1997 Mazda MX-5 Miata: BHJ Dynamics Crankshaft Damper
April 26, 2013
That's not an underdrive pulley above, it's a crankshaft damper by BHJ Dynamics. I want to be clear about that, because friends don't let friends install a solid underdrive pulley on the crank of a production-based engine. It's one of the brain-deadest things you can do to an engine. Forget that the reduction in inertia will be essentially nil in light of the manhole cover (a.k.a. flywheel) bolted to the opposite end of the crank. The bigger deal with solid underdrive pulleys is the big gamble they place on the durability of your crank, bearings and oil pump.
Many Miata owners learned this the hard way when their engine's oil pump shattered to pieces shortly after installing an underdrive pulley on their turbo Miata. To understand why this happens, the nerds at BHJ Dynamics have written one heck of a .pdf technical whitepaper for you. If that reading's too dense this early in the morning, try this: imagine what it's like to be a crankshaft.
Every time a cylinder fires, a mighty wallop is delivered to the crank's corresponding crankpin. Then, quickly, the force dissipates. Then it happens again, this time from a different cylinder, and again it dissipates. It's like the crankpins are being struck by hammers, again and again.
During each hammer strike, the crankshaft is rung like a bell, experiencing a brief torsional windup and release. It's a heavy chunk of metal, but in operation the crank actually does twist, bend and distort. In the meantime you've got other, lesser forces acting elsewhere on the crank as it does the business of pushing pistons up on their compression and exhaust strokes, and down on their intake strokes.
The various intertwining frequencies of all that monkey motion excite the crankshaft's natural frequencies at certain points in the rev range, and in these instances the crank gets pissed off, the magnitude of its twist rapidly multiplies. This twist manifests as severe vibration that somehow must be quelled.
Enter the crankshaft damper (a.k.a. "harmonic damper"), a deceptively simple-looking device that is tuned to absorb the twist induced by the torsional vibrations of the crankshaft. It's an elastomer layer sandwiched between an inertia ring and a hub.
OEM engines have crank dampers. So why not just use the OEM one? The reason is because an engine's torsional vibration signature is altered when you dramatically increase its output and/or the rev ceiling. In the Miata's case, its stock crank damper was tuned around the kinds of forces induced by a puny 128 hp. Project Miata's turbocharged long-rod VVT BP engine will be making more than double this amount and revving higher. Bigger hammer strikes. New and exotic frequencies.
Stock Miata crank dampers have also been known to become ineffectual with age. The elastomer dries out and the inertia ring can slip relative to the hub. This reduces the damper's effectiveness, yes, but there's another, more insidious side effect that can arise. See, the timing marks are located on the inertia ring. If you set your ignition timing based on a slipped stock damper, you risk radically over- or under-advancing the timing.
In the two-birds-one-stone category, the BHJ Dynamics crank damper provides for a big upgrade in the crank trigger. It has a 36-tooth (minus one) crank trigger wheel (they can change this pattern for your specific needs). By contrast, Project Miata's stock '97 engine bases all of its fuel and ignition timing decisions on a sensor that's driven off the back of the exhaust cam (stock '96 and '97 Miatas also have a 4-tooth crank trigger wheel that is used exclusively for misfire detection).
These stock cam angle sensor arrangements inherently have a lot of spark scatter due to their low resolution but also the imprecision with which they guesstimate the crank angle. Spark scatter is ignition timing error, which is exactly what you don't want when you're trying to safely extract the most output possible out of an octane-limited engine. Several degrees of spark scatter is not uncommon on a normally operating stock Miata.
The BHJ Dynamics damper's higher-resolution trigger wheel, mounted right at the action, has improved inherent timing accuracy and will introduce much less scatter. Plus, paired with our Vi-PEC V88 ECU, we have the capability of making timing adjustments on a per-cylinder basis. This is a huge upgrade.
Additionally, think about what happens when torsional vibes are allowed to run free, as in the case of an engine with a slipping stock crank damper or when some bonehead installs a solid underdrive pulley instead. The crank drives the cams, and vibes that are not damped are passed directly through the cam drive. This introduces additional errors in the stock cam angle sensing, compounding the already bad spark scatter. These vibes simultaneously throw off the timing of valve events for the same reason. It's a bad situation all around.
So, yeah, don't use solid underdrive pulleys on the crank. And do use a well-engineered crank damper.
--Jason Kavanagh, Engineering Editor