HOHN

You can keep your mpg and still get a lot of the 5" rumble by only doing 5" on the back half.

I'd recommend a 4" downpipe and initial section, and a 4" muffler. Then expand it to 5" after the muffler and complete the system.

By going with a 4" muffler instead of 5", you have more (and better, imo) options for mufflers without hurting flow or going too big on the pipe.

The muffler will slow down the flow a bit, so the "overcooling" negative that is usually associated with a 5" will be lessened.


The reason that a 5" is often cited as losing power or MPG is not so much the fact that it's 5". It's that fact that 5" is so much bigger than the turbine outlet.

If you have a large single with a big housing and pinwheel (say, SPS66), then even a 5" DP will probably not cause a HP loss.


The problem comes when you have a tiny hot side on the turbo (like a stock 35 does), then you try to transition that into a huge 5" pipe right away. This expansion is too much too soon, and this is what stagnates the flow in the pipe. FYI-- Magnaflow ships thier CTD systems with a 3.5" dp, and all the rest is 4". This is done for a reason-- the precise reason I just outlined of keeping the hot gases flowing right along.

So just keep your pipe transitions gradual (and continuously enlarging, not getting smaller) and you will be fine.

Another trick you can do to muffle the exhaust while preserving the low backpressure of a straightpipe is to use a muffler with ins and outs larger than the rest of the pipe. For example, if you run 4" all the way to the back, but use a muffler with 5" ins and outs, then the system will think and behave like a straightpipe 4", only be much quieter.

jh

wcbcruzer

Thanks for that great info HOHN. Actually, I just placed an order for a 5" straight pipe kit a couple days ago. It has a 4" downpipe, then 5" the rest of the way. I haven't seen a kit that actually offers gradual transition from a 3.5" downpipe to a 5" outlet. You think the 4" downpipe is good enough? I also ordered a 5x6x18 SS tip. This is going to be one loud beast! I'm starting to think it might be too loud.

HOHN

4" dp will be fine.

jh

Hohn2

justin- my 06 is going in on Tues for a 5" exhaust. Should I tell them also to use a 4" downpipe?

01smoker

The 5" will be loud but it will sound great. It will definetely turn heads when you go rolling by. I went over to friends house real late one night and the next day his mom asked him why there was a dumptruck here last nite. It woun't really be that much louder than a 4" system. It has a very deep sound that is noticeable compared to a stock 3" and 4" system. I love mine.

HOHN

You already have a 4" dp from the factory (the "cummins 600" and newer trucks (i.e. built after 1 Jan 05) all have factory 4" pipe).

5" will be perfect on your truck. Then you're set for when you get twin turbos. You *are* twinning it, right?


I tell you, these relatives and all their money.

Mom buys a new MegaCab, Bro has a righteous new 3rd gen. What am I to do?

I'm gonna have to bomb the crap out of my truck just to keep up with the stockers

Hohn2

Well my truck is really the boss' but it will be mine as a bonus when I sell $1m in sales.

Put the 5" magnaflow system on the truck and it sounds AWESOME! That with the stage 2 cold air really woke up the turbo sound! WOW!

Just used the 4" DP and 5" the rest of the way...hope I "chose wisely" (Indiana jones line here)

Diesel Dummy

The bigger they are the louder they are, after about a month you will surely put a resonator in it. You will be just as happy with a 4 inch kit and it will be easier on the wallet.

"Loud pipes save lives"