What size combustion chamber fred head should I get?

F

fischer550

Member
I live at 5,000 ft, and I use 91 octane. opti 2 @100:1

I have a dax bottom end, fred ported cylinder and cut piston, and an nt sped carb.

I am running his 6.0cc head, have been for 500 miles with no problems.

Since I live at such a high elevation, should I opt for a smaller chamber? I learned today that compression decreases from elevation. Could I run his 5.0cc head with 91 octane gas with no problems?
 
I would say do it. If the compression gets to high, just add a couple of aluminum head gaskets. Make sure you get a compression tester too, I got one on Amazon for $30.
 
Yep no problem. Your right on. That's why your octane rating is less than lower levels
in the country. I would think 5cc would be right on for that altitude.

Fly
 
Flyman said:
Yep no problem. Your right on. That's why your octane rating is less than lower levels
in the country. I would think 5cc would be right on for that altitude.

Fly
Click to expand...

Ditto

May not make a noticeable difference but then again it might.

I run a 6cc Fred Head on my fastest current bike that has seen 48mph on the flat and I run 87 octane in that engine with Opti2 and I have no detonation that can be detected and it runs like a scalded ape.

My elevation here is 1240 ft ASL

It for sure wont hurt anything for you to go to the 5cc head
 
I'm only 600 feet ASL and I run a 6cc head as well as a very tight squish band so my static compression ratio is 12.7:1 so I run 93 octane with Opti2 at 100:1 with no issues.
I could probably get away with 89 octane in the wintertime when the engine can't get as hot but I just run the same fuel year round since I never know what the weather's gonna do from one day to the next.

At your altitude you should be able to run the 5cc head without any issues, You can get .016" and .025" thick copper head gaskets from Juice Motor Parts and stack them to get the compression ratio right where you want it or combine one of these with a stock .040" thick gasket if the compression gets too high for pump gas. They also sell the thinner base gaskets in .016" or .025 and they can be stacked to get several different thicknesses to suit just about any engine's needs, the base gasket thicknesses are more for setting port timing (advancing or retarding) and the head gaskets are more for compression ratio, but you can always raise the jug for more top end power and still use a thicker head gasket combo if compression gets too high, but that shouldn't be the case at your altitude.
 
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