Rally Exhaust?

[teamdsm95]

I have a 3.5 inch turbo back to a straight magnaflow. No cat, no resanator. But like the first 3 I.ches of the down pipe are factory. Cheap way to make exhaust, the previous owner had done. However, I love the way it sounds but the muffler was made for diesel truck.

 

[Leonthefaded]

I've decided to try a couple of different things. Firstly for my project which you can find in my build journal.
I will try the following setup first:

3" Downpipe
Highflow Catalytic Converter
2.5" Piping from the cat
Apexi N1 Muffler
Crush Bends

The second setup I will try if I am not satisfied with the above will be:

3" Downpipe
High Flow Catalytic Converter
3" Resonator
Apexi N1 Muffler
Mendrel Bends

I decided to make this my backup if the first setup mentioned is not "aggressive" enough. More than likely I'll be getting the downpipe, muffler and high flow cat

My last setup is a surprise you will only find in my build journal update coming up.

 

[PieEyedPiper]

Leon, it is a bad idea to design an exhaust that starts at 3" and necks down to 2.5" or below.
Beyond a bad idea, unless the goal is just to replace the exhaust with something heavier than stock. Then that kind of setup meets the requirements just fine.

If you must play around with non-matching exhaust sizes, then try a 2.5" DP and 3" catback instead.

 

[Leonthefaded]

What difference does it make if I go from a 3" downpipe to 2.5" piping or 2.5" Downpipe to 3" piping?

 

[jtmcinder]

Is everyone sitting down?

Good.

I agree with Leon's request for evidence and humbly submit the following link as arguing that necking down late in the system is not nearly as much of a problem as starting out small, which contrasts with what PieEyed suggested.

Technical Articles

 

[PieEyedPiper]

Is everyone sitting down?

Good.

I agree with Leon's request for evidence and humbly submit the following link as arguing that necking down late in the system is not nearly as much of a problem as starting out small, which contrasts with what PieEyed suggested.

Technical Articles

Ah, but who said were were trading one extreme for the other? I'm happy to discuss this as I've been pummeled for the same thing in the past.
So, the claim is what though? That we should neckdown at the beginning AND the end? We should neckdown only at the end? Only at the beginning?

We ought not to neck down at all, really. I'm not interested in discussing the complexities of the 02 housing but my main angle is pressure differential.
Pressure differential = flow.
Reduced pressure differential = reduce flow.

Let's ignore any potential neckdowns at the DP/02housing connection and just start with paper towel tube sealed at one end with saran-wrap. Now poke a small hole in it. Compare the flow capabilities to a papertowel roll to one without any saran-wrap. Pretty obvious, right?
So if you throw that example onto the car, how does the car application alter the example so significantly that the results are opposite?

Pretty shitty little picture, but how is the bottom example increasing flow over the top picture?

 

[jtmcinder]

Oh, I'm not about to get directly involved in this, even though I have spent the last few months reading all that I can on exhaust design because almost every exhaust for an Evo X that provides more low torque also makes the car too loud.

But I know that a lot of people think that opening up from the cat-back is the most-important first step, and, yet, both theory and the experience of the guys at Cobb - which is nothing to sneeze at - say otherwise. Even more, I have recently learned that you can put a 2.75" or 3" downpipe and a straight-thru test pipe on an Evo X and have it gain a ton of torque and remain quiet, as long as you continue to use the monstrous rear muffler (albeit with a few mods to the guts).

So all I meant to do here is point out that what appears to be common wisdom - that huge cat-backs are a great first step - might not actually be true. You might get more gains from a downpipe (with stock exhaust) than from a cat-back (with OE downpipe). For those after low torque and low noise, this might be very good news.

 

[Leonthefaded]

"The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons. Tapering the diameter does not make more power, torque, or bring on boost faster. However having smaller pipe towards the end has less effect that having smaller piping at the beginning. In other words a system that has 3" pipe for the majority, and necks down to 2.5" at the end will flow enough for more power than a complete 2.5" system. The further downstream you neck down the exhaust the better........if you decide to neck it down.

Attracting unwanted attention and not hearing your stereo or you passenger would make for an exhaust system great for a racecar, but poor for the average Joe. I like hearing the exhaust myself, but there are times I want to listen to the radio or go on a date without screaming at my passenger. Law enforcement and your neighbors do not appreciate loud exhausts either, even if you do." - Technical Articles

Its a good article I learned alot for my next NT GS

 

[LandSpeed-DSM]

Leon Because I don't think this, like most other subjects, will actually make you think as history has shown.. I'll make an almost too general statement that applies here and is able to be understood by the lowest common denominator.

With fluid flow, through a pipe or over a plane, the high-pressure areas are where you find the slowest face velocity. The low pressure areas are where you find the highest face velocity.

After the turbo, you want the least restriction, pressure or "backpressure" after the turbine so the heat and pressure from the exhaust can work as efficiently as possible and convert more of that energy into torque on the shaft instead of being used to push a column of gas in front of it to reach the atmosphere.

What you are doing is entirely counter to this principle.

A concentric, or even bell/parabolic expansion off the turbine is ideal. You don't want steps in size, especially if you start with a larger size and move to a smaller size. This will create a pressure rise and slow exhaust flow and waste more energy that could otherwise be put to use simply working on the turbine.

Stepping up in size and expanding creates a pressure drop. Doing it smoothly would be best, of course. Or just pick one ID and stick with it.

jtm I see your link and I have heard that sentiment before. The idea being that by then so much heat is out of the system that face velocity will start to fall as the exhaust gasses will decrease in volume, and dropping the tube diameter will help it pick up again. But really there is no scavenging to be done here.

I definitely agree, that the single best thing you can do with post-turbine exhaust is to open up as big as you can immediately, though done smoothly would be ideal, like with a concentric expansion or a bell/parabolic expansion. Its that big delta P that makes the difference.

But really, even in that same paragraph the author states:

The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons

Tapering the diameter does not make more power, torque, or bring on boost faster.

The further downstream you neck down the exhaust the better........if you decide to neck it down.

And while it is true that the 90% 3" system that necks down to 2.5" can flow a greater volume than a 2.5" you are really just shooting yourself in the foot to decrease it in the first place if you are not forced to by physical constraints under the car.

 

[PieEyedPiper]

It seems were somewhat on different topics. But only sort of.

I agree that restrictions near the end of the line are less harmful than those at the beginning of the system. That's not what I'm arguing. I'm saying that if you desire maximum performance, you ought not purposefully create restrictions at the end of your exhaust stream.

But I know that a lot of people think that opening up from the cat-back is the most-important first step, and, yet, both theory and the experience of the guys at Cobb - which is nothing to sneeze at - say otherwise.

I've always agreed with this notion. Well not always, I had an in-depth conversation with DSMunkown and we came to the conclusion that a system can only (loosely) flow as much as the first restriction will allow - no matter how much 6" dia. piping you have after the cat. This is the exact reason I didn't go out and buy a catback for the car when I got it.

Leon Because I don't think this, like most other subjects, will actually make you think as history has shown.. I'll make an almost too general statement that applies here and is able to be understood by the lowest common denominator.

With fluid flow, through a pipe or over a plane, the high-pressure areas are where you find the slowest face velocity. The low pressure areas are where you find the highest face velocity.

After the turbo, you want the least restriction, pressure or "backpressure" after the turbine so the heat and pressure from the exhaust can work as efficiently as possible and convert more of that energy into torque on the shaft instead of being used to push a column of gas in front of it to reach the atmosphere.

What you are doing is entirely counter to this principle.

A concentric, or even bell/parabolic expansion off the turbine is ideal. You don't want steps in size, especially if you start with a larger size and move to a smaller size. This will create a pressure rise and slow exhaust flow and waste more energy that could otherwise be put to use simply working on the turbine.

Stepping up in size and expanding creates a pressure drop. Doing it smoothly would be best, of course. Or just pick one ID and stick with it.

jtm I see your link and I have heard that sentiment before. The idea being that by then so much heat is out of the system that face velocity will start to fall as the exhaust gasses will decrease in volume, and dropping the tube diameter will help it pick up again. But really there is no scavenging to be done here.

I definitely agree, that the single best thing you can do with post-turbine exhaust is to open up as big as you can immediately, though done smoothly would be ideal, like with a concentric expansion or a bell/parabolic expansion. Its that big delta P that makes the difference.

But really, even in that same paragraph the author states:






And while it is true that the 90% 3" system that necks down to 2.5" can flow a greater volume than a 2.5" you are really just shooting yourself in the foot to decrease it in the first place if you are not forced to by physical constraints under the car.

So much of what I wanted to say.. thanks for the post.

 

[jtmcinder]

I'm sorry that I wrote what I did. I was trying to make peace with Leon and it completely backfired. I know what you're saying, PieEyed, and I do not disagree.

No good deed goes unpunished. Gotta keep that in mind.

My only contribution here is this: there is reason to believe that, if you want the system to be small at some point, maybe because systems with bottlenecks have a tendency to be quieter, it is better to have the bottleneck be very late in the system, not at the beginning. Thus, if someone said to you: part of your system is going to be 2" and part will be 3", which way do you want it, your best answer could well be "I want a 3" downpipe and cat with a 2" cat-back" than the other way around.

 

[Leonthefaded]

I will try the amended following setup first:

3" Downpipe
Highflow Catalytic Converter
2.5" Piping from the cat
Apexi N1 Muffler or a Megan Racing Muffler
****Mendrel Bends

I've also seen multiple welded sections of stainless steel piping, welded together to create bends. I have no idea what this is called. I've googled it but have come up with zip. Hopefully some of you will understand what it means and can give me a name, because I considered trying to get something like that made for a very special project on the side of my build

 

[jtmcinder]

A great example of someone cobbling together a smooth exhaust from pre-bent pieces starts at post #19 in this thread: Vorshlag Motorsports Evo X MR Build (STU, TTB, One Lap?) - Page 2 - evolutionm.net And Terry Fair is someone to pay attention to. The dude cares about winning.

 

[LandSpeed-DSM]

I'm sorry that I wrote what I did. I was trying to make peace with Leon and it completely backfired. I know what you're saying, PieEyed, and I do not disagree.

No good deed goes unpunished. Gotta keep that in mind.

My only contribution here is this: there is reason to believe that, if you want the system to be small at some point, maybe because systems with bottlenecks have a tendency to be quieter, it is better to have the bottleneck be very late in the system, not at the beginning. Thus, if someone said to you: part of your system is going to be 2" and part will be 3", which way do you want it, your best answer could well be "I want a 3" downpipe and cat with a 2" cat-back" than the other way around.

I agree with this statement. If you have to compromise for sound or space... yes, neck down as near the end as you can.

 

[gofer]

I don't know the nitty-gritty physics of the "theory" here, unfortunately the Navy has no need to school me on the subject. However, I would still disagree that, despite COBB's tech article, any diameter change in the exhaust should be done right off the o2 and then a size increase, vise decrease. Considering thermal expansion, as the hot exhaust blows past the turbine wheel, into the o2 housing, then into the downpipe it begins cooling and, in doing so, it expands as well. If you put a restriction downstream, say at the cat, the hot exhaust gases will be cooling and attempting to expand AND go through a bottleneck which would create unwanted turbulence in the exhaust thus creating unnecessary back pressure that can effect spool up.

This is of course how I picture "it working" in my brain that knows only what my 12th grade education provided it.

 

[LandSpeed-DSM]

Cooling fluids/materials contract.. but a decrease in pipe ID would create a positive delta P value and slow charge face velocity. So partial credit I suppose, gofer

 

[Leonthefaded]

A great example of someone cobbling together a smooth exhaust from pre-bent pieces starts at post #19 in this thread: Vorshlag Motorsports Evo X MR Build (STU, TTB, One Lap?) - Page 2 - evolutionm.net And Terry Fair is someone to pay attention to. The dude cares about winning.

That is super close to that I was trying to desscibe. THAT system, but the sections are smaller from 1" to 2" in legnth and 2" to 2.5" in diameter, in a "sqaurish circle" or perfectly round 2" to 2.5" in diameter . When I get home today I'll draw up what I'm talking about specifically.


gofer: I have almost no understanding of exhaust systems and until I read that article I never realized that I had it all wrong to begin with when I first put a full 3" straight pipe to muffler on the first time around.

Question: If exhaust gases expand once they hit the catalyic converter and afterwards. Why not use "pre-cooling" intercooler piping (Heatsinks on piping) after or right before the catalytic converter to help cool the gases faster and help them expand more effeciently? Wouldnt doing this in a full 3" System get benefit( In my brain I think expanded gas = less noise + better tone)

 

[LandSpeed-DSM]

Cooling fluids/materials contract.. but a decrease in pipe ID would create a positive delta P value and slow charge face velocity. So partial credit I suppose, gofer

That is super close to that I was trying to desscibe. THAT system, but the sections are smaller from 1" to 2" in legnth and 2" to 2.5" in diameter, in a "sqaurish circle" or perfectly round 2" to 2.5" in diameter . When I get home today I'll draw up what I'm talking about specifically.


gofer: I have almost no understanding of exhaust systems and until I read that article I never realized that I had it all wrong to begin with when I first put a full 3" straight pipe to muffler on the first time around.

Question: If exhaust gases expand once they hit the catalyic converter and afterwards. Why not use "pre-cooling" intercooler piping (Heatsinks on piping) after or right before the catalytic converter to help cool the gases faster and help them expand more effeciently? Wouldnt doing this in a full 3" System get benefit( In my brain I think expanded gas = less noise + better tone)

Try reading my posts again. I think we've gotten lost after gofer's post.

Also, those "heat sink" pipes do nothing, they only have fins on the outside of the pipe, and no fins inside, and the fluid moving through the pipe is spending so little time engaged with so little surface area compared to the volume that they are literally worthless.

In any event, you do not want to go out of your way cool the exhaust gas.

Exhaust noise/tone is independent of gas temperature/relative volume. How loud is an uncorked Harley? The EGT at the end of those 4-5ft megaphones is likely less than 300F

 

[PieEyedPiper]

Here's a snippet from some PM's I figured I'd just toss in here..

PieEyed :One thing I'd like to continue to discuss, however, is whether or not a 3" into 2.5" would perform better than a 2.5" into 3".

The more I think about it, the less and less I'm convinced one way or another. I might just have to physically test it to put my head at rest. I like playing with the theory-side but my imagination is having a hard time giving values to how much more you get when you have 3" piping hanging off your 2.5" restriction or how much you may or may not lose when reversed.
And after thinking some more, I think I can comfortably agree that 3" into 2.5" should be the superior design compared to 2.5" into 3" but I'm thinking in reality the results will be rather close and that at a certain RPM their graphs would cross, not too unlike hp vs torque

Jt: And let's have the discussion in public so that some of the guys who really know engines join in. From what I can tell, the extra issue is the temp of exhaust, which, of course, goes down as you move from beginning to end. Cooler exhaust is more dense (takes less room) but is more viscous (and resists flow through a pipe). That's where I get lost.

-----------

And that's something that has me wondering, too. If the cooler, more dense exhaust is thusly more viscous then you would have to be very cautious when reducing the dia. of the piping. For all I know 2.5" could be too snug and we overshot the "sweet spot" where tapering can increase velocity.

 

[gofer]

This was the "theory" on exhaust gas that I was reading through when Glenn and I were having a discussion about something similar about a month back.

As the piston approaches top dead center, the spark plug fires igniting a fireball just as the piston rocks over into the power stroke. The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke. The gas pressure is still high (70 to 90 p.s.i.) causing a rapid escape of the gases (blowdown). A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (and is dependent on gas temperature). Expanding exhaust gases rush into the port and down the primary header pipe. At the end of the pipe, the gases and waves converge at the collector. In the collector, the gases expand quickly as the waves propagate into all of the available orifices including the other primary tubes. The gases and some of the wave energy flow into the collector outlet and out the tail pipe.

Based on the above visualization, two basic phenomenon are at work in the exhaust system: gas particle movement and pressure wave activity. The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases. The pressure waves, on the other hand, base their speed on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. At all times, the speed of the wave action is much greater than the speed of the gas particles. Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together, as in a collector for example, the waves travel into all of the available pipes - backwards as well as forwards. Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. Typically this phenomenon is controlled by the length of the primary header pipe. Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done.

Gas speed is a double edged sword as well, too much gas speed indicates that that the system may be too restrictive hurting top end power, while too little gas speed tends to make the power curve excessively 'peaky' hurting low end torque. Larger diameter tubes allow the gases to expand; this cools the gases, slowing down both the gases and the waves.

Reading this ^^^ makes sense picturing it in my head however, trying to figure out how a restriction in the back of the exhaust system doesn't hurt power out vexes me.

 

[LandSpeed-DSM]

Basically you are trying to tune pipe diameter at the end where the gas has cooled and relative volume has decreased, similar to how you would tune a primary runner to take advantage of the vacuum created behind a exhaust pulse to pull the rest of the column out the end of the pipe.

You would tune how much of a step-down from the 3" pipe in this example to keep the face velocity up and pulling a small low pressure area behind it. But the pulses are pretty much ruined by the turbine, so the scavenging effect is not strong if even existent at that point in a given system.

Also similar to how 2-stroke exhausts are designed.

 

[PieEyedPiper]

This was the "theory" on exhaust gas that I was reading through when Glenn and I were having a discussion about something similar about a month back.


Reading this ^^^ makes sense picturing it in my head however, trying to figure out how a restriction in the back of the exhaust system doesn't hurt power out vexes me.

We need to mate these two, seemingly, competing ideas and put them together for the best of both worlds!

Meet the 3inch straight through exhaust. Hey, wait a minute..

 

[jtmcinder]

One other point with regard to systems with a cat. My understanding is that one of the main effects of a cat is to oxidize CO to CO2. This would be by 2 CO + 1 O2 = 2 CO2. Thus, there's a reduction in volume of 33%. Now, maybe the oxidation of the HCs and/or NOx more than offsets this in some increase-in-volume manner, but I was under the impression that the total volume of the gasses after a cat was, on average, lower. This would also fit the idea that a bottleneck post-cat is less of a problem than a bottleneck pre-cat.

As to the change in the temp of the gasses as they move from beginning to end, this is where I get lost (as that quote from me above says). I know that cooler = smaller, which again argues for a late bottleneck (if any bottleneck), but I believe that cooler gasses are more viscous and, therefore, resist flowing through a pipe of fixed size, which would argue the opposite. Is the change in viscosity relatively unimportant?

 

[PieEyedPiper]

One other point with regard to systems with a cat. My understanding is that one of the main effects of a cat is to oxidize CO to CO2. This would be by 2 CO + 1 O2 = 2 CO2. Thus, there's a reduction in volume of 33%. Now, maybe the oxidation of the HCs and/or NOx more than offsets this in some increase-in-volume manner, but I was under the impression that the total volume of the gasses after a cat was, on average, lower. This would also fit the idea that a bottleneck post-cat is less of a problem than a bottleneck pre-cat.

As to the change in the temp of the gasses as they move from beginning to end, this is where I get lost (as that quote from me above says). I know that cooler = smaller, which again argues for a late bottleneck (if any bottleneck), but I believe that cooler gasses are more viscous and, therefore, resist flowing through a pipe of fixed size, which would argue the opposite. Is the change in viscosity relatively unimportant?

I guess it depends if it's actually more viscous. Sure it's cooler, but it could also be up to 33% less dense after the cat, no?

oops. Density ≠ volume.

 

[Leonthefaded]

Can one of you compare the system I am going to be testing first. To my current stock exhaust system setup. And explain or theorize how much difference I will see?