A/F readings
04-01-2002, 01:56 PM
#2
Track Day
Join Date: Apr 2002
Location: Phoenix, Arizona
Posts: 16
Likes: 0
Received 0 Likes
on
0 Posts
Optimum air to fuel ratio by weight is 14.7 to 1 this is known as the stoichiometric ratio.
Air fuel mixture that is less than 14.7 to 1 would be called a rich mixture, lean mixture is one in which ratio is over 14.7 to 1, i.e. less fuel more air.
This is for a non-modified car, factory settings.
If your car is modified in any way then you may require a richer setting to prevent detonation.
Hope this helps.
Air fuel mixture that is less than 14.7 to 1 would be called a rich mixture, lean mixture is one in which ratio is over 14.7 to 1, i.e. less fuel more air.
This is for a non-modified car, factory settings.
If your car is modified in any way then you may require a richer setting to prevent detonation.
Hope this helps.
04-01-2002, 02:26 PM
#3
Race Director
There's two operating modes that the stock DME computer operates in. The first is close-loop operation where the computer takes a reading from the O2-sensor. Then it continually adjusts the mixture to average out at 14.7:1 air-fuel mix. Since there's a time-delay between the time the O2-sensor is read, and the correction is made and the time it takes the mixture to travel from the cylinder to the O2-sensor, the DME is constantly playing 'catch-up'. That's why you see the dithering back and forth on the display. However, the average mixture comes out to 14.7:1 so that's fine.
The second mode of operation is open-loop under full-throttle/high-RPM where the DME computer completely ignores the O2-sensor signal and uses a straight look-up table of RPM vs. injector duty-cycle. This is meters out fixed pre-programmed fuel amounts. The A/F meter would show a fixed reading on the rich side when the DME is in open-loop operation.
The second mode of operation is open-loop under full-throttle/high-RPM where the DME computer completely ignores the O2-sensor signal and uses a straight look-up table of RPM vs. injector duty-cycle. This is meters out fixed pre-programmed fuel amounts. The A/F meter would show a fixed reading on the rich side when the DME is in open-loop operation.
04-01-2002, 04:06 PM
#4
Drifting
Join Date: Sep 2001
Location: Massachusetts
Posts: 2,597
Likes: 0
Received 0 Likes
on
0 Posts
What is considered "on the rich side".
This implies that any thing over 14.7 to 1 is rich. Correct me if I am wrong, because I have been trying to find this out for the last three months, but shouldn't there be at least something like 12 to 1 or .89 to .90 volts.
John
86 951
This implies that any thing over 14.7 to 1 is rich. Correct me if I am wrong, because I have been trying to find this out for the last three months, but shouldn't there be at least something like 12 to 1 or .89 to .90 volts.
John
86 951
04-01-2002, 04:25 PM
#5
Race Director
Well, 14.7:1 is considered stoich and optimal as far as emissions is concerned because all of the hydrocarbons are burned cleanly with no waste. However, we're concerned with maximum power under acceleration and full-throttle, not necessarily the cleanest burn. So, adjusting the air-fuel mixture to the 'rich side" at 12.5:1 gives the most power possible for any given amount of air sucked into an engine:
As you can see, going too rich doesn't hurt cylinder-pressure as much as being too lean. And keeping the engine at 12.5:1 continously rather than dithering around an average would also provide the highest engine output.
However, adjusting your air-fuel ratio to 12.5:1 can only be done with a pre-programmed fuel map since the O2-sensor is not a linear device and can only distinguish cleanly between 'too rich' and 'too lean' on either side of 14.7:1
As you can see, going too rich doesn't hurt cylinder-pressure as much as being too lean. And keeping the engine at 12.5:1 continously rather than dithering around an average would also provide the highest engine output.
However, adjusting your air-fuel ratio to 12.5:1 can only be done with a pre-programmed fuel map since the O2-sensor is not a linear device and can only distinguish cleanly between 'too rich' and 'too lean' on either side of 14.7:1
04-01-2002, 05:08 PM
#6
Drifting
Join Date: Sep 2001
Location: Massachusetts
Posts: 2,597
Likes: 0
Received 0 Likes
on
0 Posts
OK. Here is another question. What is the corresponding voltage output from the O2 sensor that corresponds to 12.5/1 ratio. I have a small meter I am planning to use instead of the ones that light up.
04-01-2002, 05:23 PM
#7
Race Director
Well, you would also need to know the exhaust-gas temperature as well since the output of the O2-sensor changes with temperature:
At 12.5:1 air-fuel ratio, you can have a voltage of 0.85-0.95. But this also encompasses A/F ranges of 11.0:1-14.6:1 as well depending upon temperature.
Best to get a separate wideband O2-sensor with its associated controller and display. Do a search on this board for "wideband" and you'll have plenty of answers to your questions.
At 12.5:1 air-fuel ratio, you can have a voltage of 0.85-0.95. But this also encompasses A/F ranges of 11.0:1-14.6:1 as well depending upon temperature.
Best to get a separate wideband O2-sensor with its associated controller and display. Do a search on this board for "wideband" and you'll have plenty of answers to your questions.
Trending Topics
04-01-2002, 07:07 PM
#8
Drifting
Thread Starter
Join Date: Jun 2001
Location: The old country
Posts: 2,605
Likes: 0
Received 0 Likes
on
0 Posts
Let me run this by you then to see if i'm on the ball here!
If I start at a setting of 14.7 at idle, and then go for a righer (around 11-12) at higher RPM's, then I can't be too wrong until I get some dyno time??
If I start at a setting of 14.7 at idle, and then go for a righer (around 11-12) at higher RPM's, then I can't be too wrong until I get some dyno time??
04-02-2002, 02:33 AM
#10
Addict
Rennlist
Lifetime Member
Rennlist
Lifetime Member
A volt meter will give you very little useful information, it will not be fast enough, most aftermarket gauges have a colour readout as average trends are about all the human eye can take in.
Below is a picture of a bosch air/volt graph, and also a picture of a new product I've been working on, have a look at my web page if anyone is interested or has any suggestions for improvements.
Martin
<a href="http://members.rennlist.com/martintaylor/" target="_blank">click here for more info on a/f meters</a>
Below is a picture of a bosch air/volt graph, and also a picture of a new product I've been working on, have a look at my web page if anyone is interested or has any suggestions for improvements.
Martin
<a href="http://members.rennlist.com/martintaylor/" target="_blank">click here for more info on a/f meters</a>