What do I need to do in order to install a high compression head?

[Ckflyer13]

Hey guys!
So I'm looking at putting a high compression fred head on 66/80cc engine. The only mod I've done so far is add a HP carb. I just have a few questions about what I need to do before I install the new head. I've seen that I need a new CDI. What type of CDI and why do I need that? I've also seen that I might need to adjust the timing? I know my fair share about engines, but I'm also not the smartest when it comes to all the internals and technical stuff.

I guess the actual question is what do I need to buy and install when I get a new head?

Thanks!

 

[FurryOnTheInside]

You don't need a new CDI to install the new head. You can get one for its own sake, though.
You will need to check the clearance between the piston and head at the closest point (presumably the squish gap) when the piston is at top dead centre (TDC) and for that you just need a piece of soft solder and a vernier caliper.
Too much clearance will waste the potential high compression of the new head, and fail to utilise the squish band too. Too little clearance will risk the piston hitting the head when the engine is running and hot.
You can adjust the clearance by choosing the right thickness head gasket, or by milling or sanding the upper surface of the cylinder.
My stock gasket was 0.7mm thick and when I tried that there as 1.2mm clearance, that is far too much for a squish gap and very not good for compression.
There are copper head gaskets available 0.4mm amd 0.2mm thick. I used twisted Teflon tape 0.05mm thick to bring my squish gap down to 0.55mm.

 

[Ckflyer13]

You don't need a new CDI to install the new head. You can get one for its own sake, though.
You will need to check the clearance between the piston and head at the closest point (presumably the squish gap) when the piston is at top dead centre (TDC) and for that you just need a piece of soft solder and a vernier caliper.
Too much clearance will waste the potential high compression of the new head, and fail to utilise the squish band too. Too little clearance will risk the piston hitting the head when the engine is running and hot.
You can adjust the clearance by choosing the right thickness head gasket, or by milling or sanding the upper surface of the cylinder.
My stock gasket was 0.7mm thick and when I tried that there as 1.2mm clearance, that is far too much for a squish gap and very not good for compression.
There are copper head gaskets available 0.4mm amd 0.2mm thick. I used twisted Teflon tape 0.05mm thick to bring my squish gap down to 0.55mm.

Thanks! This was actually really helpful. On Fred's website (https://crmachine.com/shop?olsPage=products) the heads have different 'sizes' i guess you would call it. Some are 6.0cc and some are 7.0cc. What are the differences and what do the numbers mean?

 

[JerboaJohn]

the 6cc 7.0cc , etc are the chamber volumes. 7cc isn't really high compression. if you go with a 6.0cc head, you may have to use higher octane gas to prevent pre-detonation like 90 or 92.
Fred says a 6.5 or 6.6cc is a good compromise and runs on regular 87 octane, has good power on top end, overall gain in torque @ all rpms.

love to see when you decide on a head!

 

[FurryOnTheInside]

A difference in the piston to head clearance of 0.58mm is actually a difference of a whole 1cc so it is really important to get the correct minimal clearance at the squish band (the squish gap is the clearance measured at this area**).

"What do the numbers mean?"
I really don't know how Fred or anyone measures their combustion chamber volume. Is the volume measured with zero squish gap, piston placed into the head so it touches? Obviously if it is then you will actually have around 1cc more volume when you take the necessary clearance into consideration.
Is it measured with the mating surface on a flat plate? If it is the you'll have less volume when you get a domed piston in there.
It's really difficult to compare heads from different sources if no one says how they are measuring.
But you can compare between the heads from the same source.


**This is without even getting onto the issue of making the squish band effective as a squish band, for cooling purposes or for turbulence which some believe in and a few dont. On these engines the squish band is geometrically necessary to make it possible to have a low cc combustion chamber, whether you believe in the benefits of it or not.

 

[Ckflyer13]

the 6cc 7.0cc , etc are the chamber volumes. 7cc isn't really high compression. if you go with a 6.0cc head, you may have to use higher octane gas to prevent pre-detonation like 90 or 92.
Fred says a 6.5 or 6.6cc is a good compromise and runs on regular 87 octane, has good power on top end, overall gain in torque @ all rpms.

love to see when you decide on a head!

Thanks for the info! I'll definitely post a picture on this thread once I get it installed. Just need to find a head that's worth buying and doesn't burn a hole in my wallet

 

[Tyler6357]

The best heads have large fins to keep your engine as cool as possible. Most heads sold for these engines are 6cc. In order to get the extra power of using the high compression head you need to know your squish gap. If the gap is bigger than 0.060" you might as well not even have a squish band in the head, and you are not going to get the MAIN benefit of the Fred Head, the optimized squish band. Keeping the squish gap between 0.025" and 0.035" is optimum for a 6cc volume and will provide the most power from the squish band, while also providing the best longevity for the wrist pin bearing.