Exhaust drone around 75 mph

[SilverXJ]

Since I finally got the Titan 4.Fail stroker back together with the Comp cam 68-231-4 I have had an exhaust drone at 75 MPH.. kind of annoying. Any how, I don't feel like making any major changes in my exhaust. What about changing the tip? I currently have a 2.5" in 3.5" out slant pipe. I was thinking about just trying to change the tip as i have heard they can make a difference. I have also seen tips with resonators, but have no idea what that would do. And suggestions?

[aaronkeiser]

ear plugs

[SilverXJ]

Did ya think that one up on your own?

[mecompco]

You could try a stock down-turning tip, I suppose--it might help. My Dynomax muffler with custom-bent pipe (to clear the 8.8) is straight out the back. I don't find it any more objectionable than the stock muffler/pipe w/downturn was, though.

[John]

Ok, the drone is your motor driving your exhaust system to it's natural resonant frequency kinda like a pipe organ works.
You can't totally stop it, as any given length of pipe has a natural frequency, but you can change the where this occurs (RPM).
Factory exhausts are sized to put the resonant frequencies above the operating range. To raise the resonant frequency of a system, shorten its length. Reroute, or go to a muffler with a shorter internal length, some are straight through, some run a internal folded path,etc.. If you can't shorten the system, then add slightly less than one wavelength of pipe. This will also raise the resonant frequency of the system. The wavelength in a single exhaust system 6 Cyl. is around 3.1 feet at 3000 RPM. Shortening the system by a foot will move a resonance at 2000 RPM to about 3000 RPM.
To move a drone, calculate the wavelength at the RPM that gives it. Then calculate the wavelength at the RPM you could live with. The difference is the length of exhaust pipe you need to add or delete. Add length to lower the RPM, remove length to raise it. If you want to raise it but you can't remove length, add a wavelength of pipe minus the amount you calculated. This will have the same affect. If you add or subtract a multiples of a wavelength, you will not change the rpm your exhaust drones, or resonates.


In a 6-cylinder, 4-stroke engine, there are three power strokes for each revolution, meaning that there's a power stroke occurring every 120 degrees of crankshaft rotation.

Wavelength in ft.=1100X60X1/RPMx1/3x1/2
Or:
Wavelength in Ft.=11000/rpm

wavelength=standing wavelength
1100=speed of sound in air in feet per second (rounded for ease of calculation)
60=to convert rpm to revs per second
rpm=rpm
1/3=3 cylinder firing per revolution
1/2=standing wavelength is half the wavelength of a normal wave

That or you have inadequate clamps to support your exhaust from vibrating.

John

[SilverXJ]

That all makes sense... but... it never droned before. The only difference in this engine is the cam and larger valves. Which don't really figure into your equations.

[John]

Unless we consider the exhaust is in greater volume and changed flow characteristics after the build. Your build may have produced the necessary change. Don't dismiss the brackets either. If you move mounting points, change the support tension, you can affect this also.
John

[gradon]

So the muffler has a 2.5 in and a 3.5 out and you have a tail pipe 3.5 to the back, or is it an adapter after the 2.5 out of the muffler and a short piece(the "slant")? It might not drone as much if it was a 2.5 tailpipe to the bumper(could be getting a lot of reflection from the ground/body, amplifying the sound). The loss of velocity from the 2.5-3.5 could be making that drone a lower frequency.

[SilverXJ]

Only the tip (the last 6 or so inches) is the 3.5" slant.

As for the brackets, I will probably disconnect the exhaust at the header and try to shift it. I know that that can be put on slightly different than previous.

[John]

That is where I would start. keep the unsupported lengths as short as possible. IIRC you had a exhaust tip, you also might try to shorten there also. Do the easy things first.
John

[Flash]

althow my jeep is a stock 4.0L......with 300,000 on the clock, the tail pipe and muffler were shot.

removed the muffler at the cat. and installed my spare stock muffler in its place. The tail pipe was rotted as bad as my original, so i cut it off were it when over the axle.

It had a vary bad vibration that cause stuff inside to rattle.......which real didn't bother me to much. But the exhaust blasting on the fuel tank and sending unit Did!

Installed a 90* dump, no interier noise, and was much quiter (My original Cat has became a much higher flowing unit, a long time ago )

Didn't affect how it ran at all.........But i can here that noise p.o.s. tranny now!


Flash