What A/F ratio you running?

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Dartfan71

Dartfan71

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So I recently installed a wideband O2 setup on my small block street/strip setup. I followed the instructions on the placement of the O2 sensor (in the 3" collector angled downward). I know ideal A/F is 14.7. I have tuned my new Holley HP DP carb and idle is about 13ish and cruising is about the same, still haven't checked it at WOT. I'm kind of scared to run it any leaner (Just in case). I have already went from 69 jets in the primariesdown to 65 jets and adjusted the idle mixture screws on all 4 corners. Im running 91 octane ethanol free. Should I leave it as is or try and lean it out more?
Motor is a 10.5-1 360, 904 trans, kb107's, ported J heads 2.02 valves, Howard's street/strip cam around .550 lift, headers, 3" exhaust ( I know too big, but it's what I had already). With the Hughes 4k converter and 3.91 gears the car is no slouch. Thanks for the insight.
 
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Keep in mind the stoich ratio for 100% gasoline is different than for the ethanol blends. 14.7 is for 100% gasoline.
 
Thanks mopowers. Forgot to mention that I'm running ethanol free 91 octane.
 
Here is the landscape for afrs that fast XFI suggests. Obviously the yellow area is WOT for NA motors. My modern hemi is 14.5-14.9 idle, 13.7ish cruise, 12.8-13.0 WOT.

IMG_8994.JPG
 
I have a thermoquad equipped 5.2 magnum in a dart, cruise is about 15.5, idle is about 14.7 and WOT is in the 12.90-13 range...
 
I am still tuning mine but last trip was 15 idle, 13.8 +- cruise and 11.5 power. I am going to lean it out one more jet and PV size and go with that.
I am getting 15+ to 16+ MPG.
 
Dartfan71 said:
I know ideal A/F is 14.7.
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Just wanna say. That's not NECESSARILY true. 14.7 is stoichiometry air fuel ratio. That's the ratio where, with proper combustion, all the fuel will burn all the air. It is a "complete" combustion. Any leaner. And there is not enough fuel to burn all the air. Any richer and you will be "wasting" fuel because there's more fuel than is needed to burn the present air.

But that doesn't make 14.7 "ideal". You can go leaner on cruise. As others have mentioned. Even though you aren't burning all the air. That isn't necessary. Especially at low load cruise. It really only takes something like less than 75 horsepower to keep a vehicle moving at 65 mph at low load. Don't need complete combustion for that. And you actually make MORE power on richer A/F ratios. Most guys with EFI setups start the tune between 12:1-13:1 and end up around 12.8-13.2 at WOT. Generally that's where you'll land for best power. Although you could be richer or leaner from that. It depends a lot on your engine and how well it completes the combustion cycle. They will run the car at various A/F ratios until they make a best pass or the most power. Oddly enough guys have tuned for best power on the dyno and ended up leaner than before. Left it at that A/F ratio and taken it to the track and lost time than their richer ratio. So it really does depend quite heavily on the setup.
 
How in the world are you guys getting 14-15 afr's at idle & able to keep the engine stilll running?!?!?!? neatther one of my darts (one is a mild but strong hi-po 273 4-speed. The othere is a nasty 340 automatic with 3800 stall) any leaner than 13 both engine's start to miss & can watch vacuum drop on gauge even before they start to miss. Rule of thumb for carburetor motors the highest vacuum reading usually is around 12-13 ish. My cruise is around 15 & w.o.t. 12.8 -13.1 273 ides @ 750-800 with 15.5 steady vacuum. The 340 is about @ 1000-1100 with a steady 11 vacuum.
 
Hipo65 said:
How in the world are you guys getting 14-15 afr's at idle & able to keep the engine stilll running?!?!?!? neatther one of my darts (one is a mild but strong hi-po 273 4-speed. The othere is a nasty 340 automatic with 3800 stall) any leaner than 13 both engine's start to miss & can watch vacuum drop on gauge even before they start to miss. Rule of thumb for carburetor motors the highest vacuum reading usually is around 12-13 ish. My cruise is around 15 & w.o.t. 12.8 -13.1 273 ides @ 750-800 with 15.5 steady vacuum. The 340 is about @ 1000-1100 with a steady 11 vacuum.
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Just a guess but I am thinking the carburetor model and size and the intake model and type as well as the ignition system would all play a part in the mix.
But as I said, just a guess--------------------I use the A/F numbers chart as a guide and tune to it.
 
My engine has a stock cam, idles with 20 inches of vacuum. A long duration cam will require a richer mixture.
 
replicaracer43 said:
My engine has a stock cam, idles with 20 inches of vacuum. A long duration cam will require a richer mixture.
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i thought a long duration cam was harder to clean up at idle..requiring a leaner mixture.Usually a lot of the gas is burning in your exhaust with these cams because they are so inefficient at lo rpm?
 
Well with my cam I am running is only pulling about 8" of vacuum at idle. Currently I'm idling at 900 rpm in gear and a/f is around 13. I have adjusted idle mixture screws to the highest vacuum at all four corners. It seems to like a leaner mixture at idle.

How does altitude play into this? I'm at about 4,500' elevation.
 
Dartfan71 said:
Well with my cam I am running is only pulling about 8" of vacuum at idle. Currently I'm idling at 900 rpm in gear and a/f is around 13.
I am learning.i have tried to adjust the idle with long duration cams and it's probably me but the best i could get was 13.5.With short duration cams the idle mixture screws easily adjust the af mixture to 14.5 or leaner.
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I have twice found and solved carb problems quickly with my lmxt af gauge and it makes tuning much easier.Readings are instant for metering rod, jet changes and other adjustments...a great tool that can be moved easily to another vehicle...but as i said i am learning.
 
I run an AEM gauge with datalogging on my blower car. It idles at 13.1 on 93 octane fuel and 10hg vacuum, 15.2 cruise 12hg vacuum and 11.8 WOT with 11psi boost.
 
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