compression ratio..

[matt11583]

is it better to have a higher compressiion ratio of about 11:1 or a lower one of more like 8:1 for performance in a car??

[sunfire_starter]

the higher the compression ratio the more torque and HP you can run but the higher you go the higher octane gas you need. But as for turbos it is just the oppisite. You need a lower compression ratio to run turbo effeciently. The ideal compression for a turbo would probably be around 7.0:1 - 8.0:1 so you can run higher amounts of boost

[curtis73]

Higher compression has many drawbacks and many benefits.

Higher compression: more energy from combustion, more efficient burn (NOT to be confused with fuel efficiency), more tolerant airflow qualities, snappier throttle response, higher NOx emissions.

Lower compression: cheap fuel, less stress on the roatating assembly, easier to tune, fewer cam selections, much lower emissions, lower BSFC (again, NOT to be confused with fuel efficiency)

Unlike everyone else out there, I do not subscribe to the high-compression desire. I specifically seek out combinations that work fine with low (8-8.5) compression specifically for the benefits it provides. Sure, I give up maybe 8 hp, but the benefits to me are countless. Especially living in L.A. where the most octane we get is 91, and it costs $2.60 or more a gallon.

I don't mind higher compression on certain situations where they have been properly designed to still use cheap gas, but for the most part I don't need the extra few horses.

In general, however to answer your question, the higher compression ratio (if it actually translates to higher cylinder pressures) means that the gasoline that goes in gives off more of its energy than with lower pressures. More energy means more pressure available to push the piston and more power.

[CBFryman]

/\ couldnt have said it better myself

[SaabJohan]

A higher compression ratio means that the engine will be more efficient, this results in a lower BSFC and thereby increased fuel economy and lower exhaust emissions. The increase in efficiency also means that the engine can produce more power.

The only advantage with low compression ratios is that problems caused by engine knock is reduced.

Today most modern turbocharged engines used CR around 9, turbocharged racing engines can use even higher ratios, WRC uses a ratio above 10:1.

[curtis73]

Originally Posted by SaabJohan

A higher compression ratio means that the engine will be more efficient, this results in a lower BSFC and thereby increased fuel economy and lower exhaust emissions. The increase in efficiency also means that the engine can produce more power.

The only advantage with low compression ratios is that problems caused by engine knock is reduced.

Today most modern turbocharged engines used CR around 9, turbocharged racing engines can use even higher ratios, WRC uses a ratio above 10:1.

Although normally I just say "ditto" after everything you type, I have to strongly and respectfully disagree this time. Higher static compression and the higher cylinder pressures that it implies are known factors in increasing vehicle emissions, particularly NOx. Usually HCs drop some, however in some designs (particuarly newer more efficient head designs) the increase in compression changes the VE for the better. The extra HCs being drawn in sometimes negate the benefits of the increased efficiency and can (I repeat, can) increase BSFC.

I also have to respectfully disagree that increased thermal efficiency from higher compression equates with greater fuel economy. BSFC, combustion efficiency, and fuel economy are not always parallel. The three are related, but not one and the same. Higher compression does increase thermal efficiency for better, more complete combustion, but that does not equate to better fuel economy. There was an excellent contributing writer that used to write for Hot Rod magazine by the name of Clancy somebody. I forget his last name. He wrote an incredible article breaking it all down. It was just a couple years ago so I'll try to find it, but I don't keep back issues anymore.

Basically it was a response to all of the tech letters they get asking how they can get better fuel economy, as well as letters saying, "I increased my compression and my fuel economy dropped." The basic thesis was how the three are interconnected and how they affect each other, but are not consistently related given the hundreds of dimensions that you can alter on an engine.

Again, just a respectful counterpoint.

[Polygon]

Originally Posted by SaabJohan

Today most modern turbocharged engines used CR around 9, turbocharged racing engines can use even higher ratios, WRC uses a ratio above 10:1.

Yes but they also run on MUCH better fuel.