Air fuel ratio checks on the dyno

[Daniel Cragin/DC Performance Inc]

We have been doing quite a bit of air fuel ratio changes on our dynojet and have found that the after the cat air fuel ratio is not very accurate (as much a 1 to 2 points off). For best results, install a fitting before the cat and data log the air fuel from there. Results will be better and safer, especially on a supercharged engine.

Hope this helps.

 

[Jack B]

Dan:

I have probably done over 100 pulls utilizing the oem forward O2 well. I just pull the oem O2 sensor, do not disconnect the connector and insert the dyno O2 sensor. I have yet to pull a code. I don't like the way the a/f curve lags the power/rpm curve when in the tailpipe. If you are in the tail pipe you are guessing at a/f versus rpm's.

What kind of vacuum are you using at the tailpipe sensor.

 

[99 R/T 10]

So what if you have no cats? The number should then be accurate, correct?

 

[Torquemonster]

Dan makes a very valid point.

I've seen the difference between what the AEM says (via pre cat sensor) and what the tailpipe mounted dyno reading says first hand - and the diference is as marked as described - around a whole point.

However - in the case I witnessed it was confusing because the AEM was recording lean (precat) and the tailpipe was rich..... work that one out!



it should have been the other way around with fuel burned in the cat leaning the reading out so may have come down to accuracy of the sensors themselves.

So in this case the tuning centered around finding a happy compromise between the two but weighing more heavily on the rich side on the tailpipe readings. As long as the tailpipe is not reading lean it should be safe enough.

In principle however - there is no doubt that a precat reading SHOULD BE more accurate and more timely - it is closer to the combustion and the readings reflect post combustion mixture - before excess fuel is burned in the cats.

 

[Steve-Indy]

After our last dyno day, I personally place little confidence in Dynojet setup using tailpipe A/F info...useful as a "relative" value...but certainly not as an "absolute" given the numbers that we saw.

 

[Ulysses]

So what if you have no cats? The number should then be accurate, correct?

Not as accurate as taking it as close to the collector as possible. There is also a time lag due to the length the exhaust has to travel to get to the tail pipe so your A/F reading will be skewed in relation to your RPM reading.

 

[Jack B]

Torquemonster said:

I've seen the difference between what the AEM says (via pre cat sensor) and what the tailpipe mounted dyno reading says first hand - and the diference is as marked as described - around a whole point.

However - in the case I witnessed it was confusing because the AEM was recording lean (precat) and the tailpipe was rich..... work that one out!

That is the way it should be, more O2 means lean, less O2 means rich, take a look at this article: http://www.12v.org/engine/index.php?section=hw&sm=o2

 

[Schulmann]

Great Article !
Everyone interested in tuning should read it !

 

[PRVT JET]

How about a car that runs with no cats, is the tailpipe sniffer accurate?

 

[Schulmann]

No and to fix it you need a cat simulator.
Read the article, it explains everything how to tune your engine.

 

[newredrt10]

Explain cat simulator

 

[Jack B]

How about a car that runs with no cats, is the tailpipe sniffer accurate?

--------------------

It is accurate if it is a vacuum system and the sensor is 3-4 ft up into the exhaust. The problem is correlating the a/f with the rpm. The a/f could lag the rpm's by 500-700 on a Gen2. This is also dependent on the system having no exhaust leaks.

There are actually some O2 sensors that measure entities that are not affected by the cats and will show no difference before or after.

 

[PRVT JET]

jack, My car on the dyno was 10.1 a/f with the nitrous. I'm using 46/30 jets, how much lower can I go on the fuel jet without going lean? That was using the tail pipe to measure.

 

[ChicagoGTS]

Now issac don't be getting any funny ideas...
It's amazing how fast you get used to the power isn't it? You bastard you put up those numbers at 10:1, ! That's awesome!!!

More Power!!!

 

[Jack B]

PRVT Jet

First, place the dyno's O2 sensor in the forward oem well. Then try 48's, check the a/f, and keep going up till you hit 11:1. Don't exceed 11.5:1.

 

[PRVT JET]

Thanks jack, I appreciate it. Matt, let me know when your car is done. We'll take a trip to the track and see what it's made of.