Air/Fuel Ratio help

[std]

I'm wondering where you guys run ypur a/f ratio. What do you look for cruising vs wot?

I'm working on a 400ish HP sbc, iron heads, 10:1 compression, automatic trans.

Thanks

[dieselgeek]

Re: Air/Fuel Ratio help

What fuel will you run?

Good starting point is mid/high 13s at cruise, WOT 12.2-12.5 is usually what a naturally aspirated engine wants, that's gasoline...

-Scott

[std]

Re: Air/Fuel Ratio help

Just 93 octane. It's just a street car. Thanks

[calico camaro]

Re: Air/Fuel Ratio help

dang scott, i know your big into tunning and all, but you like them on the rich side dont ya. we like them lean and mean around here.lol

[DanStokes]

Re: Air/Fuel Ratio help

Stociometric (sp?) is 14.7 at sea level. A bit richer than that is safe. If you have a wide band O2 sensor and/or access to a dyno you can dial in a bit closer to that safely, but it's hard to argue with Scott's recommendation. I'm just a dyno guy and never claimed to be a tuna - er - tuner.

Dan

[std]

Re: Air/Fuel Ratio help

I need to be richer cruising and leaner WOT. I'm in the15s cruising and the low 11s WOT

[calico camaro]

Re: Air/Fuel Ratio help

if your not having any adverse effects from cruising then you will be fine there. high 14's low 15's is what i shoot for. lean the WOT out and you should be fine. alot of people error on the fat side IMO. i always shoot for 12.8-9 range WOT then tweek it from there. most the time im in the 13.3 range for max power. you will see power drop off befor it hurts anything going lean. being a carbed sbc with iron heads i would proabaly shoot for 12.6-12.8 WOT if it were me.

[TC]

Re: Air/Fuel Ratio help

Do you plan on using the Bottle??

If so, error on the rich side.

[std]

Re: Air/Fuel Ratio help

Bottle? I'm using a beer bottle right now and I used a vodka bottle the other night. Oh yea, I don't want to forget the wine bottle. None of those got me to the rich side.

We were thinking maybe a small shot at some point, but not right now.

[milner351]

Re: Air/Fuel Ratio help

I'll second the high fourteens cruising - hard to say how accurate the O2 sensor is, and depending on location - you can get some variation in readings without changing the mixture.

Power will certainly drop off quick if you go too far into the 15s, and I'd imagine you'd start to get detonation and or lean back fires at that point too... I don't have much experience tuning a carb that lean.... but that's where OEM EFI engines spend the majority of thier time.... they have to in order to keep emissions in check and not load up the catalyst.

[dieselgeek]

Re: Air/Fuel Ratio help

dang scott, i know your big into tunning and all, but you like them on the rich side dont ya. we like them lean and mean around here.lol

nah, I always tell a carb guy to start at mid 13s. because every one of them that's ever come to me saying " it seems to run great" is always something like low 11s.


Start at mid 13s in cruise, then start whittling away at it slowly to get it as lean as you can during cruise, to where it won't misfire. But make sure that "slight uphill" cruising isn't running in the 15s because you need the AFR to progressively approach mid 12s as vacuum decreases (load increases).


The other thing about a carb set up for real lean cruising, is that it can make an engine run like CRAP during cold start and warmup. Got choke?

[SpiderGearsMan]

Re: Air/Fuel Ratio help

geek sets em too lean - he fried that buick a few times

[TC]

Re: Air/Fuel Ratio help

I look at it this way, it's not like if your AFR's are off a little that's it's going to rob you a bunch of HP. I'd say what are we talking 5hp on a 300hp motor if that.

I mean everyone knows you make the best power with a mixture that's on the lean side, but running it a little on the rich side isn't going to make a huge difference.

Put it like this if you running professional racing where every once of HP is need then I'd worry about it, other than that drive the thing and have fun. ;)

[SpiderGearsMan]

Re: Air/Fuel Ratio help

a rich motor is a happy motor

[dieselgeek]

Re: Air/Fuel Ratio help

I look at it this way, it's not like if your AFR's are off a little that's it's going to rob you a bunch of HP.

Quoted for accuracy. Couldn't agree more here. Contrary to spidey-hallucinations, I run them as rich as they'll tolerate before a big power drop off.


Sometimes, though, that number is different. 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! so I tune that one almost entirely by "feel" - richen it up until it misfires, then back off just until it smooths out. Then, I let the EFI take care of the difference in atmosphere changes - we ran 12.6:1 AFRs through the pulls on the dyno at 800' density altitude. 6600' density altitude, we logged 12.6:1 AFRs all week at Bonneville.

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