Compression Explanation

[Ralph]

I learned out of this and thought it would be good to post this for everyone. F.A.Q material

Origionaly written by:wilkin250r

Compression:

It's a ratio of before and after.

There is a difference between internal volume and displacement.

For a stock 400EX, the bore and stroke is 85mm x 70mm. Do the math, and you get 397cc (cubic centimeters) The compression ratio is 9.1:1

When the piston is at the very bottom of the stroke, the internal volume of the cylinder is not 397cc, it is actually about 440.6cc. When the piston is at the top, the volume is about 43.6cc (this is called the "combustion chamber")

The compression ratio is calculated by dividing your displacement by your combustion chamber volume.

397cc / 43.6cc = 9.1

A high-compression piston has a dome on top of it that will decrease the volume of the combustion chamber. If your piston has a dome on it of 7.5cc, then your combustion chamber is now 36.1cc instead of 43.6cc. For the same displacement of 397cc, your compression ratio is

397cc / 36.1cc = 11

[Ralph]

And im pretty sure this is compression because the combustion chamber is now smaller so the pressure is greater when the piston is at the top of its stroke, Since there is less room for the air/gas, it compresses it more. Hence compression. And thats why when u start to use realy high compression pistons like 13:1 u have to run higher octane fuel because the lower octane prematurely explodes because in cannot take the pressure.

(Correct me if im wrong, i didnt look this up or anything and this is what makes sense to me, if im wrong tell me)

[wilkin250r]

Originally posted by Ralph
And im pretty sure this is compression because the combustion chamber is now smaller so the pressure is greater when the piston is at the top of its stroke, Since there is less room for the air/gas, it compresses it more. Hence compression. And thats why when u start to use realy high compression pistons like 13:1 u have to run higher octane fuel because the lower octane prematurely explodes because in cannot take the pressure.

(Correct me if im wrong, i didnt look this up or anything and this is what makes sense to me, if im wrong tell me)

This is absolutely correct. The extra power you get comes from the higher compression, NOT from the higher octane. You just need the higher octane to withstand the higher pressures.

[stupid driver]

Another way to word this is just the compression ratio is air/fuel pressure at TDC over the pressure at the bottom of the stroke.

[Bad Habit]

Good explanation Wilkins,,,,,,,,

You are a fart smeller....:D

[hawiianpwr]

Perfectly said Ralph!:)

[NTPRacing#19]

actually race fuel or higher octane fuel such as 110 is used in higher compression motors because the fuel burns cooler and compensating for the extra heat the higher compression piston makes. thats all race fuel is used for. i know this for fact.

[NTPRacing#19]

and oh the compression ratio for a 11-1 compression piston means that the compression at TDC is 11 times greater than it is outside of the motor.

[Taco]

Originally posted by NTPRacing#19
actually race fuel or higher octane fuel such as 110 is used in higher compression motors because the fuel burns cooler and compensating for the extra heat the higher compression piston makes. thats all race fuel is used for. i know this for fact. The octane rating is its resistance to pinging or predetonation. Higher compression motors are more prone to pinging therefore you usually need to run a higher octane rating.:D

[Chanman420q]

this is good, it def should be in the FAQ's

[NTPRacing#19]

ralph the race fuel burns prematurely not because it cant handle the pressure, its the temperature that the pressure causes, thats why it burns prematurely.

heres something that made me understand compression a bit more.
a diesel engine, they have no spark plugs becuase they dont need them, the compression ratio is so high say 18-1 that the heat from the compression makes the fuel burn inside the cylinder.