Compression ratio's of MC and MCS
 

Hm. the catalog shows MC to be at 10.6 and the MCS at 8.3.

something with the super charger has something to do here right? i know thecompression must be greater in the mcs when the charger is going right?

its already.... being compressed!

Engines with turbos and superchargers have lower compression ratios. The compression ratio is measured as a "static compression ratio", meaning that it is measured when the engine is not running.

The "effective compression ratio" can be calculated if you know the static compression ration, the boost in psi, and the altitude.

The added boost from the turbo/supercharger increases the effective compression ratio. . .

interesting. didnt know that the compression ratio numbers werent based upon things compressing....

tad useless in a few ways!

The compression ratio tells you how much air gets compressed by the piston in a single stroke at atmospheric pressure. As Yucca says, add boost to the equation, and things change. Too high a CR on a boosted engine, and things start going boom.:sad:

I guess it varies too so no set standard.

I think a compression ratio of say, 10 to1, means than 10 square inches of air are compressed into a space equal to that occupied by one square inch of uncompressed air.

In the case of the MINI S more air enters the cylinder than in a non-supercharged engine because the supercharger is pumping it in. If this air is compressed a lesser amount, say at about 8 to one, as in the MINI S, the resulting cylinder pressure will at times be higher than the 10 to 1 ratio engine because there is more air present to compress. The ratio of compression, that is, how much smaller an area the air entering is compressed into, does not change, but the resulting cylinder pressure can be very high due to more air entering, via the increased air volume of the supercharger, (or turbocharger).

In order for the cylinder pressure not to get so high it breaks something, the ratio of compresson in the supercharged engine has to be lower than the non-supercharged one.

Maybe.:)

sounds good enough :) must be the case actually.

a 8.6:1 engine would be pathetic.

It doesn't matter the pressure...
 

compression ratio is the ratio of the volumes in the cylider with the piston at top dead center and bottom dead center.

The post above about boost and ambient temp giving you effective is incomplete. You need AITs and volumetric efficiency as well.

But to answer the question of why that is, with more stuff crammed into the cylinder, the temps would get too high when compressed, and the charge would detonate. This would be bad....

But things are a changing. Some of the direct injection turbos are running static compression of 11 to 1. PRetty amazing.

Matt

Thanks for clearing up and correcting my confusing and incorrect post.

You were pretty close
 

and had all the key concepts. Let's just call it a refinement...:wink:

Matt