why does 5" pipes "rob" power?

[LArnold]

i have read that 5" exhaust can take hp&torque away,but if 4" is good up to 450 hp (thats @ flywheel becuase exhuast mods deel with just engine power)then making just a few modifacations can put you into the 450hp & over range. (any fueling "box"=75-150 hp,exhaust=25-30 hp,air intake 15-30 hp ,and also other things i may not have included);but NOTHING to do w/ drive train because dodge says the "600" has 325 hp @crank then the way i see it is if youre gonna any mods then its no big deal to have a bigger exhaust ;and it does lower e. g. t's ive had my truck to 108 for a 1/4 mile or so and didnt get any higher than 1100' . anyway any other thoughts?
thanks for any replys ---LA

 

[LArnold]

i have read that a big exhaust cools down so much it causes back presure in the exhaust itself,hense the power lose,and maybe explains some mpg loss?(some say)so how can a bigger exhaust cuase power lose if the cummins motor needs no back presure to "run" right?

 

[Tim]

i have read that a big exhaust cools down so much it causes back presure in the exhaust itself,hense the power lose,and maybe explains some mpg loss?(some say)so how can a bigger exhaust cuase power lose if the cummins motor needs no back presure to "run" right?

Because when the gasses cool and lose volume, they stop flowing. Then the engine has to PUSH them out, instead of them flowing out on their own.



@ 4,000 RPM with two turbos screaming and black smoke puking out of the pipe, I'm sure 5" flows very nicely, but on a street driven vehicle, it's widely accepted that it is overkill. It may or may not cost you power, but it's probably not going to gain you anything, either, and it will be louder.

 

[JLitton]

then making just a few modifacations can put you into the 450hp & over range. (any fueling "box"=75-150 hp,exhaust=25-30 hp,air intake 15-30 hp ,and also other things i may not have included ---LA

Keep in mind that generally, if not always, the power increases from multiple upgrades will not keep adding what is advertised. For example, if you add an exhaust say you would increase 25-30 HP like you said. If you add an air intake in combination with an exhaust you will not automatically increase 40 - 60 HP. It sounds good to say you have increased your power by the advertised gain for each product, but be aware that if you would dyno your truck you probably will not achieve what you think you should have.

 

[hemijustin]

what kinda power loss are we talking about?

i was going to run 5" from the bed to 6" stacks. i plan on running upper 400hp range when im done with the truck and just like the big stacks. i doubt its enough loss to notice

 

[bmoeller]

what kinda power loss are we talking about?

If there is any loss, you'll never feel it. Never did on my truck when it was otherwise stock.

 

[Luke Warmwater]

Because when the gasses cool and lose volume, they stop flowing. Then the engine has to PUSH them out, instead of them flowing out on their own.

Really? What are the physics that cause hot gasses to flow on their own?

 

[pwerwagn]

I believe it has been shown that you will not begin to see anything hp wise with 5" until you reach about 570hp. up to that point, 4" is fine. 5" is louder though. also, I believe that 4" downpipe with 5" beyond that does abolutley NOTHING for you. the downpipe is where it needs to be 5".

by going to full 5" below 570hp, you will not loose enough, if any at all, to be noticeable (other than more noise/possible drone).



The previously mentioned part about the cummins needing no back pressure to run is false. what is meant by that is the turbo provides enough (to much in most cases) backpressure for the motor. If you think about the 1:1 drive pressure of some turbos (worse than that in a lot of cases) that means that the engine actually has the same backpressure as the boost pressure (I. E. , 40 pounds of boost = 40psi drive pressure=40psi back pressure on the motor).



--Jeff

 

[Gary - K7GLD]

Really? What are the physics that cause hot gasses to flow on their own?

Well, I've been told that hot air rises...



... And that I should always carry some large rocks in my pockets...

 

[hemijustin]

how much pipe do i need to add after i get my down 4" down pipe ? im planing on adding stacks in a couple of weekes, its gooing to take me probably the whol weekend to get the stacks set up because im going to have to relocate the in bed fuel/ tool box and exstend fuel and air lines. . im ganna add an air tank inbetween the two stacks and the tool box... but im ordering the turbo and down pipe today. and i want to put it on but dont want to waist a bunch of money on exhuast im not going to keep.

 

[QWaller]

Just get a 5 inch and put a kink in it.

 

[JonKing]

If 5" creates back pressure, why is my EGT's 50 degrees cooler?

 

[Forrest Nearing]

it doesn't rob power...



just a dirty rumor started by those w/ sewer pipe envy!!! :-laf

 

[Devan Manis]

I agree with forest... however i would bet that the 5" "robs" power rumor started with gas hotrodders that probably didn't know anything at all about a turbo engine (totally different animal) or maybe even a diesel engine let alone the two combined. No anything above 4" will gain you power UNTILL you hit the higher horsepower marks where the 4" is undersized.

 

[Tim]

Really? What are the physics that cause hot gasses to flow on their own?

It's called the scavenging effect. Very difficult for me to explain. They don't flow on their own, per se, but what was exiting previously helps to "pull out" what is behind it.

 

[hemijustin]

It's called the scavenging effect. Very difficult for me to explain. They don't flow on their own, per se, but what was exiting previously helps to "pull out" what is behind it.

this sounds similar to a set of tuned headers on a gas engine. the pulls from each exhuast stroke threw the header helps to pull the next cylenders spent gas out. but i dont think that would have much affect on a turbo charged engine unless it were runing twins were each turbo only had 3 cylenders runing it... has anyone done that? i dont see any tipe of advantage to that. anyway i really dont see how 5" could posable rob power. i would like to see some dyno numers that proved it... any ways it not going to stop from using 5"

 

[BRayls]

If 5" creates back pressure, why is my EGT's 50 degrees cooler?

better yet, If the backpressure is created in the turbo why the drop in temp after it . this could go on and on. point is if its not needed why pay extra for it.

 

[Tim]

The drop in temp after the turbo is because there is nothing else creating heat after that, so the gasses cool.

 

[Devan Manis]

It's called the scavenging effect. Very difficult for me to explain. They don't flow on their own, per se, but what was exiting previously helps to "pull out" what is behind it.

lemme help explain scavenging effect. then help show how its irrelevant to a turbo charged engine. scavenging effect uses the pressure pulses of when the exhaust valve opens, creating a sudden explosion of pressure coming from the cylinder, basically creating a high pressure "plug" if you will traveling at a very high velocity; in effect the pressure wave pulse from the cylinder transfers energy into the column of gas in front of it. This "slinky" effect travels outward toward the exit pipe, which maintains a lower pressure. (atmospheric) The leading side raises the pressure while the trailing side reduces the pressure in the manifold or header. By the time the piston reaches the end of the exhaust stroke the pressue wave will have reached the end of the tail pipe(varying by rpm).



The speed of the pressure wave pulse far exceeds the gas discharge speed created by the sweep of the piston toward TDC. The idea behind headers is to tune it so that the pressure pulse wave reaches the end of the pipe as the piston reaches TDC so the low side of the pressure helps to pull or scavenge some of the remaining exhaust gasses out.



In a turbo charged engine the pressure wave will travel out of the cylinder and run right into the blades of the turbine and help to turn the turbine but its just a pressure wave, it only goes so far, the real drive for the turbo is the piston actually pushing the exhaust out and across the turbine (drive pressure) to the lower pressure side of the system (the down pipe and then on with the rest of the system) the size of the pipe after this will only matter if it is undersized and makes it so that the pressure difference between the turbine and the atmosphere isn't reached until the exhaust exits the whole system.



I hope this makes sense im kind of in a hurry so i whipped this up real quick.

Devan Manis

 

[hemijustin]

I hope this makes sense im kind of in a hurry so i whipped this up real quick.

Devan Manis

makes sence to me i just wasent sure how to explain... and didnt know for sure what caused the pulse... very good explination... i didnt think it would do much on a turbo charged engine

Justin