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Author Topic: Sizing of tailpipes  (Read 526 times)

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Offline macker

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Sizing of tailpipes
« on: October 03, 2020, 02:14:19 pm »
Looking for some advise on fitting an Akropovic tailpipe to my XR150L - Is the size of the tailpipe very specific to engine size ??
Will an Akro pipe off a XT660R work with the 150 engine or is it a mismatch.
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Offline Bill the Bong

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Re: Sizing of tailpipes
« Reply #1 on: October 03, 2020, 04:43:21 pm »
You can determine it from the total surface area of the 2 engine's exhaust valves if the power peak is at approx. the same RPM.  Realistically, if you can make it fit, you will loose low RPM torque but possibly gain max power and over-rev, but only if the valve train can be efficient at the higher RPM.  Most likely you will see some losses everywhere but gain sound.
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Offline Gérrard

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Re: Sizing of tailpipes
« Reply #2 on: October 03, 2020, 05:25:31 pm »
Yes. No. Yes.
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Offline Dux

Re: Sizing of tailpipes
« Reply #3 on: October 03, 2020, 06:58:04 pm »
Quote from: Bill the Bong on October 03, 2020, 04:43:21 pm
You can determine it from the total surface area of the 2 engine's exhaust valves if the power peak is at approx. the same RPM.  Realistically, if you can make it fit, you will loose low RPM torque but possibly gain max power and over-rev, but only if the valve train can be efficient at the higher RPM.  Most likely you will see some losses everywhere but gain sound.

True
For example I have seen an 8% drop in power , 10% drop in torque on a header pipe where the headers were too large in diameter , the best was that I showed the dyno printout to the manufacturer and they weren’t interested . And then another manufacturer has been telling me he is getting better results with smaller diameter baffle pipes .
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Offline KaTooMatt

Re: Sizing of tailpipes
« Reply #4 on: October 05, 2020, 05:24:00 pm »
It has to do with puipe length related to diameter. I will look for my notes from varsity but an exhaust is tuned to length to use the shockwaves to help with scavenging and other engine breathing issues. The shockwave speed is relative to the diameter and the length is tuned to ensure the shockwave bounces off the end of the exhaust to get back to the valve at just the right time for the desired effect.

Please excuse the garble but I have not looked at this info in 32 years.
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Offline Petrilkop

Re: Sizing of tailpipes
« Reply #5 on: October 06, 2020, 08:23:10 am »
Quote from: KaTooMatt on October 05, 2020, 05:24:00 pm
It has to do with puipe length related to diameter. I will look for my notes from varsity but an exhaust is tuned to length to use the shockwaves to help with scavenging and other engine breathing issues. The shockwave speed is relative to the diameter and the length is tuned to ensure the shockwave bounces off the end of the exhaust to get back to the valve at just the right time for the desired effect.

Please excuse the garble but I have not looked at this info in 32 years.
I did not know this goes for four strokes as well.
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Offline Bill the Bong

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Re: Sizing of tailpipes
« Reply #6 on: October 06, 2020, 03:06:39 pm »
Actually only really applies from exhaust port to header collector. For 4T singles, the rules are not as black/white for scavenging as there isn’t a collector as such.
« Last Edit: October 06, 2020, 03:09:38 pm by Bill the Bong »
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Offline 2StrokeDan

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Re: Sizing of tailpipes
« Reply #7 on: October 06, 2020, 03:44:20 pm »
Quote from: KaTooMatt on October 05, 2020, 05:24:00 pm
It has to do with puipe length related to diameter. I will look for my notes from varsity but an exhaust is tuned to length to use the shockwaves to help with scavenging and other engine breathing issues. The shockwave speed is relative to the diameter and the length is tuned to ensure the shockwave bounces off the end of the exhaust to get back to the valve at just the right time for the desired effect.

Please excuse the garble but I have not looked at this info in 32 years.

This is why even a 4stroke has some sort of "powerband".
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Offline KaTooMatt

Re: Sizing of tailpipes
« Reply #8 on: October 08, 2020, 04:15:46 pm »
Quote from: Petrilkop on October 06, 2020, 08:23:10 am
Quote from: KaTooMatt on October 05, 2020, 05:24:00 pm
It has to do with puipe length related to diameter. I will look for my notes from varsity but an exhaust is tuned to length to use the shockwaves to help with scavenging and other engine breathing issues. The shockwave speed is relative to the diameter and the length is tuned to ensure the shockwave bounces off the end of the exhaust to get back to the valve at just the right time for the desired effect.

Please excuse the garble but I have not looked at this info in 32 years.
I did not know this goes for four strokes as well.

At the time we were calculating for the 2.0 l golf 2 motor. Our lecturer did a lot of consulting to VW. They were just about to launch the 2.0 GTi. So yes multi cylinder and 4 stroke.
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Offline 2StrokeDan

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Re: Sizing of tailpipes
« Reply #9 on: October 08, 2020, 06:25:34 pm »
To demonstrate how important exhaust measurements are, my buddy has a race-spec 350 BSA Goldstar, and out of slow corners he uses his boot-toe to block the tailpipe [no silencer], until the revs has climbed

a bit and the engine has gotten into the powerband. Below the powerband the bike is dead.

His boot blocking the tailpipe provides some back-pressure, and keep more of the mixture in the chamber.
« Last Edit: October 08, 2020, 06:26:09 pm by 2StrokeDan »
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