Modern vehicles with a diesel engine use a fuel additive called, DEF (diesel exhaust fluid).
Its job is to reduce harmful tailpipe emissions produced from a diesel engine. All pickup trucks manufactured after 2010 are required to have DEF to comply with emissions standards.
Straight piping has its perks.
The main benefit is increased air flow – it’s a way to get more power out of your truck. You can straight pipe a truck with DEF.
But there are some advantages and disadvantages you need to know before going forward with this modification, which is what we’re going to dive into.
What’s a Straight Pipe?
Straight piping a pickup truck with a diesel engine means eliminating the catalytic converter, particulate filter, resonator, 02 sensors and muffler from the exhaust system – everything required for emissions control (we’ll come back to this in a moment).
The upshot is unrestricted airflow since there’s no back pressure created by these components. The more air flow you can get through an internal combustion engine, the better the performance.
In place of all these parts is one or two long straight pipes connecting from the exhaust manifold all the way to the tail pipe.
Let’s look at some of the pros and cons of straight piping a diesel truck with DEF.
You Get Better Fuel Economy by Straight Piping a Truck with DEF
One perk of straightening the pipe of your truck is increased gas mileage.
As of this writing the cost of diesel fuel is more than $4 a gallon nationwide – over $5 in states like Vermont. The average size of the gas tank on a modern full size diesel pickup truck is over 25 gallons, you do the math.
That means that straight piping can obviously save you some coin at the pump.
Of course, to enjoy better fuel mileage after a straight pipe will require a tune which we’ll get into as you keep reading.
Some people complain of fuel consumption after a straight pipe. But this is a direct result of an air-to-fuel imbalance that a tune works to correct.
Installing a Straight Pipe on a Truck with DEF Also Improves Throttle Response
Want more performance out your diesel pickup truck?
When you straight pipe, all the restrictions that was holding your truck back are eliminated.
A combustion engine like the one for a diesel truck works by drawing in fresh air through the intakes into the internal combustion chamber.
Here oxygen mixes with fuel (and DEF) which is sprayed into the combustion chamber via the fuel injectors. But instead of being ignited by a spark from spark plugs.
Diesel engines have a compression-ignited system.
Fuel and air are ignited by the high temperatures achieved when gas is compressed by the engine’s pistons. This creates a small explosion which helps the truck move. This is an oversimplification of course.
The main point is that since many components of the exhaust system are removed. The engine is able to breathe easier resulting in a more immediate throttle response.
You Even Get Increased Horsepower and Torque
A straight pipe can increase the horsepower of any automobile by 5 to 10 HP.
This is achieved through improved air flow.
With a straight pipe you’re losing a lot of the resistance that was preventing you from enjoying more power. And since there’s less pressure on the engine you also get increased torque.
Horsepower is one thing, but torque is where your butt dyno will really feel the difference. This is why often people that race their truck go for a straight pipe system.
Increased air flow allows fuel to burn more efficiently thereby increasing both horsepower and torque.
You Get a More Aggressive Sound by Straight Piping Your Truck
A diesel truck with a straight pipe will sound a lot louder which is part of their charm and the reason why many people opt for this modification.
After all you’re removing all the elements of an exhaust system that work to dampen the noise of the engine and the noise of exhaust gases moving through the exhaust system.
So, the natural result is a more aggressive sounding diesel truck.
A Straight Pipe Reduces Weight
Exchanging the stock exhaust for a straight through pipe can lighten your truck by as much as twenty pounds.
But as awesome as all this performance potential sounds, there are a few negatives to straight piping you really should be aware of before going through with this modification.
You Can’t Pass State Inspection After Adding a Straight Pipe to a Truck with DEF
Chief among all the reasons why you probably shouldn’t straight pipe any vehicle you plan to continue using on public roads is the fact that if you live in a state that requires emissions testing.
And you need your truck to perform daily driving duties. It’ll be virtually impossible to pass seeing as how straight piping cancels all the elements necessary for a truck to meet emissions regulations.
You could decide to hot dog it and ply your vehicle on the public roads anyway. But if law enforcement should happen to see more black smoke than what they deem normal billowing out of your exhaust pipe.
All of your truck’s violations – including the fact that straight piping bypasses the DEF – all has the potential of adding up to one hefty fine.
Replacing the Stock Exhaust System with a Straight Pipe Lowers the Viscosity of Diesel Fuel
Why does this matter?
Because diesel engines – even the ones built for a pickup truck – are designed to run on fuel with high viscosity.
Remember that a diesel engine works by compressing gas and fuel to create a spark. The result is high temperatures.
The high thickness of the fuel is necessary to keep engine parts from overheating.
Straight piping a diesel engine lowers the diesel fuel’s ability to keep moving parts well lubricated. That added stress on the engine leads to premature wear and tear and eventual engine failure.
Straight Piping Isn’t Always Legal, Especially on Engines That Require Extra Filters for Emissions Control
It really depends on where you live.
But while there aren’t any federal laws that can keep you from adding a straight pipe to your diesel pickup. States and municipalities may have their own laws.
Remember we said that straight piping a diesel engine involves removing the diesel particulate filter.
Its job is similar in scope to a catalytic converter in that it’s designed to filter out particulate matter from the exhaust gas of a diesel engine – this helps keep pollutants out of the air.
So, it’s very important to check with local laws before proceeding with any modification that can land you in hot water.
Straight Piping a Truck with DEF Will Result in Check Engine Lights Due to Issues with Components Like the EGR
The exhaust gas recirculation system is one of those features of a diesel pickup truck created for emissions control.
An EGR is responsible for recirculating exhaust gas back into the engine to reduce emissions, particularly oxides of nitrogen which are air pollutants.
But if it doesn’t sense any back pressure from the exhaust system (which it won’t). Then the EGR won’t function properly. This means that along with the straight pipe installation.
You would also need to modify the engine so the EGR system is no longer active.
However, by restricting the EGR the diesel engine will run hotter and exhaust more NOx emissions into the atmosphere which is no good for anyone.
Straight Piping Any Vehicle Can Be Expensive
This isn’t exactly the cheapest modification you can make to an automobile. Let alone a vehicle that’s as big as a truck.
Aftermarket exhausts can get expensive to put it mildly. Even though straight pipes are simple in both design and function, they still cost money.
There’s also the cost of removing the old exhaust system. Even if you do it yourself it can take some time to remove everything and install the new straight pipes.
And of course, there’s the tuning involved.
Straight Piping a Truck with DEF is Going to Require a Tune, There’s No Way Around It
Yes, the installation of a straight pipe on your diesel truck is going to require a tune which can be a few hundred to a couple thousand dollars.
Generally, any time you mess with how much air flows goes in and out of an internal combustion engine. You’re going to need to tune the ECU which is a modern engine’s electronic brain so that things run properly.
A straight pipe changes the fuel-to-air ratio causing the engine to run lean in this case.
A tune will likely also involve changing engine components like the fuel injectors to compensate for the increased air flow.
But in answer to your question, yes you can straight pipe a truck with a DEF since the DEF is far away from the exhaust system getting straight piped.