Author Topic: Step in the exhaust port  (Read 541 times)

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Offline Al Smith

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Re: Step in the exhaust port
« Reply #10 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 .

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Re: Step in the exhaust port
« Reply #11 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.
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Offline brokenbudget

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Re: Step in the exhaust port
« Reply #12 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...
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Offline aclarke

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Re: Step in the exhaust port
« Reply #13 on: May 28, 2014, 10:59:52 am »
Where did this idea come from initially? Four stroke tuning? Stepped exhaust pipes?

Offline KilliansRedLeo

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Re: Step in the exhaust port
« Reply #14 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.
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Offline Cut4fun .

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Re: Step in the exhaust port
« Reply #15 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.
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Offline aclarke

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Re: Step in the exhaust port
« Reply #16 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?




Offline KilliansRedLeo

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Re: Step in the exhaust port
« Reply #17 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.
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Offline adam32

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Re: Step in the exhaust port
« Reply #18 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"...

Offline aclarke

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Re: Step in the exhaust port
« Reply #19 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

 

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