Still can't see how a 4 inch pipe gives you more power than the 5. Wouldn't less restriction be more efficient? BTW love the sound nice and loud. The 5 also got rid of that annoying pop/whatever loud noise these things make when you really get on it. Happens at like 2100-2300.

 

I'm not a mechanic so I'm not entirely sure on this but I believe that engines do need a small bit of back pressure. My pipe setup came with 2 baffles - if you want to call them that - they look like round cheese grates with a reducer ring that goes from 4 inch to 3 inch - but just for a short 1 to 1.5 inch space - which does cause a bit of back pressure. I've had no popping or other noises except for the normal sound all the way upto approx 3K RPM - maybe that baffle thing stops that? The only thing I do know is that like I said in the upper post - when I drove with the back window open for approx 35 miles on the freeway, my ears were ringing for approx 15 minutes! LOL

I also forgot to mention the fact that my pipe setup came with all the pipes and connector and hooks up right from the turbo all the way back so there's no muffler nor resignator at all hooked up to the system.

 

Mine is just a 4 inch downpipe that turns 5 inch that comes out in the factory location. No muff or res. I always heard that a PSD needed a little backpressure but in thinking I did come up with this. Maybe the power diff is from the boost. Could it be possible that the 5 inch alows room for more boost without more fuel causing a heating affect. Less backpressure causes more boost creating higher intake temps? Just a thought.

 

4 strokes do NOT benefit from ANY amount of back pressure - only 2 strokes need a tuned amount of back pressure for max power..........what a good exhaust needs to achieve is absolute max velocity with minimum back pressure to promote good scavaging of the exhaust gases..........smaller tubing must run higher exhaust gas velocities to meet the flow requirement where as larger tubing will have a lower velocity to achieve the same flow.........slapping a 5" system on a stock CTD will drastically lower the velocity of the exhaust because the motor has a small flow requirement compared to a heavily bombed CTD..........remember: you want max velocity without back pressure - too large a system will kill any hope of good scavenging..........as it has already been mentioned, 4" systems flow well to 500 hp and after that you should consider a 5" system as the motor will then move enough exhaust gas to keep the velocity high for good scavenging above 500 hp.....

 

What about a 4" Downpipe to a 5" Straight? The only reason I am doing this is because I have heard that the 5" Straight is a little more mellow than running a 4" Straight...
Any comments?

 

The theory is that with a 5" the exhaust is moving too slow to create a scavenge effect. You want your exhaust staying hot and moving quickly through the system forming a slight low pressure at the incoming side.

This is similar to running too big of sewer line. First glance says bigger is better, but it's not. Go too big and the velocity drops too much to scour the pipes and you end up clogging them.