Enlarged exhaust ports

[Milt36]

An engine from a parts bike I bought has a cylinder with enlarged exhaust ports.  I estimate they are about 30 - 40% larger than stock and the porting work seems to have been done very carefully.  The ports are very smooth and symetrical.  After getting the crank redone and replacing the coil, I got the engine running on a test stand set up but can't tell much about the power it may produce.   I know from go-kart work I did many years ago, porting can be easily over done.  Short of putting it on a bike to check it out, does anyone have a feel how the servi-cycle engine should respond to exhaust porting?

[pd]

Milt ,

 Can you describe the shape of the ports ?

My M model has 2 round exhaust ports , about a half inch each .

From my other two stroke experience , it's not uncommon for exhaust ports to be widened a mm or two and sometimes the top of the port is raised by a mm or so to slightly advance it's timing . And , most two strokes use an oval or squashed egg shaped port .

Pete .

[Ricks]

Post some pictures with a rule.

[Milt36]

The ports are in a inclined (to the vertical) oval pattern, will try to get some pictures today....

[Milt36]

Attempted to add photos.....The last one is of an old rusty stock head, the only stock one I had readily available.  The other three are of the ported head.  Was not able to get a scale into the pics, but the comparison to stock tells the story.

[pd]

The person that did that had an eye for beauty .
From what I can see , they did a very nice port job .

I have to wonder about the transfer ports . Were they done to accommodate the exhaust ?
And then , how about the intake path and carb ? Stock or otherwise ?

Is the exhaust pipe stock ?

Pete .

[Milt36]

Pete, I agree, someone really skilled put some effort in doing the porting.  Hopefully he got a good result but that is unknown. 
The intake at the carb, the transfer ports in the head are stock are far as I can tell.  A muffler didn't come with the bike. The compression is only 80 psi, but I don't know if the raised exhaust port or worn out bore and rings contribute the low reading.  Wish I had a dyno set up...I could have some fun testing these engines.

[pd]

With that much bigger exhaust , I'd have thought the transfers would also have been enlarged and the whole intake path somewhat matched .
The exhaust porting has no effect on compression .
Most likely cause would be worn rings with maybe too big of end gap , provided the piston to cylinder clearance is something close to right .

Pete .

[Milt36]

Obviously, attempting to gain performance by porting would require a lot of dyno work.  Porting the crankcase at the carb attachment is possible as is changing the timing of the rotary valve, but the intake tract in the head might be the limiting factor.  You could do something at the base, but getting to the ports where they enter the cylinder would be tough.  Also, there would a lot of trial and error involved, with a lot of scrap generated by failed iterations.
Concerning the effect of exhaust port raising on compression, consider this point, if the exhaust port is raised higher and higher, a point is reached where the compression will go down, then in the limit, approach zero when raised to the top of the head. 
So many factors, so little time and money.....probably why it all hasn't been done before.

[pd]

It has all been done before . Maybe not with our antiquated 2 strokes , but , done it has .
With a bit of diligence and thought , modern technical advances applied to these motors could / should produce positive results . I have to think that an expansion chamber exhaust would help the output of these motors . How much is the big question .
Tools to reach those hard to get at ports exist , although not necessarily specific to port work .

And , your point on exhaust ports is true .
I have to say that in 60 years of messing with motors , I've never seen , read or even heard of a 2 stroke exhaust port so high that it effected compression . Good thing 2 strokes have been around long enough that a lot of experimentation has been done .

If you want to explore the inner reaches of two stoke technology , you might read these two 'pieces' by Jennings and Bell :
https://www.twostroketuning.com/two-stroke-engine-tuning/

Pete .

[Milt36]

Over many years, I've kept up with the progress in 2 stroke performance and know first hand what amazing power they are capable of.  In my last reply, I was making comments specifically about possible performance improvement of the Simplex engine.

[Ricks]

I wonder if the enlarged port would be a cause for concern on catching a ring.  One of the old timers likes to pin the rings as he said this was an issue on the aluminum cylinders.

[Milt36]

Rick, this particular head is cast iron, but one would think that the larger the port, the more likely the ring snagging possibility.  At least the bridge wasn't removed.  I've only run this engine on the test stand for a few minutes at at time, so I haven't even got it hot yet.

[pd]

More than once , I found the rings on my M wanted to locate the ends directly over the transfer ports .
So , I pinned the rings , using 1/4" long x 3/32" wide compression pins .

I drilled the landings above the wrist pin , but toward the rear , to a depth of about 5/32" . The pins had to be driven in , so it's unlikely that much , if any, outward drift will happen . I squared the outer end of the pins and called them 'good for service' .

Then the ring ends needed relieving to fit the pins . I used a jewelers square file to reach that 'end' .

That was quite a few run hours ago and haven't seen or had anything that resembled a linked problem .

Pete .

[Milt36]

You're a great craftsman, Pete.  Very nice indeed!!