compression ratios

[yelizardking]

what exactly is the compression ratio? can i get some info on that? why is it so very importan?

[94_AcCoRd_EX]

Hey man, you already asked that...

http://www.hondastyle.com/forums/showthread.php?s=&threadid=3964

[yelizardking]

i sure did but i am still unclear, so i decided to post it in the "high performance" foroum instead.

[94_AcCoRd_EX]

Originally posted by yelizardking
i sure did but i am still unclear, so i decided to post it in the "high performance" foroum instead.

What are you unclear about?

[yelizardking]

this is the story i've read the whole thing a couple of times and i don't know it's not clicking. when i get things down i am able to explain it and this is something i am not able to explain except that a higher cr id not needed for fi b/c of the turbo or sc.

[2ndGenTeg]

Alright, check it out.

The compression ratio is exactly that: a ratio. It is the ratio of the volume of the combustion chamber with the piston at TDC (top dead center) to the volume of the cylinder plus the combustion chamber with the piston at the bottom of its stroke. For example, with piston at the very top of its stroke, the volume of the combustion chamber is 1 cubic inch. BANG!! Ignition, the piston travels down the cylinder. At the very bottom of its stroke, the volume of the combustion chamber plus the cylinder is 12 cubic inches. Your compression ratio is 12:1. Make sense? Okay, moving on.

A higher compression ratio gives you more power and better gas mileage. What you're doing when you raise the CR is to make the engine more efficient. What happens is that as the CR rises, the a/f mixture is under more pressure. When ignited, it's going to push the piston down harder and faster. However, more pressure in the cylinder results in detonation. As the a/f mixture becomes compressed, it gets volatile. Compress it too much, and it ignites on its own prematurely- also known as detonation or "ping." To fight this, you have to use a higher octane gasoline. An octane rating is the the gasoline's resistance to detonation. The higher octane you use, the more pressure the contents of the cylinder can resist.

In an all motor setup, the sky is usually the limit on CR. Your problem with a high CR is usually valve to piston contact. I've got a friend running 13.5:1 in his all motor Integra. He also runs 101 octane that's $4 a gallon, gets 40 mpg, and runs a 14.2 at 5500 ft (high 12's at sea level).

In a FI setup, you have to be very careful with the CR ratio. Remember- detonation occurs because the contents of the combustion chamber are under too much pressure. Therefore, the higher the boost, the lower the CR. However, you still want to find that medium to maximize your power. This is why you can turn up the boost on race gas- the higher octane resists detonation, allowing you to put the a/f under more pressure.

One more note- how stroke affects the CR. A very popular idea in the all motor world is to put CTR pistons into a the GSR or LS block to get a much higher CR. Why does this work? The stroke on the B16A/B is so short that you have to use a very high domed piston in order to get a respectable CR. When you transfer that same high domed piston to a block with a longer stroke (such as the LS or GSR), you get a much higher CR. This is because the volume of the combustion chamber remains the same, but the volume of the cylinders has increased with the lengthened stroke.

Hope you're still with me, let me know if you need clarification on anything.

[ebpda9]

doesn't the shape of the pistons prevent pre-ignition in high cr engines ? i read somwhere theat the flat or dish pisons will not have as much predetonation, as the dome pistons ?

[2ndGenTeg]

The reason flat pistons don't have as much of a problem is because the CR isn't as high, so the a/f isn't under as much pressure.

The shape of the piston is what increses the CR by reducing the volume in the combustion chamber. SOME high dome pistons (i.e. Endyn) have a treatment to prevent detonation. The Endyn have the sharp corners sanded to be smoother so they don't get as hot.

[ebpda9]

i read about flat pistons that will do about 11:1 for my car. I think arias makes them.

[2ndGenTeg]

I haven't heard of those. In order to drive up the CR with a flat piston, you'd have to run the piston into the head, and have the combustion chamber machined out to accomodate it.

I don't see a reason for that, other than combustion wave travel. Valve to piston contact would be a greater issue because the valve would be at angle. You wouldn't be able to run it with very aggressive camshafts.