Positive displacement supercharges are popular with the modern muscle car crowd. These blowers can add plenty of horsepower with minimal modifications to the vehicle. You can unlock even more horsepower with a simple pulley change, but you can go overboard. We talk with Whipple Supercharger’s Nick Purciello about how you should approach a pulley change for your positive displacement supercharger.

We all know that an internal combustion engine is nothing more than a big air pump, and a supercharger is just another air pump that can cram more air into the engine. Now, a supercharger may seem like a simple way to add untold amounts of horsepower to an engine, but there are limits to how much power it can add due to how much air the entire package can process. Grabbing the party pulley and trying to spin the blower faster isn’t going to do what you think it will most of the time.

“An ideal pulley combination will enhance the volumetric efficiency of the engine. The two systems must work together in harmony considering other factors such as fuel octane, airflow capability and durability of the engine along with the displacement per revolution and maximum recommend input speed to the supercharger to name a few,” Purciello says.

Purciello also adds; “The faster you spin the supercharger, the more power is required from the engine to turn it (parasitic loss). Additionally, the more air you try to cram through the engine, the more the air backs up inside the intake manifold rather than being used by the engine. This is what we are measuring when we talk about boost, an airflow restriction, which, if too aggressive, may result in higher than desired intake air temperatures, making the engine combination more prone to preignition.”

If you want to optimize how much horsepower your engine and supercharger combination will generate, you need to make sure the pulley combination is correct. No two engine and supercharger combinations are the same, so the best way to get in the ballpark when it comes to your pully combination is to talk with your engine builder, and the supercharger company as well.

“There is no magic formula as each engine combination will react differently, so rules of thumb and leaning on the experience of experts such as engine builders and tuners is invaluable. Generally, you want to find the balance of airflow (boost) and spark advance to achieve the desired result. Assuming there is an octane constraint, too much boost, and too little spark could result in high exhaust gas temperatures and be on the edge of misfire. On the other hand, too little boost may not meet the airflow demand for the desired power level,” Purciello explains.

Now, before you reach for the max effort pulley right away and try to make all the boost, you need to understand that there’s a method to find the correct pulley for your application. Testing different pulleys is the only way to see what will give you the most power and the best power curve. A dyno test session will show you that getting too aggressive with your pulley selection will cause diminishing gains. This can be caused by the supercharger being maxed out, or the engine just isn’t capable of moving the amount of air you’re stuffing in it with the pulley combination.

If you see a boost curve that has an aggressive ramp at the higher RPM range when power levels off, that typically means the engine can’t efficiently move the amount of air you’re trying to feed it. So, if you try to feed the engine even more air with a pulley change to remedy this, you’ll lose more efficiency and not see any real gains. To overcome this problem, you’ll want to increase how much air the engine can move through a change in your cylinder head and cam combination, or try to move to a bigger exhaust system if you can.

“If the engine is up to the task, an under-spun supercharger on an engine that is very efficient at moving air in the higher RPM range will have a relatively flat boost curve or one that slightly drops off on the top end. This can be compared to installing a larger camshaft and/or headers on a supercharged engine. There will be a drop in “boost” but an increase in power or cylinder pressure without changing the pulley ratio. Some make the mistake of going more aggressive on the pulley ratio to make up for the loss in boost, but that makes the cylinder pressure even higher which may be too much for the fuel octane or hardware,” Purciello explains.

What that means is when a positive displacement supercharger is being used, a flat boost curve, or a boost curve that progressively increases within the engine’s RPM limits that doesn’t exceed what the supercharger is capable of is ideal according to Purciello.

Another big misconception that’s out there about supercharger pulleys is a correlation between the pulley ratio and an achievable maximum horsepower or boost number. Just because a certain pulley combination made a certain amount of horsepower with one engine and supercharger package, doesn’t mean it’s going to generate that same number with another.

“For the supercharger, we go by maximum supercharger input RPM to set a safe limit for those reaching for the sky. For our superchargers, 18,000 input RPM is max for sustained usage such as in a marine application where WOT can be for minutes at a time. In automotive racing applications, they can tolerate up to 22-24k for short bursts. To calculate blower input RPM, you divide the driven crank pulley diameter by the supercharger pulley diameter, then multiply that value by your peak engine speed. An example of this would be: (8.00/3.25)*7,500 = 18,462 input rpm,” Purciello says.

So, what does all of this mean regarding supercharger pulley selection? Well, it means you need to do your homework and understand just how much air your supercharger and engine SHOULD be moving before you try to get crazy with your choice of pulleys.

Purciello summarizes how to approach getting the best pulley combination for your application.

“In short, smaller pulleys are not always better, you can’t overcome an inefficient engine with more boost and you can overspin a supercharger without understanding how crankshaft pulley diameter, supercharger pulley diameter, and peak engine speed correlate.”

The right pulley will help your positive displacement supercharger combination generate tons hor horsepower. Remember, the best pulley is the one that will allow your supercharger and engine to work in harmony to create the perfect amount of boost.