Exhaust Header Vs Noise

[theloyal]

Hi..
As there is the rule of 110 dB noise, so our team is focusing on using Street Yoshimura Muffler. Due to some packaging constraint we have to design Exhaust headers too, so i have some confusion regarding the function of exhaust header that whether it will help to cut down noise too or it is just use for exhaust scavenging...?


Regards

[theloyal]

Hi..
As there is the rule of 110 dB noise, so our team is focusing on using Street Yoshimura Muffler. Due to some packaging constraint we have to design Exhaust headers too, so i have some confusion regarding the function of exhaust header that whether it will help to cut down noise too or it is just use for exhaust scavenging...?


Regards

[Some Guy]

Header design dfeinitely plays a role in noise. How well you tune your engine is also going to be a factor. Also the rules commiteee is currently floating the idea of lowering the noise limit to 107 (it is in the possible rules change section) so you may want to rethink the Yoshimura.

[Rex Chan]

We design our exhaust system (headers) with the intake in GT-Power, to produce the desired torque curve. We don't do analysis for noise. Our thoughts on the effect of the length of the exhaust system is that a longer exhaust will muffle more than a short one.

About noise: someone on this forum (I think it was Monash Uni) mentioned that engine noise contributes greatly to measured noise levels. Our 2010 exhaust system loops forward next to the right side of the car, then comes back, with tip near rear tyre. At comp, we partially confirmed this idea, as we got a reading of 114dB when measured on the engine side, but less than 110dB (can't remember what it actually was - maybe 107/8dB?) on the tyre side. Of course, the tyre side noise test was measured behind behind the tyre, so it blocks the sound.

We ran a Yamaha muffler in 2009, and we kept it for 2010, as it passed in 2009, and we used the same exhaust both years (only the engine calibration was changed). However, for post-comp track days, we use a Yoshimura RS-5 straight through "muffler", as it sounds better (I can hear the engine better too). However, its way too loud (117dB??), even on the tyre side. This year's engine exhaust guys wants to use the Yoshi muffler, so has moved the exit of the exhaust forwards, near the front right tyre. This way, they'll have to measure sound far away from the actual engine, so hopefully the load Yoshi will be comp legal.

So I suppose what I'm saying is that think about where the decibel meter will be positioned when you design the exhaust. It may also help to have an engine cover, to keep engine noise away from the meter (this was what the Monash Uni post was about).

With regards to the tune, a richer air-fuel mixture usually sounds smoother/deeper than a lean one, so that might help with the engine noise. However, at 11000RPM for CBR600's it going to be loud anyway!

[Wesley]

One thing I can say about headers and noise is to get all your transitions as smooth as possible. When you weld two sections together make sure there aren't ridges or slag protruding into the flow tube - if you're dedicated, get a die grinder or sanding roll and smooth out the inside after each segment you weld on. Little bumps can cause high frequency turbulence which can significantly add to the overall noise.

It's even better if you can do a frequency analysis of your exhaust note and figure out what range most of your noise is in and pick a muffler that targets those frequencies. (chamber, packed, etc)

[Mbirt]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
One thing I can say about headers and noise is to get all your transitions as smooth as possible. When you weld two sections together make sure there aren't ridges or slag protruding into the flow tube - if you're dedicated, get a die grinder or sanding roll and smooth out the inside after each segment you weld on. Little bumps can cause high frequency turbulence which can significantly add to the overall noise.

It's even better if you can do a frequency analysis of your exhaust note and figure out what range most of your noise is in and pick a muffler that targets those frequencies. (chamber, packed, etc) </div></BLOCKQUOTE>
+1

We've discussed this at length in other muffler design threads. Frequency analysis is easy to do in Audacity.

Now that comp has passed, I can also add that our 2011 muffler whose GT-Muffler model that hit 109 dBA at WOT/full load blew 109.25 dBA with no load at competition. Remember that GT-Power deals with (relatively low-frequency) order noise (1200 Hz for my simulations, IIRC), while the dBA scale is not sensitive to low frequency. So, as Wesley stated, all efforts to reduce flow noise should be taken considering our noise test is done with a-weighting.

Regarding header design, in "Design and Simulation of Four-Stroke Engines"G.P. Blair speaks of his revised megaphone exhaust on the Seeley G50 making the bike quieter while in the power band. I believe "dull-toned" was how he described it. He then backed it up with some simulations. So if your sound test speed can work as a peak exhaust scavenging rpm, it might be worth a try.