Turbochargers and the science associated with them are still regarded by some as black magic. One of the least understood components in modern turbochargers is the anti-surge ring, or as Bullseye Power Turbochargers calls it, the MAP ring.
Explained simply, the MAP ring is designed to modify airflow through the compressor blade, altering the compressor map (hence MAP ring) to prevent — or at least move — compressor surge (which is an aerodynamic stall of the compressor blades) out of the intended operating range.
Taking that basic definition of what a MAP ring does, Bill Devine at Bullseye identified that altering the design of that MAP ring for one engine combination resulted in a less than ideal compressor map for another engine combination. The logical next step in the development process was to create an adjustable MAP ring, even though it hadn’t ever been done before.
With an adjustable MAP ring, a given turbocharger now has a significant adjustment range. The adjustment not only allows the user to reduce surge in a specific area of the compressor map where their combination lives but also allows for fine-tuning of the powerband of the turbocharger. To prove this point, Borowski Race Engines took an engine and ran it on the dyno with the MAP ring in four different configurations.
Using a 402 cubic-inch LS with a Bullseye 76mm NLX turbocharger, each pull extended the MAP ring an additional 1/8th inch out from zero to 3/8th inch openings. The results were interesting in that the two highest-performing settings were in the middle of the range of travel and were only 0.4-percent apart from each other. Taking the ring out one step further dropped the result by 4.2-percent, or 42 horsepower.
For the full complement of data from the testing and a complete breakdown of the results of Borowski’s dyno session, you can find them here. What the test proved conclusively is that in the case of MAP ring sizing (or anti-surge port sizing, to extend the terminology to other brands of turbochargers) bigger isn’t always better.
The test also proved that the sizing of the opening can be tuned to a specific engine combination’s airflow requirement to eke out a significant power gain. In this test, that was 26 horsepower better than nothing at all and 42 horsepower better than the first step of too much gap.
The “Tunable Race Cover,” as it’s known in Bullseye Power parlance, is currently available on its NLX series of mid-frame turbochargers, ranging from 64mm up to 88mm inducer diameters with a wide variety of turbine wheel sizes and housing A/Rs available.