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  #1  
Old 25th June 2022
Murph Murph is offline
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Default Cutting off Mufflers...Open Pipes

Hey

Im working on my 1979 sporty....bobber build.

Currently have aftermarket pipes-mufflers. They are discolored and dirty looking.
Im thinking of cutting the mufflers off to create open pipes....then wrapping them.

Couple questions.....

Will I have to re-jet? .....if so Ill start searching threads...or will it just depend and possibly get lucky?

Has anyone figured a way to connect a bracket to hold pipes to the frame in a different spot...so pipes arent just hanging....might create-buy some hanger for it.

Lastly....how long does it take for white exhaust wrap to discolor?

Thanks!
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  #2  
Old 25th June 2022
doodah man doodah man is offline
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Just cutting off mufflers and thereby shortening your tail pipes is going to make tuning difficult and the bike will not run all that well. and pipe wrap is notorious for rusting out the pipes. The best exhaust solution is a 2 into 1 pipe. Or, if you have to have the open pipe sound get long drag pipes and use some anti-reversion devices in them for “tune-ability”.
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Old 25th June 2022
ryder rick ryder rick is offline
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32-38" is how long the pipes should be.
But they will suck at anything but full throttle over 3500 rpm
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Old 25th June 2022
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You can attempt to tune with drag pipes, but as others have said there's no way to eliminate "drag-pipe sag".

http://www.nightrider.com/biketech/exhaust.htm
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Old 26th June 2022
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Also (and this is important) check your Health Insurance policy. You need to know they will cover you for hearing aids. These are quite expensive now with good ones starting at $5,000. (Mine were $7,000).

Also you'll be ok as long as none of your neighbors are closer than two miles. Otherwise, learn how to glaze windows. (It will save you a fortune)
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Old 27th June 2022
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Straight pipes will really take the fun out of riding. There is absolutely no correlation between noise and power. Instead of smooth power delivered in a predictable way, you will get an engine that falls flat on its face at any RPM below 4000. Literally it will go "Blat-blat-blat until it hits 3500-4000 and then you will have lots of power quickly. If you are racing and know how to ride to keep it up on the cams, it does work. I will bet that you do not know how to do this and find that it is a royal pain in the ass to ride.
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Old 27th June 2022
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NOTE: No mount of knowledge, skill or luck has been able to fix the power loss, ever. This can't be "ridden" around, quite simply one does not cruise main street at 4000 rpm without inviting some reds and blues.
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Old 27th June 2022
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I tried two inch drag pipes on my 79. Bike ren terrible any where near or above 45 mph. Tried baffles did not help. Got 1 3/4 drag pipes better but not great, put 8 inch baffles in runs great now. the bottom line is I believe some backpressure (restriction) is a good thing.Drag pipes are relatively cheap.
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Old 28th June 2022
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OK.

You don't actually "lose" power with open pipes. Well, you do, but only at a certain rev range. You gain that power back in another rev range. Overall, not much in the way of total loss.

It all has to do with pressure waves created when the exhaust valve opens. This produces a positive wave which travels down the pipe (basically at the speed of sound). When it hits an obstruction it "bounces" back in the same way your voice comes back to you at echo canyon.
It also bounces back when it hits the open end of the pipe, the pressure wave effectively "bounces" back UP the pipe. Only this time, it's a mirror image of the high pressure wave that cane down the pipe, a low pressure wave.
This all takes time and timing is everything.
The time of course depends on the length of the pipe. The longer the pipe the longer the round trip journey takes.
When the negative wave arrives just as the exhaust valve is opening again, it helps to "extract" the gases from the combustion chamber. It can at times do this so well ithelps pull the intake charge into the cylinder once the intake valve opens.
Big gains can be had at this point.
But when the timing is not right these pressure waves can also attempt to force spent gases back into the cylinder. Not good.

It's not gasses you need to obstruct. It's that pressure wave. A good design will not completely obstruct it but disrupt it enough that it's action is not as pronounced.

OK, so could I "fix" this problem.

To a great degree yes.

The key here is the length of the pipe. If you change the length of the pipe we change the timing and the rpm at which a "good" wave arrives back at our exhaust valve.
So, by shortening the exhaust system in line with the increase in rpm's, we can take advantage of the pressure waves through a much wider rev range.
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Old 28th June 2022
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Here is a good demonstration of how straight pipes will improve performance: You are at the drag strip with a 55" motor and PB cams. At the line you roll the bike until it stages, you put your weight up on your toes and let it rest on the tank. Clutch in, 1st gear. You open the throttle to 6,000 rpm and hold it there. When the top yellow light comes on you dump the clutch. By the time your brain sends a message to your left hand, the tree has already gone down and green is on. If you wait until you see green, you will lose every time. You leave the throttle WFO and when the tack shows 6,000 again you leave the throttle full on and simply squeeze the clutch and kick it into second. Sometimes at this point it may start to stand up and the front wheel will stay off the ground all through 2nd gear. By this time you are hauling ass and are crouched down like you are hiding from something behind the tank. When it hits 6,000 in 2nd you blip the clutch and kick it into 3rd and then 4th same thing. You have never shut the throttle even a little bit. 1320 feet done, you roll out the throttle and sit up to help slow it down. The tag will say anywhere from 98 to 102-104mph and the ET will be good, easily in the low 13s unless you are light and then it will creep into the 12s.
This "abuse" of the transmission is entirely safe because the buttery smooth Sportster transmission works that good. The multi plate dry clutch works the same way. If you have a pre AMF Sportster, you MUST have a reinforcing ring sweated and epoxied on where the speedometer gear used to be. Otherwise you can bust the cases.

Point here being that you can get quite a bit of power using straights but it is sitting in there between 3500 and 6,000 RPM. Winding it out beyond 6,000 with a stock motor will just slow you down. If you scare easily and try this with anything but full throttle, it may lug down below 4,000 and fall flat on its face. If you are a large person like I was, the only logical next step was to stroke it to 77" and then the numbers and performance change dramatically. You won't believe the difference. Imagine doing this on the street? As Rick says, you are inviting trouble with the law.
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