At just 5 psi, Austin forecasts 320 hp without an intercooler or 335 hp with one. Sometimes you have to use your best guess and cut and try different A/R ratios." Turbonetics' Evolution wastegate, and (optional) Duo-35 BOV complete the setup. "That picks the compressor," Austin says, but "the turbine comes from a chart we have or what I know works from our tribal knowledge. After filling in the blanks-VE and desired horsepower are two big variables-Turbonetics experts take over, dialing in the boost pressure needed to meet the goals, then looking for a "turbo map" that generates the highest efficiency in meeting them. Normally, these matches are generated after a customer fills out the worksheet on Turbonetics' website. An improved descendant of the classic TO4, it has an HP-61 compressor wheel and an 0.82 A/R turbine housing with its proprietary F1 wheel. Engineer Dave Austin's compressor match spits out a GT-K 500 turbo. Turbonetics caters to the replacement aftermarket and the hard-core competition markets. Krivickas says this unit should work for similar engines in this displacement range. Use a Precision PW46 wastegate plus a BOV. Mate the unit's 62mm compressor with a 0.58 A/R T4 tangential turbine housing that includes a 35?8-inch V-band inlet/outlet turbine housing to ease installation. He is referring to products that use steel ball bearings or old-school hydrostatic bearings (although Precision does offer roughly equivalent turbos with hydrostatic bearings at a considerable cost savings). By utilizing ceramic ball bearings, we are also able to take advantage of their excellent heat resiliency properties, eliminating the need for the liquid-cooling provisions commonly found on competitor's products," Krivickas says. "These units can handle about 20 times more thrust load than their hydrostatic bearing counterparts. Premier competition turbo maker Precision Turbo's Joe Krivickas spec'd a high-tech solution: "With 250 ci and only 5 psi to work with, we need a turbo able to move a lot of air with cool charge temps and exceptional spool-up characteristics"-namely, a PT6266B. An intercooler isn't needed if intake temperature stays under 150 degrees F. The three different available A/R ratio turbine housings are sold separately use the midrange 0.82 A/R as a starting point, and fine-tune boost response versus ultimate power with either the smaller 0.63 or larger 1.06 as needed." Symonds adds that any changes in the selection criteria or engine design might alter the turbo selection. Garrett's 53mm GT3071R (PN 700177-23) assembly meets this criteria, plus it has a ball-bearing driveshaft and water-cooled housing for optimum durability and quick spool up. "The two operating points should straddle the main efficiency island on a compressor map as closely as possible. In the midrange, you're looking at about a 15.70 lb/min airflow requirement. At 5,200 rpm, 5.50-psi intake manifold pressure and a 1.38 pressure ratio are required to achieve this flow level. That means the turbo needs to flow 27.20 lb/min to make 300 hp. The target air/fuel ratio at WOT on pump gas was 11.5:1, with a 0.46 BSFC (brake-specific fuel consumption) number. Along with our 300hp, 250ci stipulations, for the purposes of the exercise, Garrett engineer Rob Symonds assumed a 3,000-rpm midrange torque peak, a 5,200-rpm power peak, no intercooler, pump gas, and Torrance, California (near sea level) ambient conditions. For the Chevy six, we asked Garrett to do a full engineering analysis, just as if it were specing a turbo for a factory job. But it also has an aftermarket division, Turbo By Garrett, that sells through performance-oriented distributors. Industry giant Garrett supplies massive quantities of turbos for OE factory applications.
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