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Compression Ratio

Engine Building Fundamentals

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Discussion and questions related to the course Engine Building Fundamentals

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if compression ratio increase i have to remove material from chamber volume to increase also ?

Sometimes, it depends.

First you need to establish if you actually need to reduce the compression ratio - often you don't.

Then, if you do, you need to calculate how much you need to increase the unswept (total combustion chamber from the top of the piston at TDC) volume. If it's significant you may need different pistons, if it's relatively low the best option is usually to use a thicker head gasket. Machining the cylinder head to remove material is normally one of the last options with removing material from the piston crown often being preferred - with some racing classes, they will even recess the valves a little if the rules ban removing material from the head.

However, if it is close, and you need to remove some material from the head to balance the chamber volumes anyway, you 'may' be able to remove additional material to benefit the engine's breathing - but it needs to be done correctly.

there is any specific limit number for the chmber volume ?

That is impossible to answer, it depends on the engine and the specific cylinder head. For example, the old "small block"Chev'engine has many different combustion chamber volumes over the years, for different reasons, from 60cc to 80cc with numerous variations between - each of which had a different casting for the chamber, and several coolant castings.

There will be limit to how mach material can be removed as doing so may weaken the cylinder head if it's taken off the wrong places, ot the casting is made too thin. Because of the way they're cast, two heads that look the same may have "core shift" (the placement of the sand cores during casting that form the coolant passages) that will affect the initial thickness.

I would repeat, the preferred method is to chose the correct pistons, with gasket thickness used for getting it more exact. Other than cleaning up irregularities and CCing after a light skim of the gasket surface, the heads chambers should otherwise be left alone unless you REALLY know what you're doing.

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