Holy compression!

[Wags]

Just got done with my every engine performance mod experiment. I took 38mm stroke with zl rod China girl (because that's what I had) and added an arrow reed valve kit with keihin carb, holed piston, third transfer port, port matched intake and exhaust, sickbike expansion chamber (soon to be kx65 expansion chamber), sickbike shifter, jaguar cdi with high output coil, iridium spark plug, the two transfer ports were matched to the cylinder, 6cc fred head and was decked for .70mm squish band. After running a compresion test I found my compression to be 179psi! after dialing in the cdi and carb it runs great and has no power issues. Now that I've given all the necessary information my question is... how do i bring the compresion down without sacrificing my squish band or power? I know I'm putting horrible stress on my bearings and wrist pin and is only a matter of time before they fail. Any suggestions?

 

[FurryOnTheInside]

7cc Fred head?

 

[FurryOnTheInside]

Just got done with my every engine performance mod experiment. I took 38mm stroke with zl rod China girl (because that's what I had) and added an arrow reed valve kit with keihin carb, holed piston, third transfer port, port matched intake and exhaust, sickbike expansion chamber (soon to be kx65 expansion chamber), sickbike shifter, jaguar cdi with high output coil, iridium spark plug, the two transfer ports were matched to the cylinder, 6cc fred head and was decked for .70mm squish band. After running a compresion test I found my compression to be 179psi! after dialing in the cdi and carb it runs great and has no power issues. Now that I've given all the necessary information my question is... how do i bring the compresion down without sacrificing my squish band or power? I know I'm putting horrible stress on my bearings and wrist pin and is only a matter of time before they fail. Any suggestions?

It is my understanding, just from reading here, and I may be wrong but iirc..
High compression damages cylinder plating; high RPM, grossly advanced timing and detonation damages the bearings. So high compression only damages bearings IF it contributes to or leads to detonation.
But you don't want to ruin the cylinder either so it doesn't change anything, you still want to lower the compression.

Estimating the result of switching to the 7cc head might not be as simple as 179 * 6 / 7 = though, as Steve Best showed us in his thread about his volume measurements of (non Fred) heads with piston in place VS filled level. I certainly would be very interested to hear about your results if you switch to the 7cc Fred head!

I think Power, Reliability and Affordability are tripartite in engines, just as Lightness, Reliability and Affordability are in cycle components. You can't get more of one value without losing some of one or both of the others.
At least, from my reading that's how I think it works anyway.

 

[Wags]

Thank you for your sugestion. You would be correct that the larger cc head would probably be the solution but in this case I am already way over budget. The list of things I have done is just on the engine. That doesn't include seat post mounted gas tank, mounts, sensors, throttle, bike, etc., etc.,etc.. I was hoping for some kind of "trick" that I don't know about yet. Or some kind of home remedy.

 

[FurryOnTheInside]

Thank you for your sugestion. You would be correct that the larger cc head would probably be the solution but in this case I am already way over budget. The list of things I have done is just on the engine. That doesn't include seat post mounted gas tank, mounts, sensors, throttle, bike, etc., etc.,etc.. I was hoping for some kind of "trick" that I don't know about yet. Or some kind of home remedy.

Sounds like modding a spare cylinder so it's ready to use when needed would be the cheapest option.
You aren't having any problems with the high compression yet, so you can just leave things as they are, enjoy the high power you getting from your high comp' engine, and replace the cheap cylinder when the old one starts to flake.

 

[Wags]

I think I will just enjoy it. The next best thing to ridding is working on these bikes and my tto temp gauge says I only got to 336 max degrees. I've got milling the Boost port in the cylinder down to a science and only an hour and a half out of my life. Ill just keep an eye on it. Ill know more today. I'm going for longer ride.

 

[skyash]

A good oil helps cool and lube .

 

[Wags]

I only use premium gas (93 or higher) and always castor927 synthetic.

 

[Deleted member 12676]

If you have a rotary tool with you can remove metal from the cylinder heads "bowl" till the compression comes down.
It would take a lot of work though. I'd just remove the squish band. Jennings said they are only needed on engines with high compression or that rev over 9000.
If you do nothing then say bye-bye to your bearings and cylinder plating.

 

[Wags]

I have a couple different size copper head gaskets. It couldn't hurt to try a larger gap and see what happens. I would rather not cut into the brand new head just yet. Also, I have to comment on your cdi. I have zero, i repeat, zero vibrations. It's like a sewing machine. All I did to the crank was true it, shave a little weight off the upper connecting rod and lightened the piston by drilling holes in it for the reed valve . Still not sure I'm getting the most out of the cdi yet but that will come.