Understanding compression ratios

[Wilson]

I was just reading rowemeisters spec in members cars, where he states compression 10.2:1, when I realised I dont understand what they mean...

Does it mean 10.2 bar to one stroke?

Don't take the p1ss if im wrong!!

[320Touring]

I was just reading rowemeisters spec in members cars, where he states compression 10.2:1, when I realised I dont understand what they mean...

Does it mean 10.2 bar to one stroke?

Don't take the p1ss if im wrong!!

it basically means how much can be fitted into the physical space between piston and head.

1= the volume of the combustion chamber at the top of the piston stroke.


the 10.2 means how much fuel and air is actually crammed into that space (ie compressed)

so rowemeisters car fits in 10x the volume of fuel and air as the size of the chamber.

hope that helps?

[Wilson]

So when the piston is at bottom, there is 10.2 x the space as when piston is at top.

So when the piston is on compression stroke, it squeezes 10.2 into 1???

Sound right?

[320Touring]

So when the piston is at bottom, there is 10.2 x the space as when piston is at top.

So when the piston is on compression stroke, it squeezes 10.2 into 1???

Sound right?

basically mate

not quite that simple, but there or there abouts!

thats why to raise compression you skim the head or use domed pistons (to make the space smaller) so you have smaller space, same amount of fuel/air mix =higher compression

[Turbo-Brown]

The compression ratio's calculated by the swept volume of the cylinder, plus the chamber volume, all divided by the chamber volume.

So a cylinder with a swept vol of 100cc and a chamber volume of 10cc would work out as follows:

(100+10)/10 = 11:1 compression ratio.

Skim the head and you reduce the chamber volume so if we took 1cc out of out theoretical chamber volume:

(100+9)/9 = 12.1:1

[Jhonno]

its how much the volume of air in the chamber is compressed..

if you take 10cc and compress it to a space 1cc the ratio is 10:1

[Wilson]

Brilliant... thanks guys, I understand about having higher compression lower compression etc... just never new what measurement was...

OK, so rowemeisters is 10.2:1, what is the standard 325i??

[rowemeister]

There are a few "standard" 325 compression ratios mate. My later cat ready engine was about 8.2:1 (or similar) but earlier engines were something like 9.2:1. Im sure someone here will have the correct ratios.