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.
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.
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HELL I might even pay Unbroken to hook me up
