We attempted to dyno test the JK this morning, more on that in a different thread.

One this that was noticed is the AFR reading was 14.5 at idle, and remained there until just before 50% throttle when we aborted the test.

Can anybody comment, and confirm is this is correct? The tester said it should be different from idling to running at 50% throttle.

 

RIPP probably has the most experience with JKs on the dyno. Hopefully they can chime in for you.

14.7:1 is a stoich mixture. Stoichiometric just means there is exactly the right number of oxygen molecules for the number of fuel molecules. In closed loop while idling or cruising most vehicles will run stoich. Although I believe some vehicles may actually run leaner than stoich while idling and cruising. I believe they do this to make absolutely sure every molecule of fuel can easily find a molecule of oxygen during the short combustion cycle. That in theory should maximize economy.

Under full throttle (open loop) most vehicles will shoot for around a 12:1 A/F ratio (somewhere between best rich torque and best lean torque). This is because that while in theory 14.7:1 is the perfect ratio, at high cylinder pressures and high RPM it's not possible to mix the air and fuel perfectly so that each fuel molecule has an easily available oxygen molecule close by. So in theory 14.7:1 could be lean because of inadequate mixing. High RPM reduces the amount of time available for each combustion cycle making the problem worse. High cylinder pressures combined with a lean mixture (even if due to inadequate mixing) can result in detonation (knock). Even if it doesn't induce knock it would be letting unused oxygen leave the engine. Getting oxygen into the engine is much harder than getting fuel into the engine (based on their relative volumes). So to let unused oxygen leave the engine is in effect unrealized performance. So for these reasons under full throttle conditions they err on the side of having extra fuel to ensure that every single oxygen molecule can be used.

The A/F ratios I've seen for the JK look weird to me too. If I remember correctly they seem to run pretty lean. I want to say the charts I've seen have the A/F ratios at 14:1 even under full throttle. As I've said, most vehicles I've seen are considerably richer than that under full throttle for performance and knock prevention reasons. If I'm remembering things correctly and those charts that I've seen are accurate (i.e. a 14:1 A/F ratio under full throttle), I'd be curious if there is some power to be had by going to a 12:1 ratio. At 12:1 I wonder if some more ignition timing could be dialed in.