Super shorty exhaust built from stockers: Bad Idea?
06-14-2009, 06:28 PM
#1
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Super shorty exhaust built from stockers: Bad Idea?
Post 'deer incident' I'm prepping the hawk as a streetfighter conversion. Bucket headlight, no fairings, etc. In line with that theme, I'm looking to cut weight and extra crap as much as possible. The stock cans and passenger pegs look like prime real estate for some weight savings, and a way to clean up the rear of the bike similar to how I'm cleaning up the front.
So here's what I'm thinking. Take a set of spare cans I have sitting around, and lop off the whole can. Cut the weld just before the pipe necks up to the diameter of the main exhaust body. After doing that, weld a short (max 4") extension that angles to point the short exhaust parallel to the swingarm. Install a baffle in that extension to cut down the noise and get some backpressure back. This would allow me to remove the entire stock cans and the passenger peg hangers, which would clean up the rear and save a BUNCH of weight.
That said, do y'all think I could get enough backpressure back from baffling the extension pipe to make this only moderately insane? I realize it'll be loud, but hopefully not stupid loud. I know what it sounds like w/ just open header, and I'm thinking if I can get to half that I'd be happy.
Ideas?
So here's what I'm thinking. Take a set of spare cans I have sitting around, and lop off the whole can. Cut the weld just before the pipe necks up to the diameter of the main exhaust body. After doing that, weld a short (max 4") extension that angles to point the short exhaust parallel to the swingarm. Install a baffle in that extension to cut down the noise and get some backpressure back. This would allow me to remove the entire stock cans and the passenger peg hangers, which would clean up the rear and save a BUNCH of weight.
That said, do y'all think I could get enough backpressure back from baffling the extension pipe to make this only moderately insane? I realize it'll be loud, but hopefully not stupid loud. I know what it sounds like w/ just open header, and I'm thinking if I can get to half that I'd be happy.
Ideas?
06-15-2009, 06:05 AM
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Would I need a larger idle or mains? The mains are already 180/185 as I had it jetted up a bit when I did the bafflectomy+ (now the pipes are back to bone stock, so it's kinda rich) Here's my thing: How is the backpressure created by a baffle in a very short exhaust any different than the lack thereof in a set of say Two Bros pipes that have a straight shot all the way through. Granted, sound attenuation may be completely nil, but as far as backpressure goes, there's really no difference.
Last edited by Galaxieman; 06-15-2009 at 06:19 AM.
06-15-2009, 06:25 AM
#5
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Example: Baffles like this...
http://cgi.ebay.com/ebaymotors/SHORT...171|240%3A1318
http://cgi.ebay.com/ebaymotors/SHORT...171|240%3A1318
06-15-2009, 07:22 PM
#6
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I did something similar with my stockers. Almost worked well except there was too much back pressure. Low end was great but she was choked on top. If I let any more exhaust escape it would be obnoxiously loud.
Aftermarket cans get their back pressure from the length. The exhaust comes out at a set flow (rpm dependent of course) and by keeping it enclosed will create more back pressure. Similar to a rifle compared to a handgun. So real short exhaust needs more baffling to control the sound, but is less responsive to controlling the dynamic exhaust volume. I imagine jetting would be a bitch too, however I never tried adjusting mine. The pipes looked great and I had them on during the winter when I wasn't riding nearly as hard or frequently. I put the MIG cans back on a couple weeks ago and she was back to full power throughout the RPM range.
If you still decide to pursue this mod, I strongly suggest the baffled part of each can be at least a foot long. Use adjustable internal baffles that you can stack or pack with fiberglass. Perhaps something like quad exhaust similar to a Z1000 would yield the proper amount of volume needed to pressurize and mute the exhaust gases.
Aftermarket cans get their back pressure from the length. The exhaust comes out at a set flow (rpm dependent of course) and by keeping it enclosed will create more back pressure. Similar to a rifle compared to a handgun. So real short exhaust needs more baffling to control the sound, but is less responsive to controlling the dynamic exhaust volume. I imagine jetting would be a bitch too, however I never tried adjusting mine. The pipes looked great and I had them on during the winter when I wasn't riding nearly as hard or frequently. I put the MIG cans back on a couple weeks ago and she was back to full power throughout the RPM range.
If you still decide to pursue this mod, I strongly suggest the baffled part of each can be at least a foot long. Use adjustable internal baffles that you can stack or pack with fiberglass. Perhaps something like quad exhaust similar to a Z1000 would yield the proper amount of volume needed to pressurize and mute the exhaust gases.
06-15-2009, 10:09 PM
#7
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I agree, system length and baffling is not the same thing when it comes to back pressure.
As for the result on the jetting, an absolute guess would be that it might need bigger jets. This sometimes can be the result of poor gas flow, which I think you will get. All speculation of course until you test it.
Race motors, especially drag engines run well with open pipes as they only have to operate at the high end of the rev range. But street engines require different characteristics. Engines with forced induction are the exception to this of course, as cylinder filling is not dependant upon the exhaust as much as a normally aspirated engine.
As for the result on the jetting, an absolute guess would be that it might need bigger jets. This sometimes can be the result of poor gas flow, which I think you will get. All speculation of course until you test it.
Race motors, especially drag engines run well with open pipes as they only have to operate at the high end of the rev range. But street engines require different characteristics. Engines with forced induction are the exception to this of course, as cylinder filling is not dependant upon the exhaust as much as a normally aspirated engine.
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