For those thinking about a "performance" exhaust system

For the last year and a half on this site, I've seen scores of posts on performance exhaust systems. I don't normally post from a soapbox about these things, but as I've posted several times in a number of threads about this particular item, if you'll bear with me, I'll try to put my thoughts down in this separate post.

The first point I would make is that if you're just looking to change the tone/sound/rumble of your exhaust and your checkbook will bear the burden, by no means am I singling you out. Go for it, and buy the one that gives you a thrill when listening to it.
The second point I would make is that if you think any of the existing exhausts is going to add to your performance you are wrong. Sorry.

If you just do the math, the current Ford system- even with the 2.25 throttle point is still larger in cross sectional area than a single 3". Why is this important? Because you can google tons of youtube videos or documented builds with 3" single exhaust systems (including mufflers of various sorts) putting out more than 800 hp. That is way more than you can achieve on the ST with any turbos that are currently available. That does not mean the stock ST system is optimal, but it's certainly more than capable of dealing with 500+ hp.

Yes, you may say, but I've always heard that the "bigger the better". Sorry that's also not true. It isn't true with normally aspirated engines (N/A) and it is even less true on turbo engines. Otherwise, everyone would be running sewer pipes for their exhausts.

In a perfect world, at around peak horsepower, we'd like to see about 100 meters/second flow through our exhaust. Above that we get restriction through the pipe because of wall turbulence, below that, the natural atmospheric pressure interferes with the flow. Remember at the exhaust exit there is still approx 14.7 psi inhibiting flow.

The second component is heat. The higher the heat in the system trapped in the pipes the higher the flow rate. In gasses, higher heat equates to higher pressure which equates with higher velocity. So it's pretty easy to understand that a shorter exhaust with a smaller circumference (smaller surface area) will hold more heat than a longer, larger circumference pipe. This should demonstrate that two pipes are not better than one.

So, in a perfect world, we want an exhaust that can both hold heat and stay somewhere around our 100 m/s velocity. We can even do the math on the size of piping (cross sectional area) relative to the mass flow of the exhaust to arrive at a theoretical best size. Of course, since the flow rate is dynamic and we haven't yet invented a pipe that can dynamically change its dimensions to adapt, we have to compromise.

One problem that I see often is the confusion between what works for an N/A (or supercharged) engine and a turbo engine as regards exhaust. No matter what you've heard, a turbocharged engine of the same horsepower requires a smaller exhaust than the equivalent N/A engine. Sorry. Why? Because of a thing called the turbine. The turbine of course, drives the compressor, but it's not free. As the exhaust energy drives the turbine, it loses both pressure and heat (as it expands across the turbine). The result is a slower, cooler mass air flow downstream of the turbine. If you don't think it's significant, take a look at compressor dyno charts which measure the amount of energy in kilowatts it takes to drive a given compressor to produce a particular mass flow at a certain pressure ratio. You'll be very surprised. I would guesstimate that the ST turbos combined at 400 hp are probably requiring somewhere between 25-30 kilowatts of energy!

So, what to take from this? First, giant exhaust pipes may sell systems, but if they're not keeping heat contained and generating at max 100 m/s or velocity are not giving you anything related to performance. Second, an N/A system would require a larger exhaust for the same horsepower than the turbocharged engine.
Third, if you would like to see something that could be a very nice setup for the ST with a minimum of parts...see my next post.

So what might work for the ST? Now I'm a DIYer, and have no problem with a loud exhaust at full song, but I have no need to wake the neighbors or alert the bulls when I'm normally driving.

I have a suggestion for those that are looking for a DIY low-cost hi-performance system for the ST (IMHO). Yes, there will be some fabrication work, but a competent shop should be able to handle it easily. Here's some rough instructions:

Where the midpipe is currently located, fab a dual 2.5 from the downpipes with a Y merge collector to a 3" straight pipe. If possible, the length of each pipe should be kept equal from the turbine exit to the merge collector. You'll have to take into account the length of the downpipes if they're different.

This gives you whatever resonant tuning is left in the exhaust (No, I did not speak about resonance in the last post, because after the turbine it's minimal, but can still help your cats flow better). Also, because the 3" has much less surface area than the stock midpipe you'll keep the heat up to help with the velocity after the merge.

Next, connect the driver's side 2.5 at the existing angle into a connector welded to the straight 3" and just past that connector put in a valve on the end of the 3".

Now, we have a way to divert all exhaust through the driver's side muffler, or to have a straight through 3". If in some fantasy world we're generating higher back pressures than 800 hp, some at least can still exit the driver side.

Next, after the valve, extend the 3" all the way back to the passenger side bumper outlet, omitting the muffler and adding whatever cool tips you wish.

With the 3" valve closed, all the exhaust flow will run through the driver's side muffler. When the valve is opened, you basically have a wide-open 3" exhaust. straight out.
Hard to beat this for minimizing the run length, while maintaining as much heat as possible. It is also very quiet or very loud depending on what you want.

You could even set this up so it opens based on exhaust pressure (offset valve with spring) or turbo pressure (electronic or plumbed for boost). Best of both worlds, and not much more expensive (if you have a fabricator) than installing a "performance" exhaust. Such an exhaust would be easily capable of 7-800 hp and still be super quiet around town.

Does this assume no cats? Or in other words "all else being equal, flow of everything from the cats back".

Doesn't the presence of a catalytic converter change the flow dynamics? I would think that replacing the stock system with SPD or FenFab high flow cats would do more in terms of exhaust evacuation than anything. i.e., a full dual 3" system with no mufflers would be more restrictive and rob potential power if it came after the stock Ford catalytic converters than an alternative setup with SPD downpipes and the stock 2.25" exhaust system.

It doesn't make any assumption about the cats. If you use a higher flow cat, you are (hopefully) reducing the velocity loss after the turbine which would be a plus. But put another way, I'd replace the cats before I'd replace the stock exhaust.

I happen to be in complete agreement with @TMac with respect in that the stock exhaust will more than support what this engine is capable of delivering with mild bolt-ons and a tune. I sincerely believe that the same can be said about the CAI system. The stock CAI will handle what these engines are doing from the factory and to some point beyond. I honestly can't say whether 500HP is that magic number or not.

There will come a point in upgrades where these two areas will become the weak link and these common/popular add-on mods from a performance standpoint, are worth the $$$'s.

Having said all that, I have zero issues with owners taking that little bit of extra pride in their vehicles and adding these mods.

Concerning the exhaust system, there are a couple of extenuating circumstances that may help justify the expense. If you're looking for the extra noise coming out of the pipes do it. In so doing you are also potentially remedying what now seems to be not unusual problems with the stock Ford exhaust system. These issues would be both the flex joints rattling/breakage and the inlet pipe at the muffler breakage (remember Ford will likely warranty these issues).

If you get the sound you're looking for and a little extra peace of mind, plus a set of exhaust tips that you like better than the stock downspouts either in color or output direction, by all means, go for it.

I’ve built exhausts for some kind of demented hobby for decades…usually changing them up multiple times just to hit a specific tone I wanted. I’ve told anyone that’s ever asked there was no need for dual 3” on my ST but I knew the bigger pipe was needed to nail the tone I knew I could build into it and not have the typical crappy EcoBoost sound…and it worked. Whipped up a pair of Helmholtz resonators to kill the drone and have been happy. I’ve only changed the rear mufflers once and tips 3-4 times. lol

Exhaust make car go burrrr. Good enough for me.

most Explorer ST customers are looking for a different exhaust note and different look for the tips to replace the odd fake turn downs equipped from the factory

That's probably true. Coming from a performance background, I'm just putting out information for those few people who are looking to optimize the platform while getting the best "bang for the buck". And as I stated in my first point, if your goal is to change the acoustics, buy whatever pleases you! For me, I prefer more of an even bank firing Ferrari sound as opposed to a v8 rumble, which is pretty hard to achieve from the ST v6. But then, some people love the "potato, potato" sound of a harley v twin. To each his own.

If Ford would put the Voodoo 5.2L in an Explorer you would have your wish (as would I).

To provoke some convo, and to make things even easier, here's another option for those who really wish to optimize performance,and enjoy a louder exhaust. Understand that I'm not doing anything to my own ST until the turbo options (which may not ever be available) are up to the task. But, for a pretty minimal investment, this will definitely support a flow rate in the 800 hp range and make it easier to adapt to a single turbo setup (if there's enough interest, I'll make a separate post about single vs dual turbos).

Following my second post, create your equal length merge collector from the existing down pipes/cats. Now just run the 3" out to the passenger side muffler. Depending on your budget, you can either have your fabricator weld a 3" adapter to the stock muffler, or use the muffler of your choice. Completely delete the driver's side exhaust and shave a few pounds. Now you have a dual 2.5" into 3" system with a minimal amount of work. Shortest path length, smallest surface area for the size, and even cheaper than what I proposed in my second post and no playing around with the "valve". Just to warn you, it will most likely sound more like a Stelvio than a Trackhawk. Just FYI.

I fabbed up dual 3" exhausts on my GTO and G8 because they sounded so deep and glorious. Both were in the 500-525whp (600ish crank) range after the tvs1900s and supporting modifications.

I do think that 2.25" is a little small once you do more than a tune and maybe even a tad small if you have an aggressive tune, but not terribly so.

Just because a car with 800hp used 3" pipe doesn't mean 3.5" wouldn't have worked better. It's incredibly difficult to fit anything larger than 3" pipe under anything outside a truck, let alone finding the pipe and mufflers.

I would rather have the non flat plane crank Coyote lol.

This right here is completely wrong. Once the exhaust has been through the turbine housing, the shorter/bigger the exhaust, the better. Prior to the turbine housing is a bit more complicated, but that's not what anyone here is changing. In regards to N/A engines, that really depends. There can be some gains with collector extensions, but most of the tuning is with the primaries and the merge.

I assume you've read my posts concerning maximizing heat retention and velocity, so exactly what theory are you basing this opinion upon?

There is literally an exhaust argument in every car forum on the planet. What works for one platform might not work for the other. I don’t think a dual 3” DP back is needed on an ST, but if someone wants it, by all means send it. Totally different platform but food for thought, my 11’ GS vette made 870ish rwhp though 1 7/8 headers to a 2 3/4” x pipe with OEM mufflers. Swapping to 2” headers, a 3” pipe and z06 mufflers with active exhaust netted me a whopping 15 rwhp.It was SC so that’s not really apples to apples. I have no doubt you could make really good power with a set of DP’s and the oem exhaust on an ST with oem turbo housings. I mean you could always just put a dump behind the DP’s for kill days. Now when someone starts building aftermarket turbos kits, that will most likely be debatable.

This is great information, for me, I would like it to just sound like it should. This is a performance SUV and should sound like it (without the damn fake interior sounds). I told my wife (this is her daily driver) that when the warranty is over, I'm putting every HP upgrade on I can get and she can get another vehicle. HELL I might even pay Unbroken to hook me up

Yeah, the “biggest is the best” is now old and out of date based on basic hardware from way back in the day. Today’s engines are well controlled and the HP versions (Ford Ecoboost) are already tweaked by Ford to give great power/flow.

There’s not going to be any significant gain in power for an exhaust system, including down pipes, while running the stock turbos.

All said the sound of the engine can be varied greatly with exhaust systems. In a previous life, I would have changed out the exhaust right away just because. With the jekyll and hyde nature of the Ecoboost when pressing the go pedal, I prefer the relatively quiet oem exhaust.

My Raptor is on all stock exhaust and turbos for now. However, I have the Stage 5 CRP turbos, manifolds, and downpipes ready to go in when the mood strikes me, but the “cat-back” section will remain stock. YMMV

Heat retention and velocity is great and wonderful pre-turbine, but after the turbine it's pointless. You want to minimize any and all pressure post turbine, and the easiest way to do that is just dump it to atmosphere as quickly as possible.

The only example I can think of that refutes this, is Formula 1 where they use the exhaust to load the rear wing, and the rear wing to actually draw and create a vacuum post turbine housing. This isn't the case in 99.99% of turbocharged vehicles.

I've seen a few posts in the last week about exhaust sizing and the "lore" that bigger is better. I'll try to address why many people believe this, and why it's understandable, but incorrect. Much is based on the idea that the "less backpressure the better". While that statement is absolutely correct, the real question is how to effect minimal backpressure. I'll try to address this as best I can without getting too technical, but if you want a Twitter explanation, you should stop reading now!

First, let's take a look at a typical system on a supercharged or naturally aspirated engine. You have some type of exhaust manifold, with a high energy gas leaving the exhaust ports- generally into an individual pipe, culminating in some type of merge into a common area (the "collector") which is then connected to the exhaust. When the exhaust valve opens, the force of the exhaust energy (consisting of velocity, pressure, and heat) flows down an individual tube into the collector, resulting in more or less "laminar flow" i.e. the fluid (exhaust gas) is flowing more or less in parallel- there aren't major disruptions in the flow in the pipe due to swirls, eddies, or changes from velocity to pressure. Increases in diameter increase pressure, decreases increase velocity.

Now consider the turbine of the turbocharger which is a completely different animal. In this case, the exhaust energy flows down the exhaust port where it comes in contact with a turbine. As the gas expands through the turbine it imparts energy to the turbine, resulting in a loss of energy to the gas and because of the design of the turbine wheel, it also exits the turbine tangentially- meaning that it begins to spin or swirl. An exhaust pipe connected to this is basically confining a hurricane inside a tube. A very high quantity of that energy is directed in a helix around the outside of the tube while the "eye of the storm" at the center of the tube contains very little energy or flow, effectively reducing the flow capacity of the tube! Is it any wonder therefore, that those familiar with N/A engines and not understanding the phenomenon advocate large diameter exhaust systems on a turbine?

The real answer is not to increase the diameter of the pipe, it is to reintroduce laminar flow after the turbine. And to do that, we need to introduce a cone shaped transition (known as a "diffuser") by an increase of diameter followed by a sharp change of direction. I know it's hard to conceptualize, but the gradual change in diameter alters the angle of the helix while lowering the velocity- thus raising the pressure. The directional change completes the "straightening" of the flow. Now take a look at an OEM cat for the ST engine and what do you see? There is a definitely increasing transition followed by a directional change leading to the converter which further enforces laminar flow. Still think those engineers are all stupid and purposely trying to limit your horsepower? Do those catless downpipes sold by some take any of this into consideration?

If you've hung with me this long, the second thing I have to address is the preceding post about having no exhaust whatsoever- after all wouldn't this create the absolutely lowest backpressure? No, and even if that were the case, it does NOT help overall performance. Why? Because number one, you need an exhaust to direct the gasses away from the vehicle, and second because such a strategy completely overlooks the concept of inertial energy. No matter what type of racing you're doing you modulate the gas pedal. You also shift and inertia of that exhaust energy helps performance in both cases.

Assume you're at WOT and you upshift. RPMS drop by 2000, and so does the exhaust energy exiting the exhaust port. Because the turbine still has a huge load due to driving the compressor, its RPM will drop precipitously. However, the residual exhaust energy that left at 6000 RPM is still traversing the exhaust pipe. The rapid transition between the two velocities creates a low pressure zone between them, helping to draw out the lower energy exhaust entering the turbine thus lessening the decrease of turbine speed. This in turn, lessens the decrease in compressor output. To put it in laymen's terms, the inertial energy of the exhaust gasses in the exhaust pipe make your car faster. Questions?