I was under my truck checking out the downpipe after reading about them, surprisingly it only has one area that is dented and in pretty good shape. As I looked further back I saw in the cat a square about 3"x3" where three sides were cut (opened) and welded back together... what the heck? I don't think the opening is large enough to remove the baffles. I'm left with the thought that it got plugged and someone cut it open to empty it. I also was confused by the offset in/out muffler. I read that diesels need straight through mufflers. Is the stock one offset?

Obviously the best thing to do would be a whole new exhaust, but I really shouldn't spend the money right now, should I be worried about the mysterious hole in the cat or the (gasser?) muffler?

thanks again

My exhaust is stock and it has an offset muffler.

I replaced my downpipe and crossover on Sunday with a Flowmaster kit.

When I took out the stock downpipe, I thought "what a rip!". The stock downpipe didn't look THAT bad. Most of the forums made it sound like the thing was smashed shut.

Anyway I put the Flowmaster kit on anyway. HO-LEE-COW. There is a MAJOR difference in sound of the turbo (I can actually hear it!). I don't know which did it, the X-over or the downpipe but, something changed.

My next step is to make my cat look like yours by cutting a big hole in the top, gutting it, and sewing it back up.:cool:

When I took out the stock downpipe, I thought "what a rip!". The stock downpipe didn't look THAT bad. Most of the forums made it sound like the thing was smashed shut.


:lol:

Guess when my pinnacle arrives (Wed I hope) I'll see what my factory downpipe looks like, but since I'm putting on a 4" system the factory pipe needed to go anyhow.

You bring up an interesting question. I wonder how much turbo pressure/spooling benefits from an aftermarket crossover? Or is it necessary to open the rest of the exhaust to give the pressure somewhere to go.

Chaser

I wonder how much turbo pressure/spooling benefits from an aftermarket crossover? Or is it necessary to open the rest of the exhaust to give the pressure somewhere to go.

I don't think there is any performance benefit with the aftermarket crossover. It is mandrel bent versus the crinkle bent stock piece, but the stock unit isn't really restrictive in the bends. Mine had to be replaced due to a rust leak.

In terms of the crossover...I really don't know. I should have started it up without the new crossover to check it. But...the downpipe looked like a bear to replace with the crossover in place.

wagonwheeler...where you too will say "what a rip!" is when you get your 4" system and see that the downpipe is still a measly 3" unit...albeit with the minor dents removed.

One could probably get a 4" DENTED unit in for the downpipe but a mandrel bent 4" downpipe...not a chance. Just ain't no room fer it.

Here's a pic of what my stock downpipe looked like and the part of the exhaust that went by the rear AC lines.

My two cents worth would be looking at the down pipe as a whole. The smallest restriction is going to be the max flow available. Even though probably 90% of the linear pipe is not significantly malformed you are only going to get the flow rate equal to that of the smallest area. If you look at the part that's kinked and then look at the smallest part of the 3" Mandrel formed (that would be 3") the volumetric flow rate is more like close to double. Now do the same for the cross over pipe. Going from stock to the after market mandrel formed is more like only a 10% - 15% gain. I look at the x-over pipe for more of an engine exhaust balance.

Here's a pic of what my stock downpipe looked like and the part of the exhaust that went by the rear AC lines.
I thought those were AC lines too at first, but they are actually coolant lines for the rear heater core. I just replaced them recently when I changed all the hoses on my 'burban. Its a scary spot for heater hoses, but the heat shields seem to do their jobs, and they weren't in too bad of shape. The worst condition hoses on my entire truck were all the way in the back going into the rear heater core, behind the right rear wheel well. Those hoses and clamps were shot!

My two cents worth would be looking at the down pipe as a whole. The smallest restriction is going to be the max flow available. Even though probably 90% of the linear pipe is not significantly malformed you are only going to get the flow rate equal to that of the smallest area.
I've done a bit of exhaust ventilation design (not engine exhaust) but the principles are the same. The maximum flow of a system is not solely a factor of the smallest restriction. A short, quickly passed restriction does not impart that much energy loss to the gas stream (unless the restriction is very small in relation to the pipe diameter). A long 4" exhaust pipe with a short 3" restriction will have a flow rate closer to a 4" pipe than to a 3" pipe.

A long 4" exhaust pipe with a short 3" restriction will have a flow rate closer to a 4" pipe than to a 3" pipe.

Right on bro!

That's what I've been trying to say in my own idiot way in other threads. The reduction in flow is not a linear equation relating directly to a small deformity. It's non-linear and the reduction grows in that fashion as the deformation becomes a higher percentage of the whole.

Or in other words...yeah...what you said.:)

It's like running 1 1/2" pipe from the outlet of a 1" water meter to your house. You'll get more volume than if all you ran was 1" line.

That's basically how I've looked at the 3" downpipe relative to the 4" system.

Chaser

I've done a bit of exhaust ventilation design (not engine exhaust) but the principles are the same. The maximum flow of a system is not solely a factor of the smallest restriction. A short, quickly passed restriction does not impart that much energy loss to the gas stream (unless the restriction is very small in relation to the pipe diameter). A long 4" exhaust pipe with a short 3" restriction will have a flow rate closer to a 4" pipe than to a 3" pipe.