The diesel world has a lot of different designs and ways of doing things compared to the world of gas and trucks. We’ve been making a ton of articles covering some of these different things and something I noticed while making those articles is that newer Duramax trucks have a funny-looking exhaust tip.
More specifically, the exhaust tip almost looks like it’s spaced out, with a fake tip and the real tip. And that got me wondering, why is GM adding this to their trucks? Is it something to do with emissions or a way to save some production cost or what? So, with that in mind, I went out and did some research. So, sit back and get comfy, because I’m going to tell you exactly why GM does this with their exhaust tips.
Dealing With Heat
So, getting right to the point, the reason GM has this feature on their trucks is very simple: exhaust heat. I know what might be thinking, exhaust gases aren’t that hot coming out of the tip, so why are they worried about exhaust? While that’s true to a point, especially with gas cars, you have to remember that modern diesel trucks have unique emissions systems that they previously didn’t have.
More specifically, modern diesel trucks have a diesel particulate filter. The DPF by itself isn’t what causes those crazy exhaust gas temperatures, but rather how the DPF is cleaned. We’ve covered this in other articles, but for those who don’t already know, let me explain what the diesel particulate is and how it works.
Diesel Particulate Filter
The job of the DPF is simple: capture excessive exhaust soot and then burn it off occasionally through a regeneration mode. It does the regeneration through two modes: an active regeneration mode and a passive regeneration mode.
Passive regeneration occurs when the exhaust temperatures are naturally high enough to oxidize the soot collected in the DPF faster than the soot is collected. This typically occurs when the temperature of the DPF is above 300°C or 572°F. This occurs during highway driving or when driving with heavy loads.
Since passive regeneration occurs naturally, it is considered to be normal engine operation.
Active regeneration occurs when the exhaust temperatures are not naturally high enough to oxidize the soot collected in the DPF faster than it is collected. This works by injecting a small amount of diesel fuel into the exhaust stream, which is then oxidized by the diesel oxidation catalyst.
It injects the fuel into the exhaust stream using the late injection method, which is simply firing the injectors late in the exhaust stroke, or by using the ninth injector method which is an injector mounted directly to the exhaust system. In the case of modern Duramax diesels such as the LML and L5P, they use the 9th injector method for active regeneration.
The oxidation of this additional fuel raises the exhaust temperatures to approximately 550°C or 1,022°F, which is needed to regenerate the DPF. And that’s where the issue lies: exhaust gases exiting the tailpipe at upwards of 1000 degrees.
What Can Happen From Regeneration
Think for a second if you were stuck in traffic in your truck and the active regeneration mode triggered, and you had a car next to you. You’ll be shooting 1000 degree exhaust heat at them, which can damage their paint. Or for example, you walked past a truck that was in active regeneration. If you walked by the tailpipe, that could result in a pretty nasty burn on your leg.
Or for example, you were out doing work in the forest or desert and there was some brush by your exhaust tip. That can easily catch on fire and then obviously burning down the forest is pretty bad. Unless you’re an arsonist in California.
So, with that problem in mind, what’s the solution? You really don’t want to add more crap to your exhaust. You already have the diesel particulate filter, ninth injector, and diesel exhaust fluid injection. So, how do you cool down a really hot exhaust system? The answer is simple, mix it with cool air before it exits the tailpipe.
And that’s exactly what GM is doing with this exhaust tip. By having those vents prior to the exhaust tip, they’re able to create a venturi effect. It’s at this point that I’d like to point out that it’s not just GM that does this. You’ll find venturi exhaust tips on all sorts of applications, including Ford Powerstroke trucks, although theirs is definitely a little more discreet.
To explain this system really simply, as the hot exhaust gas flows past the vents, it pulls in cold air from outside the exhaust system, which then mixes in that short section before the real tip, and then that cooled exhaust exits the system. There are also potential performance benefits with this system, as it can actually help pull exhaust gas out of the system, which reduces back-pressure.
But, the performance benefits are pretty minor and certainly not the main focal point for this design feature. That being said, you can swap this to a standard style tip, and your truck will work just fine, although the exhaust gas temperatures leaving the tip during active regeneration will be noticeably and measurably hotter.
Can I Delete It?
That being said, if you don’t particularly care about the exhaust gas temps exiting your truck, there’s really not much of a reason to keep this on your truck. This is especially true for those of you out there who have deleted the DPF system and your truck no longer has to work with any regeneration mode.
I think it’s also worth noting that you’ll find exhaust tips like this on commercial applications, such as semi-trucks, for the same reason. Many modern semis have to deal with the similarly strict emissions standards, and as such also use diesel particulate filters.
I hope you guys enjoyed this short article explaining why GM uses these funny-looking exhaust tips and also why other companies use similar exhaust tips. It’s for the venturi effect and allows the exhaust gases to be cooled before the. exit the tip, which then reduces the chance of burning someone or catching something on fire.