Here's a question for you guys....would changing the cam to a higher or lower duration exhaust lobe change the resonant frequency of the engine? Or is it just # of cylinders and exhaust length that affects it?

 

I believe this is the same science used in the factory 6.0 mufflers in a 2500HD, for example. dual in and single out.

 

The frequency of the sound is from engine speed and number of cylinders. 2000 rpm/60 seconds = 33.33 revs per second. 33.33 revs per second * 4 firings per rev = 133.33 hz. If the resonant frequency of your exhaust system (and everything has a resonant frequency) is near 133 hz, there's your drone.
The exhaust typically makes 2 or 3 passes through a stock muffler, which effectively adds length vs a straight through 1 pass muffler. The factory engineers know how to figure this stuff out.

 

OK, so doing a little more driving with my truck and playing with it, here is how it drones. 2000rpm, no drone, I like it----2500rpm twice as loud with drone----3000rpm, no drone I like it too. 2500 is about 77-78mph, right where I like to run in the highway. What kind of suggestions do you guys have? I really can't make the pipe I have longer, cuz it will have to be lower (less ground clearance), or it will run into the t-case, but I was thinking maybe a pipe in a pipe like the Corsa has??? How long you think I should need for that? Any ideas on what to try next?

 

i think the 90* angle is critical to cancel the drone, it can bend after the T, but the sound waves must re-enter the exhaust stream at 90*

 

Would multiple branches work just as well as a single longer branch? The spacing would have to be precise I suppose, maybe half a wavelength offset, otherwise you would be building constructive interference, negating the whole thing or possibly making it worse.

 

No, I dont think so. Quarter wave resonators dont work that way. With a certain length pipe, there is only one stable frequency that can form a quarter of a wavelength in the pipe. There is a formula: length = 1/4 wavelength

Here's a link, scroll down to closed pipes if you want to read up on it. Closed Pipes

 

kb, I'm thinking now the resonator(s) may need to be back in the tailpipe(s). If I understand from your previous posts, you have a 4" pipe into your cutout. Then after the cutout it y's into two 3" pipes, each feeding a magnaflow muffler. What do you have for tailpipes?

Everything I read over on the Mustang forum, and there's a lot to read, ( http://forums.corral.net/forums/show...hreadid=496793 ) suggests two 24" resonators in the tailpipes to move the drone out of the 2000 rpm range where they were having the problem. The Mustang guys are running true duals with an H pipe.

Before I did my resonator, I took the tailpipe off and replaced it with a 12" turndown...shortening the tailpipe by about 4 feet. That eliminated the drone, but it was louder than I cared for and I wanted a tailpipe. The tailpipe on my truck is clamped on with a band clamp, so it was easy to try. By the way, the Autolounge calculator says shortening or lengthening the pipe 4 feet will move the drone from 1600 to 7200 rpm, so it's about right there.

I'd suggest you try the resonators in the tailpipes next if you want to continue with this. I made a 1"x 2" hole in my tailpipe so I could cover it with a band clamp if it didn't work, since I don't weld. Apparently you do though, so you have some other options.

 

It's 4" off the turbo, back to about the end of the transfer case with a 3 1/2" cut out right before the 4" ends. Then it splits into duel 3" with 2, 24" center in, offset outlet Magnaflow mufflers, then over the rear axle and exits at a 45* angle out the pass rear of the truck. Here is some pics.









Any suggestions you may have would very much appreciated

 

i have an idea... what about putting a bullet muffler of sorts in the resonator, a resonator in the resonator...i guess, might be worth a shot if you had one just layin around.