ideal a/f mixture at idle?
Race Director
Joined: Oct 2000
Posts: 14,991
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From: central FL, near the beach.
ideal for what?
1. least pollution?
2. best power?(switch ac on without stalling)
3. best fuel economy?
4. cool lumpy sound?
5. smooth idle?
6. cold start, no choke, no stall?
1. least pollution?
2. best power?(switch ac on without stalling)
3. best fuel economy?
4. cool lumpy sound?
5. smooth idle?
6. cold start, no choke, no stall?
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Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Ideal AF ratio is 14.7 to 1. Most of the time with a carb that is not going to work well. Start adding big cams, large heads and it goes down. I have been able to run about 13.5 at idle and that is about it. Full power 12 to 1 seems to provide the best power. At cruise if I can get it to 14 to 1 that is great. Most of the time at cruise I am around 13.7 to 1, any leaner and it starts to miss a little.
Cold start, good luck. I run no choke and have to play with the throttle for about 30 to 45 seconds depending on how cold it is before it will idle poorly on its own. After a few minutes of running it will idle good.
Cold start, good luck. I run no choke and have to play with the throttle for about 30 to 45 seconds depending on how cold it is before it will idle poorly on its own. After a few minutes of running it will idle good.
Race Director
Joined: Oct 2000
Posts: 14,991
Likes: 79
From: central FL, near the beach.
should be 12-13 depending on if u read the cyl that is the 1st to miss, causing it to stall.
in the summer i do 14, but if it stalls when cooler, i'll richen it up. Like gordon says, u have to know how to drive with no choke.
in the summer i do 14, but if it stalls when cooler, i'll richen it up. Like gordon says, u have to know how to drive with no choke.
Advanced
Joined: Aug 2006
Posts: 54
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After tinkering with timing and A/F for two days, I ended up with a compromise at 13.5-13.9. That's about the leanest I could run without having a choke...
Don't forget that the idlemixture affects your cruise too.
Mr. Beam
Don't forget that the idlemixture affects your cruise too.
Mr. Beam
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Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
I have done some basic monitoring with my LM1, at idle it sits around 12.3, then when I rev it on driveway, reving upto around 4000 - 5000rpm, the afr will go upto 15.5, is this good or bad??
specs on engine
383 forged motor 9.1 comp
BRC 3.750" crank, bcr 5.7 H - beam, bcr 2618 pistons,hd pins
custom machined blower cam(258/646 112lsa)
custom roller lifters solid
T & D - shaft rockers 1.6 ratio
dart pro 1 platinum alum 215 heads
Weiand - team G Intake Manifold - single plain ( 2800-7200 RPM Range)
quickfuel(holley) 750cfm double pumper, mech sec
thanks
dave
specs on engine
383 forged motor 9.1 comp
BRC 3.750" crank, bcr 5.7 H - beam, bcr 2618 pistons,hd pins
custom machined blower cam(258/646 112lsa)
custom roller lifters solid
T & D - shaft rockers 1.6 ratio
dart pro 1 platinum alum 215 heads
Weiand - team G Intake Manifold - single plain ( 2800-7200 RPM Range)
quickfuel(holley) 750cfm double pumper, mech sec
thanks
dave
Last edited by corvettedave383; Sep 6, 2008 at 02:26 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,365
Likes: 6,345
From: At my Bar drinking and wrenching in Lafayette Colorado
Optimum A/F at idle and low-load cruise is 14.7:1. Most Holley and BG carbs run best with idle mixture in the high 13's. Q-Jets will idle well at 14.5 - 15.0.
However, this is on straight gasoline - most parts of the country now use oxygenated fuels. If you have 10-15% ethanol in your fuel, stoichiometric A/F is around 13.5:1, and you need to set idle mixture to this level in order to idle correctly and to avoid off-idle stumbles.
However, this is on straight gasoline - most parts of the country now use oxygenated fuels. If you have 10-15% ethanol in your fuel, stoichiometric A/F is around 13.5:1, and you need to set idle mixture to this level in order to idle correctly and to avoid off-idle stumbles.
Race Director
Joined: Apr 2000
Posts: 17,665
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
With the LM-1 you can set your carb up to get the idle, cruise and WOT exactly where you want it. Problem is, where you want it and where your motor likes it are usually different.
Don't worry about 14.7:1 or trying to run lean to save gas. Our V8's like it rich so if you can get it idle at 12.5-13:1 and cruise a little higher then WOT a little lower then call it a day. I had mine idling at 14:1 and cruise near 15:1, fuel economy was good but the motor didn't run right
RIght now with my new motor everything is in the 12's, playing it safe for now
Don't worry about 14.7:1 or trying to run lean to save gas. Our V8's like it rich so if you can get it idle at 12.5-13:1 and cruise a little higher then WOT a little lower then call it a day. I had mine idling at 14:1 and cruise near 15:1, fuel economy was good but the motor didn't run right
RIght now with my new motor everything is in the 12's, playing it safe for now
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Optimum A/F at idle and low-load cruise is 14.7:1. Most Holley and BG carbs run best with idle mixture in the high 13's. Q-Jets will idle well at 14.5 - 15.0.
However, this is on straight gasoline - most parts of the country now use oxygenated fuels. If you have 10-15% ethanol in your fuel, stoichiometric A/F is around 13.5:1, and you need to set idle mixture to this level in order to idle correctly and to avoid off-idle stumbles.
However, this is on straight gasoline - most parts of the country now use oxygenated fuels. If you have 10-15% ethanol in your fuel, stoichiometric A/F is around 13.5:1, and you need to set idle mixture to this level in order to idle correctly and to avoid off-idle stumbles.
cheers
dave
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
so if its going into 15.5 when reving engine on driveway, does this mean its way to lean?, is reving quickly on driveway, not really showing true wot under load??
cheers
dave
cheers
dave
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Reving the engine in the driveway will not show much. You need to have the car under full load and WOT to see what it is running. You will never get WOT readings sitting in neutral.
Tech Contributor
Joined: Aug 1999
Posts: 14,365
Likes: 6,345
From: At my Bar drinking and wrenching in Lafayette Colorado
With the LM-1 you can set your carb up to get the idle, cruise and WOT exactly where you want it. Problem is, where you want it and where your motor likes it are usually different.
Don't worry about 14.7:1 or trying to run lean to save gas. Our V8's like it rich so if you can get it idle at 12.5-13:1 and cruise a little higher then WOT a little lower then call it a day. I had mine idling at 14:1 and cruise near 15:1, fuel economy was good but the motor didn't run right
RIght now with my new motor everything is in the 12's, playing it safe for now
Don't worry about 14.7:1 or trying to run lean to save gas. Our V8's like it rich so if you can get it idle at 12.5-13:1 and cruise a little higher then WOT a little lower then call it a day. I had mine idling at 14:1 and cruise near 15:1, fuel economy was good but the motor didn't run right
RIght now with my new motor everything is in the 12's, playing it safe for now
Give the engine what it wants: Listen to it, smell it, and feel it - it's a total sensory experience. Set it up for mid-14's as a starting point if you have pure gasoline, and mid-13's if you have 10-15% ethanol. Then tune it and tweak it to what it responds to from there. The engine will "tell" you what it wants. Don't be afraid to go on the rich side - most performance engines respond very well to a slight rich condition. If you want mileage, old Escorts sell for about $1200...
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Give the engine what it wants: Listen to it, smell it, and feel it - it's a total sensory experience. Set it up for mid-14's as a starting point if you have pure gasoline, and mid-13's if you have 10-15% ethanol. Then tune it and tweak it to what it responds to from there. The engine will "tell" you what it wants. Don't be afraid to go on the rich side - most performance engines respond very well to a slight rich condition. If you want mileage, old Escorts sell for about $1200... also when adjusting a 4 corner screw carb, should you change all 4 screws or can you just play with the primary metering block screws for idle??
cheers
dave
Race Director
Joined: Oct 2000
Posts: 14,991
Likes: 79
From: central FL, near the beach.
.
No, u can't simulate WOT in the driveway; well maybe by smoking some old tires
Last edited by Matt Gruber; Sep 7, 2008 at 07:29 AM.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Ive had a good play with it today, what iam finding is that either iam loading up to much and ehgine slowly idles lower and lower or its to lean, getting a flame out the top of the carb, when you hit the throttle quickly on the driveway, but the a/f isnt showing lean.
what do you think?, maybe change to bigger jets in carb, ive got a hole box of holley jets sizes
cheers
dave
what do you think?, maybe change to bigger jets in carb, ive got a hole box of holley jets sizes
cheers
dave
Race Director
Joined: Sep 1999
Posts: 11,360
Likes: 382
From: Plano TX
However, this is on straight gasoline - most parts of the country now use oxygenated fuels. If you have 10-15% ethanol in your fuel, stoichiometric A/F is around 13.5:1, and you need to set idle mixture to this level in order to idle correctly and to avoid off-idle stumbles.
So regardless if you're running gas, E10 or E85, the meter will show 14.7 at stoich. It's a limitation of the meters. Maybe someone will release a wideband where you can tell it what fuel you're using but until then they will all assume straight gas.
Instructor
Joined: Sep 2005
Posts: 180
Likes: 1
LARS or SWEDE, I have always run mine around 13.5-14.0 with no problems. I put my LM1 on a mopar with a carter and it was close to 20.0, we got it down to 18.0, but it would not run any lower?? I also had it on a AAR cuda with tripower, and it was at 16.0 to 1, Any thoughts? I am using a holley 3310 780 (run in the pure stock muscle car drags). I looked at my data from the last trip to the strip and found that the A/F ratio was right at 12.8-13.0 until i hit 4th gear and then it dropped to 12.0. What do you think? Should i drill out the high speed bleeds? If so how much bigger. Car runs 12.95 @110mph, 454 71 LS6 4 speed 4.11, wide ratio M20