Re: S4 20v vs S2 10v

[Robert Phillips <r3rfp@dax.cc.uakron.edu>]

On Mon, 6 Feb 1995, Robert D. Houk wrote:

> Um, I don't think so. 7:1 compression of 2 bars is 14 bars, while 10:1
> compression of 2 bars is 20 bars of pressure. I believe that detonation
> is more-or-less a direct (if not linear) function of gross pressure. So
> higher-compression leads to quicker detonation, all other things being
> equal. At 20:1 you can dispense with ignition altogether (you have a
> diesel engine). I recall that detonation is a function of many differ-
> ent factors, not all totally understood. For example, a dirty engine
> with lots of carbon buildup can detonate when the carbon retains enough
> heat (sitting there glowing cherry red, for example) to ignite the fresh
> mixture (whose susceptability to such might also be a function of its
> pressure). Now if you had direct injection . . .

Detonation relies on a number of factors:
1. Compression ratio (the higher the CR, the more likely for detonation)
2. Ignition timing (the more advance, the more likely for detonation)
3. Boost pressure, especially turbo (the more boost, the more chance for det)
4. Engine factors, such as carbon buildup, overall thermal transfer of 
the head, etc.
	Regarding heads, the quicker the head is to dissipate the heat 
from combustion, the less chance for detonation.  This is why, typically, 
aluminum heads can hold a higher CR than the cast iron heads of yore. 
(or *your* small block CHEBBY)

The carbon buildup in the combustion chambers effectively raises the CR 
of the engine, as the mixture of fuel and air is pressed into a tighter 
space due to the buildup.  This increases the chance of detonation.

So, to summarize, more boost is possible with a lower CR on a given 
engine.  
			---Bob
		(not a rocket scientist)