Compresson info

[randomtask37]

i was told this the other day

"the hemispherical head design creates more compression than normal heads, so if you run higher compression pistons like normal motors, it will create too much compression and blow the headgaskets and stuff"

and i was wondering if it is true. i have always thought that bmw compresson seemed a little low compaired to most other cars.

[RedBaron86325]

Re: Compresson info

i was told this the other day

"the hemispherical head design creates more compression than normal heads, so if you run higher compression pistons like normal motors, it will create too much compression and blow the headgaskets and stuff"

and i was wondering if it is true. i have always thought that bmw compresson seemed a little low compaired to most other cars.

This could open up a can of worms, but I'll give it a shot based more on my working knowledge of shaped explosive charges than that of my limited knowledge of internal combustion engine.
When an explosion occurs, the pressure is dissipated through the path of least resistance. By shaping an explosive charge, the energy released from the rapid oxidation can be, for the most part, controlled. By shaping the combustion chamber in certain ways on the head, the energy released from the explosion can be directed. This aids in power, efficiency, or both; depending on the shape of the combustion chamber. By using certain shapes, the energy released can be focused in one single spot (conical shape) or over a wider surface (dome shape--ie hemi). When compound shapes are used in the combustion chamber, the result can be even better as long as they are designed correctly for a specific purpose. One company that does use compound shapes (and has for years) is Ferrari. Not only does the gas expansion push the piston down, the main shape of the compound shape combustion chamber focuses the explosion toward the entire surface area of the piston. The second shape of the compound shape in the head directs it's energy toward the very center of the piston. Shaping your combustion chamber can be fairly simple if you look at it in a basic light. What the manufacturers have to do is to keep in mind reliability. I could actually design a very powerful combustion chamber, but the life of those pistons would probably be measured in minutes vs. years. Not only is combustion chamber shape very important, piston shape is almost equally important. If an engine designer designs a head based on a specific piston, using a different piston may not be all that useful in that application. Using higher compression pistons will add power. Using a higher compression piston that has been specifically designed for that combustion chamber will yeild much better results in power and will retain more longevity than just a basic higher compression piston. To hit on your "headgasket blowing" question, any time you raise the compression in any engine, there will be a risk. By using a slightly higher compression piston, the pressures inside the combustion chamber a greatly increased (in extreme cases, exponentially). Any place that sells/makes high compression pistons will either make or work with someone who makes a stronger headgasket for that application. I tend to think of a headgasket as kind of a 'safety valve'. I'd rather have a headgasket blow than have a piston crack or crack that brittle cast iron block. In closing of this response, the hemispherical head doesn't create more compression, it simply focuses the engery more efficiently and effectively toward the piston.