I did four runs on a Dynojet dyno this past weekend. All my runs reported that my car had an A/F between 13.5 and 15. I thought the perfect mix was 12.5. Everybody in the shop was amazed my engine didnt blow.
These are my mods: Pop charger/ Z tube/RT High Flow Cats/ Fujibituso Cat Back

 

Hmm.. if those numbers are accurate that does seem quite lean. Where did they take the measurments from? I'm guessing from the tailpipe with a sniffer? Technically the perfect mix or stoich is 14.7:1 I believe where the most gas and air is consumed in the combustion process.

 

You guessed correctly, A/F was measured by sniffer in the tailpipe. I have heard the 12.5 is stoich.

 

That does seem odd. I'd take it to a different dyno and run it again just to confirm that it wasn't a problem with the original dyno.

I also wonder about the cats. I don't know what skew would result if they were not working properly.

 

I plan on doing another dyno run, should I leave my mods on the car or run stock?

 

No stoich is 14.7:1. See http://auto.howstuffworks.com/catalytic-converter1.htm

 

You are correct about stoich being 14.7, but that is only during highway cruising when the your fuel map is in a closed loop. When you mash down on the pedal the A/F ratio should drop(increase fuel) to 12.5 to 13.

 

For a NA engine and with those "breather" mods, that's about right. I was running a 15.9 with my short ram intake, hi flo cats and injen exhaust when I was NA. I wouldn't worry.

 

plus, when they take the reading for the exhaust tip, expect anywhere from .2 - .5 righer number at the exhaust manifold...the cats and exhaust system tend to lean the exhaust out before it make it to the exit.

 

Did you have a CIL light with those mods, I do. I have ordered a code reader to find out what the code is.

 

Just stop by any auto parts store like Pep Boys or Advanced Discount Auto Parts, they have ODBII scanners and will check it out for you for free and give you the print out too.

 

nope...in fact, no lights have come on due to the SC set up...

 

With around a full point difference between pre-cat and tailpipe readings, I'd say you are OK. You really need to get a pre-cat measurement to be accurate. You are likely running a tad on the lean side for max power, but still safe, IMHO.

W

 

well, last weekend i did 3 runs on a Dynojet and (with all the mods in my sig) put down 228whp and my AFR was 14.2ish at the tailpipe. so i guess that means that it's closer to 13.7-13.8, which isn't as bad.

 

OK, here is the deal, this must be the 10th time at least this topic was discussed.

Tailpipe measurements are extremely unreliable in most cases. First, if the guys at the dyno were indeed amazed by your car not blowing up, then they don't know which way is up. Now, if that was an FI car, then it's another story.

Second. the tailpipe sniffer could be reasonably accurate ONLY if the pipe that is used to screw in the o2 sensor is well designed AND if the car has no catalytic ocnverters (test pipes). Otherwise, it's hit or miss... they could be correct, but they could very well NOT be.

The only way to take reliable A/F measurements is before the cat, period. Or, if you have no cats, to make damn sure that you don't have even the slightest hint of an exhaust gas leak. A good way to make sure of that, is to weld in a bung for the o2 sensor BEFORE the cat, since the only thing upstream of that is a single flange, and a leak there is unlikely.

I would bet that your numbers are about .7-1.0 units too lean from being a tailpipe sniffer /post cat readings. You will not blow up anytime soon. A near-ideal (not perfectly ideal) mixture would be about 13.3 or so on the low end going to approximately 12.8 on the top end.