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

[drf255]

So I read Jennings 2 stroke engine book.

My interests are woods porting for personal use and friends only.

Please correct me if I'm wrong in what I gleaned from it.

Too much compression puts thermal and pressure load on engine and components and can reduce top end power from pumping losses.

Squish should be held to a minimum to burn more of the fuel mixture instead of losing what's in the squish area unburned into the exhaust.

Raising the height of transfer ports is generally a good thing and the swirl effects should keep the fresh mix out of the exhaust.

Lowering the intake port/increasing intake duration will help top end power, but will make starting more difficult.

Widening the exhaust port too much will wear out the piston rings faster or damage them. Increasing exhaust duration won't help power output much.

Some type of expansion chamber past the exhaust will help power output.

So what's incorrect in my conclusions?

 

[old-cat]

Sounds to me like your on the fast track to making power!

 

[weimedog]

So I read Jennings 2 stroke engine book.

My interests are woods porting for personal use and friends only.

Please correct me if I'm wrong in what I gleaned from it.

Too much compression puts thermal and pressure load on engine and components and can reduce top end power from pumping losses.

Squish should be held to a minimum to burn more of the fuel mixture instead of losing what's in the squish area unburned into the exhaust.

Raising the height of transfer ports is generally a good thing and the swirl effects should keep the fresh mix out of the exhaust.

Lowering the intake port/increasing intake duration will help top end power, but will make starting more difficult.

Widening the exhaust port too much will wear out the piston rings faster or damage them. Increasing exhaust duration won't help power output much.

Some type of expansion chamber past the exhaust will help power output.

So what's incorrect in my conclusions?

Of course those changes are all relative to a starting point... and the trick is figuring out for a given design which way to push the numbers to change the characteristics more to what you want. IE. if a saw/cylinder already has nice machining/surface finish on the squish band, the squish is tight enough, and that piston /cylinder has compression numbers where you want do you really want to push for "more"? That's where the good "pro" builders earn their keep... through experience they (hopefully) have found those points of diminishing returns based on what change in characteristics you want.. (And the skills to produce them)

 

[wigglesworth]

Read bells book next....

http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf


Then go back and read em both again.


Everytime I read em, I find something I missed. But keep in mind, all the things that they've written pertains to piped two strokes. But lots of good stuff....

 

[drf255]

Bell's Book is much more intricate and confusing.

Im gonna attempt a muffler mod on my 028 Super. As per the book, I need an expansion pipe 75mm from the piston. The exhaust port is already 25mm long. Im gonna try some type of horn inside the muffler case. Will likely extend the body of the muffler over the case.

 

[rmh3481]

What about time area calculations and squish velocity?

 

[jar944]

Read bells book next....

http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf


Then go back and read em both again.


Everytime I read em, I find something I missed. But keep in mind, all the things that they've written pertains to piped two strokes. But lots of good stuff....

Then read parts 1-4 of the Frits Overmars/Jan Thiel thread on pit lane (the guys behind the 52hp aprilia RSA 125) and see Where Blair, Jennings and Bell were right or wrong in their theory's and assumptions.
http://www.pit-lane.biz/t117-gp125-...jan-thiel-and-mr-frits-overmars-part-1-locked (its starts off in french but most is in English)

 

[jar944]

Bell's Book is much more intricate and confusing.

Im gonna attempt a muffler mod on my 028 Super. As per the book, I need an expansion pipe 75mm from the piston. The exhaust port is already 25mm long. Im gonna try some type of horn inside the muffler case. Will likely extend the body of the muffler over the case.

You need more (a lot more) exhaust duration than a saw comes with to take advantage of a tuned pipe

I'm also not following your muffler idea?

This is Jennings formula:

 

[drf255]

Im only gonna do the megaphone part (on the RIGHT side of your diagram) and try to fit it inside the muffler case.

Will post pics.

I know it may overscavenge the cylinder, but with the backpressure of the case the effect may not be too much if at all.

 

[jar944]

Im only gonna do the megaphone part (on the RIGHT side of your diagram) and try to fit it inside the muffler case.

Will post pics.

I know it may overscavenge the cylinder, but with the backpressure of the case the effect may not be too much if at all.

I think you are miscalculating the length of the straight section+ divergent cone (I assume you mean you are going to make L3 and L4)

also other that sucking out raw fuel, its not going to make any more power than stock.

 

[wigglesworth]

also other that sucking out raw fuel, its not going to make any more power than stock.

Oh, you've tried it?

How much fuel did yours spit out?

Was it obvious, like a mist, or more like just extra fuel smell?

 

[wigglesworth]

Im only gonna do the megaphone part (on the RIGHT side of your diagram) and try to fit it inside the muffler case.

Will post pics.

I know it may overscavenge the cylinder, but with the backpressure of the case the effect may not be too much if at all.

Honestly, if your gonna go that far, might as well finish the rest of the pipe.

Otherwise, I'd just open the stock hole and be happy.

Unless your just looking to kill some time. Lol.

Then read parts 1-4 of the Frits Overmars/Jan Thiel thread on pit lane (the guys behind the 52hp aprilia RSA 125) and see Where Blair, Jennings and Bell were right or wrong in their theory's and assumptions.
http://www.pit-lane.biz/t117-gp125-...jan-thiel-and-mr-frits-overmars-part-1-locked (its starts off in french but most is in English)

I can't get the link to work. I'm all ears though. If a man quits learning, he's in trouble...

 

[blsnelling]

Too much compression puts thermal and pressure load on engine and components and can reduce top end power from pumping losses.

I'm not a fan of super high compression. I like to keep mine under 200 PSI. My customers tend to agree with this.

Raising the height of transfer ports is generally a good thing and the swirl effects should keep the fresh mix out of the exhaust.

I like a little more transfer timing than some. It takes more fuel to feed at higher RPMs.

Lowering the intake port/increasing intake duration will help top end power, but will make starting more difficult.

I've had no issue with that, even as low as 85°. I like a little more intake than others. Again, it takes fuel to feed this higher RPMs. Some think that it makes a fuel pig out of a saw. Again, my customers disagree, finding that they very more wood on the ground than with a stock saw and the same amount of fuel.

Widening the exhaust port too much will wear out the piston rings faster or damage them. Increasing exhaust duration won't help power output much.

An exhaust port that is 70% of the bore is considered the safe maximum. I will sometimes back down to 60-65% on larger saws, just for an add safety factor. Ring wear has not proven to be an issue.

 

[jar944]

I can't get the link to work. I'm all ears though. If a man quits learning, he's in trouble...

Its working for me?

 

[Mastermind]

You need a popup.

 

[Hinerman]

^^^^

 

[Mastermind]

Keep in mind that all these books that have been mentioned deal with an engine with a longer stroke, bigger carb, and in most cases a tuned pipe.

 

[Mastermind]

Another thing to keep in mind.......if you ain't wanting to build a turd. Case compression. Some here will say it matters not.

But I redo the port work in a lot of saws......

 

[Moparmyway]

Another thing to keep in mind.......if you ain't wanting to build a turd. Case compression. Some here will say it matters not.

But I redo the port work in a lot of saws......

Case compression ................ what about it ? Can you elaborate ?

 

[Mastermind]

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.