Compression Ratios

[Richie-Van-GTi 71 2 Cars]

Can someone explain in laymans terms how these are calculated please. I feel its something I should know but dont

Id be interested in working my own spec out (see sig) but it would be more useful when reading posts TBH.

[welshpug 1,657]

compression ratio is the difference between total cylinder capacity when at BDC and TDC (bottom and Top dead centre)

 

petrol engiones being spark ignition dont need that high a compression ratio, Diesel on the other hand is a compression ignition hence the higher figures involved (boost and Compression)

[welshpug 1,657]

so going on that theory a 1905 Mi16 running 10.4-1 has a total difference of 1905cc to 183 ish cc.

[smckeown 1]

dave's good for some things.. http://www.pumaracing.co.uk/CAPACITY.htm

[petert 586]

so going on that theory a 1905 Mi16 running 10.4-1 has a total difference of 1905cc to 183 ish cc.

 

Not quite. The formula is CR=(Vs+Vc)/Vc

 

eg. for an Mi16

 

Vs = 1905/4 = 427cc (swept volume of one cylinder)

 

Vc = 45.4cc (volume in chamber, head gasket etc.)

 

CR = (427+45.4)/45.4 = 10.4

[Richie-Van-GTi 71 2 Cars]

haha, now I understand, always read it as 11 to 1 in a ratio kind of way when infact it is 11 point 1 like a decimal number. Makes much more sense. So assuming I find out the swept volume of my bottom end and the approximate clearnance volume of the 1.9 head plus a fraction for the bit in the top of the cylinder that isnt swept then I could find out a fairly accurate CR for the engine

 

thanks to all that replied, Ive learnt something new

[Normski 1]

haha, now I understand, always read it as 11 to 1 in a ratio kind of way when infact it is 11 point 1 like a decimal number. Makes much more sense.

 

No, it is a ratio (compression ratio). The example above of the Mi16, it has a compression ratio of 10.4 to 1 (10.4:1).

[Faz85 0]

Not quite. The formula is CR=(Vs+Vc)/Vc

 

eg. for an Mi16

 

Vs = 1905/4 = 427cc (swept volume of one cylinder)

 

Vc = 45.4cc (volume in chamber, head gasket etc.)

 

CR = (427+45.4)/45.4 = 10.4

 

Your maths is slightly wrong there peter, 1905/4= 476.25cc

 

and the Clearance Volume, Vc for the Mi is actually 50.7cc

 

That gives you CR=(476.25+50.7)/50.7 = 10.4

 

So assuming I find out the swept volume of my bottom end and the approximate clearnance volume of the 1.9 head plus a fraction for the bit in the top of the cylinder that isnt swept then I could find out a fairly accurate CR for the engine

 

Yep, and the exact clearance volume for the 1.9 head (assuming its an 8v) is 55.38cc

 

I dont know what the swept volume is for the 2.0 bottom end you have, but if you get it from the joke book you can bung it in the above equation along with the clearance volume i've just calculated for you, and get the EXACT CR for your engine

 

Faz

Edited March 9, 2007 by Faz85

[petert 586]

Your maths is slightly wrong there peter, 1905/4= 476.25cc

 

You're right! It's amazing how much clearer things are in the morning. Deamon drink.

[Richie-Van-GTi 71 2 Cars]

Thanks for that Faz, I work it out to be 498.5 swept volume and using 56 for the Vc I get a CR of 9.9.1.

[Sam 4]

http://www.csgnetwork.com/compcalc.html

 

Handy calculator.

[christopher 5 1 Cars]

Here are the words on the topic by Guy Croft

 

http://guy-croft.com/viewtopic.php?t=164

[petert 586]

This is my favourite:

http://www.empirenet.com/pkelley2/DynamicCR.html

 

read it, digest it, then download the calculator. You can use it for static or dynamic CR.

Edited March 14, 2007 by petert