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How does a big bore cylinder kit produce more power?

January 19th, 2010

Tuning a two-stroke engine as most of us know is relatively simple, be it a full spec Runner 172 or a Honda Melody there’s always a way to squeeze more power out of an engine.

Most of the people I meet do this with bolt-on parts from one of the many aftermarket tuning companies e.g. Polini, Malossi etc. and with very good results. A standard 50cc scooter is around 3.5 bhp, and with the fitment of a 70 kit, exhaust and carb you will see double the power. Not bad for an afternoon with the spanners! This however is rarely the case as people then have problems… Incorrectly matched pipes and barrels, not to mention very bad jetting, incorrect roller fitment and even silly mistakes can cause most bikes to under perform significantly. It’s all well and good ripping a new part out the box and just throwing it on your bike, yet people rarely understand why a power increase has occurred.

The most significant part on an engine is the cylinder or barrel. This is where the biggest power increase is going to come from for most home tuners as this is where the power comes from for the pros. Big bore cylinder kits are basically a ‘pre-ported’ oversized version of the original. In other words, the exhaust port is bigger, transfer ports are wider and open sooner. They’re also angled differently. This means they can feed the cylinder with more fuel / air. This fuel mixture is then compressed into a smaller space by the piston moving up towards the cylinder head. Inside the head there’s a compression dome. And this is the space where the fuel mixture is forced into before the spark plug ignites it.

The volume of this and its distance from the piston at TDC (top dead centre) are crucial. The cylinder’s mixture gets compressed down from one volume, say 60cc, then it’s squeezed into the head to, say 6cc, this gives a compression ratio of 10:1. Physically measuring this on your engine isn’t that simple, so we’ll explain how to do that some other time! The barrel’s external port feeds are where the fuel/air comes from, these are fed by the crankcase transfer feeds and boost port feed. The crankcase, put simply, is a pump. By that I mean it has a one way valve (the reeds) and the moving piston compresses the fuel mixture inside the case. When the piston is at BDC (bottom dead centre) it starts to move upwards, this creates a vacuum inside the case that opens the reed valve, thus drawing a vacuum from the carb to draw in the air / fuel mix.

Once the piston begins moving back down, the valve closes and the mixture starts to compress. Then once the top of the piston reaches the port feed, the pressure is let out like a popping balloon into the cylinder filling it with mixture ready to be burned. Bear in mind that a 125cc engine actually burns around 180cc of fuel / air as its compressed mixture, this is similar to the way that turbos and superchargers work. However this isn’t the case once you reach higher rpm’s as the reeds simply don’t close fully and there’s almost a constant vacuum through the case. Stiffer reeds help with this phase as they reduce flutter on the reeds and help your engine attain a higher rpm. Although the stiffer the reeds get, the harder it is for the engine to open them at lower revs and at pull off, so it’ll be alot more lumpy and jerky at low speed take-offs…

All pretty complicated really considering a two-stroke engine has three fundamental moving parts – the crank, the connecting rod and the piston!

Words: Graham Gaymer

This article was originally published in the print version of PETROL ISSUE.02