17 replies to this topic

[Retroman]

Pretty sure the MG metro was 10:1

 

 970 S  FC and FD engines were 10:1

 

1071 S  9:1

 

1275 S  12F and 12FA engines were 9.75:1

[Spider]

Pretty sure the MG metro was 10:1

 

 970 S  FC and FD engines were 10:1

 

1071 S  9:1

 

1275 S  12F and 12FA engines were 9.75:1

 

As always, yes, you are right for Static Compression Ratios on these Engines.

 

and it high-lights what I posted in regards to Dynamic CRs.

[Retroman]

Sorry spider I didn't read your post correctly

 

The Dynamic CR is always lower than the Static CR (the one always quoted)

 

The Dynamic CR is calculated using the closing point of the inlet valve, as real compression does not start until then.

 

The piston will be some way up the bore by that point

 

Compression stops when the piston gets to TDC (Top Dead Centre)

 

The longer the duration the cam the higher the closing point of the piston so less degrees of crank rotation are left to compress the mixture

 

As there is less time (degrees), the compression has to be higher to acheive a similar objective

 

The Static CR varies more than the Dynamic CR which is borne out in what Spider said.

 

Importantly;  Assume nothing,  measure accurately,  it can then be worked out and 'adjusted' if needed