Oversize Valves Theory
Installing oversized valves has been used in all types of motorsports and performance tuning since basically forever and it’s main purpose is to increase engine horsepower.
In this article we will try to explain how increasing the size of your valves will potentially increase hp, make you go faster and be happier.
First of all it is important to note that there is a direct correlation between valve sizes and ultimate engine power potential. An engine is essentially an air pump and to produce more power we need to flow more air because this is the limiting factor. We can see this both in production and racing engines and it is mostly apparent in Naturally Aspirated engines.
The following table shows the different top of the line 1000cc models of the main 4 Japanese manufacturers, their valve sizes and rated horsepower.
As you can see from the table above, there is a direct correlation between the rated horsepower of the engine and its valve sizes. This is why you can see the valve size steadily increase with the never ending customer demand for more power with each new model. This is also helped by a bore increase that allows for bigger valves to be fitted.
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However, the engine is a complex system and each one responds differently to valve size increases, even more so when you are dealing with older engines with only two valves or older style 4 valve heads with small valves (CB900F Boldor for example). In this types of engines, increasing the valve size makes great improvements and is very cost effective.
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In essence, increasing the valve sizes and other air flow improving modifications such as porting and polishing the ports will increase air flow, increasing the amount of air available to the engine for any given camshaft resulting in higher Volumetric Efficency = MORE Horse Powers!!
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Of course there are limitations to this valve increase:
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The size of the valve seat (that can easily be changed)
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The bore diameter and the fact that the valves can become shrouded resulting in decreased air flow because of obstacles near the valve edges - caused by the combustion chamber shape or proximity to the cylinder bore.
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Valve contact on overlap - when both the intake and exhaust valves are open (this depends on many factors including camshaft profile and timing) the intake and exhaust valves may clip which we must avoid.
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Position of the valve guides that are resposible for locating the valves in the head. Sometimes changing the guide position is the only way to further increase valve sizes but this involves a lot of high precision work and is reserved for the most extreme of applications.
In conclusion, regardless of these limitations that are more important when undergoing more radical projects, most moderate increases in valve sizes result in more powerful engines and that is why this is an important modification most race teams and tuners make when trying to improve the power of a given engine.