Muffler choice débâcle!

[sewlow]

Street Racing back in the '60's & '70's. Lol! I was there! Still haven't quit!
From the time I was 5, the local street track was 3 blocks away! 6 lanes wide, 2 miles long.
In the Summer, sitting on my headboard, I could listen to 'em go from my open bedroom window. Watched from the sidelines on my Mustang bike in the later '60's/early '70's!
I began to hang around all the old old farts! To them, no question was a stupid one! They taught me a lot! They'd show me how to do things. Not just tell me. Showed me! By making me do it!
Joined the fray in '75 with my own vehicle!
Nothing better than hearing an short geared SBC power-shifted @ over 7500rpm in the close distance!
It's a compulsion! It's a calling! It's a disease!
A redline a day keeps the carbon away!

Anyways..
Back when I had my BBC Nova, one of the first things I did, within 5 or 6 days of buying it, was to have an 'H'-pipe installed by the muffler shop across from where I worked.
We did the crayon trick.
Well, I got on it right outa the parking lot. Had to! Didn't drop the clutch. Just powered through the gears. The seat-of-the pants-feel was apparent immediately! The low end grunt shredded the tires 1/2 way through 2nd. (Drivin' into it, counter-steering! Yeehaw!)
The track numbers proved it! 3/8's of a second in the 1/4, averaged over 5 passes!
And it sounded tougher @ an idle, too.

 

[drewcrew]

Really good times back then. I wish it was still like that around here. Between the state, city, county and township Police street racing is dead.

 

[michael hurd]

An exhaustingly (!) long vid from an engineer's P.O.V. I like what he says about headers, particularly the part about the 4*2*1 Tri-Y's, starting @ about 6.45.

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Good video, but I would like to add that the quoted 1100 ft/sec sound wave velocity is based upon room temp, and as far as exhaust goes, is not good to assume.

As the temperature rises, the sound travels faster, and relative humidity also affects sound wave propagation. Obviously, at idle, the temperatures are low, compared to part throttle or even full throttle pulling applications.

I can tell you that I have measured an exhaust outlet temperature @ the collector of 140F on a long tube open header equipped 9.7-1 compression, LT4 hot cam equipped vortec 350 with a Performer RPM intake, 750 Edelbrock, at an idle of 750 rpm. This engine did not like to idle that low, however that was where a vacuum reading was taken according to the racing rules of a certain body.

Headers were off the shelf Hedman units, IIRC, 1.5" primaries and 3" collector, designed for the 2wd 78-87 pickup chassis.


https://www.nde-ed.org/EducationResources/HighSchool/Sound/tempandspeed.htm

"
Temperature and the speed of sound

Temperature is also a condition that affects the speed of sound. Heat, like sound, is a form of kinetic energy. Molecules at higher temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly. The speed of sound in room temperature air is 346 meters per second. This is faster than 331 meters per second, which is the speed of sound in air at freezing temperatures.

The formula to find the speed of sound in air is as follows:

v = 331m/s + 0.6m/s/C * T

v is the speed of sound and T is the temperature of the air. One thing to keep in mind is that this formula finds the average speed of sound for any given temperature. The speed of sound is also affected by other factors such as humidity and air pressure."

Wrapping headers will keep heat in, and velocity up, which is beneficial for a street vehicle, however may not be the best choice for a header in a motorhome pulling up a long grade with limited cooling, retarded timing and high EGT's. Retarded timing leads to the combustion event happening closer to the exhaust valve opening point, and a lot of heat is directed out of the port.

Two strokes are even more susceptible to changes in the pipe temperature, and for this reason, a lot of factory tuned pipes now have integral thermal treatments.

Here's a calculator to find out the speed of sound vs temperature, it's used all the time in the audio engineering field.

http://www.sengpielaudio.com/calculator-speedsound.htm

Plugging in the numbers for 1000F temperature, the speed of sound is actually closer to 1873 ft/ sec rather than the 1100 ft/sec quoted in the video.

Changing the temperature ( or rather, keeping it more constant with wrap and coatings applied to the exhaust ) will change the effective tuning of an exhaust header.

 

[michael hurd]

Wavelengths.

You can have anything from full cancellation at low frequencies with a half wave reflection ( 1/4 wave distance to a boundary + 1/4 wave distance back = 180 degree phase rotation )

Assume for a moment, that you have a subwoofer in your home, and you have a boundary that is 3 feet away from the cone ( point of sound propagation ) you will have a corresponding dip in the response that correlates to the half wave reflection. ( 1/4 wave distance )

At around 94 hz, you should have a dip in the response.

The point of sound propagation can be from the neck of the voice coil, or slightly toward the cone edge, based on how stiff the cone is, and a number of other factors.

For full summation of acoustic wavelengths, you want to be within 2/3rds of a wavelength or less.

Phase rotation.

Phase of two signals can be expressed as a point on a 360 degree circle. As two signals combine with the same amplitude and phase, the combined amplitude is double what each source was.