hi compressions

[RABBITT_Andrew at mv8.orbeng.com.au]

>Somebody smart wrote:
>>>Therefore another approach is to outrun the fuse,
>>>which fast burn combustion chambers can do. Anyone notice how high 
>the>>compression ratios of modern 4V engines (and even some 2V) are 
>getting?

>Talltom replied:
>>Yes I have and I'm pissed! Would you say that is due to combustion 
>>chamber design, or to more precise tuning capability?

>I have read that the big reason for four valves is to get the spark 
>right in the center.  Flame front has much shorter distance to 
>travel, meaning higher compression without detonation.  You could 
>probably get the same effect with four spark plugs per cyl.   BTW, I 
>believe detonation is what happens after spark but the mix explodes 
>before the flame can travel far enough to get the whole mix burning.

Mixture preparation has a lot to do with it.  If you atomise smaller, 
the burn-rate is faster, and thus you get the same torque with 
retarded ignition.  This allows you to run a high compression ratio, 
giving efficiency gains in areas where you're not spark limited for 
MBT.

Charge motion is also a big player in mixture preparation.  With four 
valves (nothwithstanding the advantages of increased valve area), you 
can arrange what is known as 'tumble' which is akin to (but arguably 
better than) 'swirl' in a 2-V design.