# 4 cylinder Boxer 2 stroke firing order

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#### cheeka

##### Active Member
Does anyone know the firing order for a 4 cyl boxer two stroke? If we use a Lycoming 4 cyl as an example, the firing order is 1-3-2-4. Since a two stroke fires every time the piston approaches TDC would a 4 cyl boxer two stroke be a "Big bang" engine with cylinders 1 & 4 firing simultaneously and then alternating with cylinders 3 & 2? Has there ever been a 6 cyl boxer two stroke?

#### BBerson

##### Light Plane Philosopher
HBA Supporter
McCulloch drone engine:

#### Himat

##### Well-Known Member
Does anyone know the firing order for a 4 cyl boxer two stroke? If we use a Lycoming 4 cyl as an example, the firing order is 1-3-2-4. Since a two stroke fires every time the piston approaches TDC would a 4 cyl boxer two stroke be a "Big bang" engine with cylinders 1 & 4 firing simultaneously and then alternating with cylinders 3 & 2? Has there ever been a 6 cyl boxer two stroke?
Is not the definition of a boxer engine that two opposing cylinders go bang at the same time?
Refer to the Ferrari Testarossa engine that was an opposed inline engine, but no boxer. The firing order was that of a V12.

#### Jerry Lytle

##### Well-Known Member
Is not the definition of a boxer engine that two opposing cylinders go bang at the same time?
Refer to the Ferrari Testarossa engine that was an opposed inline engine, but no boxer. The firing order was that of a V12.
I found that interesting so I did a little research and came up with this: https://en.wikipedia.org/wiki/Flat-twelve_engine

#### BoKu

##### Pundit
HBA Supporter
Is not the definition of a boxer engine that two opposing cylinders go bang at the same time?...
I think that's close but not dead on. My understanding is that a "boxer" has opposed pistons that reach TCD simultaneously, but do not necessarily fire simultaneously.

Taking the standard four-stroke 1/2 VW as an example, the crank throws for the two connecting rods are separated by 180 degrees. The two pistons get to TDC simultaneously, but they are separated by 360 degrees in firing order. That is, when the right piston is about to start its power stroke and the spark plug has just fired, the left piston is at the top of the exhaust stroke and about to start the intake stroke. On the next rotation (360 degrees later), the right piston is about to start the intake stroke and the left piston is about to start its power stroke.

As BBerson's post shows, with a two-stroke engine with opposed cylinders, having the crank throws separated by 180 degrees is the only way a conventionally scavenged engine will work, and of course both cylinders in each opposed pair fire simultaneously. If the connecting rods were on a shared throw, one piston would be on the up stroke while the other is on the down stroke, and the net crankcase pressure would stay at ambient and would not scavenge properly. I suppose you could make that work with a supercharger, but that is beyond the scope of what this discussion.

--Bob K.

#### cheeka

##### Active Member
Thanks for the pic BBerson, so they do fire together! I think the torsional vibration would be pretty bad with this design. Bob a "Boxer" engine never shares connecting rods on the same crank pin which is how they differ from flat engines where they do. The question is now is how do they offset the crankpins if it's a 6 cyl or more boxer.

#### cheapracer

##### Well-Known Member
Log Member
having the crank throws separated by 180 degrees is the only way a conventionally scavenged engine will work, and of course both cylinders in each opposed pair fire simultaneously.
Yup.

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#### larr

##### Well-Known Member
As far as I understand it, piston ported multi cylinder two strokes have to fire simultaneously. The pistons must rise and fall together otherwise it is not possible to maintain positive crankcase pressure.

#### Dan Thomas

##### Well-Known Member
Thanks for the pic BBerson, so they do fire together! I think the torsional vibration would be pretty bad with this design. Bob a "Boxer" engine never shares connecting rods on the same crank pin which is how they differ from flat engines where they do. The question is now is how do they offset the crankpins if it's a 6 cyl or more boxer.
I had one of those McCullough drone engines. It did vibrate horribly. It was on a Taylor Monoplane I bought to restore after it had been damaged, and found every glue joint in the firewall failing from the shaking. And everything was oil-soaked, too, since it had to have mixed fuel, and the exhaust spewed oil over everything. Impossible to repair oil-soaked wood.

Those drone engines were intended for anti-aircraft target practice. Designed for a 15-minute service life or something like that. The carb throttle shaft would wallow out its bore real quick, and the magneto was famous for failing once it got hot.

#### cheapracer

##### Well-Known Member
Log Member
As far as I understand it, piston ported multi cylinder two strokes have to fire simultaneously. The pistons must rise and fall together otherwise it is not possible to maintain positive crankcase pressure.
Plenty of V twins, V3, V4, square 4 and 2, 3 and 4 inline cylinder 2 strokes to shred that belief quickly!

It's easy to separate the any amount of cylinder's crankcases, Honda even do it without the crank having any sealing surface between the cylinders using a labyrinth seal.

#### larr

##### Well-Known Member
Ah well, I was hoping for a longer discussion of 2 stroke possibilities.
For the OP's question, the diagram shows cylinders 1 and 2 at TDC while 3 and 4 are at BDC.
This is a dual 'Big Thump' design, 1 and 2 are simultaneous ignition and 180 degrees later 3 and 4 are simultaneously ignited.

#### cheapracer

##### Well-Known Member
Log Member
This is a dual 'Big Thump' design,
More commonly known as a "Twingle" (twin firing as a single).

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Would it work to cut the McCulloch 4 cylinder in half into a two cylinder? About 35hp and 42 pounds.