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Is my exhaust too restrictive now?

THEFERMANATOR said:
I have the stock DURAMAX downpipe on mine which isn't even a full 3" with several spots where it's reduced down some, then I have the 3.5" front stock section in it back past the torsion bar crossmember, and 4" from there out the back with a straight through CORSA muffler(You can drop a 4" diameter ball straight through it if you wanted to). I know I'm currently in the 400HP range at the wheels, and have gone well beyond 500 at one time playing with a hot tune, and the exhaust never seemed restrictive to me. I know SEVERAL guys have went BEYOND 600HP on 4" mandrel bent systems, even seen one guy do over 800HP on just a 4" system. Even seen where one guy who is using a STOCK LB7 exhaust on his truck with nothing more than a muffler delete(they're 3.5" press bent), did 638HP out of his. So I HIGHLY doubt a 4" system even with that short section of 3" downpipe is ANY sort of significant restriction.
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These are good examples. However I would say that these don't prove that the 3" pipe isn't a restriction, it just shows that these trucks made a ton of power even with a 3" pipe. There's nothing that says that these trucks aren't leaving 5 or 50 horsepower on the table because of the 3" pipe. If I get a chance I will try both just to see what it shows. It could show that 3" does lose nothing, but it could also show that my truck likes it or maybe all it does is reduce EGTs....that alone would be nice. Bench racing is fun, but hopefully we can find some true before-and-after results. I'd be happy to be proven wrong, at least there would be a confirmed answer.

JayTheCPA said:
If you do decide to try this, keep notes on how you do it as others might have an interest if the dyno results are supportive ;)

One thing to watch is the transition from firewall to floorboard is typically tight as IIRC the tranny cross-member and rear heater hoses make for congestion. So a 4" downpipe will eat into that already tight space.

And . . . a 4" downpipe will likely need heat treating right away as it will get even closer to the A/C elements than the 3" pipe.
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I definitely will take notes Jay! Yeah I've looked at the path and it definitely is tight, but if done right I think it can be done. It may take a multi-piece pipe to make it work because it may be impossible to snake a 4" pipe through there....it's tight just snaking the 3" pipe through there. And oh yeah, more heat-treating will definitely be necessary. Luckily I don't have the extra rear heater hoses, just the primary ones, but yes, it's still congested. That's why I now wish I had moved the turbo forward when I made my header. Ah well, hind-sight is 20/20 as they say.

FellowTraveler said:
Round to oval then back to round.....
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Yeah I looked into that, but I think a true 4" pipe can fit. I've also thought about compromising and doing a 3.5" pipe, which is 36% larger in area than a 4" pipe, but if I can make the 4" pipe fit I should probably just go whole hog and do it. BTW, a 4" pipe is 78% larger in cross-sectional area....it's amazing how much of a difference that 1" makes.
 
With the larger exhaust on my 69 Camaro- the biggest thing I noticed wasnt a big move up in power it was a quicker rev. Even while parked, especially the Camaro. 750 rpm up to 6,000 happened quicker.

I still haven't done the exhaust on my hummer yet. But I did on my other 6.5s. 4" Made a difference in egt. Watching the egt on hills in the Vegas heat and hills is a precurser to ect, so I learned to watch it close.

Don't expect miracles from it, but a dollar to a donut says it will be worth your while.
 
Will L. said:
With the larger exhaust on my 69 Camaro- the biggest thing I noticed wasnt a big move up in power it was a quicker rev. Even while parked, especially the Camaro. 750 rpm up to 6,000 happened quicker.

I still haven't done the exhaust on my hummer yet. But I did on my other 6.5s. 4" Made a difference in egt. Watching the egt on hills in the Vegas heat and hills is a precurser to ect, so I learned to watch it close.

Don't expect miracles from it, but a dollar to a donut says it will be worth your while.
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Sold!! I love donuts, but I'd be hard-pressed to say whether I like the smokey ones or the jelly-filled ones better.

Huh I just ate dinner and yet I'm hungry again....mmmmm.....donuts.....
 
Will L. said:
With the larger exhaust on my 69 Camaro- the biggest thing I noticed wasnt a big move up in power it was a quicker rev. Even while parked, especially the Camaro. 750 rpm up to 6,000 happened quicker.

I still haven't done the exhaust on my hummer yet. But I did on my other 6.5s. 4" Made a difference in egt. Watching the egt on hills in the Vegas heat and hills is a precurser to ect, so I learned to watch it close.

Don't expect miracles from it, but a dollar to a donut says it will be worth your while.
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Your Camaro was super or turbo charged, yes?
 
It made a difference when n/a and then later added the supercharger and it needed bigger again. So I built it to everyones spec. 400cid with 6-71 and every one said dual 3 1/2" was more than enough. Later i went to dual 4 1/4" id. I did it just to be louder and more obnoxious, but found quicker rev yet again.

Not surprisingly, racetrack day was headers with about 14" of collectors...

If long exhaust with a specific sized down diameter was the best choice for power in diesels, i dont think we would see the short, fat stacks in tractor pulls.
 
Will L. said:
If long exhaust with a specific sized down diameter was the best choice for power in diesels, i dont think we would see the short, fat stacks in tractor pulls.
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Right on. N/A needs the specific size and length to allow the exhaust to help draw intake air in. Supercharged needs to just get everything out as fast as possible. Now of course at certain power levels there will be diminishing returns as the size is increased and that's the line we're talking about here. 3" is easy to procure and fit and probably doesn't hinder much, but what if we knew how much it does or doesn't hinder? If I could just do a fat stub out the hood I would, but the world around me and truth be told, I myself would hate it too.

So pages later I think the answer to my original query is that no, my exhaust is probably not over restrictive now, but since my curiosity is piqued, I'll see if I can do some testing to see if it's leaving anything on the table. Who knows, maybe I'll make a little stub to run with the hood off just to see......:jimlad:
 
Even with the loss built into the IDI diesel compared to the DI your in a realm where no one else that I know of has really gone on a street 6.5td I suspect you'll find a good solution.

Great work!
 
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