njkx said:
Rich, isn't this the stock compression ratio? You don't think it makes sense to go with a higher ratio?
The actual compression ratio is rarely as high as the spec lists in a stock bike, but yes 12.5:1 is what Yamaha claims for this model.
These bikes run great at 13.5:1 compression and can still use premium pump gas without problems but you do have to contend with some additional heat. If you are running the bike on an MX track it's rarely an issue, if you are running it slow in the woods or in places where the airflow through the radiators can get restricted it can be a problem.
whenfoxforks-ruled said:
Rich, how much shorter life does the 14.8:1 have over the 12? And the 2 rings to 1? How much performance loss is there putting the ss valves in yamahas?
Fox, I've never seen a piston from Wiseco that is higher than 13.7:1 so I can't comment on reliability. Honestly I don't think you can get to a true 14.8 CR without a lot of other compromises in that engine so it might be a moot point.
The single compression ring Wiseco YZF pistons have service intervals similar to the CRFs. They start to lose power at 5-8 hours and I personally wouldn't run them for more than 20 hours in a RACE engine with a fast rider twisting the throttle. As you well know, fast riders wear stuff out in a hurry.
I've never seen a
legitimate dyno run that I trust that showed a power loss running properly set up Kibblewhite stainless steel valves and their racing valve spring kit. I've ridden a number of bikes with Kibblewhite stainless parts and never felt a fall off of power either.
Kevin Staples did tons of dyno testing on YZ250Fs and found the Kibblewhite stainless steel setup was a bit more stable at high speeds than the OEM Titanium parts. The quality of the valve springs in the Kibblewhite kits compared to the cheapo OEM springs is likely the reason for this.
There are a lot of junk valve springs on the market that will surely kill power with stainless valves, and there are a lot of nit-wits assembling heads without the first bit of a clue. So my comments are specific to the Kibblewhite kits. I've installed over 100 of them myself and I think between Eric and I the number is probably close to 300.
Mismatched parts in a 13,500 valve train is just plain stupid but people do it all the time. Cheap springs are the usual culprit when problems arise.
Even the good parts will cause problems if they get installed by someone with no idea what they are doing. Setting up a 13,500 rpm valve train is fairly specialized work (assuming you want it to make power and be reliable), but people are convinced if they can get the parts in the bike and it runs, then all is well. :bang:
Unfortunately most of the guys who complain about stainless steel valves did the work themselves, and frankly should have spent the money to get the work done by a professional.
The sad reality of high speed four-strokes is some of the maintenance is no longer destined to be done at home in the garage.
Of course that won't stop people from doing it themselves and then getting on the internet to bitch about X companies bad parts when their do it yourself work causes issues.