Author Topic: Hi-comp head  (Read 1688 times)

Mike Pee

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Hi-comp head
« on: June 09, 2008, 09:16:58 am »
Yesterday I put a high compression head on my maxi.  I took it around the block and didn't seem to notice much of a difference, but I decided to leave it on.  Well riding to work today it proved to be a lot better than the stock head.  Acceleration is definitely much better, though I don't think it changed my top speed at all.

So, mill those heads, or buy premade hi-comp ones.  Definitely worth it.

OldF4rt

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Re: Hi-comp head
« Reply #1 on: June 26, 2008, 09:27:49 am »
Have you changed to premium fuel?  I wonder what the end result compression ratio is.   When an engine exceeds a certain compression ratio, you need to upgrade the fuel to premium or you can have pre-ignition or "engine knock."  I always thought of 9.5:1 as requiring regular gas (what's that? 97 octane?) and 10.5:1 as needing premium, but I don't know the cutoff between those two.

Did the high-compression head say what the compression ratio would end up having?  I wonder how one would figure it. 

First, you can figure displacement mathematically with the cylinder dimensions of bore and stroke.  Or you could figure it by connecting a hose to the spark plug hole and run that hose to a graduated (measurement markings on the side) glass container held upside down under water.  Then, you would run the piston from bottom dead center to top dead center and measure the air pumped into the glass.  But that does not take into account the space above the piston in the head.

Second, I wonder how accurate a measurement you would get of just the head by removing it, holding it upside down and filling it with water.  Then dump out and measure the water.  You would then need to determine if the piston makes it all the way to the top of the cylinder.  If not, you must add the volume of the space from the top of the stroke to the bottom of the head. 

The true compression ratio would be the total volume from the first measurement plus the second compared to the second only.  You would call the second measurement "one" and see how many times the second measurement goes into the first. 

Mike Pee

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Re: Hi-comp head
« Reply #2 on: June 26, 2008, 10:28:40 am »
We can't use a jankity 30 year old compression tester?  :P  I didn't test the compression before or after the new head, so I think I invalidated any real tests.  But we can at least wee what it is now.

And it's been running pretty crappily lately.  I need to go through and check the points.  It was reccomended using a hotter spark plug with a higher compression setup, so I should try that too.  And I'm about out of gas, so I will try premium in the next batch and see if it changes anything.