do you not go by your static CR when choosing an octane rated gas ? i thought dynamic was just cranking comp ...static is when the cam is in effect so it just seems to reason that static is the one to go by when choosing the gas you use ... am i right ?

 

If I understand it correctly, static is simply the volume of the cylinder/chamber when the piston is all the way down divided by the volume of the cylinder/chamber when the piston is all the way up.

The dynamic ratio brings in other variables. It is the volume of fuel/air mixture that is brought into the cylinder/chamber divided by the static volume when the piston is all the way up. This takes into account how efficient the cam/intake/head design/etc is in filling, and how efficient the cam/head design/exhaust/etc is in emptying. So things like cam overlap and timing can play a big role in how much effective cylinder pressure the engine has.

Of the two, the dynamic is a better indicator of compression pressure in actual performance and a better indicator of octane needs.

However, that is not the only determination of octane needs. Cylinder pressure is one thing, but others include how cool the engine is running, how much heat transfer out of the cylinder (example of aluminum heads vs cast iron), how much turbulence there is in the chamber, and other things that might create a hot spot such as a hot spark plug or even carbon deposits or a sharp edge in the chamber.

Dynamic compression can be more difficult to measure accurately and it will vary with rpm, so it is quite difficult to quantify with normal methods.

Static compression is simply a measurement calculation from the engine parts used, so it is easy to determine and it can give a general idea that higher compression needs higher octane, but it is not a fixed ratio from static compression ratio to octane needed. For example some of my 1960's and 1970's motors would knock at 10:1, while some newer motors don't knock at 11:1 ratios.

It still comes down to whether it has preignition or not. Sometimes you can hear it, or you can check whether the knock sensors are detecting it.

My $0.02

Good luck.

 

There is more to it as stated above, but I typically choose octane by the static CR until I learn otherwise. Dynamic is harder to figure out.

Really the best thing is to hook up the laptop DataMaster and check for knock counts, and go with the lowest octane possible.

 

It's important to remember that in a NA engine, the cylinder will never be 100% full of air/fuel mix due to various air restrictions (TB plate, air filter, etc). So the static compression ratio is not very useful in the real world, it only provides a baseline (or potential).

The correct fuel octane depends on so many things: operating temperature, fuel stoichiometry (rich/lean mix at different loads and RPMs), compression ratio, spark management system, etc. A dyno will be helpful in determining the best octane for any given setup. Good luck.