4" Exhaust
Hotshot
Joined: Nov 2006
Posts: 11,286
Likes: 263
From: North of Greenville
OK... here comes my initial thoughts as an engineer who deals with thermodynamics and flow dynamics for both gases and fluids on a daily basis... I'm sure others will chime in as well, and may correct some of my terminology because I'm just an engineer, and not really a "turbo guy". That said, forgive my wordiness.
Yes, you will get the full effect by running stock up-pipes with your 4" system. It has already been determined that a 4" exhaust will handle your engines horsepower output to quite high levels. In other words, the only thing to be gained above 4" (for the vast majority of us) is a difference in both sound and appearance.
As for the up-pipes... they are very short, and that means that the pressure drop (friction loss due to velocity) across such a short piece of pipe is minimal. If you only increase the up-pipe diameter, you are still not changing the geometry of your turbo inlet housing and will still experience the same pressure drop through the entrance "nozzle" into the hot side of the turbo. If you do, however, also change your turbo inlet diameter to something ,larger, you are going to see a decreased gas velocity entering the turbo and your "spool up" potential will decrease (in other words, "lag" on the lower end of your acceleration curve, or sometimes called "slower boost development").
The whole "exhaust size" issue has to be designed to properly manage the gases' resistance to flow (back pressure, or pressure drop through both 1- the entire system, and 2- at each component). For optimal flow, you need to keep your gases flowing in a "linear" flow pattern with minimal-to-no turbulence inside the pipes while simultaneously having a large enough pipe diameter to create minimal friction losses due to the gas velocity. Obviously, your pipe diameter never changes over the range of operating conditions when you're running your engine, and that means you will get varying conditions based on the amount of fuel being burned and the volume of gases generated by the combustion dynamics. In other words, you will end up with a system which WILL give you linear flow with minimal friction losses due to velocity at some range of performance, but that set of performance characteristics only exists within a small range of exhaust gas flow (i.e. how much fuel is being burned, or, how fast your pushing your engine). It may be ideal while hammering the throttle, but would then be much less than ideal at normal highway speeds and normal acceleration levels, or vice-versa. You have to choose whether or not you're interested in fuel economy or power performance in a racing or towing setup... one will always be sacrificed to some degree by the other, regardless of which you choose.
Just for fun, below is some additional information I got from talking with the old Pinnacle Racing team several years back.
In Pinnacle's research and testing on exhaust design details, they discovered that if you increase your exhaust pipe diameter too much and too early (before adequate cooling takes place), you can actually create a zone of turbulence inside the exhaust system and increase your back pressure. Consequently, their down pipe for the 7.3L engine started out at stock size and only increased to 4" about half way down the length of the down pipe, and that was all in order to best manage the issue of back pressure by keeping the gas velocity "linear" throughout the entire 4" system (no turbulence to create the extra resistance to flow out the back end). Their testing was done primarily on race engines, though, so we more "common folk" will probably not ever realize the downside of this reality by going from a 4" to a 5" system.
Yes, you will get the full effect by running stock up-pipes with your 4" system. It has already been determined that a 4" exhaust will handle your engines horsepower output to quite high levels. In other words, the only thing to be gained above 4" (for the vast majority of us) is a difference in both sound and appearance.
As for the up-pipes... they are very short, and that means that the pressure drop (friction loss due to velocity) across such a short piece of pipe is minimal. If you only increase the up-pipe diameter, you are still not changing the geometry of your turbo inlet housing and will still experience the same pressure drop through the entrance "nozzle" into the hot side of the turbo. If you do, however, also change your turbo inlet diameter to something ,larger, you are going to see a decreased gas velocity entering the turbo and your "spool up" potential will decrease (in other words, "lag" on the lower end of your acceleration curve, or sometimes called "slower boost development").
The whole "exhaust size" issue has to be designed to properly manage the gases' resistance to flow (back pressure, or pressure drop through both 1- the entire system, and 2- at each component). For optimal flow, you need to keep your gases flowing in a "linear" flow pattern with minimal-to-no turbulence inside the pipes while simultaneously having a large enough pipe diameter to create minimal friction losses due to the gas velocity. Obviously, your pipe diameter never changes over the range of operating conditions when you're running your engine, and that means you will get varying conditions based on the amount of fuel being burned and the volume of gases generated by the combustion dynamics. In other words, you will end up with a system which WILL give you linear flow with minimal friction losses due to velocity at some range of performance, but that set of performance characteristics only exists within a small range of exhaust gas flow (i.e. how much fuel is being burned, or, how fast your pushing your engine). It may be ideal while hammering the throttle, but would then be much less than ideal at normal highway speeds and normal acceleration levels, or vice-versa. You have to choose whether or not you're interested in fuel economy or power performance in a racing or towing setup... one will always be sacrificed to some degree by the other, regardless of which you choose.
Just for fun, below is some additional information I got from talking with the old Pinnacle Racing team several years back.
In Pinnacle's research and testing on exhaust design details, they discovered that if you increase your exhaust pipe diameter too much and too early (before adequate cooling takes place), you can actually create a zone of turbulence inside the exhaust system and increase your back pressure. Consequently, their down pipe for the 7.3L engine started out at stock size and only increased to 4" about half way down the length of the down pipe, and that was all in order to best manage the issue of back pressure by keeping the gas velocity "linear" throughout the entire 4" system (no turbulence to create the extra resistance to flow out the back end). Their testing was done primarily on race engines, though, so we more "common folk" will probably not ever realize the downside of this reality by going from a 4" to a 5" system.
Hotshot
Joined: Jan 2011
Posts: 18,849
Likes: 179
From: Puget Sound
Adding 4" exhaust will do absolutely nothing for a stock truck, except change the note.
Now... get a bigger intake, cap the wastegate control line, then do a turbo-back 4" exhaust, and you can see the boost reach the point of defueling the truck with stock everything else. Chipping makes the benefits of 4" more substantial.
As for the up pipes, they can take it. The pressures involved there make the pipe size less important than when the pressure is released and unrestricted low-pressure flow is key.
Now... get a bigger intake, cap the wastegate control line, then do a turbo-back 4" exhaust, and you can see the boost reach the point of defueling the truck with stock everything else. Chipping makes the benefits of 4" more substantial.
As for the up pipes, they can take it. The pressures involved there make the pipe size less important than when the pressure is released and unrestricted low-pressure flow is key.
Laughing Gas
Joined: Nov 2010
Posts: 1,159
Likes: 33
From: Utah
+1-as one who went to a 4" on a stock truck, there is no discernible performance gain with an AIS intake, just sound and probably EGT drop (no egt gauge to confirm). I will be adding a RR billet wheel, new UP pipes, EBPV delete, gauges and a tune at some point where I hope the exhaust will have helped.
Hotshot
Joined: Nov 2006
Posts: 11,286
Likes: 263
From: North of Greenville
When I did my EBPV delete and installed the accompanying high flow outlet, I also replaced my badly leaking stock up-pipes (donut gaskets were GONE) with a pair of slightly used IH bellowed up-pipes. The end result is that I can really hear the turbo singing now so much more then ever before, and there is a noticeable increase in my throttle response. I expect that most of those benefits came from sealing up the up-pipes, but there is now way to know for sure because I did them at the exact same time.
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Laughing Gas
Joined: Aug 2003
Posts: 836
Likes: 9
What I was looking for was lower EGT... I have a tuner with gauges and when towing I have keep my foot out of it or melt down will occur... I think If I could lower the EGT more power would result as I would not have to get out of the trottle as soon...
It's a Van Gogh
Joined: Feb 2010
Posts: 7,812
Likes: 1,335
From: Elk Grove, CA
If the up pipe donuts are original they are probably shot. Leaks there will drive up the EGTs a bunch. Bellowed uppies with the 4" exhaust will definitely improve that. If the budget allows a bigger intercooler will also help on the long grades. A lower HP tow tune will help, and if you live or tow at altitude you need tunes written for that too.
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Logistics Pro
Joined: Dec 2010
Posts: 4,738
Likes: 15
From: Temperance, MI
OK... here comes my initial thoughts as an engineer who deals with thermodynamics and flow dynamics for both gases and fluids on a daily basis... I'm sure others will chime in as well, and may correct some of my terminology because I'm just an engineer, and not really a "turbo guy". That said, forgive my wordiness.
Yes, you will get the full effect by running stock up-pipes with your 4" system. It has already been determined that a 4" exhaust will handle your engines horsepower output to quite high levels. In other words, the only thing to be gained above 4" (for the vast majority of us) is a difference in both sound and appearance.
As for the up-pipes... they are very short, and that means that the pressure drop (friction loss due to velocity) across such a short piece of pipe is minimal. If you only increase the up-pipe diameter, you are still not changing the geometry of your turbo inlet housing and will still experience the same pressure drop through the entrance "nozzle" into the hot side of the turbo. If you do, however, also change your turbo inlet diameter to something ,larger, you are going to see a decreased gas velocity entering the turbo and your "spool up" potential will decrease (in other words, "lag" on the lower end of your acceleration curve, or sometimes called "slower boost development").
The whole "exhaust size" issue has to be designed to properly manage the gases' resistance to flow (back pressure, or pressure drop through both 1- the entire system, and 2- at each component). For optimal flow, you need to keep your gases flowing in a "linear" flow pattern with minimal-to-no turbulence inside the pipes while simultaneously having a large enough pipe diameter to create minimal friction losses due to the gas velocity. Obviously, your pipe diameter never changes over the range of operating conditions when you're running your engine, and that means you will get varying conditions based on the amount of fuel being burned and the volume of gases generated by the combustion dynamics. In other words, you will end up with a system which WILL give you linear flow with minimal friction losses due to velocity at some range of performance, but that set of performance characteristics only exists within a small range of exhaust gas flow (i.e. how much fuel is being burned, or, how fast your pushing your engine). It may be ideal while hammering the throttle, but would then be much less than ideal at normal highway speeds and normal acceleration levels, or vice-versa. You have to choose whether or not you're interested in fuel economy or power performance in a racing or towing setup... one will always be sacrificed to some degree by the other, regardless of which you choose.
Just for fun, below is some additional information I got from talking with the old Pinnacle Racing team several years back.
In Pinnacle's research and testing on exhaust design details, they discovered that if you increase your exhaust pipe diameter too much and too early (before adequate cooling takes place), you can actually create a zone of turbulence inside the exhaust system and increase your back pressure. Consequently, their down pipe for the 7.3L engine started out at stock size and only increased to 4" about half way down the length of the down pipe, and that was all in order to best manage the issue of back pressure by keeping the gas velocity "linear" throughout the entire 4" system (no turbulence to create the extra resistance to flow out the back end). Their testing was done primarily on race engines, though, so we more "common folk" will probably not ever realize the downside of this reality by going from a 4" to a 5" system.
Yes, you will get the full effect by running stock up-pipes with your 4" system. It has already been determined that a 4" exhaust will handle your engines horsepower output to quite high levels. In other words, the only thing to be gained above 4" (for the vast majority of us) is a difference in both sound and appearance.
As for the up-pipes... they are very short, and that means that the pressure drop (friction loss due to velocity) across such a short piece of pipe is minimal. If you only increase the up-pipe diameter, you are still not changing the geometry of your turbo inlet housing and will still experience the same pressure drop through the entrance "nozzle" into the hot side of the turbo. If you do, however, also change your turbo inlet diameter to something ,larger, you are going to see a decreased gas velocity entering the turbo and your "spool up" potential will decrease (in other words, "lag" on the lower end of your acceleration curve, or sometimes called "slower boost development").
The whole "exhaust size" issue has to be designed to properly manage the gases' resistance to flow (back pressure, or pressure drop through both 1- the entire system, and 2- at each component). For optimal flow, you need to keep your gases flowing in a "linear" flow pattern with minimal-to-no turbulence inside the pipes while simultaneously having a large enough pipe diameter to create minimal friction losses due to the gas velocity. Obviously, your pipe diameter never changes over the range of operating conditions when you're running your engine, and that means you will get varying conditions based on the amount of fuel being burned and the volume of gases generated by the combustion dynamics. In other words, you will end up with a system which WILL give you linear flow with minimal friction losses due to velocity at some range of performance, but that set of performance characteristics only exists within a small range of exhaust gas flow (i.e. how much fuel is being burned, or, how fast your pushing your engine). It may be ideal while hammering the throttle, but would then be much less than ideal at normal highway speeds and normal acceleration levels, or vice-versa. You have to choose whether or not you're interested in fuel economy or power performance in a racing or towing setup... one will always be sacrificed to some degree by the other, regardless of which you choose.
Just for fun, below is some additional information I got from talking with the old Pinnacle Racing team several years back.
In Pinnacle's research and testing on exhaust design details, they discovered that if you increase your exhaust pipe diameter too much and too early (before adequate cooling takes place), you can actually create a zone of turbulence inside the exhaust system and increase your back pressure. Consequently, their down pipe for the 7.3L engine started out at stock size and only increased to 4" about half way down the length of the down pipe, and that was all in order to best manage the issue of back pressure by keeping the gas velocity "linear" throughout the entire 4" system (no turbulence to create the extra resistance to flow out the back end). Their testing was done primarily on race engines, though, so we more "common folk" will probably not ever realize the downside of this reality by going from a 4" to a 5" system.
EDIT: Dang it, I can't rep you yet!!
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Cargo Master
Joined: Feb 2010
Posts: 2,298
Likes: 94
From: Delta, CO
Great explanation Pete F250...tried to rep
to the O P if your exhaust is good you could just get rid of the stock muffler, search for the Big Truck Muffler (BTM) several threads on it. Personally I wish I'd have known about it before I wasted $$$ on a 4" exhaust but ...
to the O P if your exhaust is good you could just get rid of the stock muffler, search for the Big Truck Muffler (BTM) several threads on it. Personally I wish I'd have known about it before I wasted $$$ on a 4" exhaust but ...
Post Fiend
Joined: Mar 2015
Posts: 5,277
Likes: 32
From: Blairsville, Pennsylvania
Thanks, Pete!I'm quite pleased with my 4" MBRP Turbo-back with Ford AIS intake. Unfortunately, I don't have EGTs before/after to share.
Cargo Master
Joined: Feb 2010
Posts: 2,298
Likes: 94
From: Delta, CO
All I'm getting at with the exhaust is that with a truck like mine with stock injectors there really is nothing needed but to get rid of the 2 ton muffler & replace it with a straight through design. Don't get me wrong I really do like the sound of my Banks 4" turbo back exhaust but idk if I would spend that money again & cut up a perfectly good exhaust lol
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