I've got a mildly-tuned EJ20 and recently I've been noticing some small detonation events when hard on the accelerator, but this only happens when the A/C is on. This makes no sense to me, because when the A/C is turned on, the boost is actually capped lower, and the coolant temps (and therefore cylinder temps) are also lower. The detonation happens right when it hits full boost (4000rpm) and has also happened at partial throttle at that RPM. Is the A/C belt enough of a drag on the engine that it could cause timing problems? Why would it not detonate at 16.5psi/195-degree coolant temps, but detonate at 16.2/186-degree temps?

For reference: stock EJ20, stock turbo, STI intercooler, 3" turboback, tuned to a very mild 210whp w/ 85% duty cycle at WOT.

 

Thats strange. I have a very similar setup making 210whp on a ej20. I almost never run my AC though so I've never experienced this problem. I'm also boosting the same as you. I'd be interested to know the reasoning behind the detonation.

 

I ran a bottle of Subaru-label injector cleaner through it about a month ago, so a clogged injector or fuel delivery problem isn't it. The engine has 57k on it, so it may be cylinder or valve deposits, but I've never used anything but Shell V-Power 91 in the motor. In any case, the only thing changing is the A/C being on... I'm just not happy with that gravel-in-a-shaken-can sound when coming out of the corner

 

Well the difference is only a marginal 0.3 psi, right? I'd imagine that the AC does put quite a load on the motor when its running. I've seen vehicles have hp/tq loss at the wheels on the dyno between AC and no AC

Maybe run a colder spark plug? Does your detonation happen on part/full throttle applications going uphill (AC on and off)?

 

Yes, it does happen uphill (max engine load) but only with the A/C on -- that's what's puzzling me. If I had detonation at partial throttle without the A/C on, I'd dial back my boost and replace my plugs, but it only happens with A/C.