Air/Fuel number controversy

[Justicem]

Got a question, a lot of people are really concerned about having the a/f mix at 11.5 to 11.8 when they are getting dyno'ed. Now, from what i understand, the best place to read this is before the cats. If you are taking the reading after the cats(o2 bung or at the tailpipe) don't you have to take into concideration that the cats are eating up some of the fuel therefore if you are taking your reading at those points and you are at 11.8, you are actually runnig a little rich. see what i'm saying? a guy that is running straight x pipes with no cats and a guy running cat being read at the tailpipe should not both be equal in A/F readings.

:shrug:

 

[mystic1]

could be. But I don't know if anyone has done any cat testing to find out if they eat up that much on the air fuel ratio.

 

[PhillyCobra]

It's not so simple because modern cats also do chemical reduction (breaking down NO to N2 and O2) as well as oxygenation. If they just oxidized (change CO and HC to CO2 and H2O) they would result in a richer reading by using up left over O2 in the exhaust. Overall, they probably don't affect the A/F reading much (probably less than 1/2 point but variable under different situations).

 

[KiLLaBytE]

solution: remove cats!

 

[marcseidler]

Why are you trying for 11.5? I shoot for 12.0's across the board.

 

[Justicem]

so, has anyone taken a base line reading on a stock cobra and had the a/f read? if so, what was it?

 

[Bad03Cobra]

Originally posted by KiLLaBytE
solution: remove cats!

Thats what I say and also my bung is welded in the H pipe right after the manifold.........11.80 is the way to go. 12.0 is starting to lean it out a little and if you want your motor around for a while you might want to stick with 11.5 to 11.8.......

 

[jimh]

Originally posted by Justicem
so, has anyone taken a base line reading on a stock cobra and had the a/f read? if so, what was it?

It's been a while but from what I remember I think mine read 10.0 in the upper rpms. I have a Wideband COmmander and the bung is welded pre-cat. It registers as low as 10.0 though. This is going just off memory though.

 

[2K3BRA]

My turner uses 12.0 on FI cars I've seen stock GT bottom ends last for a eternity on his tunes. Another thing to look out for is timing. A/F is a half the equation of saving a motor timing is the other half.

 

[flames]

Mine was dyno'd two weeks ago-a/f was just shy of 12.
It runs just like it should.

 

[Justicem]

thread is starting to get away from what my original question was. Being, shouldn't you account for the cats when taking a reading of a/f after the cats?

 

[fiveoguy]

When I dyno'ed mine, I asked the guys at the shop the same question. They told me that they had dynoed several cars before the cats and after the cats and generally got just a little less than 1 point leaner after the cats. This was not done on 03 cobras, but should give you an idea. If it were me, I would try to stay under 12.0 after the cats. You are right, the best thing do do would be weld in a seperate bung before the cats for a/f readings. I would assume you would also get slightly different readings using aftermarket vs stock cats.

 

[Chonger]

On the week of 4/26 I had my car dyno'd 2 separate times at different speed shops. The first time the AF was measured at the 02 bung and the second time it was measured at the tailpipe. With the exact same tune and no cats the tailpipe read 1/2 point leaner after 4,500 and 1/2-2/3 point leaner before 4,500. The chart that was measured at the tailpipe reads 13:1 till just after 3,500 and drops to 12:1 from 4,500 to redline. The same tune reads 11.5:1 from 4,500 to redline and 12.2-12.3 before 4,500. The tailpipe will read leaner, especially at the lower RPM's. The question is how much. It is not safe to automatically assume .5 point or .75 point because each car and dyno is different. If you want a true and accurate reading you have to get the AF measured in the 02 bung.

I am by no means an expert on dynos or AF ratios but have seen for myself the difference.

 

[IronTerp]

Had mine dyno tuned at Amazon in March and A/F was measured after the cats ..... Read 12.2 to 12.4 from 3,500 to 6,200. Rick said this was what he wanted....that measuring A/F after the cats was .5 to .6 leaner than actual......

 

[1 BLOWN V8]

I also have heard that besides your A/F you should also watch your Timing at WOT. Very safe is 20-21 somewhat unsafe 22-23 and 24 and up your asking for it.
11.2 to 11.8 is a great set-up and safe
Ken