Exhaust opening - how does it affect the performance?

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Buber
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Exhaust opening - how does it affect the performance?

Postby Buber » Thu Apr 24, 2008 12:51 am

So, as I can't keep my hands off my bike... :lol: I had a normal exhaust. Just for fun (and not to be "outgunned" by my wife's Virago :twisted: ) I drilled couple of holes. All was well. Then, after I changed the camshaft, it went louder, so I closed the holes. And I noticed, that on the idle it was much more prone to dying. So, to take the experiment further, I took the last baffle off for today, and ... surprise! It went at extremely low idle when cold - I don't know, maybe 500?, but didn't die.
So - what's your input on that? I mean, I'm totally not after "roaring" sound - actually I would preferr it to just "purr", but if there's technical gain in it, then I'm all for it.
Is really "restrictiveness" of the exhaust so visible? What about low, and mid-range power, that is supposed to be better with little restriction?

Just stirring the pot... :D

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Re: Exhaust opening - how does it affect the performance?

Postby Volker_P » Thu Apr 24, 2008 1:25 am

I think smooth running when still cold for a CB 650 mainly depends on how many pots are actually running well. Factors like temperature, state of plugs, carb synchronization, charge of battery and so on play a role for that. I would not have thought of the exhaust for idle, however as the CB650 has the reflection type, it might have a visible effect here, too. As a restriction also can affect the flow characteristics, an "open" exhaust that helps to get away the gas at full rpm and provides some extra power there might not be advantageous for mid range power. You may also see an impact on fuel consumption. In former times, Honda usually made a good job optimizing the exhaust in this sense.
Only modern bikes which suffer restrictions from environmental laws can be expected to just run better in all ranges with "less exhaust".
As you already play around somewhere else, what about the pinking/camshaft topic?
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Buber
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Re: Exhaust opening - how does it affect the performance?

Postby Buber » Thu Apr 24, 2008 5:49 am

Weekend will see change of tires and a longer ride (300kms) so I will be able to tell something more. On the other hand this will lean my mixture more, so engine should be hotter, so pinking.... Anyway I will see and report.
Any ideas about main jets for CV carbs? Now I have 98, and plugs look more whitish than brown....

As per exhaust influence - I'm wondering myself, but facts stand - with the last element, bike would die on idle, without goes steady. Ah - and the exhaust is "era replica" looks the same, but some italian brand.

And I went back to 50hp camshaft - realy smoother running, and (as you said yourself in other post) power difference is there, but only slight. And 1 more thread to helicoil... :evil:

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Re: Exhaust opening - how does it affect the performance?

Postby Pinhead » Thu May 08, 2008 9:54 pm

When I removed the rear baffle from my '79, it got quite a bit better mileage and low-to-mid horsepower. When I opened the exhaust, I also opened the airbox so the combination of the two could be a factor.

The rear baffle outlet hole is only about 1-1/2 inches across... I can't imagine that would be able to effectively exhaust all of the exhaust (lol). Especially considering each exhaust port and header have a larger CSA.

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Re: Exhaust opening - how does it affect the performance?

Postby chucksklrst » Thu May 08, 2008 10:03 pm

"and the exhaust is "era replica" looks the same, but some italian brand."

Buber, Can you send me a picture and where it can be gotten. I am going to have to replace the exhaust on my 79 US model in the future, so I am doing the research now. Thanks
Chuck Henderson
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10 NT700 (Now & Then)
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03 ST1300 (Silver STorm)
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86 VT1100 Shadow (Yesterday's Shadows) Sold
79 CB650 (Chasing Rainbows) Sold

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Buber
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Re: Exhaust opening - how does it affect the performance?

Postby Buber » Fri May 09, 2008 2:32 am

Now you got me... :(
it looks like that:
www.bikepics.com/members/buber

make and where to find it... I will throw myself to the ground and see what it written underneath - I will let you knwo my findings...

BTW - I closed it up, closed the extra holes I did in the emulsifier tubes in thh carbs (and couple mor for goos measure) and mileage... went up cosiderably! Could it be because of CV carbs?

Still testing, though....

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Re: Exhaust opening - how does it affect the performance?

Postby Volker_P » Fri May 09, 2008 4:51 am

Sito made such pots, I got a Z-model some time ago having them mounted but I sold them already.

Buber wrote:BTW - I closed it up, closed the extra holes I did in the emulsifier tubes in thh carbs (and couple mor for goos measure) and mileage... went up cosiderably! Could it be because of CV carbs?

:lol: You are great! Drill holes in carb parts and complain about mileage and running rough problems. And then after some time close the holes again and ask: could it really be that? :lol:
I guess when you would bring back all your modifications like that back to stock it would run like a charm with the 63hp camshaft like for all others who tried that before.
However I don't know what I have to think off as "emulsifier tubes".
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Re: Exhaust opening - how does it affect the performance?

Postby Buber » Mon May 12, 2008 3:28 am

Oh no no! I won't be accused of needlessly endangering my baby! :P

I just went through some topics on SOHC forum, where such gurus as HondaMan recommended this, and since it was a reversible operation I tried it. I guess CV carbs behave differently, and that's it.
Emulsifier tubes, are the tubes which end with the main jet. Inside them the needle goes up and down, and they have in the sides a series of little holes to allow the fuel to "foam", or emulsify with the air. Theory goes that more holes will create better foaming, hence better quality air/fuel mix. On the top of that, amount of fuel shouldn't change, as it passes 1st through the main jet below, which has certain, non-adjustable flow.
That's the theory. On mechanical carbs. I did on CV carbs, and it seems it doesn't work.

And as per 63 camshaft - I'm still not sure if there's something in running CV carbs vs mechanical ones. Since I don't have mechanicals, I can't really compare, but it sounds VERY intriguing to me.

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Re: Exhaust opening - how does it affect the performance?

Postby Volker_P » Tue May 13, 2008 2:08 am

Buber wrote:... On the top of that, amount of fuel shouldn't change, as it passes 1st through the main jet below, which has certain, non-adjustable flow.

This fixed flow limit is decisive mostly at full throttle and looses impact the more the needle penetrates into its guide. In this mode the number and especially position (!) of additional holes could also impact the mixture. As your engine got hotter, you probably caught the lean side.
Another aspect is while for the piston carbs you directly control the needle position, for the CV carbs the pressure in the intake does this for you while you only regulate an air flap with your throttle.
I suppose the SOHC recommendation could be useful for those who already use larger jets in combination with less restrictive air filters because they want to operate their engine at high power and rpm, maybe even with racing ambitions. Often single measures which are only helpful in combination with others are ineffective or even contra productive. I could imagine you happened to try out one of them.
I am quite sure that 63hp with CV carbs works because 50hp was a special creation for Germany and I got a spare engine from a Z-model which was damaged in an accident (where I took out my slightly damaged 63hp test camshaft). But I sold the CV carbs soon years ago togeher with the other Z-model stuff that does not fit the RC05 without having a look inside, so I am sorry I can't report to the jetting. Neverheless I now from a phone call some time ago that for 63hp a different jetting helps to get out even more also for the piston carbs, but I did not get the data then. As the intake vacuum conditions dependent on revs change with the cams, the CV carbs may require a change of needle position and/or jetting to compensate.
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Buber
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Re: Exhaust opening - how does it affect the performance?

Postby Buber » Tue May 13, 2008 3:14 am

Exactly! So at the moment I'm back at 50hp, but i'm not selling the other camshaft :D When time and money (mainly) permits, I will try with different jetting (and removal of air filter cover, leaving air filter in place) - there's no problem with needle position - it's fixed and non-adjustable in those carbs. I like them, because they don't have teh diaphargm.. One thing less to break :)

And I have no doubt that 63 camshaft works - it worked for me, only it drew more fuel... :lol: and gave me more power... No free breakfast there...

And ultimately, lets face it - for more power I'm planning (when I will be filthy rich :twisted: ) an XJR 1300. That will solve it! :P

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Re: Exhaust opening - how does it affect the performance?

Postby Volker_P » Wed May 14, 2008 1:35 am

Buber wrote:Exactly! So at the moment I'm back at 50hp, but i'm not selling the other camshaft :D

Oh, I see, I already considered to ask about that. :roll:

Buber wrote:And ultimately, lets face it - for more power I'm planning (when I will be filthy rich :twisted: ) an XJR 1300. That will solve it! :P

That's a quite rigorous tuning measure! :lol:
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