Athena/GYTR YZ 125 - 144 Kit

Modification and Evaluation Part 1

Sat | Nov 6 2010

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. Today’s 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 it’s 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 pipe’s 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.

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