We have received a good deal of mail asking about the HKS VPC (Vein Pressure Converter) and its popularity in Supra Twin-Turbos. In those letters, many of the authors know of the VPC, but want to know more. It eliminates restrictions in the intake tract and opens the door to tuning.
The benefits of eliminating the factory mass air sensor become crystalline when you closely examine a stock Supra unit. The sensor is about 4-5/8 inches in length but the story is on the inside. The unit has an inside diameter of 2-5/8 inches but is burdened by what we call the Cone of Restriction, a 1-1/2-inch diameter cone-shaped structure situated in the middle of the sensor. The cone is open-ended with the opening leading to the hot-wire sensor array.
The opening funnels the air to the wire and the size of the opening is calibrated to the surface area of the wire to provide the proper readings. It works somewhat similarly to the screens in other mass air sensors that control, or "filter," the air prior to metering. The cone, which extends the length of the meter, is positioned by four wing-shaped arms that add to the unit's restrictive properties. The VPC unit allows the mass air sensor to be replaced by a section of intake pipe that flows 100 percent of its potential.
The tasks performed by the mass air sensor are replaced by the VPC, which works via the speed density form of engine management that uses sensor information to determine the proper fuel needs of the engine. The VPC gleans its information via its own air temperature, air pressure sensors and takes into account engine speed before calling on the proper fuel maps for the engine. The maps, which are formulated using complex algorithms, are contained within the VPC's 16-bit central processing unit. A chip provided by HKS tailors the VPC to the car it is tuning (so a Supra and an Eclipse would have different chips). Furthermore, when tuning calls for more fuel, the chip can be replaced with one programmed to control bigger injectors. The most popular Supra option seems to be 720cc HKS squirters.
Tuning is another benefit of the VPC. As a general rule, factory speed density systems can be a bit rigid when it comes to aftermarket engine performance. In some cases, speed density cannot deliver the full potential of advanced bolt-on power mods. With the VPC, the enthusiast is empowered to optimize performance. The VPC control unit has four knobs; Response, Gain, Idle and Option Output. These knobs give the user control of all facets of the fuel curve. (See details of their capabilities in the accompanying chart.)
Along with the VPC chip, the most critical component in the kit is the main wiring harness that allows the VPC to be wired in line between the ECU and the engine's various sensors. This is the other application-specific aspect of the VPC package. The VPC can be used by splicing the unit's four required wires into the factory harness, but this takes more intimate knowledge of the engine. Shortcomings due to sub-standard workmanship can greatly hinder the unit's performance. The harness makes installation much easier and proper operation is ensured. The air temp and pressure sensors are mounted in accordance with the installation manual and once wired, the VPC is more or less ready to rock.
It should be noted that a Fuel Cut Defenser is a must; it deletes any boost limits set on the engine, opening the door to higher boost levels. The VPC's tuning precision can be enhanced with the incorporation of a F-CON, GCC tag team. HKS' new Super AFR (Turbo, Feb. 2001) is not necessarily an updated VPC. While it does offer more precise tuning it does not delete the air mass sensor.
The VPC has been a go-to option for Supra tuners since day one and with the benefits it provides, it is easy to understand why. It will be interesting to see if the trend continues.