After the lightweight racing seat install, our second quickie performance improvement was to install a lightweight underdrive pulley. This helps out in three ways. First, it reduces the power loss through rotational inertia the same way a lightweight flywheel does. Second, the smaller diameter spins the accessories (water pump and alternator) slower, reducing the parasitic power loss. And finally, it takes weight off the nose of the car. The results of our install were dramatic. So much so, that we’d recommend this enhancement early on in the development of any STS2 car. It’s cheap and it works.
But first, a note of warning. Much has been said in various Internet forums about the possible downsides of these pulleys. Most of this centers on the loss of the original pulley’s damping function on crankshaft vibrations. As a dual-mass unit, the OE pulley helps to reduce the effect of these vibrations on the main bearings and the oil pump, which is driven directly off the crank. There is anecdotal evidence that suggests a link between aftermarket pulley installation and oil pump failure on the Miata, especially in forced induction motors. The “tribal wisdom” in the Miata community looks more favorably on such installations on normally aspirated engines. If considering such a modification, do some research.
The pulley we chose was from Unorthodox Racing. Note that there are two versions depending on whether you have one of the dreaded early “short-nosed” crankshafts from model years 90-91, or the updated version found starting mid-year 1991 through 1993. 94-97 cars have only a single part listing. These pulleys come with both the alternator groove and the one for power steering and a/c. Since our car is the more “manly” base model with no PS or a/c, the unnecessary second groove was just added weight. Ah, but that’s what machine shops are for!
With pulley in hand, a quick visit to our good friend, and perennial F-Prepared Z-car fast guy, Tom Holt was made at Dell Speed and Machine. Tom chucked it up in a lathe and easily removed the offending excess aluminum. 20 minutes later, our pulley was much more svelte. A stop off at the post office on the way home gave us the confirmation. The OE pulley was 4.6 lbs and the UR pulley was 1.2 lbs, as manufactured. Our single-groove version netted out at 0.9 lbs. That’s a total of 3.7 lbs of rotational weight we removed.
Installation was relatively painless since the motor had been apart recently. Stubborn crank bolts can be a real pain, but ours came right off with an impact gun running on a small home compressor. To make the job easier, we raised the radiator up out of the way to gain a straight shot at the pulley. Removal of the plastic undertray, the sway bar, the fan and one piece of the plastic intake tract allowed the radiator to be moved up and back about six inches.
When installing a new pulley, its critical to get the woodruff key back in correctly, especially if you have one of the short-nosed cranks. There are articles on Miata.Net in the Garage section that detail crank failures as a result of incorrect reassembly. Read them and heed them.
With the car back together a quick run up/down the street was in order. Wow! A big difference in the way the car revved was immediately noticed. And the butt-dyno was applauding as well. For point of reference, before-and-after dyno tests on small Honda motors have shown a consistent 2-3 lb/ft increase across the entire torque curve. We believe it.
If considering a crank pulley for your Miata, you might look to the OBX parts sold on EBay. We went with the UR pulley since we have had good experience with them in the past, and also because we needed very quick delivery to meet our travel schedule. Others in the Miata community have had good luck with the OBX parts, which are only half the price of the UR version.
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