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Will a free flowing diesel exhaust cause a loss in torque or power?

Little Power Shop Myth #1 - If I install a 4” turbo back exhaust I will lose all my low end torque.
 
This is one of those subjects with questions we get almost daily.  It takes several forms: 
 
I read on the internet forums that you will lose all your low end power if you install a 4” turbo back exhaust. 
 
My friend installed an exhaust on his truck and now it doesn’t have the same power it did on take off. 
 
A diesel engine needs backpressure and if you open the exhaust up too much it will hurt my engine.
 
A diesel engine needs back pressure and if I open up the exhaust I will lose power. 
 
Due to the relative low cost of a turbo back exhaust, and the amount of diesel owners looking for a little more power, this is a very popular subject.  First things first, a turbocharged diesel engine does not need so much as pound of back pressure after the turbo to perform perfectly.  In fact, the less restriction after the turbo, the better.  You have to understand that between the exhaust port on the head and the inlet of the turbocharger there is usually back pressure of at least equal if not more than boost pressure on the engine at all times.  Any back pressure the engine needs for proper function is provided before the turbo, period.  Pressure on the outlet side of the turbo just leads to higher exhaust gas temps.  When we install a four or five inch turbo back exhaust system, we see several things.  First is usually a large drop in exhaust gas temperatures.  A 100-300 degree or more drop is not uncommon.  Second is an increase in fuel mileage.  If the engine doesn’t have to work to push the spent exhaust gasses out of the tail pipe (we learned in the first article that an engine is nothing more than an air pump), it can use that horsepower to help turn the flywheel and transfer more power to the ground.  And third, we will usually see somewhere between 15-30 rear wheel horsepower increase on the dyno when upgrading from a completely stock exhaust. 
 
So what about the internet forums, your buddy, or even you that noticed less performance of the line after installing a high flow diesel exhaust system?  Well, you’re not crazy and your buddy isn’t a liar.  The reason for the low end loss on some trucks is from a loss in exhaust gas temps.  A turbocharger is driven by pressure and heat.  The hotter the exhaust gas, the more it will expand and drive the turbo.  If we install a free flowing exhaust that lowers those temperatures, we will see slightly slower spool up all things being equal.  Where there was a restriction in the stock exhaust before that caused the exhaust gas temps to be higher at all times, now the temps drop quickly and they have to warm up before the turbo will spool fast again.  Is this a huge deal?  No.  Most people who have added some sort of programmer that increases low end fueling will never notice it.  Also, this slower spool up is more noticeable on standard transmission trucks than automatics due to being able to operate in lower rpm ranges at will.  Also, this “loss in low end torque” is usually only noticeable when the truck is cold or has sat idling for a few minutes.  Once the engine reaches normal operating temperatures, you should never notice it at all.  What you should notice is increased horsepower, increased fuel mileage, and lower exhaust gas temps.