Part One: The Exhaust Port

[Mastermind]

Question; Lets say we have new saw that has no history and unfamiliar design as compared to what you're used to. Where to start... Mention was made that the setting of the squish band to it's desired clearance after widening the squish band to gain desired compression will yield the desired exhaust port height in degrees of crankshaft rotation. So, are we generally trying to return to our original height before other alterations were made, or are we also trying to improve this from the original factory height? So if originally the number of degrees is close to your chart at the beginning of this thread stay with that, if not close to the chart shoot for what's on the chart? So say the cylinder was dropped .040", do I want to gain that .040" back in the exhaust port automatically? Or what other factors come into play to a different decision? In other words, how do we go about picking this starting number to shoot for for exhaust port height. Also how close should we be + or - to be in the ballpark.

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.

 

[Mastermind]

Sorry if it sounds like I have a thousand questions, it's only because I really have 10 thousand questions. Lol

Question; So if raising the exhaust port height, raises the RPM of the peak power band, while also lessening the amount of trapped fuel/air mixture. Are we really shooting for this peak part of the power band, and if so, if gone past, will this seriously diminish power because of the lost trapped air/fuel mixture, or is it because the peak power band is now unobtainable or both? In the same respect the act of dropping the exhaust like in a gasket delete or base cutting, lower the peak power band and increase grunt or torque by having a larger air fuel charge to burn? This is seaming like a more serious decision for the application of the power head if any type of success is to be achieved.
Please bear with me guys, I'm almost there on "getting it", I can feel it. Lol

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?

 

[RIDE-RED 350r]

Yes there is a point where you can raise the exhaust too high and start going backwards. And yes, generally speaking higher ex is peak power and higher rpm where lower is more torquey. Take Rotax variable exhaust for example. At low rpm the exhaust guillotine is on the closed position for a lower ex roof thereby making for more torquey and quicker throttle response down low. At higher rpm (which is adjustable by different springs and spring preload) the guillotine raises to the open position raising the exhaust roof making more power up top.

 

[CJ Brown]

I use transfer height and desired blowdown more to set exhaust height any more. Some of my stuff is in the 170-176° of duration on exhaust. Some with the high exhaust pull better than low exhaust.

So if your saw has 170 degrees of exhaust duration, it's timing would be 95 degrees - correct?

 

[mdavlee]

So if your saw has 170 degrees of exhaust duration, it's timing would be 95 degrees - correct?

Yes.

 

[Termitebuffet]

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.


Lots of trail and error in this stuff.

I'm with Randy allot of times on a new saw I'm not familiar with , I'll sneak up on numbers slowly committing to a pattern as I see how the saw responds ...sometimes I get tired of taking them apart ....lol all in the name of research [emoji2]

 

[CJ Brown]

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?

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.

 

[Termitebuffet]

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 like a wider PowerBand, imho most work saws need some width , peaky saws kinda make me crazy ....but hey its different strokes for different folks , the feel is all subjective ...[emoji2]

 

[Johnmn]

So.......we really haven't found any real solid answers to the question. "What port heights should I use?"

OK........what about width?

Shape?

Outlet size?

Finish?

I think port height will never be a solid number. To me it would depend what kind of saw your building a torque monster or a fast light handed saw. I don't know much but that's my opinion

 

[huskihl]

I took .065" with the gasket out of my 346 to get the exhaust down. It opens at 107 now. Plenty snappy for square or round trees. I like a flat torquey power curve.

 

[Deets066]

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

 

[RIDE-RED 350r]

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

Please define "way higher".

 

[spencerpaving]

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

 

[RIDE-RED 350r]

This thread is giving me a better understanding of reed valves and the hows and whys of their existence

 

[Deets066]

This thread is giving me a better understanding of reed valves and the hows and whys of their existence

Atomize fuel and stop spitback

 

[Deets066]

Please define "way higher".

Work saw 066 = 100 on the exhaust

Race saw 066 = 92-94 on exhaust

 

[Redfin]

This thread is giving me a better understanding of reed valves and the hows and whys of their existence

Intakes are the next session.

 

[Deets066]

Work saw 066 = 100 on the exhaust

Race saw 066 = 92-94 on exhaust

I don't think this applies in all cases though

 

[Termitebuffet]

Work saw 066 = 100 on the exhaust

Race saw 066 = 92-94 on exhaust

How do the power bands differ ?

 

[Redfin]

Race saw 066 = 92-94 on exhaust

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