Cobra SPL

[valkcan]

I was wondering if anyone here tested the sound pressure level of their 6 x 6 cobra's? I took mine off last year and switched to OEM because of the new bike law they passed over here about loud bikes. The law is really coming down on the noisy bikes. A friend running V&H on his 1800 Boulevard just got a $160.00 ticket. I kind of miss the sound of my Cobra's but at $160.00 a pop, I won't put them back on before I get feedback.
In any event, I tested the OEM pipes today with my rat shack meter. I did it the same way the cops do it other than having the cheap meter. I put the meter 20" from the top of the pipe at a 45 degree angle and ran it to a steady 2500 rpm. I did it 5 times for consistency. It tested 95 decibels.

Now, I find this quiet compared to the Cobra's and I don't think there is anyway that the Cobra's will make it at 100 decibels under the same measurements. I sent an email to Cobra and they replied that they don't measure the SPL of their pipes. I plan on wrapping the baffles in fiberglass and maybe weld a big washer on the engine side of the baffle to quiet them down a bit. I think they will still retain their rumble and hope they will meet the 100 decibels A weighed law.  Anyone ever test theirs and what were the results? Do you think glassing the baffles + the washer mod would help?
Don

[salty1]

Don you never said what State has the 100 dB max.? I have wrapped my baffles in the past with some success of quiting the exhaust note, but not enough in my estimation. Your thought of the washer is intriguing and may work. I wonder if it would impair the normal performance of the bike? Have any thoughts as to the washer office size?

[valkcan]

Salty1
I am from the Montreal area, Qc, Canada. That law came into effect last year. The cops have a list of aftermarket exhaust and if your exhaust is not OEM, they check on their list and ticket you. This is not general of every cop here but, like everything else, it only takes a few to spoil the pot. I think those few are all concentrated here. They do not automatically test your bike. But I want to be prepared if I have to go to court and they decide to do so.
I got the washer idea reading many posts on different forums. I was thinking of tack welding either a washer or a freeze plug with a hole around 3/4" on the engine side of the baffle. This would probably increase the backpressure and may even help to re establish original exhaust performance. It's a trial & error at this point.  I would like to have a baseline of the SPL without having to remove my factory exhaust and install the Cobra's just to take readings. If anyone has the Cobra's with the baffles in and an SPL meter, it would take around 5 minutes to complete this test. Might even be interesting to have an idea as to  why  your neighbor is always complaining when you leave for work at 5am
Don
 

[pancho]

Hey valkcan,, I have a Radio Shack decibel meter and cobra pipes,, I will take some readings later today and let you know what I find.

 I have 4 inch baffles in my pipes as there were none when I purchased it, and that was just too loud, for me and everyone else.

[R J]

Many moons ago I had Cobra's on MGM.

Like you said way to noisy, especially with WOT.

It would rattle windows.

I welded a plate on the front of the baffle, engine side.     After mucho experimenting around, a 1/2 hole gave me the most back pressure and made the pipes about 15 to 20% quieter.

Made MGM run like a poor old dog with his butt creosoted.

Then I went to Super Trapps, to MarkT glass paks to Viking which are still on MGM as of this writing.

[pancho]

I just did the test from approximately 20 inches and 45 degree angle from both the side and rear.



There are two different weighing factors that can be applied, with the "C" weighing having an extended bandwidth frequency response, (down to about 40 HZ) here is what I got.

This is with 4 inch baffles installed in the Cobras,, seems close to the laws tolerance level. I know that I can ride this bike all day long with no ill effects from the sound,, no droning, nothing.

From the side 2500 RPM       A weighing   95 DB
                                       C weighing  101 DB

From the rear                     A weighing   95 DB
                                       C weighing  100 DB

Hey RJ,,  so you think that partially capping those baffles gave you a bit of performance increase??, sort of makes sense.  Might be something I need to play around with.

[R J]

I just did the test from approximately 20 inches and 45 degree angle from both the side and rear.



There are two different weighing factors that can be applied, with the "C" weighing having an extended frequency response, (down to about 40 HZ) here is what I got.

This is with 4 inch baffles installed in the Cobras,, seems close to the laws tolerance level. I know that I can ride this bike all day long with no ill effects from the sound,, no droning, nothing.

From the side 2500 RPM       A weighing   95 DB
                                       C weighing  101 DB

From the rear                     A weighing   95 DB
                                       C weighing  100 DB

Hey RJ,,  so you think that partially capping those baffles gave you a bit of performance increase??, sort of makes sense.  Might be something I need to play around with.

You do some restricting in the system, it builds torque.   Not much, but noticeable.        I could lift the front wheel off the ground going into 2nd gear.   3rd would extend the forks, 4 & 5th, nothing noticeable.

[pancho]

Hmmm,, I may need to get a second set of baffles and try that out just to see,, maybe get back some of that "Cobra 6X6 lost HP" . Thanks RJ

[valkcan]

Thank you Pancho. That is what I needed although I have a hard time believing the results. Was the mic at 20" and 45 degrees to the side from the end of the longest pipe and level with center of the rear exit?
Your result with A weighing are what I got with my OEM pipes and my Cobra's are a heck of a lot louder than that and I have the original baffles. (about 16" long). The mic should not be higher than 8" from the ground.
Don

[pancho]

Hey valcan,,, those are the results I got,,, no I did not use a tape measure and angle finder,, just approximate on both. In your first post you did not specify

" 45 degrees to the side from the end of the longest pipe and level with center of the rear exit?"   

you said 

" I did it the same way the cops do it other than having the cheap meter. I put the meter 20" from the top of the pipe at a 45 degree angle and ran it to a steady 2500 rpm."   

SO, I measured 45 degrees on a vertical incline 20 inches from the pipe as the picture shows. 

So,,,, the results are probably skewed.

Bottom line, If the limit is 100 Db, and the cop wants to give you a ticket, even from the way I took the measurements,  he surely can.

If I get a chance, I may take a measurement from the position you described in the last post,, it will be tomorrow if I do.

[valkcan]

Hey valcan,,, those are the results I got,,, no I did not use a tape measure and angle finder,, just approximate on both. In your first post you did not specify " 45 degrees to the side from the end of the longest pipe and level with center of the rear exit?"   

My bad, I did not explain very well. I tried copying the pictures and procedure but it's a pdf document and it is password protected so, no can do. I know that if they want to, they will ticket me if they feel that it's to loud. If no one says anything, and most people don't, they will continue without considering if your exhaust is legal or not. I just wanted to know how much louder these aftermarket pipes were compared to stock. I can't even get 80db at idle out of the stock ones and if memory serves me right, that is what is suppose to be stamped on most stock exhaust, independent of manufacturer. Too bad the aftermarket folks don't publish this as part of their specs because I think that event in the states that have this law or are considering implementing it, the measuring procedure is pretty close to what is done here. But the law aside, I wanted the measurements more for information purposes.
Don

[pancho]

OK, I took a measurement from the position described in the last post and here is what I got valkcan.


c weighting SPL 104 Db  Now that is substantially different as 3DB is double the energy or pressure. Keep in mind my Cobras have only 4 inch baffles in them.

[valkcan]

Tanks Pancho. That is more like it. Mine are the original baffles so maybe with a bit of fiberglass and the washer mod, I could quiet them down enough not to be bothered, unless I really crank it up hard. I  will get some gaskets and see what can be done. It wont be for now because I have so many trips planned but, I will compare measurements when I do.
Don

[Joevalk]

I believe mine will be getting a ticket in your area. Might be a smidge over a 100 decibels

[Len]

Talk with Cobra, they make a longer, over 12" baffle that is much quieter and improves low end tq. compared to open Cobras.