July '16

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68 THE SHOP JULY 2016 PERFORMANCE favor of generating maximum horsepower. Header design is all about exhaust gas velocity. Shorty or block-hugger headers provide easier installations in vehicles since they reduce fitment issues and offer less instal- lation obstructions. While not appropriate for a race engine where you need to extract maximum power, block-hugger headers— even if not featuring equal-length primary tubes—still provide superior performance as opposed to a stock exhaust manifold. The reality is that unless you're building a dedicated race car, it's simply not worth obsessing over extracting every ounce of power. Step-tube header design features changes in primary tube diameter from the cylinder head to the collector. Utilizing changes in tube diameter along the exhaust path can affect the speed of gas flow. For a naturally aspirated engine, a step header will feature smaller-to-larger tube diameter. This means that tube diameter at the entrance (from the cylinder head flange) will be smaller, with tube diameter changing to larger diameter(s) until termi- nating at the collector. This aids in pulling exhaust gas out of the cylinder head, and speeding up the exhaust pulse (improved scavenging). For a turbocharged application, a step header design will feature primary tubes that transition from larger to smaller diam- eters, creating a "squeeze" velocity that helps to spool the turbo quicker. TUBE DIAMETER AND AREA This char t illustrates how primar y tube diameter relates to percentage increases in area as tube diameter changes. (Shown here is the area increase as compared to the pre - vious tube size: 1.125 to 1.250, 2.000 to 2.125, etc.) TUBE TUBE TUBE AREA OD ID Area Increase (inches) (inches) (square inches) 1.000 0.902 0.639 - 1.125 1.027 0.828 29.6% 1.250 1.152 1.042 25.8% 1.375 1.277 1.280 22.9% 1.500 1.402 1.543 20.5% 1.625 1.527 1.830 18.6% 1.750 1.652 2.142 17.0% 1.875 1.777 2.479 15.7% 2.000 1.902 2.840 14.6% 2.125 2.027 3.225 13.6% 2.250 2.152 3.635 12.7% 2.375 2.277 4.070 12.0% 2.500 2.402 4.529 11.3% A parallel Tri-Y collector will join two primary tubes to a common collector. In an 8-cylinder engine application, a pair of Tri-Y collectors may be used on each bank. By pairing two cylinders into a common collec- tor, you're able to pair cylinders that fire furthest away from each other to improve scavenging. This should improve torque in the low- and mid-range power bands. This follows the same theory as offered by Tri-Y headers, where primary pipes are joined together (merging primaries prior to the collector). Getting Ahead on Getting Ahead on HEADERS

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