Removed the stock Duramax exhaust tip and replaced with a shorter turn down

Wayne

Administrator
Staff member
One of the things I can't understand about the factory exhaust on our Duramax is the position of the long straight back leg roasting tip. So many times I have walked around the back of the truck to open the tailgate and felt the extremely hot exhaust blowing on my leg. The position is bad, but to make it worse, there is a 3" nozzle at the end of the 4" pipe that accelerates the exhaust so it causes a venturi effect, which pulls cool air into the stream. The effect of the nozzle is that you can be a couple feet behind the tip and the hot exhaust still blows right on your leg. I'm sure there is some cool air pulled into the stream, but It doesn't work all that well, trust me. This past weekend was the last straw, when in shorts, I again got too close to the 'cooled' exhaust, so I decided solve the problem.

Factory exhaust tip

20200704_125147844_iOS.jpg


20200704_125132439_iOS.jpg


Fuel economy reducing nozzle

20200704_125157533_iOS.jpg


Turn down installed

20200706_154439903_iOS.jpg


20200706_213308226_iOS.jpg


No more burned legs
 
Last edited:

dnewton3

Moderator
Staff member
The purpose of the OEM tip is to blend cooler ambient air with the exhaust when it goes into regen. If you think it's hot in normal mode, wait until you're near it with the regen going! That venturi effect is important because of the concerns the OEMs discovered several years ago ...
Under certain conditions, the exhaust can get so hot it could melt plastic body parts or catch items near it on fire during regen. For example, let's say it's in regen and sitting parked, and the natural wind around the truck is going exactly opposite of the exhaust direction. This condition causes the exhaust to blow directly backward and cause two concerns:
- the heat can be concentrated at the point of exit, and will convect up towards the truck possibly affecting plastic parts and paint
- the emissions can be concentrated to a unhealthy toxic level in a very localized area
Hence, the industry responded with the venturi nozzle to purposely accelerate the exhaust gasses out far past the edge of the truck. The faster the gases escape via the venturi acceleration, the lower the temps. It's the thermal principle of heat-loading a mass over time. The more they dilute the hot exhaust with cooler ambient air, the more they bring the average temp down. This is very important during regen mode. Sure, it's an annoyance because that jet-stream effect is blowing with a lot of force. But it was done with safety in mind under some fairly limited conditions (regen mode + stationary position + opposing air movement). OEMs have to be concerned about this, as they are always lawsuit-leary.

I'll be curious to see how this works out for you, Wayne. The upside is that you've diverted the flow down and away a bit. The downside is that you've eliminated that melding of cooler air into the exhaust stream and removed that jet effect. What you did isn't "wrong" to do, but just be aware that you've eliminated a safety item which has limited (but important) function under some fairly rare conditions.
 

Wayne

Administrator
Staff member
Thread starter
Dave,

Yeah, I'm aware of the blending of cooler air because the exhaust can be so freakishly hot during regen. The OEM's solution was a trade-off between getting the super heated exhaust away from the truck and burning the flesh of someone who dares get close enough. The experimenter in me wants to know if this will work for me.

There are also side eject turn-outs that are vented, but they run about $100, however I got the non-vented turn-down for $20. If I feel the heat is too much while the truck isn't moving, then I'll get the more expensive vented turn-out, but my guess it this will work fine.

Time will tell.
 

Wayne

Administrator
Staff member
Thread starter
We just got done with a 200 mile trip pulling the Airstream. A couple times when we stopped I checked if the hot exhaust was rising and heating the bumper or lower rear of box. It isn’t. The down-turn directs the exhaust down and away far enough that by the time it rises, it’s about 2 feet from the truck. Will keep observing.
 

ls1mike

Member
Interesting info, but I can help but think some of buddies would have already died with there modified "Rolling Coal" modified exhaust.
 

Wayne

Administrator
Staff member
Thread starter
Interesting info, but I can help but think some of buddies would have already died with there modified "Rolling Coal" modified exhaust.
Now that's funny. I never did understand the desire to blow black smoke. I don't want to breath the stuff, as it's really really bad for your lungs. I also don't like the diesel emissions equipment for the expense and upkeep, but I also like it, as I don't want to breath the soot from the diesel in front of me on the road. It's like arguing with myself.
 

ls1mike

Member
Now that's funny. I never did understand the desire to blow black smoke. I don't want to breath the stuff, as it's really really bad for your lungs. I also don't like the diesel emissions equipment for the expense and upkeep, but I also like it, as I don't want to breath the soot from the diesel in front of me on the road. It's like arguing with myself.
Yeah not a fan of the rolling coal. Lots of it out here as there is no emission testing. The old non-emission diesels didn't smoke like guys who have a tune to roll coal. They were just fine and really not the problem when you look at carbon emissions .
 

dnewton3

Moderator
Staff member
Rolling coal looks cool to the uninformed and those with juvenile intentions.

Heavy soot is simply indicative of incompletely burned fuel. That black soot means there's far too much fuel for the available oxygen in the diesel cycle; there needs to be a lot more air in there.

I admit it looked really impressive to me when I was younger, but that was when I was a kid at the truck and tractor pulls at local county and state fairs. Knowing what I do now, it's not nearly as impressive.
 

Wayne

Administrator
Staff member
Thread starter
In my opinion, now that I'm old, rolling coal is just stupid. Spending lots of $ on parts to make more power and black smoke, which in turn breaks parts and shortens the life of your drivetrain components, but you can tow 30,000 pounds 90 MPH. What could go wrong?
 

Wayne

Administrator
Staff member
Thread starter
We just returned from a vacation, which involved 4019 miles towing the trailer. I have fuel mileage data for the last 28,000 miles for the truck. I'm going to go way out on a limb and say that removing the 3" nozzle increased the fuel economy by about 1/4 MPG.

Banks did a dyno test on the stock Duramax exhaust tip with the 3" nozzle vs. removing the 3" nozzle. They gained 20 HP with the 3" nozzle removed. This, of course, was at wide open throttle, which my truck never sees, ever. The Banks test does show that there is backpressure, so I'm not too surprised that I saw a slight increase in fuel economy.
 
Top