Exhaust size comparison
 

A clip from the latest Diesel Power magazine

Pipe Dreams

Diesel enthusiasts already know the great power potential and more efficient fuel consumption offered by a diesel truck.All that power and economy doesnt come without picking the right parts though. An after market high flow exhaust system has to do more than simply make your truck sound better. Point in case with a 4" vs. 5 or 6 inch system, how big do you need to go? To answer that first we need to go over a few basics.
On most contemporary turbodiesel engines, the exhaust comes right off the exit of the turbo and after market exhaust manufacturers offer more than one tubing size to vent the exhaust more quickly.
One advantage of moving exhaust gasses quickly and efficiently is to reduce buildup of heat. Turbo diesel engines generate alot of heat, especially when the engine is under a heavy load. A high flow exhaust actually allows your diesel engine to run cooler and adds the potential for more boost when towing.
Large diameter tubing also reduces backpressure, making it easier for the pistons to shove that exhaust out of the cylenders and resulting inmore horsepower. Less backpressure allows the turbin to spin more efficiently, therefore it will build boost quicker and run cooler.
Interestingly enough however there is a point where bigger isnt necessarly better. While engine testing indicates that going from 3-1/2" to 4" can reduce backpressure by as much as 98%, ther can be less than 2% reduction above 4" exhaust. SO while a 5 or 6" exhaust looks cool it wont give you much more than just a coolness factor

Hummm... Yeah I have read that (and heard it many times also) but just to give you a real world example, on my truck with the PDR tow twins it had a 5" down pipe that ended just about at the trans xmember. From there I reduced it down to a 4" BD exhaust system muffler and all. Last week I installed a MBRP 5" system that hooked directly up to my down pipe with no muffler and I yielded a honest 100 degree drop in EGTs at WOT? I was very surprised since I only did this mod to make the system look better and try it without the muffler. I don't imagine I gained any HP but I sure gained cooling:D I don't really know what was the biggest contributing factor was, the removal of the muffler or the opening up of the bottle neck but... one of the two really made a difference.

I just dont understand how that statement is possible...

Evil,

The only thing I can think of is the downpipe. From stock to an aftermarket one for a stock turbo.

I wonder how much it's reduced if any from a stock exhaust outlet to a 4" on an aftermarket charger.

When I put my 5" on, there was a difference in performance. I didn't notice it, but my friends did. They said that that the power felt like it came on sooner. Who knows.....

I know some folks have reported that using a 5" or bigger without enough HP caused a loss of power.

While were on the subject. Does anybody out there have a stock down pipe just waitin to be thrown away. I want one to use as a pattern and I'll be glad to pay shipping on it.

yep i had a single 5in exhaust on my truck straight pipe of coarse after i had it on my truck for 3 weeks i took it off because of the cab drone. i went down to 4in duals straight pipe again..and with the 4in there was a noticeable gain in power to the truck

Im curious about EGT's...what was your experience between 4" and 5"??

I recently did some exhaust changes and have this to report...take it for what it's worth!

Started with a Summit Racing SS system for a 2003 ram...the significance of this is there was enough 4" pipe in this kit to rid myself of the cat and OE muffler on my 04.5, I simply reused the OE downpipe. I had the "reversible" muffler turned "loud"...adding this exhaust definately helped mileage and the truck seemed to accelerate faster when rolling (spooled quicker?).

I wanted to quiet things down, so I reversed the muffler three weeks ago (after running this way for over 2 years), and while it didn't really affect noise, the mileage increased and the truck ran better (was pulling 20mpg average with a couple tanks to 23mpg).

Sold the muffler, but didn't have a replacement, so I ran basically straight piped, dumping right under the cab. Now while this was obviously LOUD, the truck's performance also went down...top end suffered (for whatever reason)...and I'm pretty sure it wasn't fuel as I went through several tankfuls of different brands. I also lost mileage (down to around 17mpg average). All-in-all, it was a complete dog, night to day difference.

Last week I swapped on a 30" magnaflow. These appear to neck down to 3.5 inches inside, but are still straight through. Not only is the truck quiet, but it is performing like it should...all my power is back, thing will scream right up to redline. And my mileage is back up!

I almost wonder if these newer 3rd gens might need just a touch of backpressure??

steved

now that i wish i knew.....i did not have gauges in the truck at that time.....

so what if you lose the muffler and keep the cat