I've seen another round of posts here and elsewhere recently where the belief is that you can calculate a "Dynamic" CR based on the engine's static CR and IVC event to determine the octane requirement (tolerance?) of an engine.

It's bullsh!t.

Shoving a big-a$$ cam in a high CR engine in order to lower the cranking pressure is nothing but a Band-Aid to lower the operating compression ratio (the engine's Volumetric Efficiency % vs the calc'd "static" CR at at a given RPM) up to the RPM point where the engine is operating at 100%+ VE. At that point, the REAL "dynamic" cylinder pressure is going to be the calc'd "static" CR + the additional cylinder filling resulting from the VE efficiency exceeding 100%.

It's not me who is trying, or has tried, to get people to understand that DCR isn't telling you what you think it does.

==> Tim Wusz (formerly of Union 76 Racing Fuel and currently with Rockett Brand Racing Fuel) was quoted in Hot Rod at least 20 years ago that you can crutch the octane needed by bleeding off more pressure with extra duration, but that it only works up to a certain RPM point for a given engine, and you'll most likely need to run the ignition pulled back from optimum for an additional margin of safety. Beyond that point, you're at a very high risk of detonating.

==> Years ago on SpeedTalk, the late Harold Brookshire (the founder of UltraDyne Cams) and Mike Jones (of Jones Racing Cams) were involved in a lengthy thread where they both tried to get people to understand how the "Dynamic" CR calculation is anything BUT dynamic, and should not be used as a parameter for either building an engine or targeting a particular octane to use with that build.

And today I stumbled across a short article (since I'm not going to go digging thru the SpeedTalk archives to revive that old thread) where the author does a pretty decent job of explaining it, IMO: http://www.matrixgarage.com/content/why-dynamic-compression-ratio-nearly-useless

Do I expect everyone is going to agree with this? He11 no! laugh2

There are still some well-known engine builders who push the DCR approach in their pump-gas builds. If it works for them... shruggy

Agree or not, IMO you should at least read about it so that your understanding of this approach -- and why it's considered seriously flawed by some -- is clearer.

Carry on! hammer