Exhaust rapping (old school bus)

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Feb 2, 2012
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Hi everyone. First of all thank you all for your fast and sincere reply.

I wasn't really sure about the term old school bus as a title. For me is much more like a Norton Manx or McQueen Corvette. I just love this sound.

My interest is to know exactly the reason of this phenomenon so I can make it happen at lower RPM.

The following link is my motorcycle producing this sound. I have heard some other motorcycles making it louder and easier, at lower RPMs.

My theory about it is that while you open the throttle more gas and air is served by the carb. When you suddenly close the slide the vacuum produces extra gas getting into the head. This fresh non exploded gas impacting the hot exhaust temperature produced this rapping effect. Depending on the exhaust type the scavenning effect increase this phenomenon in the overlap period.

In my case I use open pipes. Tomorrow I will make a test with larger thinner pipes so I can experience any changes.

I really appreciate any thought about the roots of this sound and what I am really interested in: making is louder and at lower RPM.

P.D if I can upload an mp3 I can show you another example

 
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Feb 2, 2012
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Back to the topic.

Yesterday I made my final test..

I reduced the end of the exhaust pipe...the result was a killer DB with a kind of whistling sound..SO, I started testing lengh from let say 30 inches of overall lenght until I get the rear axle and BUM....there is a point where the rapping is very loud and deep...

My conclusion..

Depending on your compresion, and cam and carb...there is a moment where unburned gasoline is trapped between the head and the first part of the exhaust pipe...the correct lenght of the pipe will make a return source of fresh oxigen reach this "dust of gas" that is actually very hot...So combustible+oxidizing+temperature BOOM, an explotion happens in the pipes and is amplified by the pipe itself.

So I am very happy with the result. Now I can ride smiling with my BSA fellow!!!

Thank you so much for your support.

All the best
 
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