Hi Everyone, I have read with interest the various threads in the past where people have fine tuned the jetting on their carb cars on the dyno by monitoring hp output as a function of air to fuel ratio (A:F). I'm curious how this measurement is made. If I wanted to determine the optimum jetting for my car with its current exhaust system, how would they measure A:F ratio with the car on the dyno? Thanks, Birdman
Stick a probe up the tailpipe on a dyno pull which their equipment will display as A/F ratio. It sniffs what's coming out and reports back.
Birdman, check the posts, Wayne at pelican parts has a digital AF meter. Bought one and had bungs welded into the header pipes, but got distracted by the front end rebuild and a few minor things like bad fuel hoses. I'll spark it up when I get back from the family birthday suaree in san diego after the 4th. best, chris
So the reading is done in the exhauster, after combustion? Hmmm, seems like it would be hard to know the air:fuel ratio reading it that way. Interesting. It's a lot easier than I thought. I was thinking that you had to get a probe into the intake someplace. Thanks Birdman
Actually, it's pretty easy to measure it in the exhaust. If you were in the intake, you'd have to ensure that you had a homogenous mixture (remember, fuel injectors only spray fuel at the intake valves, so the intake manifold isn't full of air/fuel mixture, and you couldn't even be sure it would all be the same). Once it gets into the exhaust the residuals are all the same (from any particular cylinder; cylinder-to-cylinder variation is a whole 'nother matter!). All you have to do is see how much oxygen is left over, and from that you can determine how much fuel there was based on knowing that gasoline typically needs ~14.6:1 A/F ratio!
I spent last Friday on the dyno with my Innovate Technologies AF meter, along with my friends. We had one AF installed in the collector and another I.T. using the proprietary clamp on piece for tailpipe measuring. I was quite pleased to see that the "correct" way of reading A/F was within 0.1 at any given moment from the "temporary" way. Saves me the trouble of welding fittings into my exhaust. (note: only applicable to non-cat cars) Wayne: If you are reading this, Tim S. says you need to keep working on your carbs. Jay M
Check out this thread on the Porsche board - very useful information: http://forums.pelicanparts.com/showthread.php?s=&threadid=165439 -Wayne
