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An aftermarket supercharger pulley can make power gains quicker, cheaper, and easier than most bolt-on parts in existence. It's a simple part that changes a not so simple system. On any belt-driven blower, pulley diameter changes the drive ratio, which changes blower speed, which changes airflow and boost. The faster the blower spins, the more air it flows and the more power making potential it has, that is until heat, fuel, and belt traction become the limiting factors.
Smaller blower pulleys spin the supercharger faster at the same engine RPM. On the flip side, bigger pulleys slow it down. SPE’s pulley systems are offered in sizes ranging from 2.27" to 3.2", specifically to let you move boost and power up or down in controlled steps.
Rule of thumb (not a promise): every 0.100" smaller equals roughly 1 PSI more boost (altitude and equipment dependent).
For cross-reference, Whipple publishes a similar guideline: 0.125" change equals roughly 1 PSI.
Example Using This Rule-Of-Thumb:
−0.100" = +1 psi
−0.200" = +2 psi
−0.300" = +3 psi
Why “smallest pulley” isn’t automatically “best”. Because blower speed has real limits and consequences:
1. Supercharger speed limits (mechanical + durability)
Every blower has a safe operating speed range. Exceed it and you’re asking the bearings, gears, and rotor pack to live a harder life. Even if nothing explodes, efficiency can fall off.
2. Efficiency drops as you push it
Overspinning often raises discharge temp faster than it raises usable airflow. Hotter air is less dense, so you can end up with “more boost” on a gauge but less effective oxygen (and more knock risk).
3. Heat becomes the limiter, not airflow
If the intercooler/heat exchanger can’t keep up, intake air temps climb, the ECU pulls timing, and power becomes inconsistent (the car feels strong once… then “meh” on repeat).
4. Belt slip becomes more likely
Smaller uppers reduce belt contact area and increase belt demand. That’s why wrap/idlers and pulley traction design matter when you get aggressive. (Slip = boost drop-off at the worst time.)
While not 100% necessary for most small changes (we've done the math for you on our pulley's), matching blower speed to your blower’s specifications is key especially if you’re making drastic changes. A commonly used calculation for ideal blower input RPM is below. This info would then be used, along with the blower’s specs, to get a good starting point.
Blower input RPM = (crank pulley diameter ÷ blower pulley diameter) × peak engine RPM.
1. Tune / calibration
You’ve changed airflow and load, tuning changes almost always make a big difference after a pulley change..
2. Belt length
Many pulley changes require no belt change, and many do. Always use the manufacturers recommendations here..
3. Belt wrap / slip control (especially 2.6" and smaller)
Belt traction becomes the whole game as boost demand rises. SPE’s fixed and adjustable auxiliary idlers are designed to maximize wrap (up to 270° / 270+°) and help eliminate slip, and both are ideal pairings for 2.6" and smaller pulleys especially.
4. Install details
Special tools are often required. SPE pulley kits commonly include tools like a blower shaft holding tool and custom hub socket, and note that anaerobic sealant is required.
Gains vary with fuel, heat, and traction—but pulley + tune results can be substantial. Some third party testing results:
Off Road Xtreme documented a 2023 Raptor R baseline of 532 rwhp, a tune-only jump to 622 rwhp, and then 653 rwhp / 551 lb-ft after adding an SPE 2.7" pulley (plus a thermostat) with revised calibration. Thats a 121 rwhp gain total.
On the GT500 side, Livernois has published dyno content showing 130+ hp gains with a 2.65 pulley and tuning in their testing.
Check Out Some SPE Motorsport Pulley Parts Here: https://www.spemotorsport.com/search?q=pulley*
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