My friend has a 1999 2500 Cummins. Its 100% stock at the moment except for upgraded break rotors. He just bought it and wants to make the exaust louder. We read several posts on these boards about exaust modications but couldnt quite find the answers we were looking for. So here is his situation and the questions we have.

He wants his exaust to be a deep throaty loud sound but not too loud so that is can be obnoxious or cause much cab noise. Originally he had planned to just straight pipe it and remove the muffler and cat. However after reading these forums we discovered that doing that will cause a loss in back pressure. We dont want to cause any problems by a lack of back pressure. So now hes concidering just leaving the cat for the backpressure but using a muffler delete pipe to increase the exaust sound. Its a nice truck in excellent condition and he wants it to last him quite a while, so he dosnt want to do anything that might mess up the turbo or MPG much. He eventually plans on buying a full exaust pipe from the turbo down. But for now simply wants it louder and done for cheap.

So here are the questions we have:
[blockquote]1. What difference does the diameter of the pipe make? I know many people opt for 4" instead of the stock 3" Why is this and what are the pros and cons of going bigger?
2. Utah does not require us to take emissions tests so the cat isnt nessisary. But do you need it for the backpressure?
3. I know the full exaust kits go from the turbo to a small muffler and out, do these kits loose any backpressure?
4. What are some low cost options for making the exaust louder but leaving it relativly stock.
5. What good does going 4"-6" on a stright pipe that bypasses the muffler and cat when it narrows back down to 3" or so up to the turbo?
6. How much power does the cat actually rob if any?
[/blockquote]

 

Welcome to DF!

1 - For a stock truck or one with mild mods, 4" exhaust is plenty. Any bigger than that an you are just pushing dead air. Until you start adding bigger turbo's, twins, injectors, etc, you don't need bigger than 4"

2 - You DO NOT need or want backpressure on a Turbo Diesel. This slows the spooling of the turbo and creates lag. It's not like a gas engine that needs the a little bit of backpressure to help the next cycle. It gets all the backpressure it needs just from the turbo.

3 - You want an exhaust that has the same size inlet and outlet on the muffler as the rest of your exhaust tubing. If it gets smaller at anypoint, that just causes a restriction.

4 - If you want to keep it relatively cheap, just do a muffler swap, that is the restrictive part on the exhaust. On a '99, I don't believe you even have a cat, it should jsut be a resonator.

5 - Does no good. You would still have the restriction in the 3" tubing.

6 - Cat's rob VERY LITTLE power on a well maintained vehicle. If the vehicle is blowing lots of smoke, burning oil, or you have the power cranked up, it can clog and this robs power. But, on a stock or lightly mod'd truck, you'd never notice the removal of it. Again, I don't think you actually have a cat, I think it's just a resonator.

 

i have an 03...stock from the factory there is no cat. so i just cut out the muffler last weekend with a sawzall and slapped a 3.5inch straight pipe in there. along with my s&b intake i notice the turbo absolutly SCREAMS now and i have very minimal lag. i love it. if you ask me...the truck sounds like a 747 going down the highway...it turns heads like crazy. with the straight pipe it did get louder but its not unbearable. its a nice tone i think and it was deffinately a wise thing to do. i lost mpg's a little. i was up around 16.4-16.6 now with the intake and the muffler delete im around 15.8-16.2 so its not too too bad. if you are doing the muffler delete i suggest going to an auto part store and buying a piece of straight pipe and installing yourself. if you look on genosgarage.com they have ones already made up for $96. i bought a 6.5 foot piece of straight pipe (only used about 3' 4", i bought the extra just to have) and 2 3.5" band clamps for $70. took about 1.5 hrs to do. sawzalled the muffler right off...held the tailpipe where i wanted it...measured up for the straight pipe and cut it...put a couple tack welds to hold the tailpipe straight and then welded as much as possible (some of the gaps were too big to weld, but thats why i bought the band clamps, they r 2.5" wide)...put some high temp RTV silicon on the band clamps and put them on nice and tight. no leaks and rock solid. but thats my 2 cents... Hope it helps

 

dodge didn't put Cats on the trucks until the 600 came out (04.5+) so that is no big deal on your truck

the best exhaust after a turbo is NO EXHAUST, my philosophy on this is aim the smoke where you want it to go and leave it at that.....
I have a 5" pipe thats about 2ft long hangin off my bottom turbo, thats it, thats all I need, and its not UNGODLY loud when I'm at highway speeds, it gets that way when I'm accellerating though (BIG SMILE)

I would suggest getting a Flo-Pro kit, they're relatively cheap, have a GREAT sound, and are EASY to put on..... thats what I had on my truck before I put the twins on it.....

 

Aero - Aero - Aero ....

 

Thanks for the quick response.

What exactly does the resonator do?
So a diesel does not need backpressure at all on the turbo in order to maintain MPG and midrange power?
Yet there seems to be another faction that maintains that backpressure after the turbo does not cause any loss. Its hard to know who is correct =)

Also would you just delete pipe it or get rid of the resonator too?

 

2 of those 3 sound just like what would happen in a gasser, and sound as if they don't pertain to turbo engines in the least....

backpressure on the engine is maintained by the turbo itself, if you're up high rollin on the boost and let off the fuel, your turbo will actually suck the air out of the engine (this happens in larger turbos and is more noticeable in my truck now after the twins)

I have NO loss of lowend with the exhaust hacked, it actually causes my turbos to spool harder and faster with less throttle input. from having the edge box on my truck and going from the full exhaust to the hacked, my boost comes on smoother, faster, and much sooner with no exhaust.

your turbo itself will hold backpressure on the engine at normal operating conditions, it acts as its own regulator on the exhaust side of the engine. It also works off of the principle of MORE AIR IN = MORE AIR OUT, our engines are air pumps (as all engines are) so the more air you get in there, the better of you are......

when I had the stock turbo, stock IC, and all that (no edge box either), I gained 2MPG by changing my stock exhaust to the Flo-pro 5 inch with the cat delete...... went from a best of 16 to 18 on my normal driving trips (about 3 hours in various speeds, with mountains to climb, going from central to eastern KY)
I picked up 1 MPG when I hacked the exhaust off after the twins on trips.... when I am easy on it, I can get 21 over a 4 hour trip at constant highway speed with the cruise rollin 75-80 (going from central KY to Pigeon Forge TN)

 

Nice, thanks. We talked to a local diesel performance shop near here and they just laughed at the notion that back pressure after the turbo does any good.

Its nice to hear you can bump up the performance without sacrificing MPG. Hes actually getting around 18-21mpg right now.

I think hes decided just to ditch the resonaitor and the muffler and just straight pipe it all the way back for now. Down the road we'll look into some more extensive upgrades.

Also...would putting a K&N air filter in it along with the exaust system help or hurt the MPG?

 

it helped mine, but I will warn ya, its a slippery slope that you're walkin on.....

you start here, go there, and end up goin nuts.... diesel boost is addictive as hell, ya get exhaust, then a CAI, then a box, then ya go for a single turbo, or nitrous, or propane, or somethin, the list never ends.....
modding any vehicle is kind of like that, but when you get the gains that you do out of diesels, espeically and still retain the mpg, and make an all around better vehicle.... just makes it that much more dangerous for the wallet (thank god my girlfriend loves my truck and wants me to make it faster than it already is)

I started like that, and now I'm to the point that I'm getting ready to TST box it, FASS 150, then comes the head studs, more boost, injectors, nitrous, propane........ you get the idea.....

 

Yeah I hear thats how they all start. I do know he plans on putting some pillar gauges in next, along with a K&N intake, chrome door handles, chrome gas cover, chrome exaust tip, and of course new wheels and tires. Shes Red with silver on the bottom, so hense the chrome stuff. Chrome just looks soooo good against red imo.

Once he gets her looking & sounding sweet im sure it wont be long until he starts into getting her to perform better.

Thanks for your help.