Compression question

Colette · Post by **Colette** » Thu Jun 15, 2006 4:26 pm

I'm looking at different bikes and comparing their specs, but one thing I don't understand is compression...

For example, one bike has a compression of 12:0:1 and another is 12:2:1 - what does this mean?

FrogZ · Post by **FrogZ** » Thu Jun 15, 2006 5:02 pm

Atmospheric pressure is 1
When the valves close and the piston rises the air/fule mixture gets compressed.
The less area left between the piston and the head (squish zone) the higher the compression and the bigger the explosive force.
Also the better the combustion chamber shape and fuel octaine (out of my zone now guys) needed for perfect combustion and maximum explosive force. For example if the sparkplug is shrouded (or hidden) then the flame wont get eerywhere before the piston goes down (poor explaining I know but hey its nearly 5) and all the compression in the world wont help.
The higher the compression, theoretically the more power/bigger bang.

BUT combustion efficency comes into it.
SO pulling the head gasket out of your old BSA and aralditeing the head to the barrel will just blow the base... (or spit the head studs out of your old IT175 ahem ahem)

Old bikes had lower compression for a lot of reasons (poorer fuel, technology etc.)

Did that make sense ??

Colette · Post by **Colette** » Thu Jun 15, 2006 7:31 pm

Thanks Frog, it sortof does...so does that mean the compression that's 12:0:1 is better than the 12:2:1?

YoungKwakUK · Post by **YoungKwakUK** » Thu Jun 15, 2006 9:08 pm

Hey! I think 12.2:1 is a higher compression ratio than 12.0:1. Maybe its compressing the misture by 12.2 times rather than just 12 times. As said, higher compression usually = more power so if u mean better as in more powerful, yes 12.2 is better than 12.0 thats if its same engine. obviously an engine with a compression of 12.0:1 could make more power than a different engine with a compression of 12.2:1 due to different design.... hope that helped a bit ! please correct me if im wrong guys, id like to know for sure tooooooooooo

Smitty · Post by **Smitty** » Thu Jun 15, 2006 10:51 pm

YoungKwakUK wrote:Hey! I think 12.2:1 is a higher compression ratio than 12.0:1. Maybe its compressing the misture by 12.2 times rather than just 12 times. As said, higher compression usually = more power so if u mean better as in more powerful, yes 12.2 is better than 12.0 thats if its same engine. obviously an engine with a compression of 12.0:1 could make more power than a different engine with a compression of 12.2:1 due to different design.... hope that helped a bit ! please correct me if im wrong guys, id like to know for sure tooooooooooo

yeah sorta
compression ratio is only one indication of how much power an engine
will make (you then start with head design, number of valves
camshaft lift, number of carbs etc etc blah blah)

if all else is identical, an engine with a higher compression ratio
should make more power..providing the engines does not
suffer from detonation or preignition due to the higher compression

(oops did I just open a can of worms....?

)

Rossi · Post by **Rossi** » Thu Jun 15, 2006 11:09 pm

Collette........
stop getting technical................pick a bloody colour will ya

FrogZ · Post by **FrogZ** » Fri Jun 16, 2006 7:57 am

..and to be quite honest the colour is a DAMN sight more important than the compression ratio....

Colette · Post by **Colette** » Fri Jun 16, 2006 8:23 am

yeah ok - was just wanting to know what it meant, that's all

Smitty · Post by **Smitty** » Fri Jun 16, 2006 11:40 am

Colette wrote: yeah ok - was just wanting to know what it meant, that's all

go girl...ask as many questions as ya want...

ZX10R King · Post by **ZX10R King** » Fri Jun 16, 2006 11:49 am

http://e30m3performance.com/myths/more_ ... _ratio.htm

Smitty · Post by **Smitty** » Fri Jun 16, 2006 2:02 pm

ZX9R King wrote:http://e30m3performance.com/myths/more_ ... _ratio.htm

yep.....

that certainly explains it

mike-s · Post by **mike-s** » Fri Jun 16, 2006 2:09 pm

*blink* ohkay ill ignore the formulae there, btw frogs, downsize that sig something, its frickin HUGE.

Colette, in the long run the compression ratio isn't as important as rev range, torque curve, capacity and no of cylinders when it comes to looking at the engine a bike has.

torque = level of pulling power
number of cyl's = whereabouts in the rev range it will pull and how smooth it will operate. 1= anywhere from idle up. 2 = low to mid, but it'll run out of breath higher up. 4, not much down low, more grunt up top until redline. 3, middle of the range between the 2 and the 4, best of both worlds really.

The only time one should worry about the compression ratio is when doing an engine test, 1 atmosphere = 14psi, so a compression test giving 110psi in a 12:1 engine means that it is leaking a lot of air (it should be up about 165psi if not more), (thusly a engine like my gpx has a 11:1 ratio and 140psi would be fine)

jpbikeboy · Post by **jpbikeboy** » Sat Jun 17, 2006 10:00 am

To add my 20c worth. Comp ratio is the ratio of volume in the cylinder at bottomdead centre(piston all the way down) compared to the volume at top dead centre(piston all the way up).
eg if you have a volume of 55cc at bottom dead centre and a volume of 5cc at top dead centre- 55/5=11.0. Comp Ratio of 11.0:1
The higher the first number the more compression it has and in theory more power.
You pobably wouldn't feel the difference between 12.2:1 and 12:1.

MAN WE'RE NERDS

Colette · Post by **Colette** » Sat Jun 17, 2006 1:20 pm

Thanks Josh, that's a very clear explanation, which is what I was after

p.s. How's your little bub going?

Duane · Post by **Duane** » Sat Jun 17, 2006 1:49 pm

Compression to me is measured by how much your ring hurts after you rip a fart out.