Hotter cam with longer duration that will decrease the dynamic compression if compression is an issue. For me is a hotter cam mandatory when using high CR pistons. Ported head too. Not doing that now will cost extra in head gaskets if doing it later
The bike will be very nice for general use, city or highway.
No race warning here, pinging engine and need of octane boosters. Just use highest octane from the pump and make sure that the ignition is properly set, idle and over 2500rpm. (Advancer springs must be OK then)
If OEM cam is a must together with bigger bore than 736cc , use the cheap low CR 836cc piston kit.
Yeah but.....Have we determined what he means by high compression?! To me 'high compression' is 12.5:1. Is this kit just a 10.25 or a 10.5:1? If so this really does not qualify as 'high compression' IMO, highER than stock yes but just a slight standard size bump in most of the kits.
Well, like all things Engineering, "it depends"...
In a water-cooled engine of small displacement (e.g., less than 10 liters is sort of "small displacement" in the world of engine physics), compressions over 11:1 static are "high". In air-cooled engines, where "small" begins below 2 liters, numbers over 10:1 are considered "high", while above 12:1 they start calling them "very high". The distinctions in most cases revolve around cooling, masses of metals, and how well the thermal 'stuff' can be controlled. For instance here: cast iron engines can reach up to 16:1 compression with little trouble if the water speed in their cooling jackets is adjusted to take away the heat. The poor guys who do this are usually confronted with trying to get the heat out of the PISTONS, though, with predictable results when they don't.
One of the big issues in our engines (if I may ramble, like an engineer, for a moment) is in the materials used for the pistons. A well-managed engine has pistons that cool faster than bores, if both are heated at once and set out to cool as a test (you can do this yourself with these bike parts, to get smarter...). This is needed because the rings don't transmit a lot of this heat to the cylinder wall. This task then falls to the skirt-thru-oil contact to cool off the piston crown. So, the crowns are made smaller than the rest of the piston (so they can grow without hitting something) and the skirts are made to just barely fit in the hole, so as to improve their contact. The oil is then needed to prevent metal-to-metal contact: if you've ever held a used piston in your hand you can see how starting an engine that's been sitting any length of time has lost this oil film, and the results. Those tiny vertical scratches on the pistons happened then, or when someone waited too long to change the oil and the molecule chains got broken too short to protect things. Metals that don't mix well with each other tend to also forgive each other for these infractions somewhat, so aluminum, zinc, or [certain] coatings tend to enhance the heat transfer without scraping off any shavings in the motions. But...when harder metals are put into the pistons so they can be forged, or when aluminum is made denser by this process, some of this forgiveness goes away. Then it becomes more necessary for the oil to both keep the parts separated, AND carry the heat between them. Forged metals, being denser than cast ones, hold onto their heat more jealously, so they grow bigger in the hole than cast pistons. This is why you must add clearance above Honda's spec, for example, to use forged pistons in these engines.
Enter high compression: the heat goes up by the square of the pressures in combustion. So, if you used 100 PSI at 9:1 CR and burn rate makes this 600 PSI, but switch to 12:1 CR pistons (12/9=1.33, or 33% increase) to 177 PSI (it would actually become more than this, but I'll ignore compressibility for the moment) at the same burn rate, the same burn rate (which would actually also be higher) would make the pressures over 1000 PSI. This 400 PSI change is a 66% increase alone, which means the heat likewise quadrupled AT THE LEAST.
So, it doesn't take a lot of compression increase to melt a hole in an aluminum piston, especially when a forged one hands off less heat to the cylinders than the OEM cast version did. Be SURE to improve EVERYTHING ELSE around those pistons when "going high", as the heat must be managed, or it will manage (mangle?) the parts instead.
About the Cycle X valves: after several attempts to reach Ken about it, I got an email back asking for "what actually happened", which I relayed (as above). I haven't heard anything else since then.
