Timing - again

[Todd Knighton]

Talltom,
	Yes, I've seen the same thing, max power prior to detonation.  Though
this was typically on very crappy cylinder heads.  The one that really
comes to mind is a 1.8L 4 cylinder 914 engine my brother had.  You could
wing the distributor all the way around and never get that damned thing
to knock.  So there was definately a peak power point prior to
detonation.
	One other example I found was on a racing 911 engine that had a very
poorly designed piston.  911 heads are semi-hemispherical in shape, and
this particular engine had Cosworth pistons in it, running about 11/1
compression ratio.  The piston shape shrouded the spark plug, as well as
created a ring of unburnt gasses at the base of the the combustion
chamber becuase the quench area was up the piston a ways and left this
ring open.  This engine wanted about 37 - 42 degrees timing to produce
peak power, but would run all the way to about 45 degrees with  the
power falling off prior to detonation.
	This engine was torn down, the pistons reshaped and reassembled.  Peak
power now occurred at about 32 - 35 degrees peak with detonation onset
at 37 degrees.  The power went up 30 hp, even though we had effectively
lowered the compression ratio from 11/1 to 9/1 by reshaping the crown of
the piston.  Though the better shape obviously worked a bit better.
	Our typical engines now are flat top pistons in a semi-hemispherical
combustion chamber, running 7.5 to 8.5 / 1 compression ratio up to 1.4
bar (20 psi) boost giving us an effective compression ratio in the
18-20/1 range.  Running twin-plug on these engines requires only 18
degrees advance on pump fuel for peak power.  20 degrees knocks and
destroys engines.  This combustion chamber works very effectively in
this configuration, though not very well in a high compression ratio,
normally aspirated setup.
	Point is, and I think Ed would agree with me, is that with all things
properly setup, knock and peak power will be very close to one another. 
Though with poor combustion chamber design, like in a 15/1 compression
ratio Winston cup motor that deals with tiny little restricters.  Knock,
power, and timing get very different.

Todd Knighton
Protomotive Engineering


>   Ed and Todd both described scenarios designed to set timing to avoid
> knock. I've maintained that some combustion chambers aren't inclined to knock 
> and are best timed by power. My expierience with these(Fiat and some fe 
> series Fords come to mind) suggests that power goes down significantly before 
> knock, and frequently is accompanied by surging. While it doesn't show to a 
> great extent, the latest Circle track Magazine has dyno tests on the latest 
> Ford heads that show that timing is best at some point well behind the knock 
> point. I manintain that much bigger differences than show here are available 
> in the real world.(on track) Having blown engs using the knock method, it 
> leaves me a little less than impressed.  It still seems that the trick is 
> finding the sweet spot, with knock only useful for a message telling you 
> which way to go, and that you need to go a ways.