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Old 04-18-2008, 05:36 PM   #1
WRC7732
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Default Ideal Air/Fuel Ratio at WOT

Hey guys,

I'm tuning my 77 L-82 4-speed with an air/fuel guage. One of my favorite tools. The motor is a stock bottom end with a comp cam, aluminum edel. RPM heads and intake, and a holley SA 670 carm, and Mallory HEI distributor. Timing is 38 degrees all in, without vaccum advance, with I think it's 54. And no pinging..

My question is: How lean is too lean for the air fuel mixture in the higher RPMs at wide open throttle? I've got her running pretty strong and now I just want to squeeze a little more out, but I don't want to go so lean as to risk damaging anything. So what is a good, safe number that should be good for power? 11.5:1, 12:1, 12.5:1, 13:1, etc.

I'm at about 11:1 right now, and from the a/r graphs I've been looking at, that seems a little rich. I was thinking about shooting for 12 - 12.5:1. Is that too lean do you think?

Awaiting your responses eagerly....

Will
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Old 04-18-2008, 05:50 PM   #2
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The stoichiometric air-fuel ratio (14.7:1) that is the ideal ratio for lowest emissions, but this isn't the best ratio for power. It used to be that 12.5:1 was considered the best power ratio, but with improved combustion chambers and hotter ignition systems, the ideal now is around 12.8:1 to 13.2:1. This is roughly 13 parts of air to one part fuel. It's what combustion engineers call an excess fuel ratio and is intended to ensure that all the air is used to support the combustion process. This is because air is the oxidizer in combustion. Too many enthusiasts think that adding additional fuel beyond the ideal to create a richer mixture will make more power. This doesn't work because you can only burn the fuel when you have enough air to support combustion. That's why engines make more power when you add a supercharger or nitrous--you're shoving more air in the cylinder so that you can burn more fuel. Regardless of the amount of air in the cylinder, it still requires a given ratio of fuel to burn. Add too much extra fuel, and power will decrease.


When it comes to fuel mileage and increased fuel efficiency, this ratio changes again. All new cars run at 14.7:1 air-fuel ratio at part throttle because this is the lowest emission point. But depending upon the engine, it's possible to run an engine at leaner mixtures like 16:1 or more at part throttle to gain mileage. The difficulty with this is that driveability and throttle response suffers at these ratios. Engine response is lazy and stumbles are commonplace. Each engine will be different, but there is fuel mileage to be gained by fine-tuning your carburetor. Don't be intimidated by these lean mixtures at part throttle. You won't burn the engine up since it is making very little horsepower at part throttle cruise--often less than 30 hp.
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Old 04-18-2008, 06:40 PM   #3
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gkull's info makes sense. I don't think you can go wrong following his recommendations here.
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Old 04-18-2008, 07:58 PM   #4
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Quote:
Originally Posted by WRC7732 View Post
Hey guys,

I'm tuning my 77 L-82 4-speed with an air/fuel guage. One of my favorite tools. The motor is a stock bottom end with a comp cam, aluminum edel. RPM heads and intake, and a holley SA 670 carm, and Mallory HEI distributor. Timing is 38 degrees all in, without vaccum advance, with I think it's 54. And no pinging..

My question is: How lean is too lean for the air fuel mixture in the higher RPMs at wide open throttle? I've got her running pretty strong and now I just want to squeeze a little more out, but I don't want to go so lean as to risk damaging anything. So what is a good, safe number that should be good for power? 11.5:1, 12:1, 12.5:1, 13:1, etc.

I'm at about 11:1 right now, and from the a/r graphs I've been looking at, that seems a little rich. I was thinking about shooting for 12 - 12.5:1. Is that too lean do you think?

Awaiting your responses eagerly....

Will
I don't totally agree with what George recommended you run. You asked about a good safe WOT A/F ratio, and I would not recommend that you run the leaner 13.0 or so A/F ratio's he's calling for. Our hotrod 2 valve combustion chambers are not exactly modern state-of-the-art stuff. Your motor would be safest and happiest in the 12.5 to 12.7 range, and you'll still be in the sweet spot for making good hp. Take too much fuel away and hp goes down. Remember one thing and one thing only makes hp, and that is burning fuel. You want as much fuel as you can run without going too rich. I wouldn't recommend that you go any leaner than that. If anything, its way better to stay a little conservative to save your motor. Better safe than sorry.

Last edited by 540 RAT; 04-18-2008 at 08:04 PM.
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Old 04-18-2008, 08:31 PM   #5
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Idle and cruise you can run pretty lean if your motor likes it. Mine doesn't is starts to miss and surge when I get it to 14:1 or over. So I run it at idle and cruise in the 12-13 range and like to get closer to 12 at WOT
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Old 04-19-2008, 10:55 AM   #6
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Are you going to give credit to Car Craft for this info gkull.??...

I did read that also, and found the information helpful. My concern is that seeing as I am not running a modern EFI system, along with all of its safety equipment, I don't want to risk burning up my motor by running to lean.

I'm thinking that between 12.5-12.8 should be a nice spot to be. Any other thoughts or suggestions or personal experiences..??...


Quote:
Originally Posted by gkull View Post
The stoichiometric air-fuel ratio (14.7:1) that is the ideal ratio for lowest emissions, but this isn't the best ratio for power. It used to be that 12.5:1 was considered the best power ratio, but with improved combustion chambers and hotter ignition systems, the ideal now is around 12.8:1 to 13.2:1. This is roughly 13 parts of air to one part fuel. It's what combustion engineers call an excess fuel ratio and is intended to ensure that all the air is used to support the combustion process. This is because air is the oxidizer in combustion. Too many enthusiasts think that adding additional fuel beyond the ideal to create a richer mixture will make more power. This doesn't work because you can only burn the fuel when you have enough air to support combustion. That's why engines make more power when you add a supercharger or nitrous--you're shoving more air in the cylinder so that you can burn more fuel. Regardless of the amount of air in the cylinder, it still requires a given ratio of fuel to burn. Add too much extra fuel, and power will decrease.


When it comes to fuel mileage and increased fuel efficiency, this ratio changes again. All new cars run at 14.7:1 air-fuel ratio at part throttle because this is the lowest emission point. But depending upon the engine, it's possible to run an engine at leaner mixtures like 16:1 or more at part throttle to gain mileage. The difficulty with this is that driveability and throttle response suffers at these ratios. Engine response is lazy and stumbles are commonplace. Each engine will be different, but there is fuel mileage to be gained by fine-tuning your carburetor. Don't be intimidated by these lean mixtures at part throttle. You won't burn the engine up since it is making very little horsepower at part throttle cruise--often less than 30 hp.
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Old 04-19-2008, 12:48 PM   #7
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unless u check the leanest cyl it is a crap shoot.
that said, i've run WOT 14:1 in the leanest cyl w/no problems on the street. of course i run 4 degrees less timing on the 3 lean cyl. to keep them happy.
average readings in the collector helps to make life exciting!
.
11:1 is awful
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Last edited by Matt Gruber; 04-19-2008 at 12:52 PM.
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Old 04-19-2008, 12:53 PM   #8
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You do have modern fast burn heads. If anything you should change your max timing to closer to 34.

This post kind of hashed the A/f ratio over. When we run motors on the dyno at work we really don't look at whether it is 12.5 -13.0 The BSFC are the numbers we try to get correct.

http://forums.corvetteforum.com/show...highlight=dyno

http://www.land-and-sea.com/dyno-tec...using_bsfc.htm
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Old 04-19-2008, 02:32 PM   #9
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I run Vortec head that have the kidney shape combustion chamber and I run 13 to 1 with 34 degrees timing at WOT. If you have older style combustion chamber, it should be around 12.5 to 1 with 36 degrees mecanical timing at wot.

At cruise, I run around 16 to 1 with 55 degrees total timing without stumble. Anything over 16.5 create stumble with my engine.

Finaly, my idle is at 15 to 1 with a close to stock quality idle. I like it when it's sleeper.

Stephan
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Old 04-19-2008, 02:32 PM
 
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