There seems to be this general ideal floating around that dpf back exhausts can unlock a good amount of power

This thinking I believe is carried over from petrol cars where chasing performance almost always involves larger intake and exhaust piping. On a petrol engine e.g. Honda K series, they absolutely do make a difference and quite a significant one at that, because of this i believe the same idea carried across to modern diesel vehicles with no second thought

You often see DPF back exhaust manufactures claiming extra power with some systems claiming an extra 50Nm to gain!

To the right are a few examples of this

The test vehicle

For testing today we’ve got a D23 NP300 Navara

We will run up the vehicle 100% stock, fit a 3″ straight through DPF back exhaust and then run the vehicle up again and compare data

the data

First we’ll look at the power and torque graphs, but before that let’s explain a little about heatsoak with these D23 Navara’s

Their factory intercoolers aren’t the greatest and tend to heatsoak (store heat) pretty fast compared to other vehicles we tune

The first dyno run after a long cooldown period will always perform the best as everything is relatively cool, the next dyno runs before the next cooldown period will suffer from more and more heatsoak where the intake air temperature is higher at the start of the run. This will affect power

If you take a look at the top image on the right, that graph is representing the first two runs from both the stock exhaust and the DPF back exhaust. They are making pretty much the same exact power. The peak numbers are between 2kW/4Nm of each other and funny enough the stock exhaust looks like it makes more peak kW while the DPF back exhaust makes a tad more torque on paper

Realistically, you will not feel this. The difference between the two runs is so small i would throw it up to how the vehicle was strapped back onto the dyno the second time or slight changes in ambient temperature

So how could you fudge a dyno graph? Well, remember how i mentioned heat soak? Take a look at the second dyno graph, the colours are flipped on this one so your stock run is red and the DPF back exhaust run is green. You can see just how much torque it lost due to heatsoak compared on it’s 3rd run compared to it’s first run on the graph above. And this is why i think some people are claiming gains with their DPF back exhausts. 

When is a dpf back exhaust worth it?

Simply put, when you’re removing the thing that’s actually restricting the exhaust. The DPF! However this can only be done on off road only vehicles

A good turbo back system will help free up flow and is recommended when your upgrading your turbo/chasing more power.

Some vehicles can make more room for power with just a DPF delete as well! Lets take this RAM 2500 for example. With the DPF removed, the AFR leaned out 2 points. What this has done for us is allow us to introduce extra fuel and end up at the same AFR as factory but with a nice addition to power