Hate my 3" thermal exhaust

Let me ask you, did you just come from a site like NASIOC? I notice you have an STi. I had three. That site had 14 yr old wannabes to 35 yr olds. This site is a little different. Here you are going to find 35-70 yr olds based on market of the ST. Because of that, a lot of people here are engineers with years of experience in many different disciplines, and have been around racing for a long time. And we don't make it personal.

And, I'll be glad to give you an answer to what I think you're asking as soon as you give me your definition of "scavenging".

If you're open to learning, you'll profit from the experience.

@mcglsr2 do you have any numbers to back up what you are posting? Dragy times or 1/4 mile time slips to show an improvement from a larger exhaust. I f so I for one would like to them. Before and after only would mean some thing.

No, I didn't just come from there. I have an account. I have posted before. But I typically don't frequent forums that much anymore. I came here because I had some questions about my ST, and my google searches led me to this site. So I figured I'd join. Why not.

As for where I come from, let's just say that I fit in just fine to what you describe as the demographic of this site.

As for scavenging: without typing a small essay, it's essentially sucking the burned air/fuel out of a cylinder so that it can be replaced with fresh air/fuel ready for combustion. The piston forces some of it out on the Blow stroke (but pretty typically it will not be able to push all the spent gas out before the valve closes), and if the exhaust manifold is tuned correctly, a pressure differential is created between the piston (pressure increasing due to the piston traveling up) and low pressure "waves" in the exhaust manifold. As the gases leave, they do so in waves, with each wave effectively sucking subsequent waves along behind it, which helps magnify the difference in ratio of low pressure in the manifold to high pressure in the chamber. The faster the exhaust exists, the faster the waves (the higher the velocity), the more pronounced this scavenging effect will be. This is why it is critical to design an exhaust such that it is big enough to accommodate the volume/mass of gasses that are leaving, while still keeping it small enough to encourage velocity to build. Make the pipe too big, and the flow separates off the insides of the pipe, it stagnates, and negatively impacts velocity, which negatively impacts any scavenging effects.

That is my understanding of scavenging. Is that yours?

Sure. I'm familiar with Subarus, so here's one for a 2018 WRX. What COBB calls a "Stage 1" - it is equivalent to a stock vehicle with a tune (so think of our ST with a 5Star and no other changes) vs a "Stage 2" - the only difference being an aftermarket exhaust. COBB is a very well know OTS tuner for European and Japanese cars, and a few domestic. Keep in mind these are literally Off The Shelf, and not at all tuned to the specific vehicle. Which means the gains one would see from a Pro Tune could be significant (in my case, with my 2011 STI, a Pro Tune vs the COBB Stage 2 was significant). Also, these are for 93 Octane.



Stage 1 gains 2% peak numbers over stock, and as much as 81% over stock at specific points. This is just an OTS tune.


Stage 2, which is an exhaust (and no other changes):



...gains 14% peak over stock, and as much as 98% over stock at specific points. There is a tangible increase from the exhaust alone.


From Cobb's website, on what Stage 1 vs Stage 2 consists of:

Stage1

• Intake Requirements: Stock airbox and stock air filter
• Exhaust Requirements: Stock

Stage2

• Intake Requirements: Stock airbox and stock air filter
• Exhaust Requirements:
  • COBB Catted J-pipe with high flow catalytic converter 541204
  • COBB Titanium 515140 or Stainless 515132 cat-back exhaust

Again, this is with just off the shelf. A custom tune would be better tailored to your specific vehicle. (Edit: I thought COBB was based out of California, I was wrong, they are actually out of Texas. They no longer post their Stage2 and above maps because of the EPA. So that's a thing :-/)


If you are looking for dynos of Explorer STs, I'll have to see how many of them are out there. There may not be much depending on how much people are modding it. But I can spend some time googling for Subarus, VW's, Focuseseseses. They are all out there.

Alas, I didn't do any before/after on my own ST, I honestly just didn't think to do it. But then I also don't have it turned, so in my case I would expect my gains to be minimal.

And people have accused me of being too verbose! I was happy to read your reply until you used the word "sucking". For anyone still watching this post. There is no such quantity as vacuum. There is only absence of pressure. There is no such quantity as cold- just absence of heat. And when you understand that, it begins a new way of looking at things.

My answer is that "scavenging" is two-fold. If you create a pressure differential between the source and the sink, (the exhaust valve and the exhaust pipe/manifold/header, there will be flow. The greater the differential between them determines the velocity. This is why calculations of VE must be modified by pressure. See my single turbo thread.

There is a second type of "scavenging" that has to due with pressure wave propagation which is far more difficult to explain, When a constrained high energy gas encounters a larger area- such as an exhaust pulse hitting the manifold/collector, a negative pressure wave is formed that is reflected back to its source. This wave is time/distance dependent. When reflected back to another larger area (like the cylinder), it will reverse sign and promote the evacuation of gasses from that source.

In other words, when the exhaust valve opens and hits the collector it returns, helping to draw more exhaust out. In the case of the ST or nearly all turbo vehicles built in the last few years, engineers are using a turbo mounted directly to the heads. This means that the distance = time is much shorter, as well as the heat=energy is conserved. The reflected wave being so short happens very quickly which helps to draw out exhaust gasses before the exhaust valve has closed. This, along with a 3 cylinder even fire engine bank, helps to spool the turbos more quickly.

So perhaps @mcglsr2 you aren't so far off, but none of that takes place after the collector, regardless of whether you're talking about a turbo, supercharged or NA vehicle. The only concept for the exhaust is to contain as much heat as possible, for as long as possible, and the correct diameter as possible to produce a velocity of the exhaust gasses so as to meet the torque requirements at a certain RPM.

Let me use your words. Full Stop. End of statement. Period. Ok, just kidding, I'm always ready to learn.

Okay, with you so far. But this part:

Please explain how that applies to a turbo. What benefit does that produce on a turbo-charged engine?

Additionally, what does an ideal exhaust look like, in your opinion, post turbine scroll?

Honestly dude, and STi exhaust gains have what to do with an ST? You're killing yourself, again.

Like I said, you obviously didn't read my exhaust thread post where I made very specific reference as to how the exhaust should look post turbine. Does "diffuser" ring a bell? I'm done with trying to educate you.

As another member DM'd me, I think we're looking at FireStarter 2.0, or is him?

Well, that's what I had at the moment. It's an example of an engine responding to nothing else but a larger exhaust. So I guess I'm just killing myself then.

Alright, we are done. Have a good night.

Plus one on that Tmac. My question I was looking for info on an Explorer ST as that is what I have. Not a Subaru. So to me that was a long winded post with zero value.

Okay I’m sorry then. I was trying to be helpful. I don’t have a stash of dyno charts for STs. The only thing I can offer is to go check out the dyno thread here on this site. Many of those are with aftermarket exhaust. Maybe you’ll find what you are looking for.

No chance it can be a FireStarter 2.0, mcglsr2 actually seems to have a brain on his/her/them/their/zir/zem (don't want to offend) shoulders, FireStarter just spewed nonsense.

Yah it’s an attempt to discredit me because I challenged him on something, by associating me with whomever this Firestarter person is/was. Presumably that person is banned (or at least no longer active), and I can infer (in)famous for ranting.

It doesn’t matter, I’m tired of arguing. Agree to disagree and let’s move on. I was just trying to be helpful. Apparently I botched it. My bad.

Also, predicted response (if there is one): "you are discrediting yourself."

You are conflating a 4 banger turbo with a 2.5 inch single exhaust, that can make over 300 HP with the stock exhaust btw. (Since you want to bring up STi's)

The ST has a dual exhaust that is 2.25 inches. If what you say is true then it shouldn't make 400 HP stock. Sadly math is lost on you and you can't seem to understand that a dual 2.25 inch exhaust has the same volume as a single ~3.5 inch pipe. That's almost an entire inch over the STi stock and larger than the recommended exhaust upgrade the STi forums suggest. Even then the STi doesn't put out the 550+ HP the ST does on the stock exhaust.

All ICE'e are air pumps. The more air it moves the more fuel it can burn. The more fuel it can burn the more power it can make. Suck, squeeze, bang, blow. That's how every single ICE works, even those with a turbo. What's even more fun is that Jet engines work the same principle as well, just without a piston (they are internal combustion engines).

That means that a specific HP rating will move a specific CFM of air. Volume determines how much CFM that the pipes can move before you cannot cram more molecules in. That's the science of fluid dynamics. Air and water act pretty similarly to each other in that regard, with water not compressing as well as air.

You are blatantly ignoring the actual science and factual information. You are talking about the ST like it is a single exhaust STi. You are ignoring actual dynos and draggy times posted here. You didn't botch it because you intentionally ignored anything factual that refuted your 'knowledge'.

None of us is arguing that exhaust bottlenecks are a thing. We are telling you that the stock ST exhaust is not a bottleneck like it is on an STi. To genuinely need to upgrade the exhaust you need fuel injectors and new turbos. That is the only circumstance that it's needed. An exhaust for this vehicle is strictly for the sound. That is echoed by many veteran members on this forum. Many of which have monster power coming out of their vehicles.

I am not ignoring science nor am I ignoring factual information. Furthermore, I am not comparing an STI exhaust to these STs, I merely gave that as an example. Though I do agree it's muddying the waters and I shouldn't have done that. And for the record, the example I gave was for a 2018 WRX. But regardless, my mistake there, I regret that.

Additionally, I am NOT arguing that the OEM exhaust is a bottleneck. I even stated, several times I think, in previous posts here that the OEM exhaust is sufficient, particularly at stock power. I am not arguing that it's too small.

What I AM arguing is that there are benefits to going bigger. That there are benefits to reducing backpressure as much as possible. Sure, there are vehicles here making good power with the stock exhaust (there are even more making good power with upgraded exhausts). Okay. I understand that. All that proves is that one can make X-amount of power with the stock exhaust. It does not speak to how much "effort" the engine/turbos are under to produce this result. What I am saying is that if one were to go bigger with the exhaust, those same power numbers can also be had, with less effort. The air pump that is the engine won't have to work as hard. That's all. The turbos will spool a little faster, possibly require a little less boost to reach those same numbers. One can make a turbo produce more and more boost, and get more and more power, until it melts. That doesn't mean one should. One can make X-amount of power on a stock exhaust. That doesn't mean one should. It means one can. There is a difference.

Everything that you two have said is to the effect of how the stock exhaust is sufficient (and a lot of the data is centered around an NA motor), and none of it to how bigger hurts performance (on a turbo motor). You present your calculations here to show how the OEM exhaust is okay. Fine. Not once have a said your calculations are wrong. I have said that aren't applicable to a turbo motor. You can use them to calc the min diameter needed for a certain power level. But it does not logically follow that it must also be the max. Look, I get it. The stock one can be made to work. I'm not saying that it can't. On a NA motor, going to too big has a high possibility of hurting performance. On a turbo motor, it's less relevant, and has potential benefits, especially if it reduces backpressure even more.

If people want to keep their stock exhaust and make loads and load of power, great. That doesn't bother me in the slightest. To say that a bigger exhaust is not worth it other than for sound difference is not accurate. If you want to disagree, okay man. I am so good with that. If you want to think that the case has been adequately presented and that I am just being obstinate, fine. We can disagree on that as well.

My suggestion is to drop this conversation. It has turned into a pissing match to see who can pee farther. And whatever you do don't start a poll about it.

And to be clear, you didn't specify an ST. But, I do agree that what I did come back with was not appropriate. So I'll take that one.

In what way is it a pissing match? I see it as a disagreement on some fundamentals. If that makes it a pissing match then okay, but I'm not sure I see how. Regardless, your suggestion has merit. I am weary of discussing it. And I don't do polls.

This post to me is why I call it a pissing match. Your trying to blame me because I did not specify an ST. After all this a forum for ST owners (A fundamenral) but I'm at fault in your opinion. I disagree.