Exhaust help

[truckin151]

Cut out added 10rwhp/tq open. Lowend tq numbers dropped, topend numbers increased. 0-60 times increased by about a second open. Did not do a 1/4 mile run though.

 

[truckin151]

By increased I mean they went from around 5 and a half seconds to closer to 6.5 seconds.

 

[Belgiquebasterd]

Correct, however one would think that the Pacesetter longtube headers that come with a 3" collector, the pacesetter 3" ORY pipe that is meant to go along with the kit, into a 3" cutout & a 3" SI/SO high flow muffler, that the system should have been free'd up enough to get the right amount of flow from it. When I added the cats back in, I opened them up so they were 3" in/out instead of the 2-1/2in 2-1/4out that they were. So in terms of pipe size everything was the same, so no restrictions in pipe diameter that would speed up the velocity, only difference is the slight restriction that the cat's internals made. The restriction is basically the same as someone running a baffled muffler as well (so basically one that you cant stick your arm through like mine). The result of that is pressure being built up in the system, which then increases the velocity. This is also the case when going from a larger pipe to a smaller pipe, yes you gain velocity but in order to gain that velocity pressure must be built up first.

Look at water pipes or even your hose for example. When the water is just coming straight out of the hose, no nossel, there is no restrictions in the system (minus that from the hose itself) so it flows normal and will shoot out 3ft or so, if you add a nossel to the end of the hose (smaller pipe in terms of exhaust) to make the stream more fine the velocity of the water increases dramatically and will shoot 20ft or more. If you feel the hose though, the pressure needed to make that velocity is a good amount more than that of it flowing regularly. Same goes for when you stick you finger in front of the stream coming out of a non-restricted hose (adding a baffled muffler in terms of exhaust) you can make it shoot farther but the pressure builds up in the hose. This is where we get term "back pressure" from.

If you turn the hose on higher (giving the engine some gas) you increase the velocity and flow coming from the source, this also creates pressure. Even with no physical restrictions, the hose (pipe) and any bends creates a pressure build up at the source. So while you are correct, if the pipe size is the right size for the system you dont need any restrictions in the line because it is getting its optimal pressure from the piping itself, the problem is a stock motor wont put out enough to create that optimal pressure with just the piping alone, so to get it, you either need to adjust pipe size or add a way to build up that pressure to make the velocity match the flow rate.

I do a lot of water line designs and the concept is the same.

Don't wanna go into a yes/no discussion with you about this, since I'm far from an expert, but I think you are wrong. Pressure goes down when the exhaust gas flow speed goes up apparantly. Still trying to wrap my head around as to what the reason this has, but I think it's because of instead of like a water flow wich is constant, an exhaust gas flow is all small pulses one after another, and each pulse creates a little vacuum right behind it? Or something along those lines, **** like that is way above my brain capabilities


I found a lot of good info in the following link, very interesting read! Google 'engine backpressure myth' and you'll find loads of info

The myth of exhaust backpressure [Archive] - My Pro Street

"http://www.dsmtuners.com/forums/frequently-answered-dsm-questions/168578-exhaust-straight-scoop-backpressure.html"

 

[Hemi450hp]

By increased I mean they went from around 5 and a half seconds to closer to 6.5 seconds.

I'm going to have to challenge you on this one...haha. Especially with your mods, I dont see how you were losing power with the cutout open. All the fastest hemi trucks are either running open tq tubes, or a single open cutout without cats because these setups are making the most power, especially for the guys with bigger mods...I would put you in this group with your current mods. I wish it wasnt 110* where you are right now, because I would make a small wager with you to test this out at the track to see what the ET differences show.

Dont laugh at me here, but my first day to ever race that 03, I was stock with just the cutout and removed my air filter. The truck ran a 15.6 @ 89mph. Next pass, I opened the cutout which was after the factory cat, and ran a 15.2 @ 92mph. My 1/8th ET also dropped .2, so there were still gains down low. I'm going to shoot you a PM in a few minutes.

 

[truckin151]

Mods now are different than the mods when I firstested it out. That's why I said stock engine it be a bad idea.

 

[truckin151]

Don't wanna go into a yes/no discussion with you about this, since I'm far from an expert, but I think you are wrong. Pressure goes down when the exhaust gas flow speed goes up apparantly. Still trying to wrap my head around as to what the reason this has, but I think it's because of instead of like a water flow wich is constant, an exhaust gas flow is all small pulses one after another, and each pulse creates a little vacuum right behind it? Or something along those lines, **** like that is way above my brain capabilities


I found a lot of good info in the following link, very interesting read! Google 'engine backpressure myth' and you'll find loads of info

The myth of exhaust backpressure [Archive] - My Pro Street

"http://www.dsmtuners.com/forums/frequently-answered-dsm-questions/168578-exhaust-straight-scoop-backpressure.html"

The prostreet link is basically what I am describing. Just different termenology being used. Their definition of BP is gas moving back to the cylinder while I was using it more as the pressure build up in the system. It is still the same concept though as the running water through a hose or blowing through a straw. If your piping doesnt match the outflow of the engine it hurts performance.

 

[Budakane]

Like mentioned b4, I felt no power loss down low, I actually felt more