Engine compression ratio - how does it affect performance and economy?

[soccerballtux]

Well nothing else happens in ht might as well allow it

 

[Railgun]

2 year old necro post?

Says the guy who originally responded to a four year old necro. :biggrin:

 

[imagoon]

Says the guy who originally responded to a four year old necro. :biggrin:

Hey now, to be fair I haven't been the one that raised the dead.... I just dealt with it lol.

Time to start singing "it is the post that never dies!"

 

[Alexpine]

"that engines are sometimes run at or above their max rated HP curve, or that, all other factors held constant, an engine with a higher HP rating will not consume more fuel than one with a lower HP rating, provided that both engines are loaded identically."

Is this true? I have 2 questions -

1. does this depend on the compression ratio

2. if one takes a 2.0ltr and a 2.5 or 3.0ltr engine what is the difference in the compression ratio? difference between 2.0, 2.5 and 3.0ltr

Thanks all who wish to explain.

 

[Squeetard]

You guys are missing an important point.

Static compression ratio means nothing.

All gasoline engines have overlap built into the valve timing. There is a point when the piston is hanging around at the top of the stroke where both the intake and exhaust valves are open. The is called scavenging, the piston is not going down so it is not pulling any air fuel into the cylinders. But there is suction on the exhaust side from the velocity of air blowing out the exhaust from other cylinders being on the exhaust stroke. This helps to pre-fill the cylinder with air/fuel. This overlap also occurs at the bottom of the stroke to get more of the burnt fuel out of the engine.

The greater the overlap the less the actual compression will occur as the piston is not at the bottom or top of it's stroke when both valves are shut and compression can build.

The optimal compression for gasoline is around 11:1. If you go look at a cam catalogue you will see that radical cams actually spec a minimum static compression ratio for use with them. My race cam specs 12.5:1 minimum static. This is so the actual is around 11:1. I run 14.5:1 and 112 octane race fuel though because compression is power.

 

[bryanl]

A 10% increase in compression ratio gives approximately 1-2% more power or economy.