DIR Return Create A Forum - Home
---------------------------------------------------------
Chainsaw Repair
HTML https://chainsawrepair.createaforum.com
---------------------------------------------------------
*****************************************************
DIR Return to: How To Basics and Fixes
*****************************************************
#Post#: 40596--------------------------------------------------
Step in the exhaust port
By: KilliansRedLeo Date: May 25, 2014, 11:19 am
---------------------------------------------------------
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.
#Post#: 40608--------------------------------------------------
Re: Step in the exhaust port
By: farmboy Date: May 25, 2014, 3:42 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.
Shep
#Post#: 40620--------------------------------------------------
Re: Step in the exhaust port
By: Al Smith Date: May 26, 2014, 2:44 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 .
#Post#: 40622--------------------------------------------------
Re: Step in the exhaust port
By: KilliansRedLeo Date: May 26, 2014, 7:39 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.
#Post#: 40624--------------------------------------------------
Re: Step in the exhaust port
By: rms61moparman Date: May 26, 2014, 9:06 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!!
Mike
#Post#: 40626--------------------------------------------------
Re: Step in the exhaust port
By: brokenbudget Date: May 26, 2014, 9:46 am
---------------------------------------------------------
even the lowly jonny 2150/husky 350 has one ;)
[img width=700
height=525]
HTML http://i58.tinypic.com/jb0lc9.jpg[/img]
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.
#Post#: 40627--------------------------------------------------
Re: Step in the exhaust port
By: KilliansRedLeo Date: May 26, 2014, 10:13 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!
#Post#: 40631--------------------------------------------------
Re: Step in the exhaust port
By: srcarr52 Date: May 26, 2014, 11:23 am
---------------------------------------------------------
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.
#Post#: 40633--------------------------------------------------
Re: Step in the exhaust port
By: KilliansRedLeo Date: May 26, 2014, 11:36 am
---------------------------------------------------------
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.
#Post#: 40644--------------------------------------------------
Re: Step in the exhaust port
By: srcarr52 Date: May 26, 2014, 2:03 pm
---------------------------------------------------------
[quote author=KilliansRedLeo link=topic=3591.msg40633#msg40633
date=1401122161]
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.
[/quote]
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.
*****************************************************
DIR Next Page