All of the different componets in an engine can effect the pressure in the cylinders, but very few change the CR of a motor. Chamber mods to a head, milling a head, and different pistons are about the only things that will change the CR of a motor really. Now, if you look at a turbo motor. Say they run 8.3:1 is the CR of the engine. With the added air and pressure of the turbo at a given psi its almost like they are running 15 or 16:1cr but the motor is still a 8.3:1 motor. Does that make sense. This is how I understand it from automobile engines anyhow.

 

Right you are MrMonk. Just like any given motor is going to have less compression at idle compared to their peak power rpm. Take your example of the turbo motor. Its rated at 8.x:1 but everyone knows with boost it will make much higher, say 16:1. Which number are you going to take into consideration when it comes time decide what octane to use? The higher number of course.

 

<< Compression Ratio is a mechanicial Known it will not change with porting, did you mean Volumetric Efficiency? Some heads can flow over 100%, don't ask me how, I couldn't tell you >>



<< Actually Moose is right. Cylinder fill which is dependant upon many variables will affect compression. Cylinder pressure can and is inreased by aftermarket cams >>


Don't take that answer as fact, as I said I was just theorizing on why the high Octane fuel with that compression.
It made sense that more air in &amp; cam timing would raise the commpression requiring the higher octane fuel.

I'm not saying it's a fact just a theory!!

 

or maybe the slower burning high octane was used to balance the fast burning &quot;additives&quot;

 

<< or maybe the slower burning high octane was used to balance the fast burning &quot;additives&quot; >>


Umm you're kidding right??
Cause that makes absolutly no sense!!!!!! LOL!!

 

[img]i/expressions/face-icon-small-wink.gif[/img]

 

kind of like the guy who was puting oil in his fuel because he had a &quot;dry&quot; burn. lol.

 

<<

<< Compression Ratio is a mechanicial Known it will not change with porting, did you mean Volumetric Efficiency? Some heads can flow over 100%, don't ask me how, I couldn't tell you >>



<< Actually Moose is right. Cylinder fill which is dependant upon many variables will affect compression. Cylinder pressure can and is inreased by aftermarket cams >>


Don't take that answer as fact, as I said I was just theorizing on why the high Octane fuel with that compression.
It made sense that more air in &amp; cam timing would raise the commpression requiring the higher octane fuel.

I'm not saying it's a fact just a theory!! >>



Your theory is correct. As to whether or not that was why the high octane was used, I have no idea and really don't care. I was really more interested in what you said about those variables affecting the dynamic compression. Static compression and dynamic compression must both be considered when deciding what octane to use.

 

he farted in the fuel tank before the race.

 

<< Your theory is correct. As to whether or not that was why the high octane was used, I have no idea and really don't care. I was really more interested in what you said about those variables affecting the dynamic compression. Static compression and dynamic compression must both be considered when deciding what octane to use >>


You are absoloutly correct.
I measure deck highth, cc the combustion chambers, figure in stroke &amp; bore to get static compression.
I run a 92mm bore X 69mm stroke at 9000 RPM's with big valves, ported heads &amp; dual 44 IDF webbers on my VW autocrosser so I know I need more octane than my 12.5/1 static compression requires. I use 101 Unocal when 93 pump should be enough for 12.5/1.
Since it's an aircooled motor the extra octane prevents detonation when the head temps reach 400 degrees.

I know this to be true on an air cooled VW but I'm just guessing on a single cyl 5 valve watercooled Raptor.