Chainsaw Repair

How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: KilliansRedLeo on May 25, 2014, 12:19:01 pm


Title: Step in the exhaust port
Post by: KilliansRedLeo 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.
Title: Re: Step in the exhaust port
Post by: farmboy 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.
Shep
Title: Re: Step in the exhaust port
Post by: Al Smith 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 .
Title: Re: Step in the exhaust port
Post by: KilliansRedLeo 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.
Title: Re: Step in the exhaust port
Post by: rms61moparman 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!!

Mike
Title: Re: Step in the exhaust port
Post by: brokenbudget on May 26, 2014, 10:46:06 am
even the lowly jonny 2150/husky 350 has one ;)
(http://i58.tinypic.com/jb0lc9.jpg)

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.
Title: Re: Step in the exhaust port
Post by: KilliansRedLeo 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!
Title: Re: Step in the exhaust port
Post by: srcarr52 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.
Title: Re: Step in the exhaust port
Post by: KilliansRedLeo 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.
Title: Re: Step in the exhaust port
Post by: srcarr52 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.
Title: Re: Step in the exhaust port
Post by: Al Smith on May 27, 2014, 10:50:25 pm
I'm not certain exactly what that photo is trying to depict .It's carboned up which probabley is typical .If you're going to err,mo-rich is the side you want to be on .

Oh you could run a tad richer on the oil and maybe get by a tad lean with no problem .It probabley wouldn't pull worth a hoot though .Then again it might run like crazy .It's nearly impossible to trouble shoot or analize over the internet .
Title: Re: Step in the exhaust port
Post by: 3000 FPS on May 28, 2014, 12:03:01 am
I have to say I agree with Shaun on this.   A box muffler is way to short and to small to get the timing correct for the sonic waves to stuff any fuel mix back in to the cylinder.   If you can only open the port just so much before you hang a ring then putting a step in the exhaust port that will allow the exhaust gases to expand quicker will increase flow.  Unless I am missing something here.
Title: Re: Step in the exhaust port
Post by: brokenbudget on May 28, 2014, 04:40:56 am
I'm not certain exactly what that photo is trying to depict .It's carboned up which probabley is typical .If you're going to err,mo-rich is the side you want to be on .

Oh you could run a tad richer on the oil and maybe get by a tad lean with no problem .It probabley wouldn't pull worth a hoot though .Then again it might run like crazy .It's nearly impossible to trouble shoot or analize over the internet .

al you can see in the pic that there is a 'step' from the opening at the cylinder wall to the bottom of the exhaust port. the scratched line is the bottom of the step. ;) has nothing to do with oil content...
Title: Re: Step in the exhaust port
Post by: aclarke on May 28, 2014, 10:59:52 am
Where did this idea come from initially? Four stroke tuning? Stepped exhaust pipes?
Title: Re: Step in the exhaust port
Post by: KilliansRedLeo on May 28, 2014, 11:40:39 am
That is where I first came across it. Lots of use in Harley drag bikes, usually done in the exhaust pipe itself. The term 'anti-reversion step' is what the 4-stroke guys refer to it as.
Title: Re: Step in the exhaust port
Post by: Cut4fun . on May 28, 2014, 01:46:21 pm
Adam I have forgot all of this stuff. But I recall something about Ace Morgan doing this many years ago?   

This is the sad part of the 1st Racesaws site being burned down. All of that info was there already.
Title: Re: Step in the exhaust port
Post by: aclarke on May 28, 2014, 05:06:47 pm
kev,
I remember reading that year's ago,  think it was RS site
I understand the stepped header theory and it's effectiveness when properly engineered. I'm still having a hard time grasping how the stepped cut works though.  Pressure drop in excess of what a normal exhaust nozzle taper? Or some other dynamic?



Title: Re: Step in the exhaust port
Post by: KilliansRedLeo on May 28, 2014, 05:32:49 pm
Adam, that is the way it appears to me. Perhaps as Randy mention in another post somewhere, he was experimenting with getting more performance while keeping the noise down. Another dynamic might be that the additional drop in pressure has an effect on mufflers that are particularly restrictive. Just a WAG, but something to think about, if you look at the saw models that Farmboy mentioned in his post.

OBTW, had a shot cylinder that I tried to grind the step into, across the bottom is not too bad but up the sides and across the top is a huge **** especially if you are trying to keep all sides square to each other as well as to the exhaust exhaust nozzle. In the 4-stroke motorcycle world the face of the step had to be at 90° to the flow. Had no plan in mind just trying to see how hard it is to do......I won't be trying it again.
Title: Re: Step in the exhaust port
Post by: adam32 on May 28, 2014, 05:33:55 pm
Or some other dynamic?


My guess it bad machining that they had to come up with an excuse for..."oh it's for flow, makes the saw run better"...
Title: Re: Step in the exhaust port
Post by: aclarke on May 28, 2014, 05:52:47 pm
Or an over-run of mufflers with a larger flange!!! Lol.  I worked on a P100 that had a 1/4" grove cast into the top of the exhaust port roof/cylinder plating. It was oem, never learned the reason why Partner did this but is suspect it was to start the  blowdown event earlier while still maintaining the torque associated with low exhaust height...
Adam
Title: Re: Step in the exhaust port
Post by: Cut4fun . on May 28, 2014, 05:56:13 pm
I worked on a P100 that had a 1/4" grove cast into the top of the exhaust port roof/cylinder plating. It was oem, never learned the reason why Partner did this but is suspect it was to start the  blowdown event earlier while still maintaining the torque associated with low exhaust height...
Adam

I swear I remember some folks trying that on racesaw builds too.
Title: Re: Step in the exhaust port
Post by: aclarke on May 28, 2014, 05:59:05 pm
Tim, might be worth testing this stepped port theory in the near future with some AM jugs? I'd be game on contributing a cylinder or two plus the machining if any other members would be interested in doing the work involved with swapping jugs and testing. Maybe try a few other ideas with the same jugs...
Title: Re: Step in the exhaust port
Post by: 1manband on May 28, 2014, 07:13:14 pm
panheads also ironheads to '86, had them as cast from the factory.... the exhaust pipes, were clamped on the outside of the exh spigot of the head. 

-joe
Title: Re: Step in the exhaust port
Post by: sharkey on May 29, 2014, 07:39:29 am
Here is an interesting article that explains the process.  http://www.circletrack.com/enginetech/ctrp_1008_intake_reversion/
Title: Re: Step in the exhaust port
Post by: 1manband on May 29, 2014, 04:43:05 pm
Here is an interesting article that explains the process.  http://www.circletrack.com/enginetech/ctrp_1008_intake_reversion/

good read.   

in the older bikes....exactly that.

there used to be an aftermarket part, made for evo's that claimed to prevent this.  cone shaped thing....placed into the exhaust pipe.  thing is, evo's did not have that problem.

-joe 
Title: Re: Step in the exhaust port
Post by: mdavlee . on May 31, 2014, 12:04:15 am
There's a west coast shop that puts them in work saws also. Not sure if they really help or hurt. I've not done any yet in any saws. Figured I don't need that last 2-5% for cutting firewood.
Title: Re: Step in the exhaust port
Post by: KilliansRedLeo on May 31, 2014, 04:39:32 am
Tim, might be worth testing this stepped port theory in the near future with some AM jugs? I'd be game on contributing a cylinder or two plus the machining if any other members would be interested in doing the work involved with swapping jugs and testing. Maybe try a few other ideas with the same jugs...

Adam, I'm game for that! I was just toying with another idea, and I have a saw ready for a top end. PM me with a number and I will give you a call to discuss. I can also supply a couple of AM jugs.
Title: Re: Step in the exhaust port
Post by: farmboy on May 31, 2014, 04:39:33 pm
I cut one in a Husky 55, widened and polished w/DP muffler.  Runs good but haven't run a stock one to see difference. 
Shep
Title: Re: Step in the exhaust port
Post by: KilliansRedLeo on May 31, 2014, 05:08:24 pm
I'm thinking about something else to try also, if I can figure out how to do it. I would like to try a pair of crankcase stuffers in a 365/72 saw. Sort of like the 575/76 have. Anyone have any ideas?
Title: Re: Step in the exhaust port
Post by: aclarke on May 31, 2014, 07:46:53 pm
Didn't husky add a plastic stuffer onto one of the 3 series Saw crankshafts?    You can also machine a ring that presses onto each crank half and fill the "voids"  between the Bell and Big end pin with epoxy.  Marcel Vincent and others have used this method on race saws and it works well. 

I think the 372 would respond well to this but the port timing might have to be altered slightly to deal with the higher crankcase pressure.

Title: Re: Step in the exhaust port
Post by: farmboy on May 31, 2014, 09:27:37 pm
357 XP has stuffers.
Shep
Title: Re: Step in the exhaust port
Post by: 3000 FPS on June 01, 2014, 12:27:49 am
I have never messed with stuffers before, but I have seen them before when working on a Husky cut off saw.
   I am curious as to what the advantage is.   I realize that your reducing the volume in the crankcase and in theory this gives more pressure in the crankcase when the piston is traveling down.   
But does it not also reduce how much air fuel mixture that will enter the crankcase when the piston is traveling up. 
If so it sounds like a trade off to me.
Title: Re: Step in the exhaust port
Post by: aclarke on June 02, 2014, 07:00:24 pm
I don't think a muffler motor is gonna have the ability to fill the crankcase and take advantage of the extra volume like a motor with tuned pipe, so smaller may pump more efficiently granted the ports are designed to keep the added flow from going out the exhaust . Lots of conflicting info about case volume and pumping efficiency online, hard to make sense of what's optimum
Title: Re: Step in the exhaust port
Post by: SquareFile on November 20, 2018, 09:10:25 am
Older thread but the topic interested me studying cylinders. I feel the step down is more of a "shelf" for ring support on a downward angled port. Remove it and I'm sure bad things are going to happen. JMO, it's not for performance, tho it could aid. I have no proof it does or does not.
Title: Re: Step in the exhaust port
Post by: Cut4fun . on November 20, 2018, 09:57:32 am
Blast from the past.

Funny thing is I noticed a Partner I got in has that T port deal that Adam had talked about on the P100 a page or so back.  On the P55 they said it was for this reason.   Features built in decompression system located at exhaust port. Named "T Port" system.
Title: Re: Step in the exhaust port
Post by: mdavlee . on November 20, 2018, 07:18:13 pm
Older thread but the topic interested me studying cylinders. I feel the step down is more of a "shelf" for ring support on a downward angled port. Remove it and I'm sure bad things are going to happen. JMO, it's not for performance, tho it could aid. I have no proof it does or does not.
It sure can’t hurt to leave it there when you raise the exhaust a lot. Some guys have blended it on the 7910 cylinders and they free port with as little as .030” off the base.
Title: Re: Step in the exhaust port
Post by: Al Smith on November 28, 2018, 04:34:07 am
This case volume reduction thing could get rather complex .Full circle cranks,more stuffer would raise the crankcase pressure which would increase the velocity  of transfer .Theoretically with decreased volume by lowering the cylinder to raise compression it might not require as much actual crankcase volume .As to  how much remains a good question .With a tuned pipe it's going to back charge part of it back in .With a pressure can it's going to stop the transfer when the pressure rise equals the exhaust out pressure .With a megaphone or open exhaust it would all leak out ---makes my head hurt thinking about it -----