7/8" flex pipe for 4 strokes

CrazyDan

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#1
I found a cheap source for 7/8" galvanized flex pipe for exhaust sold by the foot.
https://www.ebay.com/itm/FLEX-TUBING-7-8-in-x-1ft-Galvanized-Standard-THICKNESS-A-028-Exhaust/111999638791?_trkparms=aid=111001&algo=REC.SEED&ao=1&asc=20160727114228&meid=5e0d6d01db204f62899d0657fe8f77d6&pid=100290&rk=2&rkt=4&sd=222813585500&itm=111999638791&_trksid=p2060778.c100290.m3507&redirect=mobile . I'm about to buy a 2' section and chop my muffler off for lulz. Gonna need to get a better air filter and jet up probably...I love this hobby with endless upgrades.
Edit: this might be next

https://www.amazon.com/Cylinder-Mot...eASIN=B078J339FH#immersive-view_1517272757343 .
 
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Frankenstein

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#2
I found a cheap source for 7/8" galvanized flex pipe for exhaust sold by the foot.
https://www.ebay.com/itm/FLEX-TUBING-7-8-in-x-1ft-Galvanized-Standard-THICKNESS-A-028-Exhaust/111999638791?_trkparms=aid=111001&algo=REC.SEED&ao=1&asc=20160727114228&meid=5e0d6d01db204f62899d0657fe8f77d6&pid=100290&rk=2&rkt=4&sd=222813585500&itm=111999638791&_trksid=p2060778.c100290.m3507&redirect=mobile . I'm about to buy a 2' section and chop my muffler off for lulz. Gonna need to get a better air filter and jet up probably...I love this hobby with endless upgrades.
Lebanon pa, not far from here, and just happens to be the same exact individual I bought mine from. In fact my most recent pipe I made up is using a piece from the length I bought.
 

CrazyDan

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#3
Lebanon pa, not far from here, and just happens to be the same exact individual I bought mine from. In fact my most recent pipe I made up is using a piece from the length I bought.
Are you running open pipe or do you have a muffler on the end?
 


Frankenstein

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#5
Also funny enough I wrapped mine in red, turns out over heating it with a blowtorch makes it almost white, really like a very pale straw color. Certainly what ever color I had picked would up ended up looking much like that black one does. The tube is not incredibly resistant to letting exhaust excape along the furling, but it seems that once it turns into tar along the lining wrap it doesn't keep coming through. The wrap I had stayed silver looking on the flex tube but definitely picked up black crap from the roads and engine.

They have stainless steel zip ties and they are a pain to tighten but work well. As a side note I noticed the plain nylon ones don't melt on the wrap either. 4t runs even cooler than a 2 so I'd bet it would be ok. I used hose clamps as well on it to make it easy starting the wrapping of the muffler and left it there lol.

Soak in water first and if you have a bend in the pipe wrap it bent not straight, once you wrap it putting a tight bend can be really hard.
 

FurryOnTheInside

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#6
I never thought to soak it in water first. I made the mistake of touching the wrap with bare hands too, a mistake I didn't make twice lol.
I had to wrap over a big change in diameter at the front of my muffler so the cable ties were absolutely essential to prevent the wrap slipping off, but I also had some little loose bits and I was able to glue those down (and also prevent any slip) with a bit of leftover JB Weld which nearly matches the charcoal grey of the "black" wrap. :)
The stainless steel ties are a pain to cut the excess off after tightening (because I already had the exhaust mounted when I added a couple more for the support brackets) and then curl the sharp ends in so I can't cut myself if I pick up the bike too carelessly.
I would save the steel ties for the header only and use the nylon on the muffler, next time. :)
 

CrazyDan

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#7
Just got my pipe installed, I am very pleased with both the sound and performance. Ran circles around my buddy on his 79cc predator build. I can keep a much higher rpm on hills I previously bogged on.
 

Frankenstein

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#8
Just got my pipe installed, I am very pleased with both the sound and performance. Ran circles around my buddy on his 79cc predator build. I can keep a much higher rpm on hills I previously bogged on.
But 4 strokes need backpressure!!
 

CrazyDan

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#9



CrazyDan

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#12
From www.physicsforums.com
“Backpressure” is a fairly common term regarding the resistance to flow of some piping system (ex: exhaust pipe) when the pressure at some point downstream of this resistance is known. In the case of an exhaust pipe, the outlet of the pipe is open to atmosphere, so we know the pressure at this point is atmospheric pressure. But because there is flow through the pipe, which represents a resistance to flow, the pressure is not atmospheric pressure all the way up the pipe to the engine. If you were to measure the pressure at various points along the exhaust pipe, you’d find that the closer you got to the engine, the higher the pressure would be. This is simply because there’s a resistance to flow of any fluid through a pipe. So the term “backpressure” simply says that there is a pressure drop through a pipe and the pressure at the inlet of the pipe is higher than the outlet and we generally acknowledge that it’s the outlet of the pipe who’s pressure is knowable. This is true for the exhaust on a 2 stroke or a 4 stroke engine. Clear as mud so far? In the case of the 4 stroke, the desire is to eliminate ALL the burned gasses from the cylinder when the piston gets to TDC - so the more backpressure, the higher the pressure will be inside the cylinder, and the less exhaust gas we will be able to expel. We don’t want those gasses inside the cylinder, so we want the backpressure on a 4 stroke to be as low as possible. Here’s a decent discussion on 4 strokes: http://auto.howstuffworks.com/question172.htm In the case of a 2 stroke, the thing that’s clearing the exhaust from the cylinder isn’t the piston going up to TDC because the piston is in fact only a little past BDC when the exhaust gasses are displaced. For a 2 stroke, it’s the sweeping of the cylinder by the fresh charge of air and fuel which is pushing the exhaust gas out. If there’s no back pressure, the fresh air and fuel will not only sweep out the exhaust gasses, but they will also begin to exit with the exhaust gas, resulting in a loss of fuel and air which reduces efficiency. If the back pressure is too high, the exhaust gasses can’t all be removed by the fresh charge. So for 2 strokes, there is an optimal back pressure to produce the most efficient fuel use while maximizing power. This optimal back pressure must be tuned to a certain RPM, because above and below that RPM, one of the two problems will surface. 2 strokes have a very dynamic gas flow which is difficult to optimize because of this. Most 2 stroke mfg’s simply tune the engine at a given RPM and throttle setting which results in the best overall performance. This is a rather simplified explanation of the 2 stroke, there are other considerations. A small amount of back pressure for example can increase the total air/fuel charge in the cylinder by raising the pressure at the point the exhaust port is closed off. If you’d like to read more, I’d suggest these: http://www.indopedia.org/Two-stroke_cycle.html http://forums.mustangworks.com/330208-post7.html

Reference https://www.physicsforums.com/threads/exhaust-back-pressure-on-a-4-stroke-ice.206232/
 

Frankenstein

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#13
From www.physicsforums.com
“Backpressure” is a fairly common term regarding the resistance to flow of some piping system (ex: exhaust pipe) when the pressure at some point downstream of this resistance is known. In the case of an exhaust pipe, the outlet of the pipe is open to atmosphere, so we know the pressure at this point is atmospheric pressure. But because there is flow through the pipe, which represents a resistance to flow, the pressure is not atmospheric pressure all the way up the pipe to the engine. If you were to measure the pressure at various points along the exhaust pipe, you’d find that the closer you got to the engine, the higher the pressure would be. This is simply because there’s a resistance to flow of any fluid through a pipe. So the term “backpressure” simply says that there is a pressure drop through a pipe and the pressure at the inlet of the pipe is higher than the outlet and we generally acknowledge that it’s the outlet of the pipe who’s pressure is knowable. This is true for the exhaust on a 2 stroke or a 4 stroke engine. Clear as mud so far? In the case of the 4 stroke, the desire is to eliminate ALL the burned gasses from the cylinder when the piston gets to TDC - so the more backpressure, the higher the pressure will be inside the cylinder, and the less exhaust gas we will be able to expel. We don’t want those gasses inside the cylinder, so we want the backpressure on a 4 stroke to be as low as possible. Here’s a decent discussion on 4 strokes: http://auto.howstuffworks.com/question172.htm In the case of a 2 stroke, the thing that’s clearing the exhaust from the cylinder isn’t the piston going up to TDC because the piston is in fact only a little past BDC when the exhaust gasses are displaced. For a 2 stroke, it’s the sweeping of the cylinder by the fresh charge of air and fuel which is pushing the exhaust gas out. If there’s no back pressure, the fresh air and fuel will not only sweep out the exhaust gasses, but they will also begin to exit with the exhaust gas, resulting in a loss of fuel and air which reduces efficiency. If the back pressure is too high, the exhaust gasses can’t all be removed by the fresh charge. So for 2 strokes, there is an optimal back pressure to produce the most efficient fuel use while maximizing power. This optimal back pressure must be tuned to a certain RPM, because above and below that RPM, one of the two problems will surface. 2 strokes have a very dynamic gas flow which is difficult to optimize because of this. Most 2 stroke mfg’s simply tune the engine at a given RPM and throttle setting which results in the best overall performance. This is a rather simplified explanation of the 2 stroke, there are other considerations. A small amount of back pressure for example can increase the total air/fuel charge in the cylinder by raising the pressure at the point the exhaust port is closed off. If you’d like to read more, I’d suggest these: http://www.indopedia.org/Two-stroke_cycle.html http://forums.mustangworks.com/330208-post7.html

Reference https://www.physicsforums.com/threads/exhaust-back-pressure-on-a-4-stroke-ice.206232/
I disagree, all the Mexicans at the auto shop are swearing by backpressured engines, all 4 strokes.
 




CrazyDan

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#17
It's OK because they have so much fuel, they can afford to loose some power.
I see you trollin :rolleyes:. If you honestly believe what you say then do more research. Google 4 stroke exhaust theory. Here's the link: https://www.google.com/search?q=4+stroke+exhaust+theory&sa=Search . Over and over you will see that best efficiency is no back pressure and proper diameter pipe. Length tunes what rpm the vacuum wave hits the exhaust valve during valve overlap, basically doing almost what 2 stroke expansion chambers do, except instead of shoving unburnt fuel back in, it helps draw the intake charge in. Pick your length for the rpm it hits on. Back pressure doesn't help 4 strokes.
 

Frankenstein

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#18
I see you trollin :rolleyes:. If you honestly believe what you say then do more research. Google 4 stroke exhaust theory. Here's the link: https://www.google.com/search?q=4+stroke+exhaust+theory&sa=Search . Over and over you will see that best efficiency is no back pressure and proper diameter pipe. Length tunes what rpm the vacuum wave hits the exhaust valve during valve overlap, basically doing almost what 2 stroke expansion chambers do, except instead of shoving unburnt fuel back in, it helps draw the intake charge in. Pick your length for the rpm it hits on. Back pressure doesn't help 4 strokes.
You hate'n?
 




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