roadhor

My Feedback: (21)

I was thinking of replacing the Saito 180 in my H-9 Cap 232 with the Magnum 160 twin. Has anyone out there ran one of these before? What kind of power do they put out? I want the neat sound of a twin when I'm flying. Thanks

NM2K

My Feedback: (14)

I hate to burst your bubble, but since both cylinders fire at the same time, how is it going to sound any different than a single?

I could be wrong about this, but logic would dictate that to cancel the vibration imposed by a firing cylinder, the best way to do it would be to fire both cylinders at the same time. True?

roadhor

My Feedback: (21)

The one I heard sure didn't sound like a single, maybe because there's two exhaust notes, even if they are fired at the same time. Ya think

NM2K

My Feedback: (14)

I don't doubt your observation.

The speed of sound is so slow that a difference of a few inches between the exhaust pipes might explain the difference in sound.

I wonder if Y-ing the exhaust of a single cylinder of the same displacement with the ends of the Y-d exhaust pipe being the same distance apart as the twin's exhaust pipes would give the same effect?

I'm getting a headache...<G>

Elwood

My Feedback: (2)

Well, I don't think so. They can't fire at the same time. Just thinking about the crank for such an engine makes my head hurt!

Mendes

My Feedback: (1)

They don't fire at the same time, and the sound is pretty neat. I've only seen a Saito 300 though not the Magnum. I would guess that you wouldn't get nearly the same performance with the Magnum. The Magnum weighs 44 oz. the Saito is about 32 oz.

captinjohn

Both pistons can go outward at the same time and help balance, and still fire at different times. Or 360 degrees apart. If they both fired at came time, it would be 720 degrees between power impulses. I think- C. John How about it engine gurus??

Elwood

My Feedback: (2)

Both pistons can go out at the same time, but that is found in engines bigger than what we are discussing here. I may be wrong, since I have not taken this engine apart. I do know, however, that the crank shaft needed for this to happen would be very complex and hard to make. The connecting rods would also need to have end caps on them, just like my old Ford 460 V8.

Dugster

I was just looking at the exploded view for the OS 120 twin. It has capped con rods and a normal looking crank for a two cylinder four stroke engine, i.e. rod journals 180 degree oppisite from each other with the corresponding counter balances in the normal places. Now all this tells me that the pistons must move in the same direction (one going in, and the other going out). This allows them to fire 360 degrees apart, not at the same time.

Now I realize that I not looking at the same engine, but I can't see why Magnum would be different. In order to have a combustion stroke every revolution they must move in the same direction. Also if both cylinders fired at the same time you would defeat the purpose of having mutiple cylinders. That is to have the combustion strokes a often as possible to help smooth out the vibration, idle and surges. Not to mention the stress imposed on the crankshaft if both cylinders fired at the same time.

Elwood,

I tried to imagine the crankshaft for both cylinders firing together and/or pistons moving oppisite directions, now my head hurts......

NM2K

My Feedback: (14)

Having both cylinders fire at the same time cancels out each cylinders firing impulse AND balances the reciprocating mass. This is the ideal way to operate a twin cylinder opposed engine.

Reality, however, may be a different matter. Hey, Mike Greenshield, where are you? <G>

captinjohn

Both pistons on a 2 cycle can move out at same time and fire at same time. Best for reed valves mounted on crankase. Also some rods do not have caps, the crank is pressed together, when it dies you need whole new assy. Captin John

Elwood

My Feedback: (2)

What RC engine does this?

captinjohn

I cannot recall right now, but I think it was a converted 2 cycle gas engine. Thanks C. John

Volfy

My Feedback: (23)

Even with the optimum firing order and crankpin offset, a horizontally opposed twin (affectionately nicknamed "boxer" for obvious reasons) does not have perfect dynamic balance. You will need to go to at least a HO Six (e.g. Porsche) to achieve perfect balance. Even then, no mechanical device is perfectly rigid and so parasitic vibration will still be generated.

Anyway, the two "boxing" pistons do afford perfect primary balance, but the crankpin offset yields a secondary imbalance in the form of a fore-aft rocking vibration. This is true for both 2 or 4 stroke. A 2stroke HO twin has to fire at the same time by its very nature. A 4stroke OTOH will benefit from trading spark every other stroke. The gain in smoothness from a more even firing order (every 180 deg) is much more than from a synchronized power stroke. Plus if they did fire simultaheously, you would need a bigger carb as both cylinders will be drawing air at the same time. last time I looked at the Magnum 1.60, the carb doesn't look any bigger than the one on the .80 and .91.

I believe this is the way the model HO twins operate. If you have a twin you can verify this easily. Just open up the two valve covers and watch the pushrods as you turn the engine over. If the valves actuate identically, you know they are firing simultaneously.

Elwood

My Feedback: (2)

Or look at the crank. If both con rods share a common crank pin, then they would not fire at the same time.

jimcork1

My Feedback: (4)

I do know the pistons move on the twin.. (just funnin..) but don't expect the twin to have the power as the saito 1.8. The 1.8 is a tiger and has lots of power. The twin will not be as power efficient and therefore if they were the same size would not have the same power. Going to a 1.5 will reduce power more. If you are ok with the power loss to have the twin.. Happy flying. I to have considered the twin for a cub.. but never got off the wallet. Jim

HJmay

Just look and see if there is an offset in the cylinders. If they are offset then they would be able to fire at the same time. Im not saying this is so with this engine, but if they are inline then I don't see how they could fire at the same time.

Jazzy

I can't see how a HO twin's crank would be if the cylinders were directly opposite eachother. It makes sense to me there must be an offset at least the conrod's thickness.
Two wrist pins on a bearing surrounding a central or main wrist pin would not work. Besides, one piston would be on it's way in while the other is on it's way out. Bad juju.

I remember hearing an O.S. twin on a large clipped wing cub. Sounded pretty neat. It purred like a muffled two stroke without the high RPMs.

Anyone care to take the rear cover off their multicylinder engine for a pic of the internal assembly?

kadamstex

My Feedback: (2)

WoW! Can't believe everyone doesn't know this. Some model twins have a double throw crank and some have a single. The OS twins and their clones (like the Magnum) have double throw cranks and fire alternately. That is, the pistons travel in opposite directions; each going in and out together. One cylinder fires on one revolution and the other on the next. The new Saito twins (the FA-60T and the FA-90TS) also work this way. The older Saitos have a single throw crank (both rods are connected to a single crank pin). The pistons travel in the same direction; that is, when one is moving out, the other is moving in. There engines include the FA-80T, FA-90T, FA-100T, FA-130T, FA-182T, FA-270T, FA-300T. On these engines, one cylinder fires, the engine rotates 180 degrees, the second cylinder fires, then the engine rotates 540 degrees without firing, repeat. This setup does not run as smooth as the double throw crank, alternate firing twin.

Hope this clears things up a little.
Ken

RJConnet

Kadamstex...... I also did not realize that it this is not common knowledge. That is, a HO 4- stroke engine can have either a single throw or a double throw crank with the engine having the double throw crank being an even firing engine (one cylinder fires on one revolution and the other fires on the next revolution, one firing per revolution). The engine with the single throw fires exactly as you stated. I am sure though that the Saito 270 and 300 are double throw alternate firing twins.
By the way, all of the big gasser twins (2-strokes) have double throw cranks and are simultaneous firing (only way you could make a 2-stroke twin work). Oh yes, when the cylinders are directly opposite each as they are when a single throw crank is used, one connecting rod is split (forked) and straddles the other so they share the same crank pin.
Have a great day....... Bob C.

Jazzy

Bob C.,
Thanks. I understand it now.
It was around 4 in the morning when I was trying to think about it. Apparently I should have been in bed.

I'd still like to see some pics though...

For comparison sake, what size single would be about equal to a 1.8 HO twin? (Four strokes that is and also taking into consideration the added weight of the twin versus the weight of a larger single which would fire half as often as the twin.)

I thought about it breifly some years ago but dismissed any further contemplation because my wallet told me to .

Jeff

wcd2424

OK.........this is getting way out there! I have a Magnum 160 and it has an awsome sound and power. I have it in a 100 in Hanger 9 cub. The cylinders are offset about the width of a connecting rod. It'll turn a 18-6 prop about 9000. I love it.

Sport_Pilot

I agree that the newer twins fire every 360 degrees. However it is possible for a two stroke twin to fire every 180 degrees. Of course you use a double throw crank, this would have a journal between the crank throws, and the journal would have webbing and a seal between the cylinders. Since each piston has its own crankcase chamber it would have to have seperate carbs, or a manifold to each crankcase chamber.

RJConnet

Yes Sport, I believe you are right. Isn't this the way all the 2-stroke multi-cylinder motorcycle engines used to be made? Seperate crankcase volumes for each cylinder and a carb. for each cylinder. I don't know of any engines being made for
model use that are made this way.

Cometlover

It would be expensive!

2 stroke model twins fire simultaneously.