How to measure your pulleys

Using the Impeller Speed Calculator

Selecting Pulleys for a Powerdyne / Cool Charger / B&M / Ford Motorsports / Dinan

What the Boost Gauge is really telling you

Why Pulleys are not measured in “Pounds”

How Speed-Shifting effects the Supercharger

How to measure your pulleys

You will need to temporarily remove your supercharger drive belt from the crankshaft to the supercharger to do this. There is no other way.

Measure the diameter of the pulley where the belt runs, That is, at the top of the belt lands. Not the bottom of the vees, not the outer edges of the pulley where the lips are high to keep the belt from walking off. See picture below, and click on it to enlarge it.

A 6” caliper ($20) will do a nice job on the supercharger drive pulley, but the arms are too short to reach around most crankshaft pulleys.

A simple solution can be made with paper and a scissors. Cut a large letter “U” out of a piece of stiff paper. Try to slip it over the crank pulley where the belt runs. If it is too small, open it up with the scissors a little more and try again. When it just touches both sides of the pulley, you can measure the U with a standard ruler and know your crank pulley diameter.

You can also use this technique on the smaller supercharger drive pulley if you do not have a caliper.

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Using the Impeller Speed Calculator

Input your crankshaft and supercharger pulley diameters and your maximum RPM's into the impeller speed calculator below. Compensate for speed-shifting if you drive that way (read the paragraph on speed-shifting below).

Select your supercharger from the drop-down list OR input your internal step-up ratio directly if you know it.

Press “Calculate” to see what your impeller RPM's will be. This information will help you select the right pulley size for your bearing life, or to bring your compressor map into line with your engine.

Impeller Speed Calculator

(Crank pulley size / Blower pulley size x step up ratio) x max rpm

Note: Vortech impeller speeds can vary with the trim package. Consult the Vortech website for more details.

Selecting Pulleys for a Powerdyne / Cool Charger / B&M / Ford Motorsports / Dinan

Superchargers with internal drive belts cannot spin as fast as their gear-driven cousins. But, belt-driven superchargers have larger volutes and larger impellers, so they also do not need to spin as fast to be effective. They move more air (CFM) at lower impeller speeds.

How small of a pulley you can use on your BD-series blower depends on your shift point, whether you have an automatic or a manual transmission, and on the size of your crankshaft pulley.

Those BD (Belt Drive) series superchargers like to run the impellers at 36,000 RPM internally, and will accept intermittent peaks of 40,000 RPM. They have a 3-to-1 step-up ratio internally from the supercharger drive sprocket to the driven (impeller) sprocket.

EXAMPLE: A Powerdyne BD Supercharger is equipped with a 3" pulley and mounted on a motor that has a 6" crankshaft pulley. If that engine shows 6000 RPM on the tach, the supercharger drive pulley is spinning at twice that, or 12,000 RPM (because the 3" pulley turns 2 xs for each 1x of the 6"crank pulley). Within the Powerdyne, because the drive sprocket is spinning at 12,000 RPM, the driven sprocket (and the impeller attached to it) is spinning at 36,000 RPM.

Use our Impeller Speed calculator to select the right size pulley for your application.

Target 36,000 RPM at the Impeller if you want a long life from your Powerdyne and 40,000 RPM if you want more performance (and more frequent rebuilds). If using our high-speed bearings and belts, you can stretch this to 48,000 rpm.

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What the Boost Gauge is really telling you

A lot of people believe the boost gauge measures supercharger output. It doesn't.

The boost gauge measures pressure. Pressure is caused by a restriction to flow. The engine is that restriction to flow.

While the supercharger is spinning and making up air, the engine is also running and inhaling, and the boost gauge only shows the remainder. In other words: The boost gauge only shows WHAT'S LEFT after the engine consumes what it can.

Supercharger output MINUS engine consumption = what's left to display on the gauge.

Sometimes we get a call from an enthusiast who has just added headers, or cams, or a larger intake and now sees his boost gauge is lower than it used to be. They call thinking they did something wrong. Quite the opposite – they have removed restrictions to air flow in their motor, and although the supercharger is producing as much air as it did before, the engine can now use more of it. The HP of this engine has gone up, and the boost gauge will have gone down.

Now that you understand that, we can tell you that a low boost gauge number is either:

1) an engine that has little restriction and is breathing well (this is a GOOD thing) or

2) a supercharger that is sized too small for the size of the engine it is bolted to.

3) a supercharger spinning too slowly to match its output to engine needs.

Why Pulleys are not measured in “Pounds”

Some time ago, a marketing nerd decided it would be a good idea to name supercharger pulleys by the boost you would likely see on the boost gauge on a certain engine in a certain car.

The car was a Ford Mustang with a 5.0 Liter motor, and the pulleys were called “6 pound” or “9 pound” pulleys by what the boost gauge showed on this car.

The problem is, the moment Ford changed camshafts, exhaust, intake manifolds or anything on that motor, the gauges were now reading differently. And the rest of us who do not have that engine in that car – describing a pulley as a “6 pound pulley” is meaningless.

You are a tuner or a hot-rodder or you wouldn't be reading this. Increasing your engine's performance is your thing . We know you will be forever making changes to your motor in pursuit of your hobby and enjoyment. So who knows what engine restriction you will have at any moment, especially when you are involved in a hobby specifically intent on removing engine restrictions. (Read “What the boost Gauge is Really Telling You” above.)

So – the ONLY right way to describe a pulley is by its diameter. Pulleys are measured across the lands where the belt runs, and are referred to a “2.90 inches” or “2.70 inches” or whatever.

How Speed-Shifting effects the Supercharger

When making your impeller calculations, be sure to allow for RPM spikes caused by speed-shifting if you do this.

Speed-shifting is leaving your right foot planted to the floor while you quickly shift to the next gear.

It is not recommended. However, if you are going to do it, be sure to allow for it in your calculations.

Your shift light might tell you to shift at 6000 rpm, for example, but a telltale on your Tach will show you probably hit 6500 rpm before you completed your shift and let the clutch out again.

This driving technique is specifically violent to all centrifugal superchargers, and often the cause of snapped internal belts among belt-driven blowers.

We have supercharger pulleys in every size here!