i was trying to explain to someone why a low compression engine is optimal for turbo charging. (i guess i really don't know the answer) he brought up some very reasonable arguments which got me questioning my thinking.



how does high compression limit our power output? i am not talking about how much boost we can run. just power. example if i wanted to make 500 hp

i could start with a low compression engine, use a big turbo and big boost.



or start with a high compression engine, use a smaller turbo and less boost.



wouldn't the final compression ratios be similar?



why do we want low compression for big power?



assuming both engines have proper tune the same fuel quality

 

As I understand it, the issue is not whether high or low compression creates more power, but whether the engine can handle it. High compression will cause the more reactive molecules in your gasoline (so the smaller molecules in the case of gas made of non-polar hydrocarbons) to oxidize just from pressure alone. The pressure pushes the molecules tighter and tighter together and thus the chances of two molecules hitting eachother with enough energy and the right orientation is increased greatly with pressure alone. This means that during the compression stroke, the greater the number of smaller molecules in the gas, the more likely two will hit with enough energy and explode. If this happens before the spark, you get knock, which is the sound you get when the explosion is early and pushes the piston down with a lot of force while it's still trying to go upwards. Eventually something has to give, and the something tends to be pistons/rotors or the walls of the combustion chamber.



Now since more power comes from a bigger explosion, you need more air and fuel in the combustion chamber and this causes increased pressure. So a few things you can do to keep power but not destroy your engine is to reduce the pressure, reduce the heat or make the fuel mixture more tolerant to higher pressure/temperature. This in where low compression comes in. It allows you to create more power without changing your fuel because you can cram more stuff into the combustion chamber since the chamber is larger. The reason to do this is we're limited to what type of fuel we can get at the gas station so road-going cars running high-boost compensate for the increased heat and pressure of the air going into the engine with lower compression. It's a trade off since a high boost engine with high compression would cause the most powerful explosions, but race gas (a high octane fuel) is not exactly cheap.

 

i guess what i am asking is, isnt a high compression, small boost, smaller turbo, the same as low compression, big boost, and big turbo.

the turbo essentially increases the compression ratio by blowing more air into the chamber

 

While the turbo increases the compression ratio, ultimately a lower compression piston results in a much larger starting volume than the high compression piston. However as it moves up towards the spark plug, the volume dramatically decreases which gives the molecules of gas in the cylinder a very small period of time to explode and create engine knock. Meanwhile, a high compression piston starts with a smaller volume which gets smaller much more slowly than the low compression piston. This gives a much larger period of time for knock.



However, you could do an N/A setup which is perfectly viable and therefore would require higher compression. The difference is then in the revs. For less revs you get more horsepower from lower compression pistons plus boost (because there is a bigger explosion each time) and therefore an N/A setup will need to rev more quickly but it can still in the end create plenty of horsepower. Again, there is a limit to how fast you can make a crankshaft spin when it is getting pounded with the force of many explosions and metal does get surprisingly fragile at high enough speeds, thus keeping revs down while gaining horsepower is desirable. That's why lower compression is often favoured for road cars.

 

Keyword is Volume, Mr Lockeed...

 

?

 

compression ratio is static, what goes on when the engine running is not



9:1 compression ratio plus 10psi of boost doesnt equal 19:1 compression ratio.

 

I think that's what I said. But maybe I was just totally unclear. A chem degree does that to people :P

 

i'll speculate a little on the subject...



with higher compression you have a smaller area to put air and fuel in, there is more pressure which makes more power but because everything is closer and a thighter fit for the explotion it's more likely to detonate.



with lower compression you have alot more air and fuel but in a larger area. So the burning is more controlled but will put down the same amount of force.



I've heard pistons with large domes can block the flame front and result in less then optimal burning.



The plus side to having a higher compression aside from increasing the power is also increasing economy because less air and fuel can enter the smaller space. But detonation is increased so you are more likely to melt something.

 

Okay, I definitely said that already...clearly you all are thermodynamically illiterate :P