SuperJ

I was out at the field today flying my cub off the snow on floats and another local giant scale junkie showed up with a 1/3 300L. Plane sounded radical so after I was done I walked over to poke around. He was flying a twin I have never seen before. It had an obviously cast crankcase with electronic ignition and a pair of Homelite 33/45cc looking cylinders on it and twin tuned pipes installed in fiberglass fuselage tunnels. The only things he would confirm were: they are Homelite cylinder castings, the ignition was not CH, and the crank and crankcase were first run production pieces. He hinted that the sale of Homelite to Techtronics has led to an agreement between somebody and Techtronics to make engines and parts available for the model market. He also said the crankcases, cranks and ignition would eventually be available for sale to modelers interested in providing their own top ends. In talking to some other local guys tonight this same guy has been seen running what appears to be a Ryobi derived four stoke opposed twin. He told another guy that complete 90cc twin motors would be under $600 less exhaust systems.

power-RCU

Hey superj, did you notice how the jugs were attached? I have been staring at my 33cc for some time and I can't come up with an easy idea for an opposed twin. The bearing cavities are cast into the head/cyl making it a problem. Are we talking about the 33cc 45cc chainsaw engines? If so AWESOME!! it's about time someone takes advantage of an inexpensive powerhouse and make it available for us to destroy!This is interesting please keep us posted if you hear anything more. Thanks.

POWER

SuperJ

The tops of the cylinders looked identical to 33/45cc Homelite chainsaw cylinders. They were attached to the crankcase with four bolts. Either
1)the cylinder tooling was altered or

2)the bottoms of the cylinders were tuned down and plates welded on or

3)the cylinders were stock production parts but from some version/engine made by Techtronics not sold in this country.

Besides the Ryobi four stroke twin he supposedly has now this guy also flew a 62cc twin with two stock Ryobi cylinders on it all last summer. he claimed it was a one off but I am starting to think he has a line into Techtronics R&D. That Ryobi twin flew really nice it had what looked like a reedcage off a little motocross bike mounted to the crankcase with a single carb. The 90cc motor he is flying now has twin carbs.

captinjohn

Superj: If you can ,take a few photos and post them!! Capt,n

SuperJ

He definitely isn't going to allow me to take any pictures with the cowl off. I had to really lean on him to even get a quick look myself.

rev

I been thinking about this for some time now. I think if you off set the clys, bolt the front of one to the rear of the other, mod the crank and ( make a two piece lower housing cover )it might work.

captinjohn

Rev: Would both pistons have to travel outwards at the same time for crankcase suction? Would you use port induction or reed valves? I was thinking a vee Twin like a Harley. One throw for 2 rods...but would reed valves work? Both pistons do not exactly go up the same time???? Any Input...Captinjohn

SuperJ

I spoke to another guy who saw the motor on the bench and was able to inspect it more closely than I and he was told the cylinders were prototype chainsaw 45cc Ryobi?? He saw no sign of welding and said the transfer port arrangement precludes them from being 33/45cc Homelite derived. He believes the cylinders are a cross between a 31cc Ryobi with that engines ports for crankcase reed valve induction but with the current Homelite chainsaw engines fin arrangement and angled plug. He also said the head had a mounting boss next to the plug that he was told was for a direct fuel injection setup. I am not sure if this is a situation of deliberate misinformation or what. I am starting to think that our Ryobi vs. Homelite distinction is the problem now that they are owned by the same parent company and the motors are sharing parts from the same casting houses, suppliers, etc. So currently I believe the following:
1)Techtronics under either the Homelite name, the Ryobi name or both intends to introduce a new chainsaw engine somewhere around 45cc
2)The cylinders from this saw will look like a cross between the current production Homelite and Ryobi cylinders.
3)Either Techtronics themselves or some unknown entity intends to offer for sale Hobby oriented engines ala Komatsu with the Zenoah engines and Horizon.
My totally totally unsupported theory is that the unknown player in this is Hobbico. This Ryobi/Homelite twin motor at less than $600 retail, street price of $450-500, is the Zenoah GT80s worst nightmare. It also makes sense that only somebody big could make the quantities large enough to interest Techtronics.

rev

captinjohn, I was going to try and have both rods on the same throw and bolt the clys, together but I had one problem the rod bearing. there is not space for two bearings. I went to a local suppler with the org bearing to see if there is one not as wide ,they said no! So I was thinking about taking two bearings and cutting some off about 1/3. But you would damage the cage for the bearing . If I had the right equipment I would try it. As far as the the crankcase and pump pressure ??????? To have both cyls fire at once you would need to offset the clys and mod the crank. Just think a 66cc twin for $ 200.00 at about 6lbs.

wvarn1957

My Feedback: (3)

On a twin with both pistons on the same crank pin, to have effective pressure you would have to make it an inline twin so both pistons would be traveling in the same direction at the same time. An opposed twin with the rods on the same crank pin would effectively have no crankcase pressure. The idea of a V twin helps some, but not enough.

I ran a quick and dirty simulation and came out with the following results for the pistons traveling in the same direction at the same time to effectively generate crankcase pressure ( both positive and negative).

Inline twin 100%
30 deg V 65%
60 deg V 40%
90 deg V 25%
120 deg V 15%
opposed 0%

As you can see the volumetric efficiency necessary to draw fuel on the negative stroke and pressure it into the cylinder on the positive stroke drops rapidly. You might get a 30 deg V twin to work, but I don't think it would be satisfactory. The only thing that might help is a supercharger, but then you are getting pretty complicated.

Will Varnado

captinjohn

wvar1957: Do you think the use of 2 small carbs and 2 reed valves would help with genorous sized ports? I wonder if a vee 2 stroke has ever been made? Might sound kinda neat! Glow ignition would be good way to build it. Then gas if it worked. CaptinJohn

wvarn1957

My Feedback: (3)

I don't think it would make any difference whether it was gas or glow. However, a 30 deg twin would have some interesting mechanical challenges im mounting the cylinders on the crank case. The biggest problem I see is that even though you have piston movement in the same direction around 65% of the time, I'm not sure what the pressure profile would be. The translation of rotating motion of the crank to the linear motion of the piston is not linear. When the crank rotates 90 deg from the position where the piston is at TDC you get half of the total piston travel, but over half of that travel travel is in the last 30 degrees of the crank rotation. In the 30 deg V, the other piston is lagging by 30 degrees and would only start down when the other piston is already in the are of fastest travel. By the time the second piston is in the area of fastest travel the other one has reached BDC and is starting back up, which takes away some of the compression volume. This in effect reduces the intake volume, but it also reduces the maximum pressure( both positive and negative).

It would be important to have a pretty good sized intake port to minimize the pressure drop. Two smaller carbs with higher thruput velocity might do a better job, but I'm just guessing. I think that it would run, but I would really question whether or not you would get the equvalent power of an opposed twin of the same displacement.

Will

captinjohn

Will: I agree with you. May not be worth the effort because of low power output. My idea of glow was to cut the cost of designing a spark ignition that would fire only 30 degs apart. Maybe some engine Gurus will chime in with some thoughts. Thanks Captn

billzair

My Feedback: (29)

I dont know if this will work but ill try to explain my idea. couldnt someone out there mill a crankcase( with a cnc mill) that has the cly. offset( jusy like the "real" rc twins out there) use the oranginal crank and just replace the crank journal with a longer one(so that both conn. rod will fit on the same journal. then, if you use 2 homie cly. the carbs ,exzaust and mag are all ready there.you could even use the stock flywheel, you would just have to place spacers on the mag on the cly thats the furthest back to make sure its over the magnet when it comes "around" scince each cyl. has its own carb and such you wouldnt need to worry about the other cyl messing up the oters "pressure" but the pistons would fire 180 off from each other and that might make alot of viberation??????????? just a thought, I might be 180 off!!

captinjohn

Take a look at TKG,s handmade Homwlite Twin.....nice Capt,n

tkg

My Feedback: (1)

Long ago and far away there was a motorcycle called a Puch. It had a really weird small angle V-twin 2c, 2 cylinder.
One piston controlled the intake timing and the other the exhaust.
The 2nd cylinder drew its intake through the first cylinder and the 1st cylinder exhausted through the 2nd.

captinjohn

TKG: Far out! Did the Punch twin fire on both cylinders, or was one just used for pumping into the other? How was the ignition part done? Capt,n

Flypaper 2

Saw a picture of one something like that a long time ago but both cyls tight side by side. Int on one cyl, ex. on the other. Both pistons going up and down at the same time. Combustion chamber open to both. A real shaker I betcha.

tkg

My Feedback: (1)

Flypaper got it right... one spark plug, one combustion chamber.
By using a small V angle and a common crank pin it had differant intake/exhaust timing than could be done with a single