for planes, as I heard his better half call him, he said Sesanas/Cesena (sp) small air cooled planes, when they hang around 10000 feet they lean out and actually overheat the heads and cylinders, then on decent they cool off too quickly and crack heads and cylinders... I asked him if the problem was with the cooling dropp or with the overheating of the heads, his last answer was "the head temperature went above the metal threshold, extreme temp drop is a known constant - fact, newer model are tuned around this problem"
I suspect they richen the mixture at higher altitude.
He is supposed to call me back in the next hout to answer any question I have. He is an aviation engineer with certificates and licenced for multiple planes, he worked for a friend with smaller planes on weekends, for 10 years, If you got a plane question I can pass it on...
he drives a Miata (IMG:http://www.914world.com/bbs2/html/emoticons/icon8.gif) His is the last signature when you sit your ass on an Air Canada plane.

Sorry Mark I can't tell you more then you already know, or what the people allredy passed on, what I can tell you is that I have personally tuned 2 VR6s, side by side, same year, same everything, same file, one worked one didn't...

Good luck with it, cool thred
Later
Joe

(IMG:http://www.914world.com/bbs2/html/emoticons/beerchug.gif)

[TimT]

The Laws of Thermodynamics dont care if the engine is a 2000 RPM engine that lives at stable manifold pressures all day, or if the engine is a 10000 rpm screamers with 1.5 BAR of boost.

Maximum power can be found at a certain A/F ratio, maximum efficiency will be found at another A/F ratio, and there is an A/F ratio that produces the least amount of harmful gasses.

for a street engine that one hopes to get tons of miles out of, Id tune on the fat side while accelerating to operating RPM, and at cruise manifold pressures Id go LOP

Thats just how I do it..on a 700HP+ engine

and the Laws of Thermodyamics are pleased

[TimT]

some graphs

[TimT]

more

[airsix]

Mark,
I shoot for 3 numbers:

Cruising/low-load/high-vacuum: 13.5-13.7
High-load/low-vacuum: 12.3
Under Boost: 11.5

Those are just "my" numbers. YMMV

-Ben M.

[Mark Henry]

QUOTE (lapuwali @ May 29 2005, 11:07 AM)

And don't start going on about how lean mixtures make an air-cooled engine run hot. They don't. The hottest CHT is very near stoich, and CHT is cooler on either side of it. You have the tools, Mark, verify this for yourself.

Well... I went for a nice run yesterday and cranked my values to high 13's under load and Stoich (14.7:1) at cruise, you should have seen my temps go through the roof.
I dropped the values back to high 12's under load and mid 13's at cruise and all was well again.

I'm sure you could build an engine (cam) to run closer to 14.7, but as a rule of thumb I'd say that yes, an aircooled engine does like a richer mxture.

[cnavarro]

When I was helping Jake tune a Nickies 2563, it seemed that it liked to be around 13.8:1- it was a kitcarlson prototype running on an IR setup. Tuned with an LM-1 wideband. If I remember correctly, Jake's 2056 in his 912 liked to be at 13.2:1. Again, the 2056 was running kitcarlson with plenum/ single TB and cast iron cylinders.

[lapuwali]

QUOTE (Mark Henry @ May 30 2005, 03:37 AM)

QUOTE (lapuwali @ May 29 2005, 11:07 AM) And don't start going on about how lean mixtures make an air-cooled engine run hot.  They don't.  The hottest CHT is very near stoich, and CHT is cooler on either side of it.  You have the tools, Mark, verify this for yourself. Well... I went for a nice run yesterday and cranked my values to high 13's under load and Stoich (14.7:1) at cruise, you should have seen my temps go through the roof. I dropped the values back to high 12's under load and mid 13's at cruise and all was well again. I'm sure you could build an engine (cam) to run closer to 14.7, but as a rule of thumb I'd say that yes, an aircooled engine does like a richer mxture.

But did you try a leaner mixture than stoich? Look at the graphs TimT posted. Temps actually fall FASTER with leaner mixtures. There's less power at leaner than stoich mixtures, but better fuel economy. You only want to run at stoich if you're trying to minimize emissions. Run lean at cruise and rich under load, but never at stoich.

I'm pretty convinced when most people talk about "lean mixtures" on an aircooled engine as "running hot", they really mean "leaner than best power", which is stoich. They never go far enough past stoich.

If all you're after is best power, then richer mixtures are always better.

[airsix]

Our engines have no business running anything North of high-13s period. If you lean out the mixture to the point that the temps fall back to normal range you'll have no power, rough running, and knocking. You can try it but you'll actually say out loud "Wow. This thing runs like crap."

-Ben M.

[ArtechnikA]

QUOTE (airsix @ May 30 2005, 04:15 PM)

If you lean out the mixture to the point that the temps fall back to normal range you'll have no power, rough running, and knocking.  You can try it but you'll actually say out loud "Wow. This thing runs like crap."

yup. there are spiffy modern (and almost invariably water-cooled) knock-sensor combustion chamber designs that can pull off a 'lean-burn' regime. our engines are from a simpler, more primitive time.

[DNHunt]

Before you get too hung up on numbers remember that Wideband O2 controllers and sensors are not all the same. Same engine tuned with different WB O2's will give different values. I've tried to tune to 12.5 at WOT and the engine wants more fuel. O2 numbers are fine for light load but when you look for power they should only be a guide.

Dave