Author Topic: Step in the exhaust port  (Read 584 times)

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Offline KilliansRedLeo

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Step in the exhaust port
« on: May 25, 2014, 12:19:01 pm »
I have read in a few places about porters grinding a step in the exhaust port. Can anyone explain what the advantage of doing this is supposed to be?

Read a post on Ass Hat where even RE couldn't say definitively that the step did anything to improve performance and they are a 'hell of a lot of work'. I can see it in a four-stroke race/drag engine but that is a whole different ball game.
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Offline farmboy

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Re: Step in the exhaust port
« Reply #1 on: May 25, 2014, 04:42:29 pm »
My understanding is an anti-reversion step.  I don't really know for sure.  Randy (MM) is cutting them on some of his ported saws.  359 has one and some Jred models.  535 IIRC there are probably more.
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Offline Al Smith

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Re: Step in the exhaust port
« Reply #2 on: May 26, 2014, 03:44:38 am »
I'm not certain what "step " is refering to .

Now it could be on the order of a boost port whatever that's supposed to do .I'm not so certain some modifications that might apply to dirt bikes really benefit saw engines .

Certain little tweeks and twitters that for example might improve mid engine speed acceleration on a dirt bike wouldn't do a thing for a saw engine that essentually only uses two speeds .Namely idle and wide open for all intents .

Offline KilliansRedLeo

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Re: Step in the exhaust port
« Reply #3 on: May 26, 2014, 08:39:44 am »
Anti-reversion step is the correct term. It is common on four strokes but in a two stroke my thought is that you need some amount of reversion in order to keep from drawing too much of the incoming charge out the exhaust. All that said, IMO Al is correct in that not everything in the two stroke universe applies to a chainsaw.
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

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Offline rms61moparman

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Re: Step in the exhaust port
« Reply #4 on: May 26, 2014, 10:06:35 am »

You can bet a steak dinner that if Randy is going to the trouble of putting them in exhaust ports, there is some benefit to it.
He is WAY too smart (and busy) to be wasting time for no gain!!!
BELIEVE THAT!!

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Offline brokenbudget

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Re: Step in the exhaust port
« Reply #5 on: May 26, 2014, 10:46:06 am »
even the lowly jonny 2150/husky 350 has one ;)


do they work on all 2 strokes? don't know for sure but I'd imagine if it was set in stone every engine manufacturer would be doing it. and it would solidly depend on porting.
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Offline KilliansRedLeo

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Re: Step in the exhaust port
« Reply #6 on: May 26, 2014, 11:13:31 am »
I agree with both of you, RE knows what he is doing! The trick as Brokenbudget says is to KNOW or be able to determine which saws it will make a difference on. As of yet no one has taught this 'old dog' that trick!
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

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Online srcarr52

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Re: Step in the exhaust port
« Reply #7 on: May 26, 2014, 12:23:30 pm »
In a two stroke it's more of an expansion step.  You're letting the exhaust gases expand quickly instead of gradually through a normal tapered port.

Offline KilliansRedLeo

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Re: Step in the exhaust port
« Reply #8 on: May 26, 2014, 12:36:01 pm »
Thus creating the first cone in an expansion chamber. Then perhaps the box muffler would be the center section of the chamber and the tuned pipe would function as the stinger. Only missing the reverse cone of the chamber. If there was a way to get a cone on the inside end of the pipe you would have all three parts of an expansion chamber.

First cone would help pull the expended charge out of the jug, box would continue the evacuation of the spent charge, reverse cone would try to stuff whatever portion of the new charge that was pulled out back into the jug and the stinger would hold it there until exhaust port closed.
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

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Online srcarr52

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Re: Step in the exhaust port
« Reply #9 on: May 26, 2014, 03:03:44 pm »
Thus creating the first cone in an expansion chamber. Then perhaps the box muffler would be the center section of the chamber and the tuned pipe would function as the stinger. Only missing the reverse cone of the chamber. If there was a way to get a cone on the inside end of the pipe you would have all three parts of an expansion chamber.

First cone would help pull the expended charge out of the jug, box would continue the evacuation of the spent charge, reverse cone would try to stuff whatever portion of the new charge that was pulled out back into the jug and the stinger would hold it there until exhaust port closed.



This is way to short to work like a normal pipe off the sound pressure waves.  I suspect it's more about keeping laminar flow or at least some order to the flow pattern when the gasses are still trying to rapidly expand.  Also, once the ex port is open you are no longer trapping the gasses but things are still burning so you have two choices, let the gasses expand or let the temperature increase.  PV=nRT, somethings got to give.

 

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