Deciding between 4” and 5” is a common question we get. Bigger is not always better. Whether you have a Cummins, Duramax or Powerstroke there’s a few things you should consider. 

Sound difference
The biggest difference between a 4 inch and 5 inch exhaust system is the sound. Each exhaust system will remove the restrictions found in the stock exhaust. Naturally the exhaust is going to be slightly louder. If you travel long distances on the highway, freeway or interstate and want it quiet get a 4in. If you tow a trailer for extended periods of time and want it quiet get a 4in. If you want to have a conversation in person or over the phone get a 4in. In general, if you want an exhaust system that is fairly quiet get a 4". The 4in system will have a higher pitched sound, where as the 5" will have a deeper tone. Either system you should be able to hear an increase in noise from the turbo.

Exhaust Gas Temperatures (EGT’s) 
With a larger surface area and better flow; a 5” exhaust system will dissipate heat more efficiently then a 4” exhaust system. Your EGT’s will be slightly lower (50-100 degrees). Whether you install a 4 inch or 5 inch system, the biggest contributing factor in lowering exhaust gas temperatures will be removing the restrictions in the stock exhaust.

Performance Gains
When testing on our dyno we have found very minimal horsepower gains when comparing 4” and 5” systems. On a mostly stock truck with a tuner, intake and exhaust the 4” exhaust generally has more bottom end torque. This is because the turbo needs a certain amount of back pressure to spool up efficiently. The 5” exhaust has slightly higher horsepower at the top of the RPM range (5-10 horsepower). Its not until you get into the higher horsepower applications 500hp+ that there’s a significant difference. So if your truck is making 500 horsepower or less; its best to go with a 4” exhaust. If you truck is making 500 horsepower plus; its best to go with a 5” exhaust.

Installation
From an installation stand point. Naturally the 4" exhaust system is easier to install. You have more clearance near the downpipe and tailpipe where you need it most. In most cases if you have anything larger then a stock size spare tire and are trying to install a 5in system you will not be able to keep your spare in place.

Future considerations
Most people like to plan for future modifications. If you install the 5in system from the get go; you wont have to worry about replacing the 4in system later on to accommodate for future upgrades. Regardless, whether you decide on a 4in or 5in exhaust system it will be leaps and bounds better then the stock exhaust. Now that you have some idea what size exhaust you want/need. All thats left is deciding between stainless vs aluminized, muffler vs straight pipe and single vs dual! Too many choices...

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