There are some interesting arguments. It's interesting that these days that when we talk about CR we are still talking about the static compression ratio which in reality means very little without knowing the cam and cam timing. What is much more important is the dynamic compression ratio.
The Ecotec and L67 make an interesting comparison because at first glance the only difference between these two engine is the static compression ratio and everything else is equal because it has the same cam, however if I'm not mistaken the L67 uses a different timing chain set and the cam is timed in slightly differently compared to it's N/A cousin the Ecotec altering the cam timing will effect the dynamic CR of the engine even though the static CR has not changed. The article linked further up this thread mentions a safe CR of 12:1 on an aluminium headed engine, I say this is total bullshit because the static CR is totally irrelevant without knowing the cam/timing. The Author than goes on to compare the static CR of a V8 engine and a Hayabusa motor cycle engine which have absolutely nothing in common.
This whole topic really should be combined with discussion about knock as it is all linked, engine knock is a factor of time, pressure and heat. Increasing any one of these factors can increase power created but can also induce knock. One way to counter knock is to use a higher octane fuel which is more knock resistant and the topic of this thread but the other 2 factors are just as important.
I would tend to agree with some here that there is more to the different fuels (91, 95, 98 etc) than just the knock rating. Assuming everything is equal other than the knock rating (of the fuel) putting 98 in an engine that is tuned for 91 is a waste of money. I however am of the belief that there is more to the difference between 91 and 98 other than just the octane rating, this belief comes from nothing scientific but what I have experienced (tried running 98 in a stock ecotec) or what I have been told by experienced engine tuners and what various experienced engine builders have said.
Going back to the L67 for a second, OZ38 mentions running more boost but not reducing the timing further back in this thread, I'm going to assume we are talking about things like doing a simple 10psi boost upgrade on an otherwise stock engine/tune. The reason this works (from my point of view) is that the factory tune runs extremely rich when in PE (power enrichment mode) for what is a relative low boost engine in OEM configuration.
(can you tell my dyslexia is kicking in, if I type what is happening in my mind we will be jumping backwards and forwards between the various topics just like my mind does.....)
To go back to the original example of this thread, doing a top swap on an ecotec bottom end, the higher CR of the ecotec bottom end will increase cylinder pressures which will mean the charge will burn faster reaching peak cylinder pressures earlier (using the standard L67 ignition timing) than the lower CR L67 bottom end. This means the top swap engine will make more power but will also get into engine knock sooner than the equivalent L67 engine. There are a number of ways to get around this issue of course, use better fuel, reduce the ignition timing or add more fuel. By adding more fuel (running the mixture richer than ideal) you reduce the burn rate of the charge in the cylinder and this seems to be the way some people do things because the assumption is that more timing is always better but as has been pointed out in this thread the reality is that once peak pressure is happening at the sweat spot (about 15° ATDC) there really isn't any point in adding in any more timing as the engine will not produce any more power but is more likely to have knock issues.
I could spend ages typing out my thoughts on this but someone else has done it much better so I'm going to link the article instead (this article is actually about knock and detecting it)
Knock Control Setup -EFI University
First, here's a brief background about engine knock. On the compression stroke, a mixture of fuel and air in the cylinder is compressed and it's temperature rises. (Boyle's Law) Between 40-10° BTDC the ecu will command ignition of this mixture and a controlled burn moving out from the spark plug will start to release heat, increasing pressure even faster. If ignition timing was right, cylinder pressure should peak right around 15° ATDC to get the most mechanical work out of the burned fuel. At that point, the flame has swept across most of the cylinder but only about half of the energy has been released. At a certain temperature for a given fuel, the mixture will autoignite (knock), which is essentially a bomb going off in the cylinder causing an extreme pressure spike. Although pressure may peak around 15° ATDC (or well before or after depending on ignition timing and burn rate), temperature is still increasing because of the burning fuel, and there's still a large amount of energy waiting to be released. It's in the 0-50° ATDC region that knock will occur. Generally, knock at an earlier crank angle will be more intense since there is more fuel available to spontaneously combust. Combustion chamber shape will have an effect on required timing advance. Less timing advance is needed for a small displacement pentroof combustion chamber. More timing advance is needed for a large displacement wedge combustion chamber. This has an effect on propensity to knock due to the amount of time the air and fuel mixture has to heat up. The faster burning design being less knock prone. A term that I've often seen associated with knock is pre-igniton. Pre-ignition is the ignition of the mixture before the spark plug fires and is caused by hot spots in the cylinder. Pre-ignition will almost certainly lead to knock. Sustained, severe knock will break ring lands, blow head gaskets, and melt holes in your pistons. The heaviest hand in reacting to spark knock is ignition timing retard since this variable will have a drastic effect on cylinder pressure and temperature. Increasing fuel will also absorb more heat, so a combination of these are what we implement to combat knock. For more on this subject, check out chapter 9 of Heywood's book, Internal Combustion Engine Fundamentals.