Pulley Sizing

Deciding what pulley size to run can be difficult. This page is dedicated to giving the most information possible to ease the decision making process.

There are multiple graphs, but they all ultimately tell the same story. These graphs have shown to be fairly accurate for a "basic" Magnuson setup. ie a Magnuson intake, supercharger, and pulley. These numbers may fluctuate with different intakes, exhaust, throttle bodies, porting, etc. but should scale with those modifications as well. At the end of the day, there is so much air pressure at a certain elevation, and that affects two things: the engines base power level (before boost) goes down with altitude, and the actual airflow from the supercharger goes down with altitude. This means at a higher elevation the engine is suffocating AND the supercharger is suffocating, so a supercharged engine is getting hit twice with power loss. This is why I wanted to develop a smaller pulley, because altitude robs lots of power from a supercharged engine.

The simplest graph to understand, Horsepower vs Altitude and what pulley. With a good tune, the Tundra engine is generally estimated for ~600 crank horsepower. There are engines with more, but ring gap and rod failure seem to crop up above those power levels, especially approaching 700hp. This graph is a very rough estimation based on how much horsepower the stock engine makes at sea level pressure (14.7psi). Use it with a grain of salt.

This graph is much more vetted since it's based purely on what the boost gauge reads. This graph is saying AFTER you compensate for the altitude deficit, how much more boost are you getting? Another way to put it, how much boost/stress is the engine experiencing beyond what a stock naturally aspirated engine at sea level experiences. For example, if you have the stock 2.45" pulley and are at 10,000ft then the engine is experiencing the same pressure and stress as a completely stock engine at sea level. And if you want to make sea level stock supercharger boost (~7psi) but you live at 5000ft, then a 2.1" pulley will be the equivalent. This is why the modular hub exists, so you can have the full supercharger experience at high elevation, and in a parking lot swap the pulley out and drive down to sea level and still have a safe and usable amount of power and the truck will feel the same as when you were at home.

The final graph looks at how much boost is the supercharger actually making. These values, minus the altitude pressure loss, results in the above graph.

The next graph is the theoretical upper limits of injector sizing vs altitude and pulley sizing. You do not want your injectors at 100% (that means they physically cannot flow more fuel), rule of thumb is 80%. So if you live at 2000ft, want to run a 2.0" pulley, you can see a ~680cc injector would be completely maxed out, so you want something larger than that (680 divided by 0.80 = 850cc injector for it to be running at 80% duty cycle). For reference when I'm at home my injectors are at 83%, when I drive down the hill they are at 94%. Getting up there, but the injectors can handle it, though if I went to sea level my 715cc injectors would be pegged and I'd start leaning out with a 2.0" pulley.