Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Octane and Mileage

Originally posted by Port:

---

Is there a reason nobody has mentioned compression ratio in this discussion? You run a certain octane based on your compression ratio, not based on timing. Ocatane is the ability of the fuel to resist combustion under compression. If you run cheap fuel in a high compression engine the fuel will ignite before the ignition lights it strictly from being compressed. That is detonation. You can cause detonation by advancing your timing because it ignites the fuel before the optimal time in the engine's stroke.

9.8:1 is the cuttoff for running 91 octane pump gas at sea level. Higher compression than that you need to start thinkin about race fuel. I am not sure what these contours have, I don't have one, YET, but I doubt they require 91 octane unless you are right at sea level. The higher in elevation you are the less air going into your engine wich lowers compression in the cylinder, so you can use cheaper fuel.

If you run high octane where you don't have the compression to get it to ignite, you are wasting fuel and potential energy in the wasted fuel every time the cylinder fires. Resulting in lost power and fuel mileage.

If your car gets better mileage on the cheap stuff, and the "seat-of-your-pants" feel is better, run it, that means it works better. Higher elevation and lower humidity will allow you to run cheaper fuel.

---

Wow. This is just a simple Ford forum.
Despite this fact, you'll get eaten alive here with random posts like this. On import car forums... maybe, maybe not.

You can't just determine octane requirements based on compression.
BTW, I ran 91 octane on my 10.2:1 motor with 6 pounds of boost in 100 degree Arizona weather That was the end of May, early June to be exact. So as far as I'm concerned I've OWN3D your azz on that one.
What was that again.... oh yes, you were talking out your arse.....

By the way, fuel igniting before the spark plug ignites it is called Preignition. Yes there is a term for it and it isn't detonation. If it is just slightly off from the appropriate amount of timing then it manifests itself as pinging. If it manifests too early then it can detonate.

Detonation is caused when the gas pressure of the burning fuel elevates too soon before the piston reaches top dead center and overcomes the piston force that is compressing it when it is ignited before top dead center. This results in knock and damage. The gas pressure tries to overcome the piston pressure as it comes up...somethings gonna break.
This can be caused by preignition, high temps, or bad fuel. All of these possibilities are affected by high compression.

You can just plain set your timing too far in advance and cause detonation with no hint of preignition and still have nothing to do with compression ratio.

BTW, timing your car right means you initiate burn with the spark plug well before the piston is at TDC in such a way that the gas expansion creates the max pressure right as the piston passes TDC.