Turbo AFR numbers...

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acarlson
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Turbo AFR numbers...

Post by acarlson »

I'm throwing this out to those turbo owners who have an O2 sensor installed on their engines.

At idle (about 950 RPM) I have an AFR of a little over 11. Bob Helt has 2 different AFR tables in his book. One shows an idle AFR at 11 and the other shows an idle AFR of around 8.5. I'm wondering what other owners have for their AFR when idling. Also I'm thinking I may lean this out a little to an AFR of 12 at idle. Any thoughts on whether that is a good idea or not ??

Alec
Alec Carlson
Dahlonega, GA
1965 Regal Red Corsa 4 Speed Turbo Convertible
Restoration "In Progress"...
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Nickshu
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Re: Turbo AFR numbers...

Post by Nickshu »

Ray Sedman (who supplied my wideband setup) told me not to trust the AFR readings at idle. Set it where it runs best and forget it. Use AFR to tune your rods/jets to maintain the desired AFR when cruising and under power.
Nick
1964 Monza Spyder Convertible #435 - Rotisserie restored - SOLD ON BRING A TRAILER 4/30/2019 - Check out my restoration thread here: [corvaircenter.com]
Thanks to all the awesome CCF, CF, COG, and CORSA members who helped me with the restoration!
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thewolfe
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Re: Turbo AFR numbers...

Post by thewolfe »

8.5 afr you would be belching black smoke out. Way way too rich. Even 11 is rather rich. My quadrajet idles between 13 and 14. I say set it as lean as you can while still having a quality idle and no off idle stumble. Every carb is different in that regard so you will have to experiment with it.
Nate Wolfe
65 Corsa 180
61 Lakewood 140
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acarlson
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Re: Turbo AFR numbers...

Post by acarlson »

I leaned it out to 12 - seems to idle just fine there. Any higher then 13 and it dies. I like how the engine is running right now. It's supposed to be 57 here tomorrow so I'll get a chance to road test it. I'll report back on my road test AFR numbers - should be interesting...
Alec Carlson
Dahlonega, GA
1965 Regal Red Corsa 4 Speed Turbo Convertible
Restoration "In Progress"...
miniman82
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Re: Turbo AFR numbers...

Post by miniman82 »

Draw through engines in general tend to show more rich numbers than an N/A or injected engine, it's normal. But not 11 or even 8:1, that ain't normal for any gasoline engine!

In the VW world, we have what's called "lean best idle". In other words you want to idle as lean as you possibly can, while maintaining a smooth running quality. You can't achieve this with carb adjustments alone, you must also tweak the base timing since leaner mixtures tend to like more advance. So you lean it out first and it's probably gonna get a little rough, then adjust timing and see if that makes a difference. I've gone as far as having one hand on the idle mixture screw and the other on the distributor at the same time, goes faster that way.

Low compression turbo engines won't take very lean mixtures though, because their combustion is so poor. You'll likely find that around 18 degrees idle timing and 13:1 is a sweet spot like I did. Leaner than that and it tries to stall, richer and you just waste unburnt fuel out the tailpipe.
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