Compression?

hey fellas could someone explain to me compression of an engine and how it works wha the hell 9:1 means and all that im just trying to pick out some heads for my rebuild and i relized it might be a good time to figure out what kind of compression i want to shoot for also if you could explain what type of gas runs best of what kind of compression and why, what i guess im asking for is automotive 101 :iceslolan

The Compression Ratio indicates how much the air/fuel mixture is compressed by the piston stroke. i.e. the volume of the cylinder at bottom dead center vs top dead center.

To calculate it, divide displacement/# of cylinders + combustion chamber size by combustion chamber size

The size of your combustion chamber depends on your cylinder heads, pistons, head gasket thickness, and the height of your engine block. Sorry, but I have little idea of how calculate exactly the combustion chamber size.

A higher compression ratio wil give you more power and torque. But, since the air/fuel mix heats up as it get compressed, you have to make sure that it does not ignite on its own and screw up your engine. "Detonation" is the term for this. To prevent this, you need a higher octane gas, as more octane makes gas harder to burn/ignite.

A 9:1 compression ratio will run on 87 octane gas fine, but a 11:1 compression ratio will need a higher octane gas, such as 91 or 93.


This concludes today's lecture in Automotive 101. Any questions?

basically a compression ratio is how much the gas/air mixture in the cylinder is compressed. the higher comp you have, the more power you will make, but you will also need to run higher octane fuel and there will be more stress on your engine parts..so a 10:1 ratio means if a 100cu of air is in 1 cylinder, before the power stroke, it will be compressed to 10cu. (this results in a bigger bang, cuz it expands faster, more violently)

I'll try to make this as simple as possible...But this is for Static Compression, not Dynamic Compression.

Comp Ratio = (Total Chamber Volume + Swept Volume)/Total Chamber Volume

Swept Volume = 'pi' x (Bore/2) x (Bore/2) x Stroke

Total Chamber Volume = Combustion Chamber Volume + Head Gasket Volume + Deck Clearance Volume + Valve Relief Pocket Volume + Crevice Volume

http://gmhightechperformance.com/tech/0609htp_77_z+stroker_engine_build+total_chamber_vo lume.jpg

http://gmhightechperformance.com/tech/0609htp_78_z+stroker_engine_build+chamber_volume.j pg

in basic terms, no math and confusin stuff, take a cylinder and divide it into 9 equal sections as if it were a 9 story building. now imagine that 9 equal sections being squeezed down into one (you squeeze that 9 story building into one floor). thats 9 to 1 compression

now it gets advanced, not really but a little. you take that building, at is same total height, and turn it into a building for short people or something, so now you make it a 12 story building, but its still the same height as the 9 story one, and you squeeze all those 12 floors into one floor(which would be alot smaller of a floor) thats 12 to one compression. basic class complete. and im not to sure on this but i think around 12 to 1 you need high test, 93 octane