3.5" verses 4" down pipe? - Ford Powerstroke Diesel Forum
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Old 01-24-2010, 03:47 PM
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3.5" verses 4" down pipe?

I am looking into a MBRP 4" exhaust and curious what type of performance differences may be seen using a 4" verse a 3.5" down pipe? Not planning on hooking up a tune or serious engine mods as of yet.
Other then instal pains over the two is there a huge gain to be seen from one to other?
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Old 01-24-2010, 06:21 PM
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Also interested. If I get a 4" downpipe with a full 4" exhaust, would there be any reason to upgrade to a 5"?
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Old 01-25-2010, 04:30 AM
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I have a 4" diamond eye down pipe now and before I had the mbrp 3.5" down pipe and I felt a slight decrease in torque with the bigger dp. If I could do it all over again I would have kept the 3.5 b/c there was more torque an it peaked louder than the 4". I hope this helps!
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Old 01-25-2010, 06:51 AM
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Yea I have a straight through 4 inch and a ford tech was telling me that i would get more out of a 3.5 in downpipe because its provides slightly more back pressure which is good for your turbo.
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Old 01-25-2010, 07:42 AM
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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.
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Old 01-25-2010, 07:46 AM
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Quote:
Originally Posted by joe sinacola View Post
I have a 4" diamond eye down pipe now and before I had the mbrp 3.5" down pipe and I felt a slight decrease in torque with the bigger dp. If I could do it all over again I would have kept the 3.5 b/c there was more torque an it peaked louder than the 4". I hope this helps!
if you still have your 3.5" DP you should put it back on and sell me the 4" .
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Old 01-25-2010, 11:06 AM
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The most bang for your buck comes with the removal of your cat.....engine runs smoother and cooler.

Not really a horsepower gainer, but worth 80 bucks.....engine seems to be much happier. I felt a very negligible difference in my buddy's truck w/ 4" exhaust but nonetheless a difference........
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Old 01-25-2010, 01:15 PM
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4" will be louder. 3.5" will have better performance. Some slight back pressure is good forthe VGT TURBO. This is not conserning the standard turbos. Some people notice the differenct some dont.

3.5" is better than the stock 3" dp.

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Old 01-25-2010, 02:35 PM
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not to hijack the thread, but whats the part # for the MBRP 4" dp? also, does anyone know the part # for the 4" cat delete? ive only got the 3" cat delete with stock piping right now, and want to go 4" dp, 4" cat delete, and 5" back.

thanks!!
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Old 01-25-2010, 03:09 PM
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Get rid of the stock muffler!!! its heavy and it is far more restrictive than the cat. MBRP in My sig fit precisely. Very happy w/same.
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