This depends greatly on the cam. Specifically the intake closing event, overlap has nothing to do with it. I've got a cam for a 10:1 306 (my personal toy) that runs just fine on pump gas but would require race gas in a 347. If you go custom you can do it no problem. Keeping the cranking compression around 185 is key and that is done by delaying the intake closing event and thus dynamic CR.
What you've written here makes a lot of sense to me, and I can definitely see how the intake valve's closing event is the most important part with its relative effect on detonation. However, I have to respectfully disagree that overlap "has nothing to do with it," though.
While you're right in that the dynamic compression ratio is affected by only the intake closing event, and is not affected by the overlap. Overlap still significantly affects volumetric efficiency, proportionately with RPM. And volumetric efficiency is a factor in determining the detonation threshold of an engine. The higher the VE, the more prone it will be to detonation.
However, detonation is most likely to occur at high load and low RPM, where low piston speed and yet a constant flame front velocity conspire to create higher peak cylinder pressure. Conversely, engine's are less likely to detonate at higher rpm. High overlap cams have lower VE at low rpm, and actually a better VE at high RPM, which helps to offset the tendency of the motor to detonate at its lower rpm without restricting the ability of the engine of the motor to make power at high RPM.
It was a confusing bit to type out. Did any of that make sense?
Chris
FWIW - there are a number of additional factors that enter into the whole detonation picture that haven't been touched on in the thread. Detonation is a complex phenomenon and often misunderstood. The areas that haven't been touched on yet include - 1) intake air temperature, 2) quench dimension, 3) fuel/air dispersion and 4) combustion chamber design.
Google detonation or ping and there are lots of good technical articles about what actually happens. While we think we have a homogeneous air/fuel mixture in the chamber just before the plug sets it off - we actually don't. There are an almost infinite number of different a/f ratios in the chamber during the compression stroke - and they're moving around. Detonation occurs when one of these small pockets of LEANER a/f mixture sees the right conditions to self ignite, usually before the spark plug sets off the entire mixture. So at the heart of reducing the chances of detonation under any operating condition are trying to assure as homogeneous an air/fuel mixture throughout the chamber as possible.
Newer OEM's are running 10.5-11-even 12:1 static CR's these days. Direct injection has made this possible. Fuel is injected directly into the chamber (not the port) at pressures that are 80-100 times higher than what our cars use (3000-4000 psig). This much higher pressure does a MUCH better job of evenly atomizing the fuel throughout the chamber. Timing the injection closer to the ignition timing also allows less time for detonation to occur. With this approach, the mixture is much more homogenous -- which reduces the chances of detonation. These newer engines also have knock sensors which allow timing to be run/real-time-adjusted to get right on the ragged edge of detonation. In fact, some systems run a small current through the spark plug after it's fired to measure the effectiveness of combustion in each chamber - and with distributorless ignitions can alter ignition timing BY CYLINDER to optimize things. Yes - our 25 year old efi and ignition system technology is a dinosaur compared to what's available today.
We can't go to direct injection on our engines -- however, within reason we should do whatever we can to atomize fuel more effectively. Fuel pressure on the high end of the acceptable range for a particular set up helps; and running the smallest injector possible that will still meet the flow needs of the engine helps. At high load/small throttle openings not much fuel is moving through the system. And a smaller injector does a more precise job of metering small amounts of fuel than a larger one does.
In our cars, in addition to the things already mentioned, several things can be done to help. Be sure the air intake is delivering the coldest air charge possible. The cooler the chamber - the less the chances for detonation. Keeping coolant at 180F helps. Keeping the air charge cool helps too. In fact, cooling the fuel would help as well. I don't think anyone has mentioned material choice in the heads yet. Aluminum heads pull heat out of the chamber more effectively than cast iron heads do. This feature also helps control detonation - all things equal, you can run a higher CR with an aluminum head than with a cast iron head. Not to mention the weight savings.....
Keep the quench dimension as tight as possible. This is the distance from piston to head OUTSIDE the combustion chamber. Ideally - .035" or so. Keeping that dimension tight forces every bit of the air fuel mixture into the chamber as the piston comes over the top. As it's all forced into the chamber at TDC (note - the ignition process has already begun at this point) it becomes very turbulent in the chamber -- which further homogenizes the air/fuel mixture. This is a detail that's often overlooked by people. In fact, sometimes folks will put a THICKER head gasket in thinking that reducing overall static CR will help with detonation - only to find that the big quench dimension they've created significantly reduces turbulence in the chamber resulting in detonation potential being even worse.
Combustion chamber --- and even port/valve configuration plays a huge role in how turbulent the mixture is before combustion. Later model chambers/intake designs are MUCH more effective at introducing turbulence/air-fuel mixing than older stuff was. The reason it came to be was to reduce emissions and increase efficiency (fuel mileage). But that also helps hp/torque production. One of the benefits of the GT40P head is it's efficient combustion chamber design. So the shape of things is important.
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