whats the real difference in stock 3.5 stright pipe exhaust vs 4 inch stright pipe? - Ford Powerstroke Diesel Forum
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Old 04-26-2007, 09:51 AM
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whats the real difference in stock 3.5 stright pipe exhaust vs 4 inch stright pipe?

I know i need to get rid of the flat down pipe on my obs. I'm wondering if i should just delete the cat & muffler on my truck?

If that 1/2inch really going to make any real difference?

I went threw all this on mustangs and that would be like going from stock 2 1/4 duals to 2.5 inch. Not really any difference unless you move more air/fuel threw it.

p.s Unlike a n/a gas engine the cold air intake on the truck actually made a noticeable difference.

I have a tymar intake on my truck. I know that unlike a gas engine since i have a turbo it is enough back pressure and the stock muffler/cat are chocking it.

Only other mod my truck will see is a chip down the road.

I'm stuck stright pipe the stock exhaust and get a down pipe or go with a complete 4 inch turbo back?

Thanks guys

EDIT :Who would have know i spelled straight wrong twice in the title lol

Last edited by 95PDXLT; 04-26-2007 at 11:11 AM.
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Old 04-26-2007, 10:16 AM
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Pipe Dreams

Diesel enthusiasts already know the great power potential and more efficient fuel consumption offered by a diesel truck.All that power and economy doesnt come without picking the right parts though. An after market high flow exhaust system has to do more than simply make your truck sound better. Point in case with a 4" vs. 5 or 6 inch system, how big do you need to go? To answer that first we need to go over a few basics.
On most contemporary turbodiesel engines, the exhaust comes right off the exit of the turbo and after market exhaust manufacturers offer more than one tubing size to vent the exhaust more quickly.
One advantage of moving exhaust gasses quickly and efficiently is to reduce buildup of heat. Turbo diesel engines generate alot of heat, especially when the engine is under a heavy load. A high flow exhaust actually allows your diesel engine to run cooler and adds the potential for more boost when towing.
Large diameter tubing also reduces backpressure, making it easier for the pistons to shove that exhaust out of the cylenders and resulting inmore horsepower. Less backpressure allows the turbin to spin more efficiently, therefore it will build boost quicker and run cooler.
Interestingly enough however there is a point where bigger isnt necessarly better. While engine testing indicates that going from 3-1/2" to 4" can reduce backpressure by as much as 98%, ther can be less than 2% reduction above 4" exhaust. SO while a 5 or 6" exhaust looks cool it wont give you much more than just a coolness factor.

Cheers,Kevin
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Old 04-26-2007, 11:04 AM
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1/2 increase in pipe diameter can = as much as 98% reduction in back pressure. I wonder how little it can be.

It makes sense on a 500hp/850ftlbs engine 3.5 vs 4 inch = as much as
98% decrease in back pressure.

On a basically stock engine i'd like to know the how little reduction figure.

No Pun intended but that sounds like a write up from an advertisment for aftermarket exhaust.

Last edited by 95PDXLT; 04-26-2007 at 11:09 AM.
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Old 04-26-2007, 11:18 AM
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I think the big thing with aftermarket exhaust at least a good one is they are mandrel bent so the bends are smooth unlike the stock exhaust. I guess you could always keep the stock and delet the cat and exhaust and then go from there. Probobly would be pretty inexpensive to do that first to see if you like it or not. I think if you just got yourself a downpipe and did the rest like you said you would be happy.
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Old 04-26-2007, 11:52 AM
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Well i've found

downpipe $100
cat/muffler delete $50

$150 plus shipping and the time to monkey it up.


I found complete 4 inch turbo back straight pipe for $ 245

complete 4 inch turbo back w/ muffler for $269

By the time you figure the time lost to monkey up the delete pipe for $100 more its complete with no welding or cutting.

Guess i answered my own questions lol.
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Old 04-26-2007, 12:26 PM
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Brad, I pondered the same question and was told by a local shop to buy a downpipe, delete the cat and that would open it up enough to let it breathe and get the gases out with a good intake. After looking at the cost I decided to go with a 3" downpipe which opens into a 4" pipe and a muffler and it works great. It was easy to install but you have to bend the firewall a little to make the 3" downpipe fit good. I used a powerspreader from work and it worked great. The other plus were the mandrel bends.
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Old 04-26-2007, 12:31 PM
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Quote:
Originally Posted by Chad T View Post
Brad, I pondered the same question and was told by a local shop to buy a downpipe, delete the cat and that would open it up enough to let it breathe and get the gases out with a good intake. After looking at the cost I decided to go with a 3" downpipe which opens into a 4" pipe and a muffler and it works great. It was easy to install but you have to bend the firewall a little to make the 3" downpipe fit good. I used a powerspreader from work and it worked great. The other plus were the mandrel bends.
So you kept the stock muffler and just deleted the cat and added the aftermarket downpipe?

The stock giant tomato soup can looks restrictive no?
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Old 04-26-2007, 05:41 PM
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I have ran all of the above, other than an aftermarket complete kit. I could tell a big difference in just the cat delete and downpipe with the stock muffler. I prefered the sound of the muffler to the stock straight pipe, but it was a little more peppy with the straight. I ran 4" straight for a while, but had no low end power. It took too long for the turbo to spool, but when it did it was great. I then added a high-flow muffler and got the spool-up back, and it sounds better in my oppinion.
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Old 04-27-2007, 04:57 AM
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Thanks for all the input guys.
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Old 04-27-2007, 04:16 PM
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FYI, the 4" turbo back is actually a 3" downpipe then 4" from there back. You can't fit a 4" DP on the obs trucks without ALOT of firewall "relocation".
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