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Stock header question

Old 02-19-2007, 09:48 AM
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Stock header question

I have a question I can't determine from the searches. I have read about removing the restriction in the exhaust header before. Does anyone know for sure if this improves performance objectively and if it has any implications for rejetting?

I currently have slipons with some other engine mods (HC pistons, stage 1 cams, lightly ported heads) and have the exhaust off for a rear shock rebuild so thought it would be a good chance if genuinely helpful. I figured it might be useful in an engine passing more wind than normal but also realize there are some benefits in back pressures etc that would be hard to predict the outcome - so, looking for some experience. Looking at the restriction it looks simply like a manufacturing issue rather than performance design, but who knows?

thanks

bill
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Old 02-19-2007, 11:49 PM
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Yup, per Bob Hayashida, this is a manufacturing shortcut. If you're trying to decide whether to do it or not, look at it this way...it can't hurt, especially with the mods you have. Though with those mods you're best getting a full system to be honest.
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Old 02-20-2007, 04:04 AM
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I agree, the full system would be an advantage with your set-up.

The best I have seen is an M4, although Moriwaki and Akrapovic make good full systems too. Not sure what is available new any more though, so you may have to search for second hand.
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Old 02-20-2007, 07:37 AM
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thanks guys. that is what I was looking to hear - makes sense from the looks of it. I'd love a full system, but very rare to see used ones around for sale.

bill
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Old 02-20-2007, 08:25 AM
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I questioned Roger Ditchfield at Revolution Racing about the "restriction" and he suggested leaving it in place. He explained at length why it was there and, givne his credentials and knowledge in all things VTR, I chose to follow his advice and have not tampered with it. Now granted that is with a basically stock engine, your case may be different. A full system would definitely help your performance though and allow you to get the ful gains from your mods (did I mention I was envious of them? lol)

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Old 02-20-2007, 11:50 AM
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Originally Posted by mikstr View Post
I questioned Roger Ditchfield at Revolution Racing about the "restriction" and he suggested leaving it in place. He explained at length why it was there and, givne his credentials and knowledge in all things VTR, I chose to follow his advice and have not tampered with it. Now granted that is with a basically stock engine, your case may be different. A full system would definitely help your performance though and allow you to get the ful gains from your mods (did I mention I was envious of them? lol)

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I'd be EXTREMELY curious to hear his theory on this... Perhaps he's thinking it'll affect torque output or something, but man....it's pretty rough in there.
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Old 02-20-2007, 01:34 PM
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It is rough for sure, but when you grind it out, or put on a full system you will find out that the r/h muffler has a lot more gas flow than the left. And I mean a lot.

The Moriwaki street system I have, in between the collector and the r/h mufller, is a restrictor built in to reduce the pipe size. The left side is left full diameter. I can only think of one reason for this, and that is to even up the gas flow between each side.

So Roger may be thinking along these lines.
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Old 02-20-2007, 01:42 PM
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Now that's a good point...I can see one cylinder flowing a lot more than the other playing some havoc with tuning the bike...
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Old 02-20-2007, 02:37 PM
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The plot thickens - very contradictory thoughts on it - usually means none of us knows definitely one way or another. I think I'll leave it as is for now. I have to rejet anyway and if I'm really feeling ambitious while its being dyno'd I'll do a pre-post drilling out the restriction run.

Unless, of course anyone know where I can buy a good full system compatible with the mild engine mods noted.

bill
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Old 02-20-2007, 02:42 PM
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I could really test this as I have 2 sets of stock pipes....but one is dented badly on the bottom and it'd cost me more cash than it's worth to try and fix it....shame I'm not a machinist or welder, it really just needs to be cut and have a new piece of pipe welded in. :/
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Old 02-20-2007, 05:14 PM
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Hi again,

from memory, you're pretty close there Shayne. Roger mentioned that some early full systems (by, I beleive Two Brothers, and one other he mentioned) had failed to take this very fact into account and later systems were redesigned to take this flow differential into account.

If you really must know the actual reply, you can always e-mail him, he is a very helpful fellow

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Old 02-20-2007, 07:57 PM
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Well, it seems we have two highly experienced tuners (Ditchfield and Hayashida) saying different things. Unless we can get both of them on here to argue the point I suggest we agree to disagree. As they say, YMMV!

But FWIW (and my attempt at the last word), Moriwaki never made a slip on system for our bikes. It's my hypothesis that they only made a full system because the stock header is such a POS.
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Old 02-22-2007, 03:55 AM
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You would think that Moriwaki would know though wouldn't you?

When I discovered the set-up, I used the baffles in my Staintune mufflers to balance each side by modifying each one until they flowed evenly ie more restrictive on the right. Now each side flows close to the same gas, at idle anyway.

Who knows whether it was worth the effort or not?

Gave me something to do I guess!!
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Old 04-27-2007, 07:58 AM
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I relaize this is a bit late in the game but here is the reply/reasoning provided by Sir Roger in regards to the exhaust restriction (copied from vtr1000.org site):
> > ----- Original Message -----
> > From: "revolution racing" <[email protected]>
> > To: <[email protected]com>
> > Sent: Thursday, October 06, 2005 9:32 AM
> > Subject: Re: [VTR_List] Stock header clean up
> >
> >
> > The restrictor in the standard headers is there to
> > assist the pulsing which
> > helps to scoure the front cylinder and to create a
> > turbulence vacumn thus
> > accelerating the gas speed at that point above certain
> > revs. If this is
> > removed the left hand exhaust can will discharge a
> > greater proportion of the
> > gas emmisions.as did the first Akropovic and Two
> > Brothers full exhausts did,
> > until they were modified, to the detriment of the
> > engine performance. Later
> > full systems were designed to overcome this effect by
> > changes in their
> > length, merge angles, tapers and taper length.
> > Honda, Moriwaki, Akropovic and others could have
> > saved themselves
> > millions in R&D if they had only listened to some AU
> > guy and cut away the
> > "excess" to improve power output !!!!
> >
> > Regards
> > Roger

cheers
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