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 Post subject: optimum AFR for power
 Post Posted: Fri Oct 05, 2012 9:13 pm 
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Got a question about AFR , and power. I know most engines like or make most power between 12.5-13.1 roughly. My question is, is that for sea level or is there a specific DA that that comes into play? Like last week it was 80 and my engine ran better leaned out to 13.1-13.4 then at 12.7. so this sunday it willl be like -200 in the morning, will that 12.5 come into play or does the engine need to be even richer?

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 Post Posted: Fri Oct 05, 2012 9:36 pm 
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The optimum ratio for power is 12.8 in an engine with full vaporization and complete burn of available oxygen. Many engines make maximum power at a different ratio and the reason for that is there not burning the mixture optimally.
Many times I see engines that make more power with an increase in air temperature rather than an increase in density. The reason for that is the particular engine lacks heat from dynamic compression.
Some methanol engines are reactive to humidity, that's reasonably normal for alcohols as there is a substantial percentage of water made in an alcohol burn compared to a gasoline one.
To figure out the 'personality' (as i call it) of an engine you have to record the atmospheric conditions and compare them as individual conditions rather than a DA calculation.

If your engine is requiring the large range change your saying then the engine could do with reconfiguring. Your comments are typical of a low power per cube design. Stock engines are low power per cube but they are the result of years of development to eliminate the issues you are experiencing. The camshaft choice is the main culprit.
Its not enough to say the engine has a certain compression pressure you have to also consider the compression time. It's the compression pressure and rate and time of compressing the mixture prior to ignition that's important.

Prior to running in -200 air you should probably richen it to in the area of 12.5 but I wouldn't go richer. It would probably be wrong to think that the AFR should be say 12 for -200' because eventually you will run into not enough energy (heat) to vaporize the fuel load. This is where you have to fudge things and rely on octane to get you through.
One main jet size increase in a barrel will change the AFR on a Holley by approx 0.5 ratio in that barrel.


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 Post Posted: Sat Oct 06, 2012 7:16 am 
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Okay thanks, The reason I ask is cause after i have a higher float setting in front and raised the back just a tad too, my meter yesterday on a 3 sec WOT hit now showed 11.9-12.3. I will leave it where it is and watch my meter and the mph tomorrow on the first run. i do know now if i need to lean it i could just change out those back bleeds, and it will give be about .75 on the afr change from what i seen last saturday when testing .

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 Post Posted: Sat Oct 06, 2012 7:51 am 
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You realize that with having a float setting that different front to rear you may have an e-hole covered in the rear and not in the front O:) or vise versa.......

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 Post Posted: Sat Oct 06, 2012 7:55 am 
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Beretta wrote:
You realize that with having a float setting that different front to rear you may have an e-hole covered in the rear and not in the front O:) or vise versa.......



Yes I do :).. That is why i wanted to get the front up from before cause i knew it was under the top emulsion up there hence my float thread, and could be effecting my metering, actually the front and back are very close to one another now, I meant I raised the back also to now match my front? I also have pressure set now at 6.5. Sound better \:D/

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 Post Posted: Sat Oct 06, 2012 8:01 am 
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Darby,Did you read this??? Very helpful when testing different float levels..
viewtopic.php?f=6&t=7865
One other thing. If you set the floats with a 3/8 drill there going to be real close to each other and go from there with the Nut "flat" method for adjusting..

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 Post Posted: Sat Oct 06, 2012 9:00 am 
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Location: Montgomery, Alabama
Quote:
To figure out the 'personality' (as i call it) of an engine you have to record the atmospheric conditions and compare them as individual conditions rather than a DA calculation.


That's what Vapor Pressure or Water Grains is for.

Relative Humidity means nothing by it self because it's just that. "Relative"\

http://www.altalabinstrument.com/glossary.html#vp

http://www.altalabinstrument.com/glossary.html#r

http://www.altalabinstrument.com/glossary.html#gr

http://www.altalabinstrument.com/glossary.html#d


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 Post Posted: Sat Oct 06, 2012 9:20 am 
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Beretta wrote:
Darby,Did you read this??? Very helpful when testing different float levels..
viewtopic.php?f=6&t=7865
One other thing. If you set the floats with a 3/8 drill there going to be real close to each other and go from there with the Nut "flat" method for adjusting..


Barry I did read it, and I know tom from my pontiac board, and have spoke with him on the phone when i was setting up my 850 :)

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 Post Posted: Sat Oct 06, 2012 9:23 am 
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Scott Smith wrote:
Quote:
To figure out the 'personality' (as i call it) of an engine you have to record the atmospheric conditions and compare them as individual conditions rather than a DA calculation.


That's what Vapor Pressure or Water Grains is for.

Relative Humidity means nothing by it self because it's just that. "Relative"\

http://www.altalabinstrument.com/glossary.html#vp

http://www.altalabinstrument.com/glossary.html#r

http://www.altalabinstrument.com/glossary.html#gr

http://www.altalabinstrument.com/glossary.html#d


I have always went by the DA for my car and recorded the Performance accordingly. My car usually will pick up 3 tenths from hot 90 degrees to 50 or colder, but then the track comes into play also for traction, and my track is known to be loose :(. It sucks they spray if they spray only out to 60-90 ft thats it! the rest is like take your chance on keeping it stuck!

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 Post Posted: Sat Oct 06, 2012 9:14 pm 
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looking more like -500 by morning now!!!

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 Post subject: UPDATE!
 Post Posted: Sun Oct 07, 2012 6:29 pm 
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All new bests today Had great air, and I Am still trying to see what this car likes for shift points etc, and by the end of the day car ran, 10.60 on the brakes at 1100 ft mark, was a 10.56 Pass according to what the car runs at the eighth using my 1.575 for the calculation for 1/8th mile. Ran 6.71 to the eighth at 102.11. 60 ft on that run was 1.49. Lifted and carried the wheels nicely! Car does not like timing only 27-28 total!! Still only shifting at 6000 too. Jetting appears to be a tad lean yet cause in the morning air was under sea level and couldn't run much past low to mid 60's, at 125.67 but as the day went on and air got worse even tho it was great it started to pick up even more. I totally believe there is a high 10.40 in it yet, The meter showed it was low 12's for AFR , But is about 12.8 in the middle and kinda spikes to low 13's on each shift change. Will post the graph from one of the later runs for you to look at.

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 Post Posted: Mon Oct 08, 2012 7:30 pm 
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So How Do I straighten that fuel curve out above? Is it possible the power valve has something to do with it in that area? Cause its solid in high gear and pretty flat.The car did run 126.67 sunday! Probably would have been 127+ on that last run of the day. With the air in the mrning under sea level shouldn't have my 12-2-12.5 been rich enough , or is it possible my scoop is pulling in alot of cool fresh air? Cause as the weather turned to about 500 ft above that is when the car really started to like the 6000 shift point in the morning it wasn't liking it as much, indication of being lean? Looks like the race this saturday will get rained out and we will race sunday, but weather will be much worse than what we had sunday, looking like mid to upper 50's with 40 dew points etc.

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 Post Posted: Tue Oct 09, 2012 8:49 pm 
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Location: Auburndale, Florida
Refresh my memory. Is this a powerglide car?

Next, pay no attention to what the optimum full power AFR is. The car will tell you what that is. You have to ask it what it wants. That gets done by adjusting the main jets. In this case I would jet down and see if the MPH picks up. If it does, jet down again, and again until it slows down. At that point return the jetting to the point it made to best MPH. Then you can look at the graphs and see where your AFR was when you were in the sweet spot.

When that you know where that sweet spot is we can tailor the fuel curve. But, I'll tell you now. Without an RPM trace you're pretty much going to be lost. Without it you won't know at what RPM the ups and downs in the curve occur.

The problem with these AFR thingies is that we become slaves to them. They'll drive us off into the weeds if we ain't careful. At that point we have to get back to basics.

You can do this. You're already one of the baddest cats at your track.


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 Post Posted: Tue Oct 09, 2012 9:47 pm 
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nomad wrote:
Refresh my memory. Is this a powerglide car?

Next, pay no attention to what the optimum full power AFR is. The car will tell you what that is. You have to ask it what it wants. That gets done by adjusting the main jets. In this case I would jet down and see if the MPH picks up. If it does, jet down again, and again until it slows down. At that point return the jetting to the point it made to best MPH. Then you can look at the graphs and see where your AFR was when you were in the sweet spot.

When that you know where that sweet spot is we can tailor the fuel curve. But, I'll tell you now. Without an RPM trace you're pretty much going to be lost. Without it you won't know at what RPM the ups and downs in the curve occur.

The problem with these AFR thingies is that we become slaves to them. They'll drive us off into the weeds if we ain't careful. At that point we have to get back to basics.

You can do this. You're already one of the baddest cats at your track.


Hey Bruce No its a t-400 stock gearing in the trans. Yea I have a rpm convertor that came with my LM1 Just never looked into how to hook it up, and with only two times left to get out probably will tune the ole fashioned way by the jetting for mph way til spring comes. I hear what your saying about the AFR, I have a fiedn that has a 565 chevy and it runs way lean ,but that is how the motor likes it and it doesn't hurt it.

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 Post Posted: Wed Oct 10, 2012 6:18 am 
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I brought the car to work today its like 30 degrees, DA -700 and i stepped it down and it was 12.5-12-9 range then i let off in high gear. cruising is now around 12-13. range which i figured it would be rich with a 94 jet compared to a 86 jet in front. I just wanted to see where the AFR was with the 94's and the bigger 24 MAB in back, so now the carb is all square. I only took it up to 5000 grand tho , shift at 6000. Well see at the track.

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