Compression Ratios

[ShakeyC]

Having a thick day today cannot get my head round what it is help

[maxfield]

Look here:

http://www.carbibles.com/fuel_engine_bible_pg2.html

Should explain what it is.

[Jhonno]

its the ratio of the volume of when the piston is at bdc : tdc

ie 100ml @bdc - 10ml @tdc = 10:1 compression ratio

[ShakeyC]

Ah get it now, what baffles me when people talk about going low ratio for turbo engines and people going higher for turboless ones so why does 1 go down and other up?

[Jhonno]

higher compression ratio is more efficient but leads to higher combustion temps and internal pressure therefore when you factor a turbo into the equation it all goes higher again (dynamic ratio i believe) leading to bits not being upto the task ie head gasket and failure occuring, or pre detonation due to the high temps.. hope that makes sense..

[Turbo-Brown]

Indeed, so 100ml at BDC and 10ml at TDC would mean a clearance of 10ml and a swept volume of 90ml.

So out CR would be (Swept + clearance)/clearance=CR

Giving 90+10=100

And 100/10=10, that being the compression ratio.

That's the static compression ratio, but things like cam timing and lobe profiles, bore:stroke ratios etc will also have their part to play in the compression ratio which the engine actually sees while it's running.

[ShakeyC]

Thank you everyone i been learned lol

[CHRISP]

Look here:

http://www.carbibles.com/fuel_engine_bible_pg2.html

Should explain what it is.

Good reading mate