Carb spacer effect on a piston ported cylinder?

[Terry Syd]

Rotary valves would be cool, just bulky.

Take a look at the Dolmar 6100, it isn't a rotary valve, but the design does have the potential to feed the crankcase like a rotary valve engine would. Maybe the next version from Dolmar will have a boost port in the back of the cylinder with a single VERTICAL reed rather than the two small horizontal reeds.

Some of the newer cars now have separate plenum chambers on the intake. I worked on one (can't remember the model) that shut off the plenum chamber when the RPM increased.

 

[awol]

Not necessarily. Take a look at the induction "forest" on a big-block light-alloy engine of a McLaren Can-Am car of days past. Point is, each cyl has its private intake tract. Chrysler had some interesting "cross-fire" manifolds back when, again with each cyl having its own long induction tract.
Just too bad that piston-ported 2T engines have to have symmetrical port timing about TDC/BDC. Rotary valves would be cool, just bulky.

The McCulloch saws of the 40's and 50's used a rotary intake valve. They were king of the woods at that time, even though weight was around 40 lbs. I wonder how one would respond to timing tweaks? hmmmmmmm........

 

[Terry Syd]

The rotary valve is the King of two-stroke induction systems. A broad powerband can be achieved by opening the valve a few degrees before the transfers close. This helps at low RPM (under peak delivery ratio) to reduce the suction in the crankcase caused by the rising piston from pulling mixture back out of the cylinder. However, a reed valve induction can open even earlier, as soon as there is a pressure drop in the crankcase - that's why reed systems are known for good low-end torque.

However, a reed has mass and at high RPM it reacts slower and the actual timing is closer to that of a piston port engine. Plus, the reed presents an obstruction in the intake tract. The rotary has no obstruction and at high RPM is opening by the time the transfers close and is full open and flowing as the piston hits maximum piston speed - it can really flow the mixture. The rotary valve can get full crankcase filling by 65-75 degrees ATDC.

The Dolmar 6100 has TWO intakes, one is the reed and the other is the piston port. The reed can improve the low speed torque and the piston port can provide a straight shot into the crankcase without any obstruction. The combination of the two systems presents an intake that closely approximates the rotary system. With a bit of work, it may be able to duplicate it without the weight disadvantage of the rotary.

 

[Deets066]

Boost bottle would need to be after the carb, they used them in snowmobiles. I had a custom boost bottle in my triple 700. It equalizes pressure in the intakes and stores atomized fuel (spitback) from when you let off the throttle and get back into it, throttle response was instant.

I don't see the need for it on a saw

 

[rogue60]

The rotary valve is the King of two-stroke induction systems. A broad powerband can be achieved by opening the valve a few degrees before the transfers close. This helps at low RPM (under peak delivery ratio) to reduce the suction in the crankcase caused by the rising piston from pulling mixture back out of the cylinder. However, a reed valve induction can open even earlier, as soon as there is a pressure drop in the crankcase - that's why reed systems are known for good low-end torque.

However, a reed has mass and at high RPM it reacts slower and the actual timing is closer to that of a piston port engine. Plus, the reed presents an obstruction in the intake tract. The rotary has no obstruction and at high RPM is opening by the time the transfers close and is full open and flowing as the piston hits maximum piston speed - it can really flow the mixture. The rotary valve can get full crankcase filling by 65-75 degrees ATDC.

The Dolmar 6100 has TWO intakes, one is the reed and the other is the piston port. The reed can improve the low speed torque and the piston port can provide a straight shot into the crankcase without any obstruction. The combination of the two systems presents an intake that closely approximates the rotary system. With a bit of work, it may be able to duplicate it without the weight disadvantage of the rotary.

I've got a rotary valve 2 stroke Pope mower from 1959 I restored. Very simple design, it sure has got some grunt and spins some high revs, massive crank weights compared to other little industrial 2T engines I've worked on and thick heavy blade disc with no decomp it takes some pulling over!. I like using it with side discharge wound up it pretty much throws clippings over the neighbour's fence lol.

 

[rogue60]

To the OP basically yes intake length on a 2T can be shortened or lengthened for different performance whatever it may be the turner is chasing..I forget what year but Kawasaki moved the carby back on one there KX125's about a half inch and picked up 1.2hp with that mod alone.
It all comes down to the particular engine one is working on and experimenting what works and what doesn't, for me trying different mods is not about 1st place trophys just the enjoyment I get from tickering on 2T engines just a bit of fun. The RC guys and the crazy mods they do to whipper snipper engines are worth checking out very interesting same as the Moped forums they are always trying new things.
Chainsaw's forums are pretty stagnant as far as 2T mods go, the race guys might be trying new things but that's all hush hush stuff.

 

[Al Smith]

There was an early motorcycle,name starting with V .Could be Vellicette,Villers,Vickers ,something .Any way some time prior to 1930 they had a production 'cycle with a tuned intake track .

I think the main disadvantage was the power band was too narrow .