Part One: The Exhaust Port

[Deets066]

I know of a couple 66 work saws that blow that theory.

Me too

 

[RIDE-RED 350r]

How do the power bands differ ?

Think of a drag/hill shooting port job on a 2-stroke dirt bike or wheeler

 

[Deets066]

I'd like to get my mitts on @Definitive Dave's 660 white top and have a peak inside

 

[huskihl]

I know of a couple 66 work saws that blow that theory.

And a few 346 race saws that blow it as well

 

[spencerpaving]

Work saw 066 = 100 on the exhaust

Race saw 066 = 92-94 on exhaust

Close...

 

[spencerpaving]

My 084 I'm working on has 202* of ex dur...it's still a work in progress. ..it runs good but something just ain't quite right yet...think it needs more intake to support the rpm I think it should turn

 

[Deets066]

And a few 346 race saws that blow it as well

346 like a low exhaust

 

[Deets066]

My 084 I'm working on has 202* of ex dur...it's still a work in progress. ..it runs good but something just ain't quite right yet...think it needs more intake to support the rpm I think it should turn

 

[XP_Slinger]

Great thread guys, thanks for firing this up Randy.

Here's how my brain thinks on exhaust height: "torqy" , wide power band saws with low exhaust ports exhibit this kind of power because the piston is being forced down by the expanding combustion gases for a longer period of time before the intertia of the crankshaft takes over the process. The resulting peak hp rpm is gonna be lower but peak power will live in a wider rpm range.
On race saws that deets mentioned: with exhaust ports in the low 90's the combustion push is quicker therefore handing off the rest of the job to crank inertia sooner making its power more "peaky". Another part is those guys are feeding a lot more fuel to their saws. Not sure what their blowdown numbers are but it will be interesting to read when this thread gets to that topic. The other part of racing is tuner pipes, all in all race saws are getting a lot more fuel than our woods ported saws. Again this is all theory in me brain and I've never even touched a race saw.
As far as a port height? I can't answer that, I have to parrot what a few others have said. First look where you're starting taking into account how the saw runs before you mess with it.
Hope what I've said makes sense because now that I've wrote it I'm doubting it...lol!

 

[Stump Shot]

Many times I use the machine work to lower the exhaust port to less duration than it was to start with.

I'm gonna say that there is no replacement for experience.

I've done close to 40 MS661s........and still haven't decided to stick with any one "recipe" yet.

Lots of trail and error in this stuff.

A question with a question.......

If peak power is made at 14,000 RPM, will the saw be able to maintain that RPM in the cut.....under load?

Or........would it be smarter to build the engine so that peak power was at 10,000 so it could be loaded heavier?

I'm going to pick 10,000 RPM. How do I go about finding it? Maybe this is an unanswerable question, I'm just looking for a place to start at, not looking to go straight to the finish line. Also I made a fresh apple pie, this does make it better.

 

[Mastermind]

Ideally you want to be cutting at peak horsepower, but if your power band is very narrow, it will be difficult to keep your saw "on the edge" in the cut. So while you may be able to achieve a higher peak horsepower by porting to achieve top rpm's, it may be more practical to port for a slightly lower rpm but flatter horsepower curve.

Or not - that's why I'm here to see what I should un-learn.

I think you picked up what was laying down perfectly.

Let's talk race saws then, exhaust height is way higher

I don't know a damn thing about race saws. Not that I'm not interested in them, I've just not had any time to tinker on em.

I love threads like this where we can learn...the only problem is I start second guessing my guess's

Well Bret, you build some of the best running saws I've held in my hands. If you start second guessing yourself I'm screwed. lol

 

[Deets066]

I don't know a damn thing about race saws. Not that I'm not interested in them, I've just not had any time to tinker on em.

Me either, but I'm tryin to learn

 

[Mastermind]

I'm going to pick 10,000 RPM. How do I go about finding it? Maybe this is an unanswerable question, I'm just looking for a place to start at, not looking to go straight to the finish line. Also I made a fresh apple pie, this does make it better.

You gotta run the saw stock.......see what it feels like. Is it already sorta high strung, or is it something that you can really lean on?

I think @tree monkey said a lot when he told me that he studied the saws that loggers really held in high regard. Why did those saws run the way they did? How could he make other less desirable saws run that way?

These days I'm sorta locked into a production type of build. I spent a lot of time finding a "recipe" that was not too high strung, nor too torquey. Not too hard to start....and not prone to hang a ring or kill a crankshaft. I'm not looking for the fastest saw anymore, but I still wanna know I can build that mean ass machine when I need too.

BTW.......I love pie.

 

[drf256]

I tried this design based on the old megaphone exhaust on snowmobiles in the early 70s....it works well...but it takes a ton of fuel to feed it...oh and very loud....not good for a work saw...but it gives a good idea

Sent from my SCH-I545 using Tapatalk

Not trying to criticize at all. Just trying to learn.

Wouldn't that roof be considered too flat?

 

[Deets066]

Not trying to criticize at all. Just trying to learn.

Wouldn't that roof be considered too flat?

It's flat for a work saw, but it will make the most power

 

[spencerpaving]

Not trying to criticize at all. Just trying to learn.

Wouldn't that roof be considered too flat?

Good question. ..I don't know...seems fine so far...if the ring ends on the ex side..if would definitely go with a rounder roof

 

[Termitebuffet]

Good question. ..I don't know...seems fine so far...if the ring ends on the ex side..if would definitely go with a rounder roof

I go pretty rounded on mine , I get a little worried I might flatten to much...[emoji2]

 

[Fruecrue]

Good read.

 

[XP_Slinger]

So on exhaust shape. Flat roof is best for power because more of the port is opening at once thus allowing the combustion pressure to force more exhaust out quickly creating kind of a vacuum to pull out the rest? Instead of oval which vents highest combustion pressure through the small gap at the peak of the port when it begins to open. Which doesn't "pull" as much exhaust out with it. Just making sure I'm straight on that. Fully understanding that flat is risky for the rings

 

[Deets066]

This one looks pretty flat, it's one of my better running saws though too