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

[srcarr52]

I do run the piston backward......the pins end up on each side of the exhaust.

I used a 272 piston in the last one I did. You give up a little on port width because the skirt is narrower but you're giving it up anyway on the exhaust side to support the rings.

Oh, if only Stihl would have stuck with their original 066 non-decomp port timing. What an easy saw to port.

 

[Chris-PA]

I'm a long way from being an authority on two stroke theory........but I have spent a bunch of time, energy, and money testing what works, VS what doesn't.

For instance, I just ported a few 064s. I did up a "special" jug to test alongside the "normal" ones.......

View attachment 392147

Just gotta grind a little more and those transfers will look like a McCinderblock engine!

 

[foggysail]

OK, I know ziltch about mod'ing a two cycle engine although I have a few questions. The first, why would one even want to? If greater power is required, why not get a saw with greater power?

Along the same thought track, why would the OEM not try to get his product to market with the best bang for the buyer's bucks? And here I will make another assumption and that is the OEM's design engineers weighed trade offs of mechanical strain on things such as bearings, crankshafts, pistons and all the other components that go into a chainsaw or for that matter, any combustion engine.

This reminds me of when I was a kid. A friend changed his measly 100HP Ford V8 for an Oldsmobile V8 engine. Spent months making the conversion along with motor mounts and an adapter plate so the transmission could mount the engine. Great job! Got his work of art out onto the road, burned rubber for the few seconds it took to blow his transmission. It was a never ending battle, fix this, change that, break something else.

Anyway.......just my thoughts--

Foggy

 

[old-cat]

The high performance work saws that most people here build are really pretty mild or conservative compared to some automotive engine modifications.
Also most builders agree that these modded chainsaws need more oil in the gas to live.

 

[Mastermind]

OK, I know ziltch about mod'ing a two cycle engine although I have a few questions. The first, why would one even want to? If greater power is required, why not get a saw with greater power?

Along the same thought track, why would the OEM not try to get his product to market with the best bang for the buyer's bucks? And here I will make another assumption and that is the OEM's design engineers weighed trade offs of mechanical strain on things such as bearings, crankshafts, pistons and all the other components that go into a chainsaw or for that matter, any combustion engine.

This reminds me of when I was a kid. A friend changed his measly 100HP Ford V8 for an Oldsmobile V8 engine. Spent months making the conversion along with motor mounts and an adapter plate so the transmission could mount the engine. Great job! Got his work of art out onto the road, burned rubber for the few seconds it took to blow his transmission. It was a never ending battle, fix this, change that, break something else.

Anyway.......just my thoughts--

Foggy

I agree completely.

These guys are freakin nuts.

 

[Chris-PA]

OK, I know ziltch about mod'ing a two cycle engine although I have a few questions. The first, why would one even want to? If greater power is required, why not get a saw with greater power?

Along the same thought track, why would the OEM not try to get his product to market with the best bang for the buyer's bucks? And here I will make another assumption and that is the OEM's design engineers weighed trade offs of mechanical strain on things such as bearings, crankshafts, pistons and all the other components that go into a chainsaw or for that matter, any combustion engine.

This reminds me of when I was a kid. A friend changed his measly 100HP Ford V8 for an Oldsmobile V8 engine. Spent months making the conversion along with motor mounts and an adapter plate so the transmission could mount the engine. Great job! Got his work of art out onto the road, burned rubber for the few seconds it took to blow his transmission. It was a never ending battle, fix this, change that, break something else.

Anyway.......just my thoughts--

Foggy

Because we can.

 

[woods works]

made me Laugh Out Loud

I agree completely.

These guys are freakin nuts.

 

[komatsuvarna]

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......

yup, it matters just as much as the compression on top of the piston.

...Low pressure down stairs, less charge up stairs and some lazy transfers.

Never understood a huge intake either...the crankcase cant hold but so much, then it spits some back out the intake before the piston closes it off...the longer the intake port is open more than it needs to be, its blowing the charge back out the intake port, making more spitback. Atleast in my mind

 

[Mastermind]

On one hand, I feel like I need to learn to keep my trap shut........but on the other, I wanna see anyone that wants to take the time to look for it to be able to get solid info.

I reckon I'll just keep on sharing what little I've learned.

 

[nmurph]

On one hand, I feel like I need to learn to keep my trap shut........but on the other, I wanna see anyone that wants to take the time to look for it to be able to get solid info.

I reckon I'll just keep on sharing what little I've learned.

Randy, you've revolutionized the sharing of porting info on AS. It was a black art bf you came along where you had to befriend one of the few builders here that would give you dribs and drabs of what they knew, but would never come close to disclosing as much of what they know as you. Most of what you do is an open book. I've been thrown some bones by some, and a couple have shared in depth stuff, but you just ripped the covers off for everyone to see.

Thanks...(and I hope the check is on the way to me)!!!!!

 

[Mastermind]

So much of it was wrong too Neal. That's what got me to start putting it all out there. I'm not sure if it was wrong by design.....or if it was just......well, wrong.




check sent

 

[drf255]

I THINK I degreed my 028 Super.

Lots of reading glasses and flashlights.

Holy crap are those transfers hard to see.

The numbers I got which make little sense to me. Lots of dwell at the mid path.

Exhaust
Opens at 100* ATDC
Starts to dwell at 178*

Intake
Opens at 110* BTDC
Dwells at TDC

Transfers (really tough to see)
Open at 121* ATDC
Dwell at 180*

So what's that mean?

 

[drf255]

Sent from my iPad using Tapatalk

 

[Mastermind]

What do you mean by dwell?

 

[drf255]

Starts to stop moving, but sits there for some degrees of crank rotation

 

[Mastermind]

Well OK....

 

[Chris-PA]

Never understood a huge intake either...the crankcase cant hold but so much, then it spits some back out the intake before the piston closes it off...the longer the intake port is open more than it needs to be, its blowing the charge back out the intake port, making more spitback. Atleast in my mind

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.

 

[blsnelling]

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°.

 

[wcorey]

OK, I know ziltch about mod'ing a two cycle engine although I have a few questions. The first, why would one even want to?

Can’t help you there, it's complicated I guess… If you don’t get it, you just don’t get it.

If greater power is required, why not get a saw with greater power?

For some people, power to weight matters..., to others not so much.

…why would the OEM not try to get his product to market with the best bang for the buyer's bucks? And here I will make another assumption and that is the OEM's design engineers weighed tradeoffs of mechanical strain on things such as bearings, crankshafts, pistons and all the other components that go into a chainsaw or for that matter, any combustion engine

I get the impression that these questions and observations are based primarily from an engineering perspective.

For a more balanced view you need to at least factor in the R&D budget.
Then on to the environmental guys to see what you can get away with there, noise, emissions, safety…
Then there's manufacturing, have to see if it can be built within the budget put down by Accounting.
Then after that there’s Sales, where they tell you to either tone it down or delay it because it competes too closely with the bloated inventory of another existing product that needs to be gone from the shelves first.
Next the Marketing/alchemy guys throw a wrench in all of the above because they envision multiple product cycles that span many years with only small incremental improvements (need to save some for later) and don’t want to start an arms race with the competition that’s not good for any of them…
And of course engineering isn't made aware of half of this until the things almost done.

Finally everyone begrudgingly agrees on a huge mass of compromises and something gets tooled up and built but the thing mysteriously fails in field testing… Then the company’s stock takes an unexpected hit and accounting steps back in… A new CEO with a new vision... a merger, then a buyout.. Get sued for patent infringement... And on and on…


We’re probably lucky to even get the outdated technology that’s available now…

 

[LegDeLimber]

Speaking of reeds: Back in the early 70's, the Yamah pistons had a couple of small~ish holes in the lower area of the skirt on the intake side. The intake tract was shaped/configured so that
on the down stroke, the case charge flowed under and out of the piston.
The intake tract had an area that allowed that back flow to pass around the upper part of the piston and then into the combustion chamber.

Basicaly, anytime there was enough negative pressure to lift the reed petals,
You began to draw in another charge of air/fuel.

This let you get pretty agressive with porting
and not kill the bottom & mid range that made a dirt bike so much manageable in the woods
or hitting a hill with no place at the bottom, for a running start.

But of course you traded a bit of top end breathing/power for this.
Personaly I found it made the power in a hell of a lot better power band
and rpm range for off roading.
You didn't need to ride with one finger on the clutch lever anytime you got out of a straightaway.
You could make creek crossing, without destroying the banks in three passes.

And as could be expexted, Some folks liked to take things a bit beyond the stock openings.
There's a fairly nice cutaway cylinder, about 2/3s down the page here
and you can see all of the bits and begin to see where the air flowed.
http://www.blasterforum.com/threads/porting-an-apex-cylinder-and-cases.60416/
I strongly recommended that you have a detailed talk with some of the experinced ' builders here, before you consider making any changes.

Not sure what's going on with these patent drawings, as Yamaha was definitely building engines
that were ported & reed valved like that, before 1973.
http://patentimages.storage.googleapis.com/pages/US3905340-1.png
http://patentimages.storage.googleapis.com/pages/US3905340-4.png