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AFR's guess work or science?

Old 08-22-2006, 04:37 PM
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Default AFR's guess work or science?

I know a little about tuning, so bear with me as i fill in the gaps.

I know that dumping fuel into the cylinders reduces cylinder temps and reduces the risk of dreaded knock. But i've always been told ignition is the difference between a good and a bad tune.

I am guessing the main area's where afr's are key is during transition, ie going from closed to open throttle, if you aren't careful the car will just bog down as it can't burn off the extra fuel?


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Old 08-23-2006, 11:59 AM
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Default Re: AFR's guess work or science?

Ignition is really what affects power. Any AFR between something like 13.0-16.0:1 will get you within 5% of your maximum power, you really just need to make sure that it stays at very least within that range. For boosted cars you want more fuel for 'thermal management' purposes as you mentioned. Changes in throttle are a bit tricky because it is the rate of change in vaccuum that determines how much air will end up in the cylander at that moment. Usually when a car bogs in this case it is not getting enough fuel.

A car may appear to run quite well on a very very small and fatal amount of fuel.
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Old 08-23-2006, 01:34 PM
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Default Re: AFR's guess work or science?

Originally Posted by Gavin
Ignition is really what affects power. Any AFR between something like 13.0-16.0:1 will get you within 5% of your maximum power, you really just need to make sure that it stays at very least within that range. For boosted cars you want more fuel for 'thermal management' purposes as you mentioned. Changes in throttle are a bit tricky because it is the rate of change in vaccuum that determines how much air will end up in the cylander at that moment. Usually when a car bogs in this case it is not getting enough fuel.

A car may appear to run quite well on a very very small and fatal amount of fuel.
what he said!
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Old 08-23-2006, 05:26 PM
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Default Re: AFR's guess work or science?

Originally Posted by Gavin
Ignition is really what affects power. Any AFR between something like 13.0-16.0:1 will get you within 5% of your maximum power, you really just need to make sure that it stays at very least within that range. For boosted cars you want more fuel for 'thermal management' purposes as you mentioned. Changes in throttle are a bit tricky because it is the rate of change in vaccuum that determines how much air will end up in the cylander at that moment. Usually when a car bogs in this case it is not getting enough fuel.

A car may appear to run quite well on a very very small and fatal amount of fuel.
yes.. so i guess with wideband you will know that youre runing wrong..
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Old 08-23-2006, 07:13 PM
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Default Re: AFR's guess work or science?

Or at least with some way to see whats coming out of a narrow band you could make sure youre NOT lean, but for a boosted car you want to make sure its well below 14.7:1 for a lot of the base fuel table.
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Old 08-23-2006, 07:14 PM
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Default Re: AFR's guess work or science?

if you boost your car and you havent a wideband and you like to tune it.. forget it!
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Old 08-23-2006, 07:25 PM
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Default Re: AFR's guess work or science?

Agreed.

Here are the guidelines from the EFI University textbook.
This is in lambda, to convert to AFR multiply by 14.7.

Idle - .90-1.0
Cruise - 1-1.05
Overrun - 1.09+ (I believe overrun refers to being in the higher RPM areas with the throttle closed)
Transition (high load, not boosting) - .88-.90
Low/Medium boost - .82-.87
High boost - .80-.85
Dead zones (areas of the table not normally seen) should be reasonable values which follow the contour of the rest of the table.

This is a GUIDELINE. If you were running, say, an air-cooled & turbo Porsche motor you would want much more fuel to help counter the heat.
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Old 08-24-2006, 01:58 PM
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Default Re: AFR's guess work or science?

Read this about thermal management with Fuel.

http://www.innovatemotorsports.com/resources/rich.php
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Old 08-24-2006, 05:15 PM
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Default Re: AFR's guess work or science?

runing too rich could tofuck the o2 sensor..
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Old 08-24-2006, 05:29 PM
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Default Re: AFR's guess work or science?

Very good article.
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