Thunder Tiger PRO 1.20 Gas Conversion
The Thunder Tiger Pro 1.20 is a well suited engine for gasoline conversions. It's a ringed piston in a chrome cylinder and has Schnurle porting. These features are desirable for high-performance gasoline engines. What makes this engine a particurarly good conversion is its power to weight ratio. While I will be adding some weight with the electronic ignition and Walbro carb, it will still be about 20 ounces lighter than a Zenoah G20 and it will produce 30% more horsepower.
The conversion is relatively easy: I'll replace the carb and add a CH Electronic Ignition. While most all the parts are available to make this conversion, I will machine my own components to give the engine a factory finished look.
FIRST: The Walbro Carb adapter
This adapter is desigened to fit perfectly where the factory carb installs, so there's no modification to the engine crankcase. The factory carb can be used with gasoline however it requires a fuel pump and regulator to run properly, which the Walbro already has. The Walbro is easier to tune up also. I actually ended up using a Walbro WT 340 carb which is smaller than the one in the photo. The WT 340 bore is identical to the factory carb.
SECOND: The Electronic Ignition components
Here is the little sparkplug that replaces the glow plug. They are cute but expensive!
Here is the magnet installed in the prop hub.
This is my custom-made Timing Sensor Ring (made of T-6 Aluminum) and has the Hall Effect sensor installed.
CH Electronic Ignition modules are available for "generic" engine installations and they work well. But I prefer to make the parts so it will look as good as it runs. It takes two components to time and fire the electronic ignition. You need a small magnet on the prop hub, and a Hall Effect Sensor which fires the ignition as the magnet passes by the sensor.
The magnet is set into the prop hub which is 1/8" diameter and about 1/8" deep. I use epoxy to secure the magnet. It really doesn't matter where you locate the magnet on the hub since the hub is taper fitted on the crankshaft and can be rotated anywhere you like. I made a timing ring out of T-6 aluminum which slips over the front bearing support. It uses 2 4-40 set screws to lock it in place. The timing ring can be rotated 360 degrees so timing the engine won't be a issue. I used a 1/4-32 NGK Sparkplug to replace the glow plug so there's no modification to the head.
The ignition timing needs to be around 30 to 32 degrees advance to get full power from the gasoline. I ran the engine on the factory carb first then switched to the Walbro. The factory carb actually ran faster than the Walbro however, the factory carb was difficult to tune and keep tuned. With the Walbro you just set it, and forget it.
Here is the complete setup with the factory carb. It runs faster than the Walbro but it won't keep a good tune.
Here is the complete setup with the Walbro Ready To Fly.
Even though the RPM was impressive, the factory carb was a real pain in the ass to keep tuned.
Here you see the Pro Gasser running on the Walbro and a 15x8 Graupner 3 blade prop. It takes a lot of horsepower to spin this prop! With an APC 15x8 two blade prop it ran up to 8970 RPM. To establish a reasonable benchmark, I used the Zenoah G20 gas engine to compare it to.
Zenoah uses an APC 15x6 prop to benchmark the G20:
The G20 turned the APC benchmark prop at 9400 RPM. This translates to 1.82 horsepower and 13.43 pounds of static thrust.
The TT Pro 120 conversion turned the same prop at 10,140 RPM, making 2.26 horsepower and 15.63 pounds of static thrust.
That's nearly 1/2 HP (actually .44 HP) more than the Zenoah G20. The G20 Ready To Fly weighs 50.6 ounces whereby the TT Pro conversion RTF is 36.6 ounces, making the TT Pro 14 ounces lighter! The real kicker is the ignition! The Zenoah electronic ignition pulls massive current thus it needs a high (amperage) capacity battery. The CH uses an average of 350 Mah so a smaller and lighter battery can be used. I used a 4.80 volt Triple A 850 Mah Nickel Metal Hydride and it ran for several hours and still had lots of battery power left.
Now for the bottom line: A Thunder Tiger Pro 1.20 can be purchased new for $174.95 plus a CH Ignition at $89.00 plus 30.00 for a new Walbro carb. TOTAL COST=$335.00+tax. The Zenoah G20 sells for $329.95+tax. It's pretty much a wash in the cost dept., but the conversion is substancially more powerful, a lot lighter, and runs longer on a single ignition charge. The last BIG PLUS to the conversion is... it will install directly on a 60/90/120 airplane using standard engine mounts! You can't say that about the G20.
THIS IS A SUPEREME GASOLINE CONVERSION!