2 stroke exhaust port with before needing a bridge

[digiex-chris]

Is there a formula to figure out how wide your exhaust port can be in a two stroke before you need to have a bridge? And is there a guideline on how big that bridge should be?

 

[Niels Abildgaard]

Hello Two Stroke Friend

If the piston rings are made unable to turn round,, racing two strokes have run with 60 degree.
If rings are free professor Blair from Belfast adviced 25 degree as maximum.
My feeling is that bridge width is not important but bridge cooling is.
Real high power two strokes have water channels drilled along the bridges.

 

[John Rudd]

If the piston rings are made unable to turn round,, racing two strokes have run with 60 degree.
If rings are free professor Blair from Belfast adviced 25 degree as maximum.

Ahh but what if the piston is ringless as in an aero engine?...

Most of the 2t engines I have ( aero that is) have a bridge on the e port and are ringless....
I think that the port dimension should be such that there is the minimum of restriction to ensure fastest gas flow through the port to facilitate engine breathing ensuring max performance...

So unless I'm missing a trick here how does a bridge aid cooling and cooling of what?

In search of enlightenment...........

 

[digiex-chris]

I heard it was the opposite, that the bridge has little cooling so you have to careful about it seizing on the bridge.

That's great that a pinned ring can go across 60 degrees, I don't need a bridge! Thanks, that info is surprisingly hard to find on the net.

 

[hammer2100]

I hope you wouldn't mind my two cents. Bridges do sieze I have seen hundreds. I have built many high horsepower motorcycle engines. The best don't have bridges. If the port is designed with a good expansion chamber the pipe size will be alittle smaller than the port. It is important to keep the port an oval shape to keep the rings from trying to expand out into the port. Which is why the bridge trys to sieze when the exhust gases heats up the bridge. We tried cutting back the brigdes but you had to watch going to far or you start having ring promblems.
Good luck
Hammer2100

 

[digiex-chris]

due to the pipe I want to use I'm restricted to a 76 degree port. If I make it oval shape, I can get away with no bridge? How does an oval shaped port change the timing curve?

 

[hammer2100]

Exhust port timing will start at the highest point of the oval. Because of the loss of compression down the port. I don't know what your working on but have you thought about what are called boost ports. Thease are placed higher than the larger port but are there to aid in getting exhust savaging started earlier in the exhust pipe. Is your pipe a true expansion chamber?

 

[digiex-chris]

I'm doing a conversion, replacing a brass/chrome liner with a lapped aluminum piston with a cast iron liner and a ringed piston. I don't really want to modify the motor itself much. I think I'm comfortable enough to pull the trigger on a design now, your help is much appreciated. I'll look into boost ports if I'm not getting enough power. I'm not quite sure what you mean by a true expansion chamber

 

[tomrux]

true expansion chambers will have
from the port out

a pipe section
a divergent cone
a plain section
a convergent cone
a small exhaust outlet in the shape of a pipe (normally called a stinger)

about a foolscap page of math is required to work it all out.

If you pass me details like port size, engine size (bore X stroke) ant a full timing spec I can run it through a program i have and it will ouput some design criterea for a pipe to acheive increased engine performance in areas of interst.
Be aware tho that going for max output will have a narrowing efect on the useful rev range. only really needed if you are looking for max power out of an engine.
My motor program will also give tuned pipe lenght if that is all you need.

Tom R