Athena/GYTR YZ125 - 144 Kit
Modification and Evaluation Part 2
Mon | Nov 8 2010
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
-Good Power Spread and Part throttle performance
-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.