Air/Fuel Ratio help

[TheSilverBuick]

Re: Air/Fuel Ratio help

Remember running on the lean edge at cruise will also bring new barometric compensation concerns.

Even with a second barometric sensor for real time atmospheric readings? (aka the MAPDaddy chip, which appears to work perfectly in my MS2, one guage read the atmosphere and the other the engine vacuum).

[TC]

Re: Air/Fuel Ratio help

On our bonneville Studebaker, which runs that crazy c118 stuff from ERC, if you try to run any richer than 12.5 or 13:1, the mixture won't even burn!

DG do you think that dropping the octane would allow you to run richer?? Meaning that a lower octane fuel burns easier.

[dieselgeek]

Re: Air/Fuel Ratio help

Remember running on the lean edge at cruise will also bring new barometric compensation concerns.

Even with a second barometric sensor for real time atmospheric readings? (aka the MAPDaddy chip, which appears to work perfectly in my MS2, one guage read the atmosphere and the other the engine vacuum).

sorry, should have clarified - I was talking about guys running Carburetors. Setting it up for "ideal lean" when cruising with a warm engine, will cause them barometric concerns if they drive where there is a lot of DA change.

a MS with a Mapdaddy will give you OEM-grade compensation. That's the same combo that runs inside the EMS-Pro and we have driven Bill's mustang over 5000' elevation changes without even shutting the engine down... never notice a difference in AFRs, but you can see the correction happening - it's cool to watch. You'l have a lot of fun logging this as you drive back and forth from Ely to SoCal.


TC, yes, the engine will definitley run "more like a normal one" on lower octane fuel, but I don't blame the behavior entirely on the octane of the fuel. There's a lot more to it than that... burn rate is slower and cooler though, for sure.

-Scott

[calico camaro]

Re: Air/Fuel Ratio help

its crazy how much different some "gas" burns than others. changing brands makes it hard to read the plugs too, with the different dye's and such. i thought you might like it richer due to having more corn in your gas out there, since you live in a corn field scott. lol jk

[JeffMcKC]

Re: Air/Fuel Ratio help

If you read a plug with the dye from the plugs its fat still, I know mine idles at 19 and cruise is at 13.8 I beleive we where at 12.7 or so on my runs but leaning it down got cut short.

[dieselgeek]

Re: Air/Fuel Ratio help

mine idles at 19

this merits an explanation.

Engines with the kinds of cams that are bleeding off cylinder pressure at low RPMs, make a wideband sensor USELESS for tuning idle and part throttle. Jeff's motor is passing so much unburnt oxygen at idle, all that we can do is tune for max vacuum and by "ear."

Under load however, and at higher RPMs, the wideband sensor starts working as intended.

[SpiderGearsMan]

Re: Air/Fuel Ratio help

On our bonneville Studebaker, which runs that crazy c118 stuff from ERC, if you try to run any richer than 12.5 or 13:1, the mixture won't even burn!

DG do you think that dropping the octane would allow you to run richer?? Meaning that a lower octane fuel burns easier.

low octane fuel burns faster

[MadmanMark]

Re: Air/Fuel Ratio help

Is reading the plugs the best way to figure out if you're rich or lean? I keep debating adding a sensor, but keep getting confused about where it should go and how good it would be for telling me what's happening in the chamber.

I use this article for the plug reading: http://www.dragstuff.com/techarticle...ead-plugs.html

Is this probably OK? I know I'll never know if I'm at 15 or 11, but I can tell when the plugs get black.

19 at idle? That sounds kindof weird.

[dieselgeek]

Re: Air/Fuel Ratio help

On our bonneville Studebaker, which runs that crazy c118 stuff from ERC, if you try to run any richer than 12.5 or 13:1, the mixture won't even burn!

DG do you think that dropping the octane would allow you to run richer?? Meaning that a lower octane fuel burns easier.

low octane fuel burns faster

not because of the lower octane.

[horsewidower]

Re: Air/Fuel Ratio help

mine idles at 19

this merits an explanation.

Engines with the kinds of cams that are bleeding off cylinder pressure at low RPMs, make a wideband sensor USELESS for tuning idle and part throttle. Jeff's motor is passing so much unburnt oxygen at idle, all that we can do is tune for max vacuum and by "ear."

Under load however, and at higher RPMs, the wideband sensor starts working as intended.

Thank you for that, I about fell out of my chair.

[Eric68]

Re: Air/Fuel Ratio help

Scott is dead on with his cruise in the high 13's recommendation as a starting point. Depends on where you live, but if you have lots of hills to go up and down that is where you want to be -- a carb will definitely lean out pulling up a hill and richen up going down a hill. If you live where it is dead flat then you can get away with pushing the lean limits at cruise, especially with a small cam / mild combo.

One other thing to be aware of regarding cruise. If you see a big change in AFR when going up hil VS going down hill when cruising chances are that your high speed bleed is too big. I found that a lot of aftermarket stuff comes with huge high speed bleeds (lots of emulsion air looking for big power numbers at WOT) that make cruise AF ratios oscilate and in general get whacky on you. .028-.030" seems to be a sweet spot for most of the stuff I play with . . .just something to keep in mind.

At WOT mid-12's usually makes best power. With modern combustion chambers and good intake manifolds (modern race stuff) best power sometimes is a little leaner (like 13.0:1). Blower stuff will often make best power way down in the mid 11:1 AFRs.

Don't forget about idle too. Lean idle might save gas and spark plugs, but I've found that 13:1 seems to provide the best stability. Leaner and the engine may want to stall during hard braking, and richer it may tend to load up and foul plugs.

By the way, my wide band seems to work fine at idle. Maybe they don't with real big cams, but reversion at idle doesn't pass fresh air out the exhaust, it passes exhaust gasses into the intake creating an EGR effect. Burnt exhaust gasses in the intake are mostly inert and should not effect what the wideband is reading -- unless the engine is misfiring and then the unburnt oxygen would in fact show as lean on a wide band. If you richen up the idle mix enough to prevent misfiring the wideband should read OK.

PS. Dang Scott. You turning into a carb guy on me? ;D

[calico camaro]

Re: Air/Fuel Ratio help

well, there is more to carb tuning than mixture screws, air bleeds and main jets. non flat ground is the reason we have a selection of power valves, pump cams, and squirters to work with. not to mention making the timing jive with all of the fuel changes.
but big cams will definatly give you a false lean condistion on your sensor. thus the reason open loop idle is pretty much a must for the big donkey dick cams out there for the lsx stuff these days.

not saying any of the suggestions are wrong, i just like to go as lean as possible without any adverse effects. some seem to work from the fat side up and i guess i work from the lean side down. both work, just my way of doing things

[milner351]

Re: Air/Fuel Ratio help

Great thread guys - thanks for sharing all your thoughts. I'm still mostly a carb guy - but someday I'll run an EFI set up.