So I read Jenning's Book.....

[drf255]

Check your intake timing again. Measure how many ° BTDC when the piston just begins to open at the bottom the port. It should be 7x°.

Pretty sure it's 70*. I'm using an auto Degreeing wheel. In this case, I read the larger and not the smaller number.

I deleted the base gasket, so my intake is opening sooner in the cycle.

So it would be 180 - 110 = 70*

Looks like I can lower the floor of the intake a bit. What should I shoot for, 74*?

The exhaust looks ok from what I've read.

I have 21* of blowdown IF my numbers are accurate. It doesn't appear that I should raise the transfers any further.

This OEM 028 Super jug looks factory "ported" compared to the regular 44mm WB jug. The passages and ports are much larger. The transfers are already opened towards the exhaust so much that there's maybe 3/16" of cylinder wall between where they end and where the intake opens. Opening them any further toward the intake would force me to flip the piston around.

Both 028 jugs have a ring retaining finger projecting down from the intake roof.

Also, FWIW, a Stihl flywheel puller make a decent mount for a degree wheel.

 

[hotshot]

If only we could put some kind of one-way valve between the carb and the case, and then we could get rid of the piston porting and that symmetrical intake duration.....

Reed valves really make a lot of sense. With porting events that are symmetric about TDC, it seems like one side of the port opening is always a problem.

Hence the rotary valve, but you'll not (yet) find one on these relatively inexpensive 50-100cc chainsaw engines we use.... motorcycles yes.

 

[wigglesworth]

Pretty sure it's 70*. I'm using an auto Degreeing wheel. In this case, I read the larger and not the smaller number.

I deleted the base gasket, so my intake is opening sooner in the cycle.

So it would be 180 - 110 = 70*

Looks like I can lower the floor of the intake a bit. What should I shoot for, 74*?

The exhaust looks ok from what I've read.

I have 21* of blowdown IF my numbers are accurate. It doesn't appear that I should raise the transfers any further.

This OEM 028 Super jug looks factory "ported" compared to the regular 44mm WB jug. The passages and ports are much larger. The transfers are already opened towards the exhaust so much that there's maybe 3/16" of cylinder wall between where they end and where the intake opens. Opening them any further toward the intake would force me to flip the piston around.

Both 028 jugs have a ring retaining finger projecting down from the intake roof.

Also, FWIW, a Stihl flywheel puller make a decent mount for a degree wheel.

Don't touch the exhaust height, widen if room allows, raise the transfers to 118'ish and widen the intake. I wouldn't lower it.

It WILL impress ya....

 

[rburg]

Don't you need to be working?

 

[wigglesworth]

Don't you need to be working?

Probably....

 

[Mastermind]

Don't touch the exhaust height, widen if room allows, raise the transfers to 118'ish and widen the intake. I wouldn't lower it.

It WILL impress ya....

Well yeah........but what do you know?

 

[wigglesworth]

Well yeah........but what do you know?

I knew that one thing that one time....

Does that count?

 

[nmurph]

I knew that one thing that one time....

Does that count?

You're giving yourself too much credit.

 

[Mastermind]

Yeah way too much.

 

[Mastermind]

 

[hillwilliam]

Case compression, or also known as primary compression is what drives the transfer charge. The more you lower the floor of the intake port, and/or raise the transfer ports the less case compression the engine will have. After a certain point the transfer flow becomes lazy......and the engine will have excessive spit back from the carb.

What I can learn from you is never-ending, Randy. I'd never thought of case compression in terms of degrees. Got me curious - it's probably irrelevant, but how would one calculate case compression in a reed valve saw? From TDC (or just after) until transfers open? I always figured reed valve saws would have better case compression, because the seal is pretty much immediate when positive pressure begins.

If only we could put some kind of one-way valve between the carb and the case, and then we could get rid of the piston porting and that symmetrical intake duration.....

Reed valves really make a lot of sense. With porting events that are symmetric about TDC, it seems like one side of the port opening is always a problem.

I do like my reed valve saws. And Johnny Cash too.

 

[Mastermind]

I don't know much about reed engines........another thing I need to study.

 

[Chris-PA]

I don't know much about reed engines........another thing I need to study.

Me neither, and I've never had one. I'd like to learn more.

The idea of having an asymmetric intake event is appealing when I think about what happens from a low speed or static point of view, but then it is much more difficult to visualize what is happening with piston porting when you try to account for things like the momentum of the moving air columns. Reed valves have their issues too.

 

[hillwilliam]

Some of the old reed valve saws are pretty gnarly, even by modern standards. BIG Macs, with which I have no experience, would be an example. And some of the bigger Pioneers have a lot of snort after a bit of tinkering. And a 655BP doesn't even need tinkering. Reed valves are archaic and redundant, since piston ported saws can run just dandy, but I like 'em anyway.

 

[Milkman31]

I've still not put it on a saw. I have three jugs sitting here I want to try Dennis. Yours, one with just bridge ports, and the one I posted a pic of.

I need to clone myself.......then I could get some stuff done. Hey Randy #2......get that damn saw together ya slacker.

I've often thought I'd like to clone myself. Then I think it wouldn't last 15min and I would have myself's a$$ whooped.lol!!!!!

 

[jar944]

A reed valve will give you the widest power band, a rotary valve will give you the most peak and the piston port is the most compact.

 

[Terry Syd]

A reed valve will give you the widest power band, a rotary valve will give you the most peak and the piston port is the most compact.

I'll disagree with that statement. A rotary valve engine can be set up for a very broad power band. The power characteristics is mostly determined by the transfer time/area as to where the band is - and a rotary can feed what ever you build.

By opening the rotary valve a few degrees before the transfers close you can help prevent the backflow that occurs back down the transfers when you are lugging the engine - ie: more low end torque. It will also allow the valve to be full open when the piston is hitting max piston speed on the way up - about the time that a piston port engine will just start to open. The rotary feeds the crankcase so well that a closing timing of 65-70 degrees ATDC is all you need to make good top end power.

I think the best system for a chainsaw would be the combination reed/piston port design that was used by Suzuki back in the early 70s. A Dolmar 6100 is a close reproduction of that system and it should be possible to put a boost port in the cylinder behind the reeds. If someone did some development work on a 6100 (different reed cage and reeds, boost port), it could be a real strong running engine.

 

[hillwilliam]

with all that great case compression I think they would respond very well to some extreme porting. In the motorcycle world they responded well to custom reed valves also. Tapered & different materials of construction with different spring rates. Just gives you more to play with. I think a guy could do a lot with carbon fiber reed valves and reed blocks optimized for flow. You won't make the peak power of a modern engine, to much pumping loss but the power band could be exceptional.

I think you're right. I could never do much with the Pioneer reed blocks, but I have two-stage Boyesen reeds on three saws and I think a single-stage carbon fiber on a fourth one. The saw in the avatar has been running the same reeds for over 30 years.

 

[jar944]

Institute of TwoStrokes a écrit:
Frits are you happy to explain about after the exhaust port closing? Torroidal heads, turbulent eddies spin rates etc etc?
Frits Overmars:
I would be happy to explain about a lot of things. But I work 24/7 and the rest of the time I sleep, so there is not much time left to do some explaining.
Combustion in an ideal homogeneous air/fuel-mixture has a flame speed of maybe 2 meters per second. So how is it possible that combustion in a 125 cc engine, running at 13000 rpm, is completed within 50 degrees of crankshaft revolution?
The answer is turbulence. Burning lumps of mixture are blown across the combustion chamber and then simultaneously start their own fires in every corner. The blowing is taken care of by the squish effect that pinches mixture (red on the drawing below) from between the piston (yellow) and the outer area of the cylinder head (blue).
There are programs that calculate the squish velocity, and they are all ********. Why? Because none of these programs take into account that at high rpm the crankshaft bends, the conrod stretches and the piston moves higher by about 1 % of the crankshaft stroke. And it makes a lot of difference for the squish velocity, whether the minimum distance between piston and head is 0,6 mm or 0,06 mm...
The higher the flame speed, the better is is for power and for the thermal load on the engine. So make your squish gap as narrow as possible: 1 % of the stroke. And do not be afraid that the slightly higher compression ratio will cause detonation; effective squish will do just the opposite.

In days of old the ideal combustion chamber was considered to be a sphere with the spark plug in the center, so that the flame path was as short as possible in all directions. And because that was impractial, the next best thing was a semi-sphere. But that is ancient history; we need turbulence!
Toroidal cylinder heads are shaped so that the inital turbulence, created by the scavenging streams of fresh mixture and still present in the cylinder after the transfer ports have closed, is enhanced as the squish effect moves the mixture towards the spark plug. And after that, the toroidal turbulence takes burning lumps of mixtures from the plug area towards all corners of the combustion chamber.