Detonation is all about time - the octane rating gives an indication of how long it will take for heat to decompose the fuel into something that will spontaneously ignite, and the engine characteristics such as ignition timing, bore, plug location, chamber shape, temperature, and rpm determine how much time is available for that decomposition to occur after the spark plug fires until the flame front reaches the farthest wall of the chamber and all the fuel is consumed. For a given set of circumstances let's say that the fuel is consumed just as the last bit is just starting to spontaneously ignite, so the engine is right at the threshold of detonation. The combustion process is heavily influenced by pressure so it tends to take about the same number of degrees of crankshaft rotation, because the crank rotation determines the piston location and thus the combustion chamber pressure ignoring the combustion process (I know that sounds a little circular but it accounts for the major changes in pressure versus crank position). Now look at it in terms of time - double the rpm and you halve the time needed for the same number of crankshaft degrees. Take that situation where the engine is right at the detonation threshold, and cut the rpm in half, now there is twice as much time for fuel ahead of the flame front to decompose into something that autoignites, and voila!, lots of detonation. Everyone knows about turning the boost down and backing off the ignition timing to reduce detonation (both of which reduce the chamber temperature and peak pressure which slow down the decomposition reactions), but you can also "outrun" it by turning up the rpm. That's one reason motorcycles 20-30 years ago could run 11 and 12:1 compression way before cars could: they operated at double the rpm (and had small bores). In "our" case with the higher compression ratio and high boost and high timing the engine may not detonate at 8000 rpm but may well experience detonation at 4000 rpm unless you keep the boost down or retard the timing until the rpms are up. A second factor is that peak torque happens at whatever rpm it happens at because that is the rpm where the cylinders are filled most efficiently which means they get the biggest charge of air and fuel and thus will have the highest combustion pressure, compared to other rpms, and thus will be more likely to detonate if everything else is the same. And, that peak torque usually happens down between 3500 and 5000 rpms, which is another reason it's more likely to get detonation down there.
About the LSx's, yes, they are high compression and can run some pretty impressive boost numbers, but they have somewhat smaller bores than the typical 4"+ of a 4.1L-based Stage II motor, and heads that give much more swirl and turbulence to the intake charge than Buick heads do (think 70's small block Chevy heads, not 2000's, when you picture a Buick head
). If you ran 12-13 deg of timing with a Buick a good chunk of the combustion would still be going on in the exhaust, and you wouldn't make much power and you would kill the exhaust valves pretty quickly. To get the same "completeness" of the combustion process at the same point in the exhaust valve opening cycle (to make up a phrase) the Buick heads need 5-15 deg more advance than the current LSx heads. It's not that it's good that we CAN run more advance, it's great that the LSx's DON'T NEED TO.
Oh, while I'm typing, someone said something about "cylinder pressure is cylinder pressure, right?" Well, not exactly. If you think of the turbo and cylinder as two separate compressors, it turns out that you make more power by letting the turbo provide most of the compression, since it is a more efficient compressor than the piston is and you can do intercooling to cool the air down after the first stage of compression in the turbo before doing the second stage in the cylinder. At any given chamber pressure you will have a cooler, denser charge if the turbo did more of the compression. If all you care about is the peak hp number you want the lowest compression ratio that will barely let the engine start and run, and then a huge turbo. Problem is that turbo will take days to spool so you can get great test-n-tune et's where you wait and wait for it to spool, but you will never win a race because your reaction time will be pathetic. That's why people go to higher compression ratios, to get faster spoolup but at the sacrifice of some peak hp.
