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Post by crash and burn on Oct 7, 2008 4:11:10 GMT -5
other than noise what are the downsides of running with nothing but a head pipe?
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Post by joneill4 on Oct 7, 2008 11:56:23 GMT -5
Roscoe P. Coaltrain kee kee
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Post by crash and burn on Oct 8, 2008 2:28:43 GMT -5
Funny but the fuzz 'round here have better things to than gripe 'bout pipes on a bike. What I was wondering is will it hurt anything? I have been told it can lead to a burnt
exhaust valve. Is this true?
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smoris
Junior Member
Posts: 31
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Post by smoris on Oct 8, 2008 4:19:45 GMT -5
if your pipe is to short, you might risk your exaust valve being burnt, but when it is about 1m or longer, you won't have any problems (besides your ears will start to hurt after some time i think  ) |
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Post by joneill4 on Oct 9, 2008 11:54:29 GMT -5
I have always heard the same thing about exhaust valves, but never from the guy it actually happened to. Always from the urban legend types. I've also seen a lot of those race type reverse megas that end 5 or 6 inches past the header. I'd like to get caferacercarl's opinion on this.
I'm sorry that your cops are so busy. The cops have absolutely nothing better to do than write inspection tickets where I live, but I guess in the grand scheme it's a good thing.
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Post by colinjay on Oct 11, 2008 17:06:19 GMT -5
The only engine that I have ever burnt out exhaust valves in is my tractor engine, but I don't think that counts.
Back in 76 - 77, my elder brothers friend, Peter, ran a TORUQE Engineering exhaust on his XT500C. This pipe was a very short with a small reverse cone megaphone ending just behind the riders footpeg (it was made in the US and I think intended for 1/2 mile dirt track use). If ridden hard for a few laps of the local MX track and then turned off for for about 10 minutes or so, quite often the engine would not have any compression and therefore would not start. After a few kicks, the compression would return and the engine would start. We eventually worked out that the problem was due to the exhaust valve sticking open and the reason for it doing so was the short exhaust system. Peter added a couple of inches into the header pipe and the problem was virtually cured. (Later on when I worked for our local Yamaha dealer, I found out it was not uncommon for for the exhaust valves to stick open on the early XT/TT500s even with std exhaust systems if the were run hard for a long period and then stopped and left to cooldown.) Anyway, after lenghtening the pipe, the only times he had problems with the valve sticking was after long/hard road rides and the bike had stood overnight, which, as I said, was not uncommon with many of the early XT/TTs. While he had the bike, Peter never had the head off the engine, so I can't say if there was any burning of the valve, but for it to stick open so frequently is a definite sign of the valve overheating and I am sure if he persisted with the pipe at its original (short) length he would had eventually had more problems with the exhaust valve.
CJ
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Post by solo2racr on Oct 11, 2008 18:48:00 GMT -5
This is simply speculation but, I think what may happen when the exhaust is shorter than the recommended tuned length is that the lack of back pressure will cause the incoming mixture to drawn out the exhaust during the cam overlap and burn at the valve and out. As the engine revs up, there is enough velocity in the gasses to keep pressure in the exhaust. As you lift the throttle and the gas velocity decreases, a "vacuum" is created in the exhaust and drawns out the incoming charge. This would result in "backfires" and a burned exhaust valve. Comments??
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Post by caferacercarl on Oct 19, 2008 2:03:00 GMT -5
The sticking of valve to guide is heat related and pre 85 exhaust valves will drop the head off them as they are fused together at the head, the valve will "burn" the sealing face [and even blow a path through the head of the valve] because of leaning out up high more than other factors but they do go better with an exhaust with a perf core muffler and length like the picture above of the race SR in solo2racr's lovely life combo!! beaver/music/hot rod single!!
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Post by phoebeisis on Oct 19, 2008 12:40:21 GMT -5
Big aside here, but I've never understood why a too lean motor would run hotter. To run hotter you have to be burning more fuel, or burning what is there more completely. This- a good efficient burn- is a good thing for power and FE?
If the mixture is actually "too lean" then it won't burn much at all. Now, if it doesn't burn in the combustion chamber, does it then burn when it gets into the exhaust track/ head pipe-thus maybe burning on both sides of the exhaust valve? I could see too lean causing problems in that case.But, just how does it manage to ignite so well in the exhaust track, if it is too lean in the CC?
Just how does too lean create too much heat for an exhaust valve? Heck, it is designed to take the heat of combustion?
Sorry about the aside.
Charlie
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Post by solo2racr on Oct 19, 2008 13:30:26 GMT -5
It's been awhile since I have studied this so if anyone whats to elaborate............
Lean = a higher air/fuel ratio were as rich would be a lower air/fuel ratio. One answer is that more fuel helps to cool the engine. The other is that with more air available (lean) the mixture will burn hotter. Think of it this way. Working with a acetylene torch, as you add O2 the flame gets hotter. Simply opening up the acetylene more just gives a bigger flame, not hotter.
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Post by phoebeisis on Oct 19, 2008 16:56:00 GMT -5
solo2racer- thanks for the info. I keep forgetting that these-the Yamaha 500's- are 30 year old air cooled motors, not modern water cooled understressed car engines that are tuned to run as absolutely lean as possible.
Yeah, I can see that running near peak power would overload the heat dumping capacity of an SR500. Yep, you might very well have to waste some fuel to cool it.
Thanks,
Charlie
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Post by cafekid on Oct 22, 2008 22:43:50 GMT -5
solo2racr is correct and i agree roscoe and boss hog will be on you like white on rice.
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Post by StewRoss on Oct 23, 2008 6:20:22 GMT -5
In the aviation World, running an air cooled piston engine really lean for a time burns away the sides of the piston crown etc. Not good...can eventually burn right through to the rings etc... SR  |
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Post by davedunsboro on Oct 23, 2008 7:09:02 GMT -5
Yes I learn't about that when I was a 16 year old , the hard way. A mate put a smaller main jet into my Elsinor as he reckoned the power band came in at lower revs .I didn't really notice the difference but I did notice when the piston blew up & the engine seized ! So how cool is this forum ,Whenever I do anything now all good/bad info is at my fingertips & the help is invaluable!Cheers chaps ,Dave.
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Post by solo2racr on Oct 23, 2008 9:50:28 GMT -5
In the aviation World, running an air cooled piston engine really lean for a time burns away the sides of the piston crown etc. Not good...can eventually burn right through to the rings etc... SR Wouldn't that be detonation? Probably can't hear it over the prop noise. Good reason for a CHT gauge. And just so every one knows, detonation is where the fuel in the engine explodes due to excessive heat/pressure. Unlike pre-ignition where the fuel in the engine burns (from a piece of red hot carbon or a metal burr, etc) before the spark occurs. From the outside, both sound similar. Like a rod knock only sharper. But it really doesn't matter that you can tell the difference as both detonation and pre-ignition will destroy an engine. |
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