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I have a original 4into1 exhaust on my CB750F, if i remove the muffler end(right passed the junction of 4into1, will i lose performance or gain performance? High end torque/low end torque? any help
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Administrator
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You might want to specify the year you have. The F was used for 2 different models.
Anyways, a 4-1 pipe actually lowers your bottom end. You get more at the top instead which is why it is the standard for racing on these old bikes.
If you drop off the muffler, You might not gain anything, you may lose. Contrary to redneck belief, straight pipes do not always result in power gains, especially on bikes. A racing bike is different as the power output is greater but most still have some sort of muffler.
Another thing is that without the muffler, you have very short pipes and you can burn a valve, though that is probably unlikely. Next, you will have no fans in your neighborhood cause of the noise, cops will love you though when they can write a ticket for it.
The ride IS the adventure. The destination is just to get gas!
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Its a 1975, I figured all of that was true just wanted someone to clarify that. i plan to keep the exhaust on but im going to fab something up thats for sure. Actually the low end was great with it off, but i lost my top end power. so back on they go!
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The valve burning thing is an old wives tail . -- You could run it right out of the ports if you wanted without any affect to the valves . -- You would however be hurting the power output while only making alot of noise . -- Great research went into the design of these exhaust systems . -- Run it the way it was designed and you get the performance it was intended to give you . --
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Administrator
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If it was an old wives take then it would not happen. I have seen it happen. Also, it continues to circulate around virtually all motorcycle aspects. This is not like the synthetic oil argument. This can and does happen.
The ride IS the adventure. The destination is just to get gas!
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I'm close enough to Missouri to say , " Show me ." -- I've been an auto mechanic for 37 years , and worked on bikes as well , off and on ,for alot of those years . -- Never seen what you speak of , ever . -- The mechanical explanation for a "Burned valve " is that the valve margin becomes thin enough or weakened enough by heat to start "torching" a spot on the valve , and the high pressure within the combustion chamber pushes the superheated combustion gasses through the small fissure , opening it up like a cutting torch . -- They burn from the INSIDE , not the outside .
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Administrator
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Then you are misunderstanding it. The issue occurs when cool outside air comes into contact with the valve. You get hot/cold temp changes. regardless, just cause you have not seen it does not mean it has not happened. Did I take a pic that the valve I saw? No but do YOU take a pic of every bad valve you come across? I doubt it.
The ride IS the adventure. The destination is just to get gas!
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Sounds like you're looking for a pissing match , and I'm just not interested . If you feel that strongly about it you can be right . -- you win . --- Here's a scenario for you . -- Vehicles with air injection systems forced cold air directly into the exhaust ports . -- No valve problems .
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Administrator
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Ah but we are not talking about engines DESIGNED to operate that way. I don't care if you do not agree, if you would bother to even try a small simple google search, you will find MANY links on burnt exhaust valves and their causes.
On an air cooled bike such as an sohc or dohc, the cool air can cause carbon to build up around the valve seat and prevent the valve from shedding heat to the head like it is supposed to. This will cause the valve to get too hot and burn.
Now as for pissing match, I won it before it started. Now are you that lazy where you need links or can you manage it by yourself?
The ride IS the adventure. The destination is just to get gas!
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This post was updated on .
Like I said , 37 years and have never seen it . I guess if you look hard enough you can find anything on the internet . Your computer skills are obviously better than mine , so why don't you set up a poll about this debacle and see what the consensus is among the CBrs . -- Rerun , I do agree with you on one thing , which is that excessive HEAT is the main cause of burned valves , but there is usually an underlying problem that causes the heat to begin with .-- Short exhausts isn't it . -- I would also like to add that I apologize to the author of this thread for it becoming what seems to be a personal debate .
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Administrator
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I can't tell you to see burnt valves but I can say to do a simple google search and just type in burnt valve. There are pages and pics to see. If you do not want to do that, I can't force you but you should know it IS something that happens and well documented on small and large auto and truck engines and even aircraft engines. And yes, exhaust length can have an impact. Maybe not on all engines but I DID find links on it.
But regardless, a short exhaust or no exhaust is detrimental to engine performance, back pressure is needed.
The ride IS the adventure. The destination is just to get gas!
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I have 37 years , in the field , hands on , knuckle busting , back breaking experience in this very field . Yes , valves do burn , and the internet is full of information on all sorts of things that was posted by one guy , who got the data somewhere else , from somebody else , who got it from his step-sisters uncle twice removed . -- Do you see what I'm saying?? -- I don't doubt that there are numerous pictures and pages of text , but at best they only tell part of the whole story . -- Airplane engines is a prime example . -- How many hours of flying time does it take to burn a valve , and were there any mitigating circumstances that preceded it . ( i.e. underlying problem ) -- The valves in our bikes are nitrided to help them survive in their harsh environment . If the burned valve you saw had been refaced , that would be a contributing factor as well . -- The manual says replace , not reface . -- Just another possibility in an endless sea of them . -- We now agree on two things . -- Valves do burn , and it is caused by excessive heat .
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On the subject of mufflers... My 79 CB750K has a non-stock 4-into-1 exhaust on it; it ain't real loud, but what would that be doing to the performance of this bike? high end? low end? Thanks
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I had a Vance / Hines on my 81 custom , and the " Seat-o-the pants" feel was that it lost a little on the bottom , but gained on the top . -- Performs better overall with the stock exhaust unless you re-jet for the bigger diameter headpipes and flow increase . -- If you're looking for more performance , put 900 cams in it . That wakes it up !! -- Easy job too.
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