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Exhaust for KZ engines: characteristics and tricks to optimise performance
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A exhaust is a fundamental accessory, capable of greatly influencing the performance of a 2-stroke engine . Those for the shifter classes, in particular, have different characteristics that you can use to try and get the best from the engine according to your needs. Eligio Torreggiani, the owner of Elto Racing, a leading company in this field, explains the basic requirements of exhausts for KZ and how to alter the performance of the engine by varying the curve between muffler and silencer or using a round silencer rather than one with an oval section.
When choosing a exhaust, you should always keep in mind acceleration. Improving the engine at high revs by increasing revs is pointless, in my opinion. The goal is to be as quick as possible when exiting a bend: if we were to gain even just 10 milliseconds on our opponent by doing so, multiplying the gain by all the bends on the track would result in major progress at the end of a lap. Vice versa, an engine that is optimised at high revs can only make gains on the final section of the straight.
A KZ engine exhaust has to comply with the model that each manufacturer has had approved. However, with those values as a starting point, you can work on tolerances in order to obtain different engine performance.
Keep in mind that the expansion of exhausts for KZ classes consists of several cones welded together and each cone has a tolerance of +/- 1 mm compared to the approved size. The diameter also has a millimetre of tolerance: by using these differences you can establish "rules" (valid, in general, for all engines) with which to vary engine performance. To this end, the most important part of the exhaust is the front half, which goes from the engine to the side pod. If we need higher revs, the exhaust should be 3-4 mm shorter than the one normally used and have a 1 mm wider diameter. Vice versa, to improve low revs and acceleration, the exhaust should be 3-4 mm longer and the diameter of the cones should be 1 mm less.
Of course, improving the engine at high revs by changing the exhaust can make performance worse when accelerating and at low revs. Every modification should be optimised in line with the preparation of the engine, working on the range of use of the engine through modifications on the reed valve pack, for example.
Not less important, it’s just that an engine is less affected by modifications made in this section compared to those made on the first section. Usually, if you need to increase the arc of use of an engine, the second section of the exhaust should be lengthened. If, on the other hand, the engine lacks "bite" at high revs, a exhaust with a shorter second section is best.
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