THE SHOP

July '16

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JULY 2016 THE SHOP 67 PERFORMANCE Larger-diameter primaries lend them- selves to increased top-end power, but if too large, insufficient backpressure will result in a loss of power, especially at bottom-end and midrange operation. As far as primary tube length is con- cerned, longer tubes will provide increased torque, while shorter tubes may aid in quicker exhaust gas scavenging. As noted earlier, bigger is not always better. Rather than choosing the largest- diameter exhaust headers available for your engine, it's important to match the tube diameter to the requirements of the engine, and to tailor the engine's power band within the target operating rpm band. If you want peak torque at, say, 4,000 rpm for a street car, the header tube diam- eter needs to accommodate this. If you're dealing with a racing engine where peak torque needs to be in the 6,000-rpm range and peak horsepower will be in the 8,000- rpm range, you need to match the exhaust system to work optimally at the anticipated engine speeds. A street engine generally prefers more torque at the bottom and midrange, while a race engine (depending on the applica- tion) can sacrifice bottom-end torque in An example of a typical 4-1 collector. A group of four primary tubes are joined in a common formed collector. Depending on the model variation, the tubes may be inserted and welded, or a merge "spike" may be first installed onto the exit ends of the primary tube group before the collector is installed. TUBE LENGTH & PERFORMANCE Primary tubes too small in diameter ..................................................................... This can choke the engine by creating excessive backpressure. Primary tubes too large in diameter .................... While this may improve top-end power, low-end torque and mid-range power will be reduced. Shorter primary tubes ..................................................................................................................................................Suited for power band at higher rpm. Longer primary tubes .....................................................................................................................................................Suited for power band at lower rpm. Long small-diameter tubes ....................................................................................................................................Theoretically more torque at lower rpm. Shorter and larger diameter tubes ...............................................................................Theoretically more horsepower, with torque at higher rpm. This small-block Ford is destined for a replica GT40 installation, requiring an overhead- swept design. Due to the location of the tubes, radiated heat is a concern. This is an ideal candidate for the use of stainless steel, or ceramic-coated stainless or mild steel to reduce under-hood temperatures. Instead of using bolts, the use of exhaust studs is a great idea where space allows. Studs provide a convenient guide for header positioning. Also, for situations where the headers will be removed and installed frequently (as with race car use), the use of studs eliminates thread wear that would be possible with continued removal/installation of bolts. Install studs finger-tight, with a slight preload. Never double-nut a stud and attempt to overtighten the stud. The clamping force of the nut will secure the flange.

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