HP YZ144 Long Haul Update 2
Wed | Nov 3 2011
The YZ144 is dead! Yep, we killed it, but not how you would expect! It now has about 100 hours since the rebuild/144 kit addition, and what a fun 100 hours it’s been! At a pro challenge in north Texas on practice day, we managed to suck a rock through the chain - and put a hole in the cases where the clutch actuation rod goes through.
All of a sudden the bike’s clutch started to slip, and the clutch pull became nearly impossible. Assuming a toasted clutch and somehow messed up cable, the bike was brought back to the pits, put on the stand, only to notice a pool of oil quickly accumulating on the stand!
Quick inspection showed the issue clearly - looks like new cases are in order, as welding then re machining the actuating slot is looking unlikely...
But the failure is really a blessing in disguise, it was really becoming time to replace the crank - 100 hours on any two stroke crank ridden hard is pushing it, let alone on a crank that has seen less filter changes than ideal, more hp than intended, and hard hours and long motos in the Texas heat.
However being who we are - and never satisfied, the ideas were already starting to churn from a prospective customer email who wanted our best effort on his yz125, size no concern/restriction, cost no real issue.
We have decided to embark on a 150 long rod set up. Basically the same 144 piston size (+4mm currently) but coupled with a +2mm stroker crank, and a 7mm longer rod.
-More case volume to help supply and support the +25 cc’s of displacement
-Less angularity of rod, technically speaking less friction
-Slightly less peak piston speed on long rod vs shorter rod
The goals for this engine are to hit stock 250 two stroke peak hp numbers, with an additional 1000 RPM of usable power spread over the current 144.
While this is a huge undertaking, and the goals are certainly lofty, we are fairly close already in power peak numbers, and with the aforementioned changes, we think with a custom ignition curve, custom pipe to suit, and a lot of R&D time, we will meet the goal.
Some issues that will have to be overcome include:
-Increased piston speed due to longer stroke
-Exhaust port casting room becoming limited
-Intake becoming slightly restricted, from the reed size, to carb, to air boot and air box
-Pipe design must fit chassis but still meet modern design criteria to achieve desired power width and power levels
The custom ignition curve should actually help reach our goal of wider power, as a better retard curve up top will improve over rev instantly, so we feel it will be mostly a game of finding creative ways to flow more air through the same size hole on the exhaust side!
In all likelyhood you should be seeing a full cnc head set up for this engine to meet our squish and compression goals.
Work has begun already on the crank, the design for the head is ready for cnc, and we have begun some flow testing with the exhaust port to try and improve efficiency! Can’t wait - will keep you tuned.