Recently I was asked what engine stroking and boring is all about. Does it really work? I honestly have to say: Sometimes it works extremely well and sometimes it fails miserably.

There are many design parameters that engine builders refer to when starting a new engine project. But the two main objectives are to get more air/fuel mixture into the combustion chamber and then to burn it more efficiently. That's how you produce more horsepower.

An internal combustion engine is essentially an air pump. With its upward travel, the piston creates a low-pressure area in the crankcase. This in turn pulls the air/fuel mixture (in vaporized form) from the carburetor into the crankcase. When the piston reverses in a downward direction it pressurizes the crankcase until the transfer ports open. Then the air/fuel mass is transferred from the crankcase to the cylinder/combustion chamber.

So how does a bigger bore or longer stroke create more horsepower? Some gain comes from the engine's increase in cubic displacement. But the majority of the gain will be from increased port area. The guy who bores out his 600 triple (700 big bore piston kit, re-sleeved cylinders, larger pistons, and machined heads) but fails to change port area from the 600, more than likely will be very disappointed with the way his big bore engine performs. Typically an engine built like this will lose horsepower. The port area was not increased to match the bigger bore diameter.

We need more port area in the big bore or the stroker engine to allow for more air/fuel volume to enter the cylinder and combustion chamber which leads to more horsepower. So will the big engine need higher port area percentage than it had when it was in stock form? Maybe not if the stock engine was designed as a high output engine with good flowing ports and large port area for the engine's displacement.

The reason it may not be necessary to increase the bigger engine's port area percentage is because it is a bigger engine and we are working with a port area percentage for a given engine size. For example, on a stock 600 triple with the 700 big bore kit, the 600 bore size is 60 mm and the width of the exhaust port window is 39.6 mm at its widest point, which is 66 percent of the bore diameter (60 mm x .66 = 39.6 mm). So if the 700 big bore has a bore diameter of 70 mm and the same calculation is used (70 mm x .66 = 46.2 mm), the same 66 percent on both bore diameters gives the 700 big bore a 6.6 mm wider exhaust port and more area. This is a simple example to show the point that a bigger engine needs larger ports to breathe properly but the actual port area percentage for a given engine size may not change.

"Stroking an engine is altering the crank pin location to increase or decrease the piston travel in the cylinder which then increases or decreases the engine's displacement. In most snowmobile engines it is an increase in stroke and displacement that is most popular in trail applications. Shortening the stroke of an engine is usually done to meet the rules of a specific racing class. The maximum distance the crank pin can be moved to increase the stroke depends upon the engine design. Typically on a snowmobile engine it will be 3-4 mm from the crankshaft center line which translates to 6-8 mm piston travel in one revolution of the crank shaft.

Like the big bore engine needing more port area because of the bigger bore, the stroker will also need more port area than it had with the stock crankshaft to really get the full potential it may be capable of producing. Rather than calculating port area to a bigger bore, it will be calculating port area around the longer stroke. Technically there are some advantages and disadvantages in stroking an engine versus increasing the bore size. One big advantage for trail applications is the mechanical advantage the longer stroke has. The piston has more leverage to rotate the crankshaft. (An example of this would be like using a longer end wrench to break loose a stubborn nut.) This usually will give an engine better low end and mid-range response.

One disadvantage for some applications would be increased piston speed. Excessive piston speed can cause ring seal problems and lubrication problems, which decreases piston life. Piston speed will obviously increase with rpm and it will also increase with an increase in stroke. For a reliable engine, keeping the piston speed around 3500 to 3800 feet per minute would be a good idea. As an example of the effects of stroke on piston speed: Engine stroke 68 mm or 2.679-inches running rpm 8200 will travel 3647 feet per minute (.166 x 2.679 x 8200 = 3647). Increase the stroke 8 mm to 76 mm or 2.994 inches will equal 4075 feet per minute (.166 x 2.994 x 8200 = 4075). This might be a little high for the best reliability.

A stroked or bored two-stroke engine can deliver impressive power gains, but only if the whole engine package is well designed and the exhaust system has been designed and dyno tested to fit the engine requirements.

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