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Discussion Starter · #1 · (Edited)
Going to build some pipes for my pre-unit, but I'm really torn and could use some feedback.

Here's the specs...
650 motor, T120 head, dual 32mm Miks, stock everything else.

Right now it has 1-3/4" short drag pipes. First off I'm going to build something totally different in style to what I currently have. Straight down the left side sort of like scrambler high pipes but with no mufflers. I would like to increase the back pressure a bit to boost the performance. Should I go with a 1-1/2", 1-5/8" or just keep the 1-3/4" and make them as long as possible? I'm really leaning towards the 1-1/2" construction, but will this give me too much? Should I step it up to 1-5/8"?

What do you guys think?
 

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scootermcrad said:
I would like to increase the back pressure a bit to boost the performance. Should I go with a 1-1/2", 1-5/8" or just keep the 1-3/4" and make them as long as possible? I'm really leaning towards the 1-1/2" construction, but will this give me too much? Should I step it up to 1-5/8"?
I've always been taught that the exhaust diam. should match that of the exhaust valve.

It's not a matter of creating pressure in the exhaust, it's about improving exhaust scavenging by getting greater velocity in the pipe as the exhaust gasses exit. Smaller diam = greater velocity = more negative pressure signal at the exhaust valve for scavenging.

jason
 

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Discussion Starter · #3 ·
JasonMcElroy said:
I've always been taught that the exhaust diam. should match that of the exhaust valve.

It's not a matter of creating pressure in the exhaust, it's about improving exhaust scavenging by getting greater velocity in the pipe as the exhaust gasses exit. Smaller diam = greater velocity = more negative pressure signal at the exhaust valve for scavenging.

jason
I think I get what you're saying... so 1-1/2". Where is the line drawn though? What's to say 1" isn't good (besides commond sense and it would look weird)? how does the theory apply to overall efficiency with stock valves? There must be an efficiency range based on relating output to diameter and volume and ultimately coming out with a number "X" that will fall in a range. Right??? :confused:
 

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I guess I'm retarded, because I never even think about that shit. I just use whatever pipe size looks best for what I'm building, although I wouldn't use something too extreme either way (i.e., 1" or 3"). The differences in performance from one size to the other never made any difference to me. I just tune the bike as best I can with my pipes, and that has always worked fine.

If I was building a drag bike or something, then I might worry about it.
 

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Discussion Starter · #5 ·
Tha Nutz said:
I guess I'm retarded, because I never even think about that shit. I just use whatever pipe size looks best for what I'm building, although I wouldn't use something too extreme either way (i.e., 1" or 3"). The differences in performance from one size to the other never made any difference to me. I just tune the bike as best I can with my pipes, and that has always worked fine.

If I was building a drag bike or something, then I might worry about it.
I was sort of thinking this too. I'm not racing Bonneville for the land speed record OR hitting the drag strip on the weekend either. I may go with the 1-5/8" to tighten it up a bit and they're going to be long enough anyway to provide some decent back pressure, even though they're going to be straight through... I just figure if I was going to start from scratch, it may as well be benficial to dialing the bike in good too.
 

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there's issues with valve overlap too. cylinder scavenging is a timing thing with both pressure waves that revert back up the pipes and small amounts of pressure from the intake charge. what's in question is whether there is enough energy from the wave inversion to help scavenge. getting it wrong is pretty easy, we can bet most pipes are wrong because of the amount of trial and error involved in the testing. but how wrong do you have to be in order to really hurt power? that's what i am trying to learn right now. i'm actually reading a pretty good textbook on exhaust design right now. its for work, not exactly light reading if you catch my drift. if i bring anything away from it that seems to be related to bike stuff i'll post some later.

otherwise make them loud! loud is better right? right.
brandon
 

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Discussion Starter · #7 ·
stolenmojo said:
if i bring anything away from it that seems to be related to bike stuff i'll post some later.

otherwise make them loud! loud is better right? right.
brandon
Absolutely! Share knowledge and enlighten us all with what you gain from that book.... and YES.... LOUD!! Especially in Vegas where morons on their sell phones need to hear you coming up on them!! :eek: Scary here!!

Back to pipes though... straight through is just about going to be loud no matter which way you slice it... maybe some of you can just tell me what you've built your pipes with.
 
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