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The G38 Benchmark is an APC 18x10 and turns it at 7400 RPM.

This engine as illustrated weighs 4 pounds, 5 ounces. After the removal of the magnito ignition, it weighs 2 pounds, 12 ounces plus the weight of the new BCMA Ignition (3 ounces). The TOTAL weight savings will be 25 ounces! Or, approximately 1 and a half POUNDS lighter.

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Here are the components removed. Everything from the right of the engine will be replaced, including the prop hub. This G38 is the "older" model engine which used a steel prop hub, the newer G38's use a bolt-on aluminum hub.

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I will also be replacing the carb with a NEW WT540 Walbro. The WT540 is greatly superior to the OEM carb. While I am at it I will also replace the exhaust as well. Another big plus to using EI is that the cooling fins normally blocked by the magnito are fully open to air flow now.

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Here you see the BCMA Electronic Ignition Module which is actually made by RCEXL. It weighs a mere 3 ounces less battery, and it is equiped with an 18 degree micro-processor spark advance.  While the conversion parts are available, I will machine my own parts to fit a specific application. The machining will include; a new prop hub, magnetic timing ring, and sensor support, all made of  T-6 aircraft billit aluminum. Before any machining takes place I have to decide where I want the sensor parts to be located. My options are; the front using the prop hub or the PTO on the rear. I'm leaning towards the prop hub but using the PTO would make a cleaner (and well protected) installation. I energized the ignition with a 4.8 volt battery and found the BCMA to have a super hot spark. Much more so than CH.  

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If I use the PTO, the sensor (Hall Effect Switch) and the magnetic timing hub will be enclosed in the engine mount. This will protect the parts from the elements. There is also more places to anchor the sensor. The downside is, timing the engine must take place with the engine removed. I'm not too crazy about that, but once its timed I shouldn't ever have to mess with it again. I'll study this a little longer...

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WT 540 Walbro

   The WT 540 Walbro carb incorporates some special features not found on RC engines. The biggest upgrade is the Accellerator Pump.The Pump injects fuel through the highend nozzle when the throttle is opened. This provides smooth throttle responce.The WT 540 also uses a special spray nozzle for precision fuel delivery on the topend, and it has a brass fuel inlet. Last but not least, it has long needles. The WT 540 is available with an optional automatic choke release which unlocks the choke when the throttle is slightly opened. This carb is 1/2 mm bigger than the OEM G38 carb and is a perfect bolt-on upgrade.


After a long and intense fondling of the engine, I have decided to install the Ignition Components on the PTO. There's really a lot more options for placement back there. So I'm off to the drawing board to design the timing hub, sensor support, and prop hub. This genreally doesn't take too long in AutoCad, a few days tops. The prop hub and timing hub with be turned on my lathe while the sensor support will be turned & milled.  

Next day...

The new ingition components took about 2 hours to machine, and they fit the G38 perfectly. Because the parts are within the engine mount, the timing it will have to be set prior to mounting the engine. This is your classic "set it , and forget it" application. 

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The ignition sensor parts are 98% done. All I'm lacking is mounting the magnet in the PTO hub, and installing the Sensor Bracket in the engine mount. It doesn't matter where the magnet is installed as long as it passes directly under the Timing Sensor. The PTO hub can rotate 360 degrees on the crankshaft. With a little minor re-engineering, I could set the timing with the engine mounted. Instead of using a jamnut, I can use a #10-32 set screw then bore an access hole in the engine mount. This will give me access to the timing hub externally through the hole. I think that's the way I'll go. The Ailenco rare-earth magnet is very small put powerful. It's 1/8" in diameter and 3/32 thick.   

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The mechanical timing is set by rotating the PTO Hub on the crankshaft and locking it down with a #10-32 set screw. Full advance will be mechanically set at 30 degrees BTDC, then the micro-processor will retard the timing to12 degrees advance at idle. As the engine RPM goes up, the ignition will make its way to full advance. The micro-processor is programmed to have the ignition at full advance at 4000 RPM.

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I used a dial test indicator to find Top Dead Center then set the Degree Wheel to ZERO. I use the sparkplug to see and hear the spark while observing the degree wheel. The PTO hub (with the trigger magnet) can be rotated to set the exact degree of ignition (spark) timing. This is a mechanical full advance setting of 30 degrees before Top Dead Center. The module will take care of the retarded timing. I will re-check the timing again after the PTO hub is locked down to be sure it didn't move.

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The timing is set and now its time to install the new WT540 carb. This is nothing more than a bolt-on.

Moving on to other changes...

The original prop hub is steel and is intended to be the flywheel bolt. Later on, Zenoah began using an aluminum prop hub which bolted to the flywheel instead of threading on the crankshaft. This was a major improvement. CH Ignition makes a replacement hub for the flywheel and it works great if you want to keep the same prop stand-off. My G38 needs a shorter stand-off, around an inch shorter. A prop hub from an FPE 1.3 engine is close and I could make it work, but I believe I can make one that's perfect. I have a 1 5/8" diameter T-6 aluminum billit that will make a first class hub. It will take a lot of machining though. I also want to upgrade the muffler. Someone modified a weed-whacker muffler to fit the G38. It has too much back pressure because the stingers are too small, plus their welding technique leaves a lot to be desired. I will work on both the hub and muffler and try to finish them at the same time.

The prop hub will have to be machined in a single setup to ensure everything is on absolute dead center. This mandates how the machining takes place and the sequence of cuts & boring.

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This is the new prop hub profile. It will move the prop back about 1 1/4" from the original location. The major diameter will be 1 3/4" and the minor diameter somewhere around 1", and the prop support is 1/2". There will be a 6 bolt prop anchor using #6-32 capscrews. The hub will be seated by threading on the crankshaft and locked with 2 1/4-28 setscrews. The image is only a concept drawing and not to scale.     

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There is the new Prop Hub 90% done. The setscrews and drilling the Prop Flange is next. Except for the taper, the concept drawing was very close!

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The new prop hub uses 6 each, #8-32 Stainless Steel capscrews to anchor the prop. It won't be going anywhere! The engine is a gnats ass from being fired up.

Running Summary: The timing components are mounted on the PTO (inside the engine mount) and it's timed at 30 degrees BTDC. The new WT540 carb is installed directly to the jug, leaving the plastic spacer off. Most RC gas engines don't require a heat barrier on the carb because of the massive air flow. It also makes the throttle connection easier on this particular engine. The new prop hub is done but I let it "cure" to ensure it stays on. The hub is threaded on, and JB Welded for thread lock, and 2 setscrews on top of that. I don't want is an 18x10 APC prop flying off! I checked the prop hub for run-out to be sure it's turning on dead center, and its perfect. I mounted the engine, primed it, and it fired up on about the 21st flip. The engine didn't kick back so the spark advance must be working OK. I let it run for a few minutes to warm up and I stayed way behind the engine just so in case the hub gives up. I hadn't tuned the carb yet but it was running good. I set the idle around 2490 to be sure it stays running. The engine throttled up smooth with good power. The first full throttle yielded 7540 RPM which is about 140 RPM more then the Zenoah spec. I killed the engine, checked the prop hub, and leaned the high-end about 1/8th turn. It fired up on the 2nd flip this time. It throttled up faster this time and peaked at 7590 RPM. I re-tuned both the lowend and high-end got it down to 2100 and up to 7610 on the top. The throttle transition is awesome! The WT540 carb (with the accelerator pump) is doing its job.

I'm very nervous about the un-proven hub and staying out of harms way. APC props are heavy and deadly if they come off. Today I will run the engine 10 or 12 times and push it to the limit to be sure it's safe.

The engine is calculating at 3.30 horsepower and 18.26 pounds of static thrust. I can clearly see by its current running, there's more power available. The muffler is someone’s attempt to keep it quiet and it's not the original Zenoah muffler. My guess is, there's too much back pressure here. After I get past the prop hub reliability tests I will re-work the muffler.

My conclusion thus far is; the BCMA Electronic Ignition appears to work excellent. The battery consumption is low as compared to CH and similar ignitions. Since it has a lifetime warranty I'm not too concerned about durability (just yet). The WT540 carb is the best of the best, so I doubt that it will be any sort of problem other than normal wear & tear. The engine is, well Zenoah... they are tough and reliable. Once I am confident the hub is safe I will install the engine in my Ultimate Pitts. It will replace my G26, which is electronic ignition as well.

In COMPARING the G38 to an OS 1.60 FX:
Pound for pound, the OS 1.60 FX is the stronger of the two. The FX is just as reliable as the Zenoah. In fuel consumption, the Zenoah wins "hands down". The FX throttles up the 18x10 a little faster and is a little quieter than the G38. The FX is 10 ounces lighter than the G38 also. The bottom line here is: The G38 sips gasoline at a rate of .6 ounce per minute at full throttle costing about $2.30 a gallon (including oil). The OS 1.60 FX consumes mass quantities of 10% nitro at the rate of 2.0 ounces per minute at full throttle. At $13.00 per gallon. If you are looking for tons of power and light weight, the FX is great! If you want excellent power and great fuel economy, the modified G38 is the ticket!


After getting rid of the original muffler and replacing it with a Brison 3.2 muffler, it lowered the Back Pressure Rate, and the G38 really came ALIVE with the new ignition & carb! It's turning 8550 RPM on the Factory spec Benchmark prop. That's 1150 RPM more than a stock Zenoah G38 is rated at. The accurately timed Electronic Ignition coupled with a high quality Walbro WT540 brought my G38 to a whole new level of high-performance!

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I installed the engine in a Lanier Ultimate Pitts which was powered with a modified G26. After some minor tuning, the G38 purred like a kitten. It starts easy, has lots of power, and is smooth as glass. I saw a massive improvement in the flying. I checked over the biplane after 5 flights and everything was perfect. This conversion was well worth the trouble and worked better than I would have imagined! I did change the Brison muffler over to a Bennett Pitts muffler to make it a little quieter.

You can get COMPLETE Ignition Conversion kits from BCMA Engines  for several engines including the Zenoah G38.

Copyright 2007 M. B. Fuess