Contour Enthusiasts Group Archives Forums General General the old topic of rasp back again

After a recent removal of the resonator I came upon the issue of rasp. I read through many of the archive posts and discussions about rasp and how people feel about it but one thing that I noticed and was also noticed by others was that after about 5-10 minutes of warm up time the rasp went away. Now I got to thinking, why it goes away when it warms up and what can be done to the exhaust to keep the resonator out and not add anything like a high flow resonator or a muffler in its place to give the sound of the warmed up non-resonator sound and not have to deal with the warming up of the car/exhaust and the rasp.

ANy suggestions has anyone else thought of this and maybe had ideas.

dan

Nothing can be done other than add a hi flow resonator or just deal with it.

It's always been my experience building cars for many years, that rasp was always a problem when the exhaust diameter was too small. 2 1/4" pipes on V-8 cars for example always were raspy. Then if you went to 3" exhaust the rasp was prettymuch gone. Another factor that always was important (w/ dual exhaust anyway) was it would rasp a lot until you welded in a crossover tube between the headers and the mufflers to connect both bank's exhaust pipes (so the opposing side's sound waves would cancel out each other since the cylinder's firing pulses are closer together), but that isn't an issue with single exh. Contours though since all 6 cylinders are firing into one pipe. But if you just increase the diameter of the pipe all the way from front to rear, the rasp should go away, since you will have much less airspeed in the pipe at higher RPMs.

Also, the reason why it is more raspy when cold is because IR energy (heat) absorbs soundwaves, which is why a hot exhaust system is always quieter than when cold

Sound and heat are related in that they are both forms of electromagnetic energy.

You need to do some reading in order to understand the concept:

http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Electromagnetic_spectrum#Infrared_radiation
(scroll down to InfraRed heading)

http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Sound_wave

http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html



In the diagram above, if you were to keep going to the right beyond radiowaves, you would eventually get to a point called VF which is voice frequency, which is the frequency in which humans talk and hear at (20hz-20,000hz).

IR (InfraRed energy), or more specifically, Mid-InfraRed energy (30 THz to 120 THz) is known as 'heat' to us. When these mid-IR rays are bouncing around inside the exhaust pipe (the medium in which the sound will travel), it will change the characterisctics of the sound traveling through it.

See this page:

http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Speed_of_sound

Sound travels differently based on temperature. Under high heat, soundwaves travel faster, thus making the higher pitch frequencies (i.e. rasp) bounce around with faster frequency early on in the exhaust pipe and thus their frequency widens to lower frequency within the pipe before it even makes it to the outlet of the pipe (sound waves always widen as they travel distances and bounce around off things- thus lowering the frequency in hz). So for this reason, many high pitched sounds are dissipated inside the pipe when its hot.

This is the best explaination I can come up with to try and explain this concept while still keeping it in simple enough terms so it will be understood

this is exactly the kind of info i love.....guys, please continue to explain things in as much detail as you see fit to use. if we (i) get lost we (i) can always ask or look it up. thanks