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OS FS-40 Surpass Big Bore

March 18, 2012

I've had this idea of doing an OS FS-52 big bore (.61) with a liner from an ASP 61 four stroke. ASP already did this a long time ago when they made the 61 in the 52 size case. I had an OS FS-40S on the desk the other day while doing a diesel conversion when the idea of the FS-40 big bore came to mind. I grabbed a liner from the FS-48/52, and looked the parts over. It became obvious it could be done and the resulting displacement would be 7.64cc or .47ci. Then, more thoughts came to mind. The head would accept valves from the 48/52 as well, with some work. The ports could be enlarged, but there is not much room. So, this is the plan.

Stage 1: Big bore from FS-48/52.
Stage 2: Two exhaust valves from FS-48/52.
Stage 3: Increased compression ratio 11:1.
Stage 4: Intake vavle from FS-48/52.

My only concern is the strength of crankcase casting around the cylinder.
This is the FS-40S crankcase mounted in the lathe for boring to fit the FS-48 liner. The hardware is big, but the work light.
The new liner in the crankcase.

August 22, 2012

The head was milled to accept the larger cylinder.

August 23, 2012

I ran a stock FS-40S for base figures and the FS-40 Big Bore (.47). Initially the results were a bit disappointing. All test were done today at 31°C(88°F), Graupner Super Nylon props, and 82/18 fuel(no nitromethane).

FS-47 Big Bore
Stock Carb
9x612300- - -

These results are expected. If the intake tract was made no larger than needed for the .40 then we can only expect gains when the intake flows enough to fill the cylinder, ie., at low speeds. The 400 RPM gain at 9000 with the 11x7 is good and represents a 14% increase in power. The engine displacement is 17.5% larger now. I didn't expect such small gains above 10kRPM.

 FS-47 Big Bore
FS-52 Ex Valves
FS-47 Big Bore
5.8mm Carb
9x6- - -- - -
9x7- - -12,800

I installed exhaust valves from the FS-52. These are identical in size to the stock FS-40 valve with a relieved stem in the port. I expected some gains, but saw no results, things even seemed worse. This indicated the carb was limitng the air flow. I installed a stub intake adapter from the FS-40S-C (car version) and a 5.8mm slide carb. This carb was slightly too large and fuel draw suffered. Flying would be impractical without a pressurized, regulated fuel system like a Cline. I had to run a piece of tubing on the carb inlet as an intake stack due to the amount of blow back from the carb. This time, the engine came to life! There was a 900-1000RPM gain at with the 11x7 prop for a ~35% gain in power. A 1200 RPM improvement was obtained with the 10x7 for a 38% increase in power. Again, these are 0% nitromethane figures. If I get an opportunity, I'll run the stock engine with the large carb to compare.

The next step is to investigate porting the engine. There is little room left for larger ports, but if air flow can be improved, more power may be available.

July 4, 2014 This is the beginning of a new connecting rod to increase compression ratio to ~11:1 from ~8:1.

June 2, 2015

I decided to install a manifold and carb from the FS-48. This required a new backplate to mount the carb.

 FS-47 Big Bore
FS-48 Carb
FS-47 Big Bore
FS-52 Carb
10x6- - -12,600

The muffler from the FS-48 was used. It has a larger exit than the FS-40 muffler. Modifications also include porting the head. The exhaust was port was enlarged to match the FS-48/52 dimensions with no trouble. The intake was enlarged as much as possible hoping not to break through any walls. I was too optimistic and ended up with holes to the glow plug recess and the left rear head bolt counterbore. These were sealed with aluminum solder. Sharp edges were removed in the head and manifold. The FS-48 carb has a 5.4mm bore, slightly smaller than the last test run. Power did improve over the stock carb, but not much. The temperature today was 19°C(67°F), so power for any given RPM is slightly greater than the hotter temperatures before.

The FS-52 carb has a 5.6mm bore, only slightly larger than the FS-48. The power was tremendous exceeding the previous 5.8mm carb with the straight intake manifold. I thought with the numbers from the FS-48 carb the intake tract was holding it back, but this run proves otherwise. It runs well with exhaust pressure. RPM increased 1400 over stock with the 10x6 for 45% more power. The 11x7 did peak at 10kRPM, but sagged to 9.9k quickly. I think it was just getting too hot. I did make fresh fuel for the run with the FS-52 carb, so the old fuel maybe have been bad.

This arrangement is practical and can be flown. I don't have a suitable airplane right now, but will finish something soon. Once again these figures are with no nitromethane in the fuel. 15% showed about 500RPM more with the 11x7.

The new rods are still being made. Another porting option is to modify the head to run the larger FS-48/52 intake valve. This modification would remove the bronze valve seat. I'm not certain the valve would run on the bare aluminum head for long.

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Copyright 2012- G.Kamysz