High compression ratio

[Stangman]

And with the ignition systems we have today it’s harder to read plugs.

[cjshaker]

And with the ignition systems we have today it’s harder to read plugs.

Not when it comes to pre-ignition. The tell tale signs of peppering on the porcelain are actually easier to spot with less sooting. The engine will tell you when it's not happy, you just have to read the clues.

I know a dyno session isn't cheap, but it's well worth the money. A lot of things can be determined in one dyno session, much easier than trying to hash things out in the car.

[Barry_R]

DCR is a fun tool, but it's not the end-all-be-all.  You also have to have dyno experience to know what number correlates to "safe" and "not safe" with a specific octane rating.  I do say "dyno" because a dyno is harder on an engine than a vehicle can ever be.   You also have to take into account how efficient the engine is.  Some engines are over-achievers, in that they have over 100% volumetric efficiency.  That's when DCR kinda goes out the window.  You just can't keep increasing the compression ratio and putting a bigger cam in (or retarding cam timing) to make it work. 

I have been preaching that for years now...

[Falcon67]

DCR is a fun tool, but it's not the end-all-be-all.  You also have to have dyno experience to know what number correlates to "safe" and "not safe" with a specific octane rating.  I do say "dyno" because a dyno is harder on an engine than a vehicle can ever be.   You also have to take into account how efficient the engine is.  Some engines are over-achievers, in that they have over 100% volumetric efficiency.  That's when DCR kinda goes out the window.  You just can't keep increasing the compression ratio and putting a bigger cam in (or retarding cam timing) to make it work. 

I have been preaching that for years now...

The DCR calc on my 351C says I'm good for 91~93. Plugs say "no, you're not" - hence the heavy dose of 110 to the fuel cell.  Plus - the VP "settles" the car and makes more consistent runs than plain pump fuel.  The 302 has 9.5:1 and when that engine is pressed into bracket race service, it gets the same 60/40 mix of VP/pump with the same result.  The seasonal blends plus 10% ethanol is hell on dial ins LOL.