Compression Ignition Addict
Join Date: Nov 2008
Location: Houston, TX
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no offense, but that makes no sense. you do not need back pressure with a turbo. What you need is a smooth flowing exhaust, which depends on a few factors like the size of the turbo and how much boost your running, how much fuel, etc.
the turbo creates all the back pressure a gas and diesel motor will ever need. the exhaust after the turbo needs a smooth free flowing exit. The reason you may notice less torque with a 4" dp over a 3.5" dp is because with 3.5 the exhaust gas is flowing smooth and free flowing out of the turbo and out the tail pipe, thus causing more horsepower because the motor is not working to push the exhaust gasses out of the tail pipe. When going to a 4"dp you may run into the problem of having too much area for the exhaust to flow, therefore instead of the gasses flowing smoothly out the tail pipe they're getting bunched up and pushed out, therefore causing the lower tq.
an example would be, blow into a small coffee stirrer, then blow into a mcdonalds straw, and then blow into a straw that is 1" inner diameter. the mcdonalds straw is going to be the easiest to blow through and keep a constant steady flow. You cannot blow hard enough to do the same with the straw that is 1" ID.
However, a 4" DP will give a deeper exhaust note and will lower EGT's even lower. So take what you can get, its a trade off. a little more torque with slightly higher EGT's with a 3.5". Or a little less torque and slightly lower EGT's with a deeper tone.
'07 F-250 PowerStroke - 5" MBRP Straight Pipe, Edge Insight, Eric Tuned SCT, HID's, 2.5" Newton Lift w/ 4" rear block and Icon Caster Sleeves, Bilstein 5100's, 18" XD Monster's w/35" Toyo M/T.
'01 BMW ///M3 - Weekend Toy
'17 Chevy Malibu - DD and Company Vehicle