I know some things, like higher compression requires higher octane fuel, and running a SC you need low compression. What does compression do, does it have to do with the air that enters a motor?

 

compession raises the temp of the air in the cyl to start...but maily makes the "explosion"....more like a burn..much faster...it is like explosives...if they were just left open it would just burn but when inclosed they burst...violently....ne way...i am not quite sure.

 

An engine's compression is the the combustion of oxygen and fuel on top of the piston. Compression ratio is equal to the cylinder and combustion chamber volume divided by the combustion chamber volume. Then you also have static compression and dynamic compression. Just think of the piston getting pushed down or "compressed". You don't need lower compression to run an SC you need it to avoid detenation (bad) unless you are great at tuning.

 

To give you an oversimplified explanation think of "compression" as the air between the top of the cylinder and the head.

The head has a chamber of a certain volume. Let's say 64 cc. And lets say your compression is 10:1. That only squeezes the air/fuel mixture in that chamber so much. Lets say now you mill the head and the chamber is 58cc. Now the air space is smaller (6 cc less) but you have the SAME quantity of air/fuel coming in. Therefore you're forcing that air/fuel mixture to "fit" in a smaller space. Now you're increased your compression. When you ingnite this mixture of air and gas that's now squeezed more then before it will explode with more force, therefore forcing the piston down quicker, creating more horsepower.

A lot of factors can change the compression ratio: head gasket thinckness, top of piston design, chamber volume, how the rings are placed on the piston, deck height.. etc

 

Let me try to get technical, but still understandable here.

Compression ratio is the ratio of the volume of air in the cylinder at Bottom Dead Center (BDC) or the bottom of the stroke of the cylinder to the volume of air in the cylinder at Top Dead Center (TDC) or the top of the stroke.

Therefore, if you were to put the piston at the top of it's stroke, measure how much it takes to fill the cylinder at this point, that would be the 1 in the ratio. Let's say, for example, that it takes 100 cc's to fill it here. Then you put the piston at the bottom of it's stroke, and see how many "1's" or what the volume is that it takes to fill it. Let's say it take 960 cc's to fill it here. that would be 9.6 "1's". Your compression ratio would be 9.6:1.

Now, as for what compression does, it's pretty much like Blackened said, light a certain amount of fuel and oxygen in open air, and it will just burn. Compress it, and it will explode. Compress it more, and it will be a more intense explosion.

I hope this helped, and didn't confuse anybody!

 

why does the lesser gas cause detonation though??

what exactly is detonation?

 

Lesser fuel (Lean mixture) causes detonation because the air gets warmer than it would with more fuel in it.

Detonation is when part of the mixture doesn't burn when it should because of a poor mixture, not enough turbulence, or poor fuel distribution, then the part that is unburnt, burns late, and collides with the flame from the part that burns correctly.

Then you have pre-ignition, or ping, and that is when something in the combustion chamber causes the fuel miture to burn, instead of the spark plug, such as a carbon deposit, or valve edge, or anything else that get's hotter than it should.

Detonation occurs after the start of normal combustion (ignited by the spark plug), and is much more damaging, and pre-ignition (as the name implies) occurs before the normal combustion starts.

Again, I hope not to confuse!

 

You guys are smart thanks for all the info

 

How do detonation and pre-ignition damage an engine? I don't know much about them except it sucks. I crossed up the 5th and 7th cylinder plug wires once, and it wasn't too pretty.

 

We're going back a ways here, but as I remember by eng theory;

Under normal combustion, the flame front moves away from the spark plug and across the combustion chamber at abt 700 ft/sec.
This is a "burn" that causes the air/fuel mixture to burn/expand at the correct rate and produce a usefull pressure to push the piston down.

During denotation. The flame front moves away from the spark plug at over 4000 ft/sec. This is more like a BANG than a "burn".
Pressure builds and drops way too fast and is not really of any use. It's like hitting the piston with a hammer.

Kinda like trying to push your car by hitting it with a big hammer on the rear bumper. A lot of energy is released but, too quickly to do any good.

The BMEP (brake mean effective pressure) is the pressure the cyl sees during the "burn". The higher the BMEP, the more likely detonation is to occur. Higher octane fuels help out here since they are more stable at elevated pressures and temps.
Running rich also helps due to the cooling effect a rich mixture has on the combustion chamber.
Water injection also helps but I've only seen it used on large (5000+ cu in.) piston-type aircraft engs; never on cars.

Entire books have been written on detonation, hope this condensed version helps ya understand what going on in there a little better.