Pipemax exhaust design

[Warpspeed]

I agree, an absorption muffler, even a very good one is no substitute for dynamically throttling the exit area of the expansion box.
The flow restriction located right at the very back of the system is a CRITICAL feature of achieving significant noise reduction, especially the classic heavy droning at particular road speeds.

Tweaking the system back pressure up and down is about as effective as turning the volume control knob on your stereo.
There is also another less well known effect going on here.

A positive exhaust pulse is reflected from the end of an open pipe as a negative pressure wave reflection.
We all know that, and use it to tune our induction and exhaust system lengths to specific rpm.
A positive exhaust pulse is simply bounced back from a dead end blockage as a positive reflection wave.

The trick is to just have sufficient opening area right near the end of the pipe to completely absorb the pulse so there is no reflection at all back up the pipe.
This is known as impedance matching, and will absorb and damp out any buildup of pipe resonance if the opening size exactly matches the exhaust flow. Too big a hole looks like an open pipe, too small a hole like a dead stopped end.
You can tune the hole size to completely null out any pipe resonances, even multiple resonances !!
The practical problem with doing this is, that the perfect optimum exit hole size varies with the rate of gas flow.

A spring loaded flap or maybe a controlled poppet valve can open and close to maintain a constant (low) back pressure.
This not only drastically reduces the whole noise spectrum, it also prevents the buildup of the classic resonant drone at particular road speeds. So the droning is not just made a bit less, it virtually disappears altogether.

Another point about all this.
If you have a single exhaust system on a V8, a single exhaust expansion box x8 individual cylinder displacement will work fine.
That does not mean that a single cylinder engine can get away with a x1 expansion box.
It still needs all the x8 volume for each exhaust "pop" to dissipate into.

So if your V8 has dual exhausts, you really need two expansion boxes each x8 or larger.
An H pipe will certainly help a lot if this is not possible, as each exhaust "pop" is split between both expansion boxes. If your system has to be made small, definitely use the H pipe.

If you have a ton of room, and big dual expansion boxes are practical, then the H pipe is probably not going to make such a big difference.
If fitting it across the drive line is really difficult, leaving it out is not going to hurt.
I suspect an X pipe would work better than an H pipe for dual expansion boxes, but I have never compared the two for noise reduction. It may make no practical difference, I really do not know.

[swatson454]

I can tell you from 'sperience that twin boxes with an h-pipe is much quieter than no boxes with no h-pipe. I found that out after blowing the boxes apart for the last time and finally cutting them out.

Vizard never mentioned how stout they have to be to stay together

[Rick Finsta]

Necropost, but I wanted to see if anyone on here has current experience with the Dynomax VTs? I'm putting exhaust on the car in the next few weeks and if these will work for a mid 11 second street car and kill the drone I am all for them.

Lots of older posts about clunking noises that were supposedly fixed but I'd like to know if these are still out there working fine after several years before I weld a set of them into a brand new exhaust system!

[David Vizard]

belgiquebasterd.

Walker Mufflers - one of the largest muffler companies in the world, quotes the flow on the boxes the mufflers are in and on their web sites.

Of note is that the flow numbers are very realistic and usually a couple of percent more than on the box.
DV

[Truckedup]

belgiquebasterd.

Walker Mufflers - one of the largest muffler companies in the world, quotes the flow on the boxes the mufflers are in and on their web sites.

Of note is that the flow numbers are very realistic and usually a couple of percent more than on the box.
DV

Is that on all their mufflers or just performance? I recently installed a 18823 OEM replacement Walker Sound FX on a Chevy truck and there was no flow numbers on the box or their website...

[Rick Finsta]

I've got conventional mufflers that work great I'm specifically looking for feedback on the VTs mostly because I want to quiet down the drone a bit if possible. Driving 55 miles each way to the track at 3500rpm gets a bit rough.