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| Joined: | Mon Sep 18th, 2017 |
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You pretty much nailed it. A slippy pipe has an internal, movable convergent (rear) cone with the stinger attached. The big end of the cone is supported by the large diameter straight section of the pipe and the stigner is supported by the small end of the outer rear cone. When the kart accelerates from a standing start or out of a slow corner the engine rpm will be at the stall speed of the clutch which is generally set just a little below the peak torque rpm of the engine. To optimize the acceleration the rear cone must be further from the exhaust port at the lower end of the rpm band and for maximum top speed the cone needs to be closer to the exhaust port. The relationship is pretty linear as far as the distance so you would expect the range of adjustment to be in direct proportion to the width of the rpm band of the engine. However, that was not always the case because the volume of the pipe is also important to the performance of the engine. Exhaust pressure will result in the rear cone position pushed as far from the engine as the design allows and that is where the cone needs to be for the hard pulls off the turns. as the engine speed increases the driver pulls the rear cone toward the exhaust port thereby reducing the tune length of the pipe and the volume simultaneously. Typically on a road race course the pipe is pulled all the way in on the long straights and the driver will release the handle when he lifts to brake for a corner. As the kart accelerates off the corner and gains speed the pipe is pulled back in. As you imagined, if the driver is too early with the pull he can cause the motor to overheat and seize up. Too late on the pull and power is lost. Use of the slippy pipe is an art and just like tuning the needles on the fly some drivers were better than others at sensing the needs of the engine and developing a feel for when and how much to pull the cone in.
Steve O'Hara
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