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Supercharger boost pressure controls how much additional horsepower your engine will make from using the supercharger. One of the simplest modifications that can increase supercharger performance is increasing the supercharger drive ratio to allow the supercharger to spin faster and move more air, thus creating more boost pressure.

First an overview of the supercharger drive ratio. In a typically supercharger installation, the supercharger is driven by the engine’s serpentine pulley and belt system. Because the supercharger is connected to the crankshaft on the same belt, there is an obvious drive ratio between the supercharger and the engine based on the respective pulley sizes. 

 

Illustration shows crank diameter (A) to supercharger pulley diameter (B), it also shows a well designed system where the belt wraps around most of the crank and supercharger pulleys with great placement of the idler pulley for maximum belt traction.

Illustration shows crank diameter (A) to supercharger pulley diameter (B), it also shows a well designed system where the belt wraps around most of the crank and supercharger pulleys with great placement of the idler pulley for maximum belt traction.

For example, if the crank pulley is 4″ in diameter, then for every rotation of the motor the belt moves 2*pi*r which is about ~25 inches. If the supercharger had a snout pulley that were 2″ in diameter, and there was NO BELT SLIP, then 25 inches of belt rotation will cause the supercharger to spin by 25/(2*pi*r) = 2 complete rotations. To simplify things, the supercharger drive ratio is exactly the ration between the diameter of the crank pulley to the supercharger pulley or in our example 4:2 which is 2:1. 


It becomes obvious that to increase the rotational speed of our supercharger, and therefore to increase the amount of air (and boost pressure) produced by our supercharger system we have one of two options:
1- Increase the diameter of the crank pulley: 
For example swapping to a 4.4″ crank pulley will over drive all our accessories connected to the serpentine belt by 10% (including our supercharger). In a positive         displacement supercharger this will typically result in a 10% increase in power. In a centrifugal supercharger this will typically result in a 10% increase in peak boost.

2- Decrease the diameter of the supercharger snout pulley:
For exampel swapping the 2″ supercharger snout pulley to a 1.8″ diameter pulley while keeping the 4″ crank pulley will achieve a 10% increase in supercharger RPM (this time without affecting any of the accessories connected to the serpentine belt because their ration with respect to the crank pulley has not been affected) and the same expected result of a 10% increase in flow for positive displacement superchargers or a 10% increase in peak boost pressure (and a faster onset of boost) for centrifugal superchargers.

A few things to note here:

Increasing the diameter of the crank pulley may show some high RPM power loss due to overdriving the accessories (water pump, power steering, alternator…etc) and if those accessories are driven beyond their maximum recommended RPM’s then their frictional losses may increase and you may lose high rpm power. One way to correct for this is fabricating or purchasing a complete pulley system that oversizes both the crank pulley as well as the accessory pulleys to maintain their drive ratio, while overdriving only the supercharger.

Decreasing the diameter of the supercharger pulley decreases the length of the belt contacting and driving the supercharger, and with the decrease in contact there is an obvious decrease in drive force for the supercharger. In some cases the supercharger belt may slip at high RPMs causing a decrease in boost, a loss in peak power and possibly damaging the belt. There are a few ways of preventing this from happening, one is to use a higher rib belt, for example swapping from a 3 rib belt  system to a 6 rib belt system gives a belt that is twice as wide and thus much less likely to slip around a small supercharger pulley. Another possible solution is a re-routed belt path that gives a larger contact area between the supercharger pulley and the belt (where the belt wraps around a larger portion of the pulley before moving on to a relocated idler pulley (that holds the belt tighter to the supercharger snout). Lastly and typically preserved for racing, the typical v-ribbed belt system can be replaced with a cogged pulley and a toothed belt that does not slip. 

httpv://www.youtube.com/watch?v=HxXu6cAtbmk


Upgrading both the supercharger snout pulley as well as the engine crank pulley has a multiplicative effect on the drive ratio. For example. a 4.4″ crank pulley installed with a 1.8″ snout pulley, compared to a 4″ and 2″ system will have the resultant drive ratio:

New drive ratio = 4.4/1.8 = 2.44 which is a 22% increase over stock. Thus the combination of two 10% pulleys (one at the crank and one at the supercharger) gives a compound and multiplicative result rather than a simple additive result. 

Last thing I want to mention here is it is important to know the maximum reliable RPM for your supercharger before you overdrive it. For example if we had started with a car that has a redline of 7000 rpms, and a 4″ crank pulley and a 2″ snout pulley, and an Eaton MP62 supercharger that has a peak RPM of 16000 rpms then:

The supercharger drive ratio is 4/2 = 2.0
At 3000 RPMS, the supercharger rpm = engine rpm * drive ratio = 3000 * 2 = 6000 RPMS
At 7000 RPMS, the supercharger rpm = engine rpm * drive ratio = 7000 * 2 = 14000 RPMS

Now since the supercharger still has 2000 rpms to go before it is maxed out then we know that we can overdrive it a little for more boost and more power. Typically overdriving teh supercharger to 16000 RPMs from 14000 RPMs would be a power increase of 14% or about 30 HP on an Eaton M62 (good power for the money for a simple pulley swap). 

With the new pulleys:

The supercharger drive ratio is 4.2/1.8 = 2.44
At 3000 RPMs, the supercharger rpm = 3000 * 2.44 = 7320 RPMS which means we are probably making 15 more hp at 3000 rpms!
At 7000 RPMs, the supercharger rpm = 7000 * 2.44 = 17080 RPMs, which is 1000 rpms higher than the maximum recommended RPM from our charger.
At this point the supercharger being overdriven will have larger frictional losses, a higher power consumption to drive it, and much higher outlet temperatures. Definately not safe for sustained operation.

If we do the math backwards then 16000 supercharger rpms will occur at 16000/2.44 = 6557 engine rpms. Meaning that with this gearing you will gain power from 0 to 6500 rpms, but from 6500 to 7000 rpms you will lose power.

The warning here is that even though most people would be willing to trade off 500 rpms of high power (especially if it’s on a car that has it’s cams optimized for peak power at 6000 rpms for example and isn’t building power past 6000 and forcing you to shift early), for a low rpm torque and power boost which helps alot with in town driving and mid gear passing conditions. Aftermarket tuners know this preference and are not hesitant to overdrive your charger, so it’s up to you to make sure you are aware of this when you make the decision to do so. 

A little history: 

Supercharged planes have been used throughout history in warfare and in luxury aircraft to help the plane’s engines produce consistent power in spite of low density air at higher altitudes. Superchargers compress air and so they increase air density. Since these planes operated at a wide range of altitudes it was sometimes necessary to have two different supercharger drive ratios that would give you two different boost levels or two different ‘density ratios’ depending on what altitude range the aircraft was to be operated at. These planes were equipped with a dual supercharger drive system with redundant pulleys and belts that had a different drive ratio. Furthermore, they had cockpit controlled clutches on the pulleys such that they could switch between one drive ratio or the other ‘one the fly’ quite literally, or disengage both chargers and produce less power and save fuel for increased range.

The quick change system: 

Quick change supercharger pulleys.

Quick change supercharger pulleys.

This technology did not transfer directly into cars, however we have something similar. Sometimes supercharged cars at high boost can be a handful on the street due to the increased torque at lower RPMs. This becomes very evident for those of us whole live in areas with unpredictable climates that see lots of rain or snow fall. In a street/strip compromise some companies have come up with two piece supercharger pulleys. The inner piece of the pulley is an inner sleeve that is pressed onto the supercharger snot (as you would press on a one piece solid pulley). The outer piece of the pulley is an outer ring that bolts onto the inner sleeve using some high grade bolts. This design allows a ‘quick change’ of supercharger snout pulleys between your low boost street pulley and your high boost ‘race’ pulley in a matter of minutes. You just release your belt tensioner, remove 4 bolts, swap pulleys, tighten 4 bolts, and then re-tension your drive belt and viola a 15 minute quick change. This is a very nice option for large displacement motors running positive displacement roots or twin-screw type superchargers that have more than enough torque to overwhelm their street tiers. This also works well with an assortment of other street/strip products such as drag radials, water injection, exhaust cutouts where you can – on a prepared track – swap pulleys, swap tires, open an exhaust cutout and run open header, and switch on your water injection system for added cooling and fueling. Mr. Jackal and Mr. Hyde mentallity.

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