To Cool head or not to cool head?!?

[SLORYDER]

Upon further thought, what Daj is saying makes sense.

 

Although you lift the E port 2mm say, its relation to the entire trapped volume is likely small in relation to the .010" you take off the head and its associated reduction of the compressed volume at TDC.

 

So, to be safe

 

.025"=Pump gas

.030"=50/50 race fuel, pump gas

.035"(squish allowing) race fuel

Methanol=Re-chambering

 

This should be a safe medium timing and other things considered.

 

OK, after much thought it is clear to me. This statement that I said holds some truth, but is really irrelevent to compression vs. octane relationship.

 

I now realize why it is UCCR that determines octane.

 

Cranking compression, like I mentioned before is affected by port timing, UCCR is not; this is due to the fact that when you are kicking over the engine, at that low of an RPM there is not pressure being built up inside of the cylinder prior to the exhaust port closing; all the pressure goes out of the port. This is why the height of the port is important when measuring compression w/ a guage.

 

Now, the UCCR (un corrected compression ratio) is calculated by determining the entire volume above the piston at BDC, compared to the entire volume above it at TDC. You may wonder why we use the entire volume above the piston at BDC, since the piston doesn't seal the cylinder until over half way through it's upward stroke..

 

The reason is that at the RPM when the exhaust pipe is performing its duties most efficiently (peak hp rpm) the pipe is returning a pressure wave that is compressing the cylinder before the port seals, then the piston further compresses the mixture until it reaches TDC.

 

So basically the piston draws in mixture from the carb to the extent of the displacement of the cylinder (175cc) then it transfers this mixture to fill the cylinder above the piston. At this point the pipe is actually sucking even more mixture from the carburetor, which ends up being even more cc's of mixture than the 175cc that the piston draws.

 

Where does this excess mixture go? Out the exhaust port into the pipe. So you have mixture filling the cylinder (175cc) PLUS a good deal in the pipe. Well as the piston travels upward, unlike at the low rpm you get from kicking the engine over, your tuned pipe is not allowing that mixture to go back out of the exhaust port, rather it is shoving the contents of the pipe back into the cylinder with the pressure wave that is returned from the reverse cone of the pipe, giving you compression BEFORE the piston seals off the exhaust port.

 

 

 

Sooooo bottom line. I stand corrected. Even if you go higher w/ your exhaust port, it is not safe to assume that you can go lower on your octane.

 

I believe the rule of thumb stated in my previous post should hold true though. If anyone sees a flaw in this please feel free to correct me.

Edited June 3, 2010 by SLORYDER