Athena/GYTR YZ 125 - 144 Kit Modification and Evaluation Part 1 ***********************************
The What: The Harris Performance evaluation and modification process to the Athena produced yz144 engine kit for 2005-2010 Yamaha yz125 engines - with strong relevance to earlier yz125 engines. Our study begins with a brand new Athena 144 kit. Included are a 58mm piston kit, complete cylinder casting, new power valves, and a head casting along with gaskets. Retail pricing is in the 850-900 dollar range. Overall manufacturing quality is on par with OEM yamaha parts upon first inspection, and no defects were found in plating or mating surfaces. Most consumers purchase a 144 kit for one of two reasons: More power, or as a budget replacement for old or damaged stock components. Modern rule structure allows the two stroke to be modified for larger displacement in an attempt to put power output on par with the current crop of 250 four strokes. The Concept: A nice, new stock yz125 typically shows around 32-33 hp on a popular dyno set up. Todays modern 250 four strokes are typically in the range of 35-36 on that same scale. People say torque tells the whole story, and while we would agree to some extent, our opinion of the REAL key area is the torque spread, or as we define it, the usable RPM range in which the torque provides maximum acceleration. The primary difference between a two stroke and a four stroke of twice displacement comes in torque spread, with the 4 stroke often providing more than double the usable RPM range per gear, or around 7000 RPM of usable range. The two stroke often struggles to provide 3500 RPM of usable range, however a good rider can use the transmission effectively to keep the engine in this sweet spot. One area of design consideration when developing an engine is the RPM drop due to each gear change. The more speeds you have, the less the RPM drop possible per gear change. Modern race bikes typically drop a maximum of 3000 RPM in the most used gears (3rd - 5th) A 144 is 15.2 percent larger than a 125. If you apply the concept that if all engine elements are working equally as well as a stock 125, but at 144 capacity, a 15.2 percent power increase is within reasonable expectation. 15.2 percent on 32 hp is 4.86hp, bringing the stock 125 up to just shy of 37hp peak. This certainly bridges the gap in peak power to a four stroke, however the RPM spread would not match that of its four stroke counterpart. In order to be competitive, the two stroke is going to need to make MORE power within its usable range than the four stroke. Port Evaluation: First things first, we measured the cylinder as delivered. First glance, without even pulling out our measurement tools, it was evident the cylinder was NOT developed in port arrangement as it should be. It was clear that Athena merely took their 125 offering for this bike, and bored it to 58mm up from 54mm, then plated it with no port modifications. The stock yz125 cylinder comes in with the following timings: Exhaust Port: 187.5 Degrees from deck - 189 from timing edge of piston* Main Transfer: 126 degrees from deck Secondary Transfer: 126 degrees from deck Boost Port: 126 degrees from deck *Piston employs chamfer on timing edge - which must be considered during measurement. The exhaust port has a circumferential width of 30.5mm per side, total width including bridge comes in a 65.8mm. We will relate this to a percentage of bore circumference compare each cylinder. At 54mm bore, the circumference is Pi * Diameter - or 3.14 * 54mm = 169.6mm 65/169.6 * 100 = 38 percent. The stock athena 144 cylinder measures as follows: Exhaust Port: 194 degrees from deck Main Transfer: 118 degrees from deck Secondary Transfer: 118 degrees from deck Boost Port: 92 degrees from deck The exhaust port has a total circumferential width including bridge of 66.7mm This works out to 36 percent of total circumference. Both engines employ use of a 5mm bridge. Clearly, to the casual observer, huge variance can be seen between the stock yz125 design, and that of the athena 144. It should also be obvious that the intake ports show the largest discrepancy...and a quick glance at the cylinder should show why! When the cylinder was bored, the ports with steep angles were changed the most! The boost port experienced the largest relocation, as once bored its role in the scavenging process has been greatly altered. To go further in depth one could measure each port area. This has been performed, but will not be discussed here. However basic concept...the greater the port area for a given duration, the greater its flow potential. It is related in the terminology time/area. Two strokes are about balancing each aspect of the scavenging process perfectly. Too much exhaust time/area for a given intake time/area, and your wasting energy that could be used to produce more torque per stroke. Too little exhaust time/area for a given intake time/area, and the engine can not evacuate the cycle thoroughly...leading to mixing of fresh charge and spent charge, finally resulting in non optimum power output. Engines with too little exhaust time/area are often very peaky, as without the pipes influence, the design elements do not lead themselves to efficient operation. Unlike four strokes, often the biggest struggle is finding this balance between intake and exhaust ports. The Athena cylinder has one positive aspect going for it as delivered. Plenty of exhaust time/area for the supplied intake time area. Our prediction would be that the Athena cylinder would have decent power spread (power off the pipe) with excellent over rev characteristics. That said peak power production will not be great, as the intake time/area is grossly under matched in comparison to the exhaust time/area.
Athena/GYTR YZ125 - 144 Kit Modification and Evaluation Part 2 ***********************************
The Head: The supplied Athena cylinder head has obvious differences from the OE yamaha head. Primarily the squish area ratio, and chamber volume. One must remember that to obtain the same static compression ratio as a stock bore engine, a big bore engine requires a larger head volume. The stock yz125 head measures out at 10.5cc of volume bringing static compression ratio to 12.9:1 with a squish area ratio of 50 percent. The Athena head measures at 11.5cc of volume plus the volume added by the head gasket of 1cc of volume for a static compression ratio of 12.5:1. We feel Athena missed the boat on their set up, with a head using approximately 42 percent squish area ratio, at 1.1mm squish clearance using thinnest base gasket. Our ideal head design would utilize around 50 percent area ratio, and 16:1 static compression ratio at around .9mm squish. This would provide a nice compact combustion chamber along with an optimum MSV. We also dislike use of a head gasket instead of head o-rings, as experience has shown a higher percentage of coolant leaks while using a gasket. The Modifications: In order to get a true baseline for the Athena - we felt it best to list our design constraints for normal users: -Commercially available budget conscious Exhaust Pipe -Pump Fuel -Good Power Spread and Part throttle performance -Good Reliability -Worthwhile Power for investment
We have developed two set ups, one for most consumers, and the other for serious racers. The set up for most consumers is as follows: The exhaust port is widened to match the stock yz125 percentages, ensuring good reliability qualities. The exhaust port shape, cross sectional area, and progression are also be modified for optimum flow both forwards and backwards to suit the larger bore and port size. The transfer ports are modified in both their timing (degrees open) and widths to achieve optimal scavenging process and time/area for the engine. Due to increased bore size and subsequent reduction in available port area per displacement, additional blow down timing (and less transfer timing) is used than in the original yz 125 design. With this particular set up - the engine achieves excellent power and over rev characteristics. The boost port is radically modified to reach correct timings - as well as a less steep entry angle in order for slightly better midrange power. All ports are chamfered correctly for long ring life, with extreme attention and care applied on exhaust port. We use a chamfer on the exhaust that greatly improves ring life and longevity of power by carefully shaping how the ring feels the port upon entry. The cylinder is then cleaned thoroughly and prepped for installation. One major concern was to make sure the set up works well with commonly available pipes and that the engine still revs where it should to match the pipes.
After countless motos, a few loaner days to some customers, about 70 gallons of c12, and 3 top ends, we feel its time for an update on the HP modified Athena 144 kit. So far everything has gone great, with no failures yet (knock on wood). Power has held strong up to the 15 hour mark with each top end, at which point the bike begins to loose ring seal and acts rich.
But my oh my what a ride it is when fresh! There is something to be said for a motorcycle pumping out well over 40 horses in a bike that weighs around 200 pounds ready to ride. The joy of keeping it pinned into corners, slamming the brakes, and then rocketing back on the gas and out of the corner defies expressionand the ability of the light bike to float over bumps and change direction at a whim is truly astonishing.
Every time I think, maybe these 250fs are better here, or maybe they would let me turn a quicker lap, I hop on and remember, wait a second! They feel heavy, sluggish to turn, slower to stop, and dare I say it, SLOW! A stock 250f truly feels like a tank compared to the 144 and ironically some obstacles that require some finesse and timing almost seem ten times easier on the 144 due to its huge weight advantage.
While the bike has been insane amounts of fun, there are some instances I feel it lacks a little. On really deep starts, you HAVE to catch those shifts precisely and quickly, or you loose out big time. On dry, slick, turning acceleration, it requires a lot of practice with gear selection, clutch application, and throttle use to be effective at putting power down. And you better be able to use the clutch! Not always to keep it in the power, rather sometimes to tame the delivery!
But all in all, its a bike I will never sell. Its tons of fun to ride, literally eye watering fast, and sounds so good it will make a grown moto man cry! Its a bike everyone should own if budget were of no concern.
100 HOUR UPDATE **************** HP YZ144 Long Haul Update 2 She's Dead!
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 its 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 bikes 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. Basic concepts: -More case volume to help supply and support the +25 ccs 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! Cant wait - will keep you tuned.
Aye technically its a 150 Athena kit, but what they've done with that is done the same with a 170 kit. And remember the barrel is Nicosil coated, and that dissipates better than steel lined, so is more for distance / endurance at mid range revs, rather than top end performance as the steel lined cylinders are.
