Adjusting Supercharger Boost Pressure

   This article is written mainly for my personal notes based on my experience with my truck. 1987 S-10 60 3.4 V-6 with a Powerdyne BD-11 Supercharger, Holley TBI, bored intake, headers, dual exhaust and an Air-to-Air Intercooler. Most of this information is common across the board with centrifugal Superchargers. It may be of some help for someone tuning their present Supercharger system. (Update: I have since went to a single exhaust catback system and deleted my intercooler)


   When I first put my Supercharger on the 2.8L motor, I was only getting a couple pounds of boost. I was surprised because it came with a 4 pound pulley. The main components that effect boost levels need to be considered. First off, with an Intercooler installed I didn't realize how much that would effect the boost. The Intercooler will act as a restriction and reduce boost pressure a few pounds depending on its size and design. Also, with all the piping needed to plumb it, it's a lot of extra volume to fill. Of course the compression of the motor will effect boost pressure as well. Besides that, with centrifugal superchargers, the faster you spin them (RPM) the more boost they make. With 2 pounds of boost I was ready to bump it up.

   Hopefully you have a boost gauge and know what boost you are starting with. You need to use a reference point. I would run through the gears and try and hit 5000 RPM (or whatever) and use that number. Make your adjustments and compare them. The easiest way to change boost pressure is by a simple Supercharger Pulley change.   A smaller pulley simply spins the Supercharger faster at a given RPM. There are three available for the Powerdyne: 

2.70" Part# D10045-270 (estimated 11psi auto/ 9psi truck)
2.93" Part# D10045-293 (estimated 9psi auto/ 6psi truck)
3.12" Part# D10045-312 (estimated 6psi auto/ 4psi truck)

   If you need to go smaller than the 2.70" you will have to go with a company like Auto Specialties Performance. They have a 2.62" available and can make a 2.5" if needed. If you had bought a Supercharger Kit, the Supercharger Boost is rated using the factory setup on your vehicle. A lot of times someone installs a kit and finds they are short on the boost claimed by the Supercharger company. 

   Boost can vary depending on your setup. I ended up going with a (10psi) 2.5" pulley to get my boost up on the 2.8L. After I put in the 3.4L motor I had a lot more boost and went to a 2.62" to achieve the same boost. This is due to the different compression ratios of these two motors. The 2.8 is rated at 8.5:1 and the 3.4 has 9:1 compression. After I removed the intercooler I went with a 2.93" to get the same boost. Pulleys, compression, exhaust system and intercoolers all have an effect on the final boost numbers. There are a lot of different variables. Your setup could be completely different than mine. The only way to know is with a boost gauge. You do have gauges don't you?

Summation: 3.4L crate motor, intercooler, headers/dual exhaust, Holley 400 CFM TBI, bored intake, 2.62" pulley = 6 lbs boost @ 5500 rpm.
3.4L, no intercooler, headers/catback exhaust, Holley TBI, 2.93" pulley = 6 lbs boost @ 5500 rpm.

FOOTNOTES:

This is the 2.62" pulley I got from Auto Specialties Performance. The pulley's size is measured by diameter in inches from rib to rib. FYI: This is really a Vortech Pulley ASP cuts down to fit a Powerdyne. The Vortech has a longer neck which makes it stick out farther on the shaft of a Powerdyne and would not line up with the belt correctly.

A good source for pulleys: superchargersonline.com

Keep in mind most Superchargers have a maximum RPM recommended. You will have to check with your specific application. Some Supercharger companies will void their warranty by changing to a smaller pulley.

UPDATE: "I removed the intercooler" I needed a 10psi supercharger pulley in order to get 6psi of boost with the intercooler setup. After I removed the intercooler and plumbed the supercharger straight to the TBI my boost shot way up. I now have a 6psi pulley and can actually get 6psi of boost. It should also be noted I deleted the dual exhaust and went with a catback system.

This setup makes much more sense to me. By using a smaller pulley to get the boost up with the intercooler meant the motor was using more horsepower to make up for the parasitic drag. 10psi of drag versus 6psi of drag now for 6psi of boost. And, believe it or not, it even smoothed out my idle a little since the supercharger isn't working as hard now. (Think about the drag you feel when you're AC compressor turns on.) This should also help my gas mileage too. See before and after pics. 

RELATED:
Tuning a Supercharged TBI fuel system
Supercharger Bypass valve
Holley/GM TBI notes

Tuning 1987 Chevy S-10 Blazer Supercharged 3.4 V-6 TBI