Muscle Mustangs & Fast Fords
Vortech V3 SCi-Trim Supercharger Kit - Boost Basics
Cranking Up The Boost Involves More Than Just Spinning The Blower Faster.
MSD references manifold pressure in a different format than the normal pounds per square inch (psi). The program calls it psia, or pounds-per-square-inch absolute. The simplest way to think of it is boost with the normal earth's atmosphere of 14.7 psia added in. Zero boost on a psia gauge would read 14.7; anything above represents boost. For example, a reading of 15.7 psia represents 1 psi of boost in an engine. It's important to understand the simple conversion because MSD references manifold pressure in this form.
As we mentioned earlier, increasing boost is as easy as adding a smaller blower pulley. Of course, spinning the blower too fast can cause heat and impeller overspeed. This kills efficiency. But for this test, we knew the blower was well within its limits since this was an upgrade kit was from Vortech.
Another bump in the road with spinning a supercharger harder is belt slippage. Upping the impeller speed comes at the cost of having less belt wrap on the smaller pulley. Less contact means less belt traction, and ultimately, slippage. To remedy this, Vortech includes an eight-rib blower-pulley system, which is a step up from the base kit's six-rib belt, and Vortech supplied us with wider blower and crankshaft pulleys for the eight-rib upgrade. The six-rib drive is acceptable for up to 6 or 7 psi; after that, it's recommended that you add an eight-rib drive to prevent belt slip.
Ricky Best also points out that there is another critical component included in the H.O. kit. He highly recommends a bypass valve, which is easily described as a pressure-relief valve for when the throttle blade is closed and the engine isn't ingesting all of the extra airflow from the blower. Without the relief of pressure, the air will back up in the inlet system between the throttle body and the blower discharge. The reversion will find its way to the impeller, causing it to skip and jerk, causing belt problems in minor cases. The jerking can lead to a broken supercharger head unit. The reversion situation is most prevalent when the engine goes directly from WOT to a closed throttle blade. The sudden shutting of the door to the manifold will cause a major problem in blower applications at 8 psi or higher.
Vortech routes the vented boost back to the inlet elbow on the impeller side of the blower. The boost cannot be vented to the atmosphere or downstream of the MAF sensor. The MAF sensor has sampled the air and therefore it needs to be recirculated upstream of the MAF sensor. The ECU reads the sensor and determines the engine is ingesting a certain amount of air. If the sampled air is released to the atmosphere, then the engine will run overly rich, if it can idle at all.
Vortech provides a plastic elbow with a nipple on it so the bypass valve can be connected back to the inlet system upstream of the MAF sensor. We aren't using the elbow with our UPR pipe. The UPR inlet pipe feeding our supercharger was installed because the Pro-M 80mm MAF sensor wouldn't mate with the Vortech inlet system due to its size. The blower kit is designed around a stock 55mm MAF sensor housing, not the massive 80mm unit on this car. Luckily, UPR molded the inlet pipe with provisions to route the bypass valve hose into the inlet pipe. The standard Bosch bypass valve included in the Vortech kit is rated up to 12 psi. For applications above that level, Vortech sells properly sized valves for race and street use.
The smaller lower pulley (6 inches versus 6.75 inches) brought total boost up from 6 psi at 5,500 rpm to 10 psi at 5,800 rpm. Output rose from 405 rwhp to an impressive 460 rwhp-above what we like for a stock short-block. This was the absolute maximum power Burcham was willing to put to the stock internals. These mods netted us a gain of 55 rwhp from an additional 4 psi of boost.