OKay this is on a 1971 El Camino with a mild 355. The owner has 1.75" primary Hooker Super Competition headers(full length of course). He wants to replace his exhaust from the collectors back. Currently he has 2 1/4" true duals on it with a pair of flowmaster 3 chambers. Alright my buddy knows he wants to run 2.5" exhaust with an x-pipe. Heres the question: We have heard that if you want to run an xpipe, to get maximum scavenging you need to run the pipe from the collectors to the xpipe the same size as the collectors. What this means is he would need to run 3" pipe from the collectors to the xpipe then 2.5" from the back of the xpipe to the mufflers. What is the best way to go about this?
Thanks

 

what you want to do is keep any kind of change in diam of the pipe to a min


if you don't want to use a 3inch pipe don't get a 3 inch collector

 

I've always heard that you just don't want the x-pipe diameter to be smaller or only slightly smaller then the exhaust pipe. In this cause that would be 2.5" but you could even use a 2.25" x-pipe and probably be fine.

 

It doesn't matter where you go to 2 1/2", it's still going to be a dual 2 1/2" exhaust.

The engine doesn't car where you put the restriction. You can reduce it to 2 1/2", 6" from the tip of the tailpipe and the engine will see the ENTIRE exhaust as 2 1/2".

Just make sure (as said earlier) the the x-pipe is not smaller than the pipes going into it.

AJ

 

Actually the engine does care where the restriction is. If you put the restriction closer to the engine, while the exhaust velocity is still high the engine will not be affected as much. But if you put the restricition later when the exhaust velocity drops, the engine will experience a larger backpressure.

 

The only thing that drops velocity is a restriction. It doesn't matter where it is.

Perhaps what you're thinking of is the length of the pipe? The closer you put the end of the exhaust to the engine, the less restiction there is. Plus it alters the pulse waves, which can change scavenging and sound level.

However, an exhaust is like a chain, it's only as strong as THE WEAKEST LINK!!! GOODBYE :sillylol:

AJ

 

It slows down as it cools.

 

That is why you want the restriction as close to the engine as possible, because lowering the velocity increases backpressure. That is why it is better to have smaller diameter exhaust pipe closer to the engine and bigger pipe farther from it. So that the backpressure is reduced as the velocity decreases. This is simple fluidmechanics and thermodynamics.

 

wouldnt having a smaller diameter pipe would increase velocity?. i mean forcing an amount of exhaust through a 2 1/2 inch pipe would have to move faster becuase of less area. just because you have a 3 " system doesnt mean that same engine doesnt put out the same amount of exhaust out of the valves on a 2 1/2 system. i dont think the engine puts out more exhaust from the valves when you change systems. maybe im wrong. if i am, well im sorry and dont worry about anything i just said.

 

True. Forgot about that.

AJ

 

Well fluid this and thermo that... blah blah blah...

You're obviously not understanding what I am saying. Quit overlooking the obvious. You don't "want" a smaller pipe ANYWHERE.

True, reducing the size of the pipe does increase velocity, BUT the only way that'll happen is from the engine having to push harder to get the exhaust out. That's called "pumping loss"

All I'm talking about is the area of the pipe. I'll say it again; The place that you put the resriction in is irrelevant. It's still a restriction. The engine still has to push harder to get the exhaust out.

If we were talking about the diameter of the collector, or the length of the collector, or even the angle of transition from the primaries into the collector, then I can understand your point about the heat differences, and velocity and scavenging, etc.

Lets use a water pump for example (since you wanna talk fluidmechanics) and say you were pumping out a swimming pool to empty it for the winter. The suns been beating down and the water is 80* F (since you wanna use thermodynamics). You're using a 3" pipe that's 8' long. You need the run to be 10 ft to get it to the sewer drain in the road. You also have 8' of 2 1/2" pipe.

So you're saying that by cutting the 3" pipe down to 2 ft, and adding the 8' length of 2 1/2" is going to put less strain on the water pump because it's 6' closer and is going to keep that water warmer? Or should I say because the water will be warmer at the restriction point that it's good and will make a big difference?

BUT...If I cut the 8' length of 2 1/2" pipe down to 2' and added that to the 8' section of 3" pipe, then it will be harder on the pump? And my time it takes to pump all the water out will go up because the water is cooler and more dense and won't flow as well through that 2' of 2 1/2" pipe there as it would if the joint was only 2' from the pump?

Man... I could probably save .5 seconds (out of 6 hours) next time if I do it your way.

I'd say that the advantages and disadvantages to each would cancel each other out there.

What I'm trying to say is it would be a whole lot better if I had just bought the 10' of 3" pipe. That would make a WORLD of differdence on how easy my pump would do it's job AND it would pump a whole lot more water in the same amount of time, or IE, take less time to pump out my pool. Which ever way you preffer to look at it.

AJ

 

Oh wait... I just read your post again and forgot....

You're also saying that if I were to put the 8' length of 2 1/2" pipe right on the pump, and then use a 2' length of the 3" pipe, then that would be the best way!!!

I totally understand now. That would be the best way because then all the pump would see is the 8' of 2 1/2" pipe. The 2' of 3" would be 'invisible' to it, right?

WELL THAT'S THE SAME THING I WAS SAYING WHEN I ASKED YOU IF YOU WERE THINKING OF THE LENGTH OF THE PIPE!!!!!

But that has nothing to do with what brody's asking!!!

AJ

 

All i'm saying is that it is better for the restriction to be closer to the engine when the exhaust gas velocity is higher and the backpressure is lower. Because if the restriction is farther from the engine when the exhaust gas velocity is lower and the backpressure is greater. The exhaust exiting the engine will be backing up against the greater backpressure caused by the resitriction located in an area of low exhaust gas velocity.
Yes i agree that any restriction is worse then no restriction at all, i am simply trying to explain why the restriction closer to the engine is more advantageous, then a restriction farther from the engine, from a physics standpoint.

 

Maybe you mean "less disadvantageous" rather than "more advantageous".

Where does this come from? Why do you say the backpressure is greater because the exhaust gas velocity is lower?

 

im wondering now. if it is better to have mufflers(aka restrictions) closer to the engine the why arent everybodys mufflers welded to the headers? i know in thirdgens its tight, but what about an older chevelle. just because those cars have more room underneath does not mean the principles change. if it is better then why arent people running mufflers welded onto their headers then pipe from the muffler(which is now beside the tranny) all the way back to the back of the car?

HERE IS A POST FROM CHEVELLETECH.COM - its about exhaust

posted by: onovakind67
"What is 'backpressure'? Any fluid flow system will have a pressure drop across it, else there would be no flow. I don't know where the concept of backpressure emerged, or where you measure it and how it is measured in a system that has as many pulses, waves and reflected waves as the typical V-8 exhaust system. An exhaust system design is basically a compromise between velocity and volume. Too much volume reduces the velocity and the scavenging effects, reducing the VE of the motor. Too much velocity restricts the volume we can flow, limiting the amount of air we can pump through the system. Added to this are the wave lengths the headers are designed for and sharp discontinuities in the system that create undesirable reflected waves.
If you are going to neck your collector immediately down to 2-1/2", why not just run a 2-1/2" collector and enjoy superior scavenging? On my 406, I run a 1-5/8" primary system with 2-1/2" collectors mainly because a 3" collector with 1-5/8" primaries is so inefficient at scavenging. A 1-3/4" primary will scavenge better with the same 3" collector, giving the impression that the bigger header works better. An even better impression is generated by keeping the 1-5/8" primaries and using a 2-1/2" collector.
After the collector and crossover, the exhaust has cooled considerably, occupies less volume and the pressure is more homogenous, so you can run a smaller pipe with little penalty. Placing the mufflers as far away from the engine as possible also reduces their effect on the system, minimizing flow limitations, etc.
Here's an interesting article on exhaust system design: http://www.accesscom.com/~knliao/exh_theory.html "

what do yall guys think about that?

 

I agree no restriction is better then a restriction

but wouldn't putting that restriction close to the engine put a higher strain on the motor.

the exhaust gas is at a higher velocity correct but if you put a restriction there it would cause more turbulence in the gas kinda like a spoiler or something on a car might not do create much drag at low speeds but at higher speed cause all kinds of drag and turb.

since you have that turbulence you are going to slow down each exhuast pulse there after and being so close to the exhuast port it has no time to regain good flow before the next pulse comes out and so it just creats one big fubar event.

when you put it near the end of the exhuast tip though doesn't being that it is so far away the exhaust gas doesn't really have much time to effect anything?


ok I know what I am trying to say I just can't put it out there.....

hope you understand my thoughts here


hehe good luck

 

This is why they make stepped headers with a smaller pipe "restriction" closer to the engine and a larger pipe farther form the engine.

 

the hooker super comps that he has though arent stepped. i get what you are trying to say, but i totally agree with rx7speed. i dont want a retriction in my exhaust, but since i have to have one(muffler) i want it as far back as possible, and thats where it is at.

 

isn´t the reason they make stepped headers to prevent reversion?


this way if you are at low rpms the exhuast doesn´t revert back on itself?

do you think they would make stepped headers just to make a restriction this way they put it close to the exhuast port... again only to make that restriction?

 

The idea behind stepped headers is to flatten out the torque curve. The smaller initial size of the primary isn't there to increase backpressure close to the engine.

Everyone say it with me:

:lala:BACKPRESSURE IS BAD:lala:

 

backpressure is bad......




do I need to dig up an old backpressure post from a while back


mark you know what I'm talking about

 

I'm am saying that smaller pipe closer to the engine doesn't create backpressure. But if the smaller pipe is farther from the engine, when exhaust gas velocity is slower it will create backpressure.

 

Keep telling yourself that and maybe it'll come true.

AJ

 

Why do you think that backpressure is increased when velocity is lower?

 

let me ask you this

your throwing a punch

what would have the effect of slowing your punch down more

putting some kind of restriction right at the start of your punch or something at the end?

putting the restriction right up on the exhuast port is not a good idea

yes the gas has a higher velocity at that point but that is why it is NOT good.

why do you think they put the mufflers in the back of the car and not right off the header.

yes they put cats right off the header on soem cars but that is to light off the cat quicker without regard to making power


when the exhuast gas is movng right at the port and gets slowed down then each pulse there after is not going ot have as much pull put on it to help it out of the exhuast system. but when you put it near the exhuast tip there is, for lack of better term, a buffer that will allow some slack in the speeds b/c of all that distance between the exhuast port and the part that is slowing down the exhuast g asses


you might want to start reading up on scavenging and exhuast tuniing and you might find you are wrong on this one bud

 

yep.

 

Oh, and rxwhoreslut ( ) you're not a better ***** than me, you're just cheaper.

 

 

how would I be cheaper


you just give it up way to easy