6 Rib M122 Supercharger Pulleys For Department Of Boost GT450/550 Kits
(in the Continental U.S.)
The GT450 kits come with a pulley and a hub. But at some point, you may want a different pulley for various reasons. These reasons may be more/less boost, you damaged your pulley, you are converting to a 8 rib belt system, etc.
The GT450 kits come with a 3” pulley. That will get you 10-11psi of boost with a M122 supercharger.
If you want to make more or less boost you will need a different pulley. There is no hard and fast rule for what size pulley will get you what boost (exactly). There are too many variables in the combo. But a good rule of thumb is if you change the supercharger pulley 1/8” (.125) you will see a 1lb change in boost. The smaller the pulley the higher the boost, the bigger pulley the lower the boost.
Pulleys available in the following sizes:
**We make most of the pulley sizes to order. This can take 10-14 business days. We do our best to get them out sooner than that though.
Pulley Size/Blower Speed Calculation Tutorial
We cannot safely make a supercharger pulley any smaller than 2.52”. If you want more boost than a 2.52” will get you you’ll need to run a bigger crank pulley/balancer in combination with a correctly sized supercharger pulley to achieve your desired blower speed. This is where things get a little tricky. When you start playing with crank pulleys you need to start calculating your blower drive ratio so you know what you are doing and so you don’t over spin the blower. The upper limit for the M122 GT500 blower and the TVS’s is 18,000rpm. You can spin them faster. A lot faster. But they start to drop out of their efficiency range after 18,000rpm.
First A Standard Type Ratio
The supercharger pulley is 3”. The crank pulley (stock) is 6.5”. You take the crank pulley/balancer number and divide it by the supercharger pulley number. Then you multiply that number by how many RPM your motor will spin.
6.5 divided by 3 equals 2.16. This is your blower drive ratio. You then need to multiply that by your 6,000rpm engine speed which gets you 12,999. This is how many RPM the blower is spinning at a 6,000rpm engine speed. The math looks like this.
Now for a more aggressive ratio
If you are going for more boost chances are that you have built you motor to handle it. That will allow you to run more boost and will allow you to spin the motor faster. You will probably need a over drive (OD) crank pulley/balancer to get to the blower speed you want. Most OD crank pulleys are advertised as 5% over, 10% over or 15% over. But that doesn’t help you much in the math department. Mostly because they aren’t exactly the percentage they are advertised at. So, you need to know the diameter of the crank pulley. You will have to get that from whom ever makes the one you will be using. A popular one and the one we will be using for this example will be the Innovators West 10% OD crank pulley/balancer. They are 7.25”.
If you use a 2.65” supercharger pulley and a 7.25” crank pulley and spin the motor to 6,500rpm the math looks like this.
This is pretty close to the max blower speed.
The big question is how do you calculate for a specific boost level? Well, that involves some fuzzy math. There are a lot of variables when figuring boost out. Too many to be able to figure it out on paper. Engine displacement, cam size, CAI and TB size, etc all have an effect on the boost number. So really all you can do is give it your best guess, see what other people are running for combo’s and throw a dart at the dartboard. That said you can “fuzzy math” your way into it…………a little.
Let’s say you want 18lb of boost. If you use the rough guestimate that you get 1lb for every 1/8” of supercharger pulley size change you will see that you would want to run a supercharger pulley that is about 2.15”. But you can’t run one that small so you need to figure out the drive ratio and calculate what you would need to run for a supercharger/crank pulley combo. Here is what the math looks like.
3.02 is the drive ratio you are looking for. To figure out a blower drive ratio first start with what crank pulley/balancer you will run. In this case we will use the 10% OD/7.25” one. You divide the crank pulley/balancer diameter by the drive ratio. That will get you your ideal supercharger pulley size.
Now we know that we can’t run a 2.4” supercharger pulley so we would need to run an even bigger crank/balancer to get what we wanted. In this case it really doesn’t matter though. You can’t spin the blower that fast anyway. This was just an exercise. Hopefully I have explained this well enough so that you can do the math for whatever your goals are.