OS Speed 91HZ-R 3C/3D Differences
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First impressions you will see is the color of the box, going away from the traditional OS Engine blue box, OS Speed uses the black and red box. Up first is a look at the 3C version.
Upon opening the box, pilots will find a color manual and a nice red heat sink to distinguish the looks between the 3C and the 3D versions.
Accessories included in the box are a 0.2mm cylinder gasket, a one-way fuel filter, a OS #8 glow plug, and a needle control arm.
Equipped on the OS Speed 91HZ-R is the DRS system. This regulator allows the engine to run smoothly through every aspect of flight by providing the correct air/fuel mixture.
Another of the differences is the carburetor. The 3C version uses the 61F whereas the 3D version uses the 61G.
On the backplate there is a hole to mount the BPS-1 Speed Sensor.
Now a look at the 3D version. At first glance, the big difference is the black heat sink for the 3D version. In the box, the manual’s color is different and there is no needle control arm.
Now it’s time to dismantle the engines to see the internal differences and similarities. Starting with the crankcase. Both versions have a slightly different intake than previous 91′s. Similar carburetor openings and crankshaft bearings.
A look at the backplate reveals an arc groove to accommodate the bottom of the piston.
Close inspection of the heat sink shows a hole pattern to increase airflow, thereby increasing heat dissipation. Also, installed is the 0.1mm head gasket, suitable for use with 15% nitro fuel. If you are using 30% nitro fuel, then use the included 0.2mm head gasket. The other option is if you live in a dry climate, then use both the 0.1mm and the 0.2mm head gaskets. Keep in mind other factors will need to be considered, but use these recommendations as a guide for you to start with.
The piston felt similar between the two versions.
The crankshaft itself is redesigned to help with better combustion chamber mixture.
Both versions use the DRS System and at a glance look similar. Aside from the carburetor number, the throttle arm is marked differently, 1 notch for the 3C version and 2 notches for the 3D version. Another difference is the carburetor opening, the 3C version is 10.5mm x 12.5mm, whereas the 3D version is 10.5mm x 11.5mm version. Why the difference? 3D stressed torque and 3C stressed speed.
Other differences can be seen with the cylinder liner. Ports are opened differently. Also, there are notches on the liners to distinguish between 3C (1 notch) and 3D (2 notches), matching the throttle arm.
The OS 91HZ-R 3D version looks like will be available as a combo with the matching OS Powerboost 91 Pipe.
So there you have a closer look at the similarities, but more differences between the 3C and the 3D versions.
Source: RC Mania