Should you run a 4 inch exhaust?

[Livernois Motorsports]

Recently we have had a rash of tune inquires focused around 4" exhaust systems on the F150 trucks, specifically about trying to tune back in some lost power they have developed with these systems.

From the factory, the stock exhaust is engineered to create the proper back pressure for the size of the stock turbo, and stock calibration. When opening up this airflow, there is a considerable loss in low end power and torque because of the loss in back pressure, especially without running a performance tune to help compensate for these changes. Specifically, this is due to an increase in turbo lag.

With a 4 inch exhaust specifically, we see a much more exaggerated negative impact due to increased turbo lag, which results in a decrease in low end power. Additionally, we are seeing people complain of poor gas mileage, poor driving feel, and reduced towing capabilities, all because of having to lean on the truck harder to do the same things it did before with less throttle input.

For these reasons, we decided to properly size our Thunderstorm exhaust, so as to find that balance of back pressure necessary for quick spool up times, yet ensuring enough flow at high RPM/Boost to ensure no restriction of power capabilities. So, in short our Thunderstorm exhaust will keep the responsiveness you love while still increasing hp/tq!

If you have any more questions or would like to contact us about exhausts to fit your truck, give us a call or visit our website.

(313) 561-5500
info@livernoismotorsports.com
www.livernoismotorsports.com

[MiWiAu]

From the factory, the stock exhaust is engineered to create the proper back pressure for the size of the stock turbo, and stock calibration. When opening up this airflow, there is a considerable loss in low end power and torque because of the loss in back pressure, especially without running a performance tune to help compensate for these changes. Specifically, this is due to an increase in turbo lag.

(313) 561-5500
info@livernoismotorsports.com
www.livernoismotorsports.com

I'll start by saying, I'm no expert, but I struggle to see how reducing back pressure downstream of the turbo would create lag and a loss of power.

If that were true, wouldn't larger diameter downpipes hurt performance, when in fact results have shown that low end power is helped? Why would turbo drag cars dump exhaust with the least restriction possible?

I've seen excerpts from the link below in various places, but I've never seen the information debunked with any good evidence. http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

I could see how you might stop seeing gains by going bigger and bigger (especially with our stock tiny turbos), but again, I'm still struggling to see how backpressure actually hurts.

Thanks for any help in explaining this!

EDIT: I'm not trying to start an argument, just trying to understand the theory.

[Gjkrisa]

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

https://youtu.be/RWTARjxiqlo

Sent from my Nexus 6P using Tapatalk

[Gjkrisa]

I'd think a three inch would be good I thought fords 2.5 sucked but I haven't seen anyone go to a three although our cars are twin pipe 2.5 and most four cylinder turbo cars do a three inch.

Sent from my Nexus 6P using Tapatalk

[FoMoCoSHO]

I put PPE catted on and took them off in a matter of 2 weeks.

They didn't fit right. I could have blocked off the original bung and had a new one made but quite honestly, there was nothing in the butt dyno or logs that justified keeping them. Load absolute actually dropped a bit. Like the CAI I felt like part throttle response suffered as well. I do a WOT blast whenever I can but the fact is the majority of my time is spent at part throttle or cruising and I'm not willing to sacrifice that for 10-20 HP on a car that weighs 4700lbs.

I can just as easily add 1/2 psi and a degree of spark. Or maybe turn down the boost a bit so the turbos are operating in the sweet spot of the efficiency map. You might not run the same peak power but you may carry power longer by avoiding the dreaded high IAT2 temps. If only we had those maps.....My suspicion is the 13 ish psi I hear from many of the LME customers may be the right answer.

I have read that any in or out air change can affect speed density calculations. I don't know if that's a fact but for me the car is best as built with just the tune and 3 bar. I don't feel like power is being limited in any way whatsoever. May not even need the 3 bar before its all over.

[MiWiAu]

I'd think a three inch would be good I thought fords 2.5 sucked but I haven't seen anyone go to a three although our cars are twin pipe 2.5 and most four cylinder turbo cars do a three inch.

Sent from my Nexus 6P using Tapatalk

For the life of me, I can't find an actual spec for the Explorer Sport exhaust diameter. I measured it a while back, and think I remember it being 2.25". I would have assumed that the SHO would have used the same pipe diameter stock, but I could very well could be mistaken.

Looks like the Magnaflow and LME exhaust are 2.5" (for the XSport), while the Borla is 2.25".

[Livernois Motorsports]

From the factory, the stock exhaust is engineered to create the proper back pressure for the size of the stock turbo, and stock calibration. When opening up this airflow, there is a considerable loss in low end power and torque because of the loss in back pressure, especially without running a performance tune to help compensate for these changes. Specifically, this is due to an increase in turbo lag.

(313) 561-5500
info@livernoismotorsports.com
www.livernoismotorsports.com

I'll start by saying, I'm no expert, but I struggle to see how reducing back pressure downstream of the turbo would create lag and a loss of power.

If that were true, wouldn't larger diameter downpipes hurt performance, when in fact results have shown that low end power is helped? Why would turbo drag cars dump exhaust with the least restriction possible?

I've seen excerpts from the link below in various places, but I've never seen the information debunked with any good evidence. http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

I could see how you might stop seeing gains by going bigger and bigger (especially with our stock tiny turbos), but again, I'm still struggling to see how backpressure actually hurts.

Thanks for any help in explaining this!

EDIT: I'm not trying to start an argument, just trying to understand the theory.

No offense taken at all!

While the 4inch isn't detrimental at WOT like stated in other threads, on the low end you can lose torque due to the size of the turbos and the required pressure to actuate sensors and valves.
The majority of driving is done with frequent stop and go situations and the larger exhausts can cause the vehicles to seem sluggish off the line. This is info from customers with both tuned and non-tuned vehicles, and in some cases personal experiences. Slight increases in size can be beneficial but I wouldn't recommend going above 3" in any case. Our exhaust is 2.5" and works well, the 4" doesn't adapt to the tunes well, regardless of company.

[SHOdded]

Would be interesting to see a dual 4" exhaust on any of these transverse Ecoboosts.  I didn't thing there was room to fit those in without risking damage to the vehicle.

[derfdog15]

I'd think a three inch would be good I thought fords 2.5 sucked but I haven't seen anyone go to a three although our cars are twin pipe 2.5 and most four cylinder turbo cars do a three inch.

Sent from my Nexus 6P using Tapatalk

For the life of me, I can't find an actual spec for the Explorer Sport exhaust diameter. I measured it a while back, and think I remember it being 2.25". I would have assumed that the SHO would have used the same pipe diameter stock, but I could very well could be mistaken.

Looks like the Magnaflow and LME exhaust are 2.5" (for the XSport), while the Borla is 2.25".

Stock SHO pipe is 2.25", I think the biggest I would go is 3", but 2.5" is a happy medium. Then again, it doesn't really add anything, except maybe a few peak hp numbers from everything we have seen. Mainly noise fluff. I know for the eco F150 there are 3" kits that have wonderful results.

[BOT_ROCKET]

From the factory, the stock exhaust is engineered to create the proper back pressure for the size of the stock turbo, and stock calibration. When opening up this airflow, there is a considerable loss in low end power and torque because of the loss in back pressure, especially without running a performance tune to help compensate for these changes. Specifically, this is due to an increase in turbo lag.

(313) 561-5500
info@livernoismotorsports.com
www.livernoismotorsports.com

I'll start by saying, I'm no expert, but I struggle to see how reducing back pressure downstream of the turbo would create lag and a loss of power.

If that were true, wouldn't larger diameter downpipes hurt performance, when in fact results have shown that low end power is helped? Why would turbo drag cars dump exhaust with the least restriction possible?

I've seen excerpts from the link below in various places, but I've never seen the information debunked with any good evidence. http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

I could see how you might stop seeing gains by going bigger and bigger (especially with our stock tiny turbos), but again, I'm still struggling to see how backpressure actually hurts.

Thanks for any help in explaining this!

EDIT: I'm not trying to start an argument, just trying to understand the theory.

No offense taken at all!

While the 4inch isn't detrimental at WOT like stated in other threads, on the low end you can lose torque due to the size of the turbos and the required pressure to actuate sensors and valves.
The majority of driving is done with frequent stop and go situations and the larger exhausts can cause the vehicles to seem sluggish off the line. This is info from customers with both tuned and non-tuned vehicles, and in some cases personal experiences. Slight increases in size can be beneficial but I wouldn't recommend going above 3" in any case. Our exhaust is 2.5" and works well, the 4" doesn't adapt to the tunes well, regardless of company.

Are you sure it's not just a result of not transitioning to 4" in a way that doesnt cause excess turbulence? What sensors are there that need back pressure? It just doesn't make sense that Ford would implement something that would make it run worse by optimizing the exhaust for turbos.

[EcoPowerParts]

This is where boost activated cutouts work REALLY well.
That being said here's some technical information from a Garrett engineer:
http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

[AJP turbo]

Wow!...so the larger exhaust the better...no ifs ands or buts according to garret

Looks like the customers butt dynos need recalibrated

Loss of back pressure which leads to a loss of torque seems to be a myth on turbo cars

[BOT_ROCKET]

I have never seen a reputable source say that back pressure is a good thing in any car, turbocharged or otherwise. Turbos hate back pressure, and unlike NA cars, turbo cars dont need to scavenge exhaust out of the cylinders. The need for back pressure is a myth in ALL cars, but apparently Ford decided they need some sort of sensor that runs off back pressure...

Would love some sort of confirmation on that, tho.

[MiWiAu]

I have never seen a reputable source say that back pressure is a good thing in any car, turbocharged or otherwise. Turbos hate back pressure, and unlike NA cars, turbo cars dont need to scavenge exhaust out of the cylinders. The need for back pressure is a myth in ALL cars, but apparently Ford decided they need some sort of sensor that runs off back pressure...

Would love some sort of confirmation on that, tho.

I've seen some articles that indicate Ferrari uses some "variable back pressure" system to restrict more flow at lower RPM, but I wonder if that's more for sound than performance. This was a NA engine as well.

I'm with you on the turbo stuff - I've seen multiple sources say you want relatively small diameter/high velocity in the header to spool the turbine faster, but once it exits the turbine, minimal back pressure possible.

This is where boost activated cutouts work REALLY well.
That being said here's some technical information from a Garrett engineer:
http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

Yeah, that's the same link I provided in my first reply in post #2 and what leads to my confusion regarding back pressure.

Also, here's an excerpt from a 2012 SAE article about the "new" 2013 Ford Explorer Sport:
According to Carl Widmann, Explorer Sport's Vehicle Engineering Manager, "the cold end exhaust system is a true dual exhaust with pipe diameters and packaging optimized to minimize back pressure."
http://articles.sae.org/10815/

If that is an accurate quote, they were trying to MINIMIZE back pressure, not OPTIMIZE back pressure. Surely there were some "optimizations" made for mass production and cost as well.

Wow!...so the larger exhaust the better...no ifs ands or buts according to garret

Looks like the customers butt dynos need recalibrated

Loss of back pressure which leads to a loss of torque seems to be a myth on turbo cars

Brad, have you seen any negative impacts on your end with tuning that indicate downpipes and/or larger/free flowing exhaust hurt low end performance?

[BOT_ROCKET]

I doubt anyone's butt dynos are out of spec, but I also doubt those big exhausts are designed in a way that promotes a smooth transition to the bigger pipe. They might legitimately perform worse, but not because turbos want to be choked.

Sent from my A1P using Tapatalk