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.
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. |
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
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.
QUOTE (kwales @ May 29 2005, 10:06 AM) |
Go over stoich and you can overheat- even a watercooled. |
I knew this would be good
Maybe this thread should have classic status?
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.
Just to confuse things more...I am running mine at 2.35V. this is somewhere around 13.2.
My crossover point is 2.5v..
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. |
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
some graphs
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more
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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.
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. |
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.
QUOTE (Mark Henry @ May 30 2005, 03:37 AM) | ||
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. |
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.
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." |
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
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