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oilman's Avatar
 
Join Date: Jan 2015
Location: Edmonton, Alberta, Canada
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Bisimoto Pulse Chamber - Sound Measurements

I purchased a Bisimoto pulse chamber exhaust for my 79 911SC in December, and love the sound!

I had researched it on Bisi's website, looked up a few YouTube videos, and reviewed a number of posts on the Pelican Parts forum. There certainly were mixed reviews as to how loud the system is….. I was intrigued, so I just purchased one... and it did not disappoint! Beautiful design, great quality construction.. Unfortunately its winter here in Edmonton, so the car is confined to the garage . But I installed it anyway, to hear the new sound… and it is Loud!! Beautiful sound, but very loud. So since the car is sitting for a few more months, waiting for the snow to melt, I decided to test sound levels in a few configurations.

My SC is largely stock with a Bitz Racing EFI and a converter bypass pipe.

I decided to take a few sound level readings with a cheap meter from Amazon, in following configurations.

With and without the sound arrestor
With the Cat Bypass and with the Original Cat reinstalled.
I tested at idle and at 3000 RPM (no load), exterior and interior measurements

Conclusions:

• I think I understand the mixed opinions on sound levels. If you have a converter installed, the sound levels are reasonable. With no converter, it sounds like an all-out race car.
• The pulse chamber is effective at managing interior sound levels at 3000 RPM
• The optional sound arrestor effectively drops sound levels by >7 dB, as advertised

I've attached a photo of the results, hope that works... just in case it doesn't, here's a summary.

PC w/o Sound Arrestor, Cat Bypass (dB)

1000 RMP - exterior 91.0 dB
3000 RPM - exterior 116 dB (that's loud!)
1000 RPM - interior 76.0 dB
3000 RPM - interior 93.3 dB

Adding the optional sound arrestor did not impact the exterior idle, but reduced the 3000 RPM by 6 db. The sound arrestor reduced the interior sound 4-7 dB.

Reinstalling the converter made the biggest impact, with a 7-8 dB decrease across the board.

The pulse chamber / sound arrestor combo seem very effective with the converter installed.

PC w Sound Arrestor, Cat installed (dB)

1000 RMP - exterior 83.5 dB
3000 RPM - exterior 97.7 dB
1000 RPM - interior 75.7 dB
3000 RPM - interior 79.8 dB

In the end, I reinstalled the converter, and am really happy with the sound and the look. I think I may even remove the optional sound arrestor.

Curious if anyone has dyno tested this combo with and with out the converter installed.

Hope this helps folks who are considering the Bisimoto Pulse Chamber.




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Dave

79 SC coupe
Old 03-04-2018, 11:15 AM
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Nice feedback using numbers.
Old 03-04-2018, 11:43 AM
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So what exactly is the advantage of this over a normal muffler bypass?
Old 03-04-2018, 01:06 PM
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Quote:
Originally Posted by oilman View Post
I purchased a Bisimoto pulse chamber exhaust for my 79 911SC in December, and love the sound!

I had researched it on Bisi's website, looked up a few YouTube videos, and reviewed a number of posts on the Pelican Parts forum. There certainly were mixed reviews as to how loud the system is….. I was intrigued, so I just purchased one... and it did not disappoint! Beautiful design, great quality construction.. Unfortunately its winter here in Edmonton, so the car is confined to the garage . But I installed it anyway, to hear the new sound… and it is Loud!! Beautiful sound, but very loud. So since the car is sitting for a few more months, waiting for the snow to melt, I decided to test sound levels in a few configurations.

My SC is largely stock with a Bitz Racing EFI and a converter bypass pipe.

I decided to take a few sound level readings with a cheap meter from Amazon, in following configurations.

With and without the sound arrestor
With the Cat Bypass and with the Original Cat reinstalled.
I tested at idle and at 3000 RPM (no load), exterior and interior measurements

Conclusions:

• I think I understand the mixed opinions on sound levels. If you have a converter installed, the sound levels are reasonable. With no converter, it sounds like an all-out race car.
• The pulse chamber is effective at managing interior sound levels at 3000 RPM
• The optional sound arrestor effectively drops sound levels by >7 dB, as advertised

I've attached a photo of the results, hope that works... just in case it doesn't, here's a summary.

PC w/o Sound Arrestor, Cat Bypass (dB)

1000 RMP - exterior 91.0 dB
3000 RPM - exterior 116 dB (that's loud!)
1000 RPM - interior 76.0 dB
3000 RPM - interior 93.3 dB

Adding the optional sound arrestor did not impact the exterior idle, but reduced the 3000 RPM by 6 db. The sound arrestor reduced the interior sound 4-7 dB.

Reinstalling the converter made the biggest impact, with a 7-8 dB decrease across the board.

The pulse chamber / sound arrestor combo seem very effective with the converter installed.

PC w Sound Arrestor, Cat installed (dB)

1000 RMP - exterior 83.5 dB
3000 RPM - exterior 97.7 dB
1000 RPM - interior 75.7 dB
3000 RPM - interior 79.8 dB

In the end, I reinstalled the converter, and am really happy with the sound and the look. I think I may even remove the optional sound arrestor.

Curious if anyone has dyno tested this combo with and with out the converter installed.

Hope this helps folks who are considering the Bisimoto Pulse Chamber.




Thank you for a very interesting post. Your measurements indicate that the PC does nothing without the sound arrestor. Actually, it may even increase the dB's?

Would be very interesting to see a back-to-back dyno of all your setups.
Old 03-05-2018, 05:02 AM
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thanks for the numbers and a scientific approach

Old 03-05-2018, 08:10 AM
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I bought mine in the falls, not installed yet but I can't wait to hear it. Mine came with the sound arrestor so I will start with that first and see how it goes.
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1976 911s targa
Old 03-05-2018, 08:20 AM
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Quote:
Originally Posted by old man neri View Post
So what exactly is the advantage of this over a normal muffler bypass?
The only difference is they have a resonator welded to the side to try and cancel out exhaust drone.
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Derrick
Old 03-05-2018, 08:22 AM
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Location: San Francisco & San Diego CA
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Wow. 116 dB is VERY LOUD! I've run my Lotus Elise fully uncorked / open exhaust at Laguna Seca (unlimited sound days) and it blew 115+ dB at the sound booth. Definitely required the use of earplugs.

I am considering giving the Bisi PC a try.

Thanks for the data.
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Frank Amoroso
1987 911 M491 Coupe
1988 911 M491 / M470 Cabrio
WTB / ISO: G50 911 M491 Coupe or Targa
Old 03-08-2018, 07:43 AM
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Eva
 
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Join Date: Apr 2012
Location: CT
Posts: 4,047
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If you just email Bisi for the sound readings, he'd send them to you.

The guy is an intensely detailed engineering mind...all this data already existed
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'78 SC Targa ~Eva~
'13 Suburban ~Birdie~
"The car has been the cave wall on which Industrial Man has painted his longings and desires." -Eddie Alterman-
Old 03-08-2018, 09:08 AM
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