After getting inconsistent misfires in different cylinders, I've decided to go ahead and replace the coils and plugs (a '97 M3 Sedan with less than 90k miles). Anyone tried the high-performance coils from Bavarian Auto? The product description says they will improve low-range torque and high-range bhp, as well as improving fuel efficiency. Is it true, and are they worth the additional $120 over standard coils? By the way, this is my first post, though I've been lurking for years. Thanks! Christopher
No experience with those, but my coils on my M52 motor has +240,000 miles on them and the motor pulls like it did when I first got it 10 years ago. Assuming you don't have a problem other than the coils, I'm pretty damned pleased with whoever BMW used. Are you getting error codes? Have you swapped the coil packs from one cylinder to another to see if the problem moves? A 12 year old M3 with 90K miles? It's not even broken in yet.
I installed them in my E46 as part of a 120K(-ish) mile tune-up and ran them nearly another 100K before parting with the car. In my experience: Any HP and torque increases are indetectable by a butt-dyno (the rest of the tune-up did more for that). Improvements in fuel economy were likewise immeasurable (once again, fresh plugs no doubt mattered more). The hotter spark will chew through plugs, cutting plug service life nearly in half compared to stock coils. They are not as well-shielded against RF leakage as the stock coils and produce plenty of static in the AM band (not generally a problem, but irritating when I needed traffic reports from the local AM news outlet, especially in the fringe areas where the noise would often smash the signal to nothing). Being red and black, they looked reasonably cool on the rare occasions when the trim panel was removed from the valve cover. They definitely produced a hotter spark, as advertised (as evidenced by the accelerated wear on my spark plugs). While that does have benefits, they're miniscule and simply keeping the engine well-tuned doubtless does more. If I had it to do over, I doubt I'd bother.
Thanks, guys. Fortunately, most of my work during the week requires either flying or rental cars, so my baby can sit warm and snug in the garage, waiting for the next track event! I've been getting intermittent misfires in several different cylinders, so I haven't gone to the trouble of swapping them around, etc. I suspect ethanol more than anything, and was going to go on ignoring it, but a few times now, the car has started running very rough, as if more than one cylinder stopped firing at the same time. Shutting the car off and restarting cures it, but it has happened more than once and I thought coils and plugs would be the easiest place to start. Actually, the one guy I spoke with at Bav Auto said about the same thing as Stig, that there was not much improvement and they probably weren't worth the extra money, but I wanted to get some expert opinions from actual drivers (you guys). I think I'll stick with the standard set, new plugs, and see how it goes. Thanks again for the input.
As a technical note, an ignition coil only outputs the voltage required to arc the sparkplug gap. That can tyically be from 12-20 Kv for normal ignition. More coil "potential voltage" output does not provide any performance change in the combustion process. A higher output coil can often fire fuel fouled sparkplugs, improper AFRs, etc. From actual dyno and track testing I can confirm that a higher output coil does not produce more power unless the OE coil is insufficient to fire the intake charge.
