# Thread: v twin 2 stroke

1. ## v twin 2 stroke

Hi everyone,my first post so go easy if I am asking a daft question.
I'm thinking of making a v twin 2 stroke,both pistons firing at same time,would I use one rod pin ala harley or not on the cranshaft,but having the rods side by side and not like a harley inside each other?

2. Firing takes place near top dead center. With a shared crank pin, if the V-angle is 90 degrees then only one piston will be at TDC, while the other is leading or lagging by 90 degrees (mid-stroke).

If you split the crank pins by 90 degrees you can have both pistons at TDC center at the same instant in crank rotation. If your V-angle is 45 degrees, split the crank pins by 45 degrees to match.

Split rods or side by side don't make a difference here, other than a split rod won't work on two crank pins 90 degrees apart...

It sound like you are aware a 2-stroke needs both pistons at DTC together for crank case compression.

3. Thanks for the reply dynosor,I think I 'm understanding it now the conrods have to be at the same angle as the cylinders,whatever the angle.
What I dont understand is if the fuel air comes in when both pistons are coming down,how is the mix distributed evenly to both cylinders if the crank isnt seperate?

4. Originally Posted by gjs
What I dont understand is if the fuel air comes in when both pistons are coming down,how is the mix distributed evenly to both cylinders if the crank isnt seperate?
Not sure I understand your question, but here goes:

You will battle to make a 2-stroke V-twin that uses a shared crank pin because the average crank case pressure will be near atmospheric, as in no pumping because the displacement of the two pistons cancel each other. Such engines are 4-stroke and don't use displacement volume below the piston for induction.

Making the crank pins offset at the same angle as the V-angle will increase the requirements for balancing masses, similar to a larger displacement single cylinder engine, and the firing interval and sound will be similar too. You will not get the off-beat Harley sound unless the crank pin is shared, in which case you need valves to make it a 4-stroke or it won't work.

5. Resparking this old thread as i didnt want to create new one for a query in direct relation to opening post,

For a v-twin 2-stroke (say 45 deg cyl angle, shared case, 45 deg offset pins) with pistons firing at same time, what difference does it might have in comparison to an equal capacity single in terms of -

1/ vibrations- more but reasonable ?
2/ characteristic- power or torque which gets pronounced increase?
3/ response- spins up quicker ?
4/ rpm limit- increase or decrease ?
5/ which would you prefer, if both are home built ?

I ask considering the smaller bore-stroke multis spins fast w.r.t big bore-stroke of single which rather thumps.

6. Originally Posted by apRRentice
For a v-twin 2-stroke (say 45 deg cyl angle, shared case, 45 deg offset pins) with pistons firing at same time, what difference does it might have in comparison to an equal capacity single in terms of -

1/ vibrations- more but reasonable ?
2/ characteristic- power or torque which gets pronounced increase?
3/ response- spins up quicker ?
4/ rpm limit- increase or decrease ?
5/ which would you prefer, if both are home built ?

I ask considering the smaller bore-stroke multis spins fast w.r.t big bore-stroke of single which rather thumps.

You have covered all the right aspecs to consider this question, with the key being that the crank offset is equal to the V-angle.

Assuming the V-engine and single cylinder unit have the same displacement, the twin would have a shorter stroke, or a smaller bore, or both, compared to the single. Assuming the bore and stroke are both reduced on the twin, the engine should rev higher and the balance should be slightly better than a single - not as good as a 90 degree V or 180 degree horizontally opposed engine.

Torque is a function of displacement, volumetric efficiency and mechanical efficiency. Each cylinder of the V-engine has less air to flow per unit time, but the air in the crankcase is split between them. It may be possible that the twin would fill better and provide more torque. If the twin could also fill better at high revs, it will make more power. On the other hand, mechanical friction may be higher in the V-twin engine (much less of a problem than a V-12), reducing its power and torque over the single.

A short crankshaft running between ball bearings would help to reduce friction. Piston (and ring) friction would take more manufacturing precision to reduce with a smaller bore – machining capability may be small compared to the larger bore, but becomes more significant as bore size is reduced. If piston and cylinder are of lapped ring-less design, it may be possible to scale friction down with bore size.

If the twin can get away with smaller counterweights on the crank, thereby reducing rotational inertia, it should be more responsive than the single cylinder engine.

The V-engine would be more interesting to build, to look at and to run, but the sound will not differ much from a big single...

7. Hi dynosor, for the detailed response.

So its not too bad to try one such engine, its position will be between a single cyl & proper Vtwin in terms of performance or rather its nature. Think an engine with intake aside the crank axis (like kawasaki Gto 100) provides much ease in mechanical construction.

Well, my next main Disc valve or Reed valve, which would serve better in view of shared compression in crank chamber.

& one wicked Q, consider the pin offset slightly lesser at 35 deg (cyl's 45 deg). Advantage i see is the ease for crank to tick over at TDC's than 45 deg with both pistons at TDC together. I hope a small reduction in output due to gas transfer issues might be there but this will be countered good by the smoother crank spin. Is it right to say so?

I know am asking bit too many but there's no better place than here :-)

1. ###### 2 stroke v twin
03-20-2013, 05:00 PM