I hear a lot of talk on here about backpressure. What should I keep in mind when selecting a system for my truck? Pipe diam.?

Brian
2004 f150 supercab XLT
4.6, 4X4

 

dont go bigger than the stock diameter. which is 2.5"

 

OK I posted something of the same nature.

Check this link. I got plenty of replies...

https://www.f150online.com/forums/sh...hreadid=176534

 

The one thing nobody mentioned is the fact that increasing the diameter of the exhaust system not only increases performance, to a certain extent, but it will also change the RPM point of max torque. This may sound good at first, but what it's like in the "seat of the pants" is the truck now feels like a dog at low speed. Moving peak torque up in the power band leaves less down low where it's needed for accelleration from rest, towing, mud bogging, and such.

My F-250 has a 40 series Flowmaster, single 3" inlet with two 3" outlets exiting in front of the right rear wheel. The pipe leading in is the original 2.5" system with an adapter to fit the 3" inlet. I didn't install this, the original owner did. Now, after 120,000 miles the Flowmaster is rotting and needs replacement. I will go with a 2.5 in and dual 2.5 out.
Why?
Just like I stated above, I need the torque down low. I regularly tow a 5000 lb. boat and trailer. Some times I have to rev the **** out of it to haul the load up steep boat ramps or up hilly roads. I usually wind up engaging 4 wheel low to get up steep ramps. Not a good thing to do on concrete or pavement.


This just my .02 worth of opinion.

 

That's why people say they lose power on the bottom-end. You are correct, in reality the torque curve is moving up, so you need more revs to get to the peak.

 

Your all wrong in your thinking!
First, no engine "needs" back pressure. It kills performance. If you have back pressure at low rpm it surely will increase as rpm goes up and kill even more power.
Second, using a larger diameter pipe or larger crossection slows gas flow. The slowing decreases cylinder scavanging that does lower torque.
Third it does not raise the torque rpm. The cam timing and intake breathing controls it. That doesn't change.
There is a lot of missunderstanding here that keeps getting repeated so much that it is taken as fact because no one counters it with the technical truth.

 

Bluegrass, you'll like that link posted above. It has more real info in it. No one is disagreeing with you much of what you are stating is noted in that post also.

But flow rates can have an effect on where peak torque occurs. Here is just one article on the subject: http://www.circletrack.com/techarticles/73598/ Not as much as internal modifications, but a slight effect nonetheless.

One quote from this article:
Since peak torque (volumetric efficiency) is associated with specific exhaust gas flow rates, the sizing of primary pipe diameter affects the wave speed (engine rpm) at which these critical or “mean flow” velocities. Stated another way, the sizing of primary pipe diameter can be used as a tool to selectively influence where in the rpm range torque boosts are placed or influenced.

 

Well yes it can influence torque peak but the change is so small that it is not an effective means to tune with except with various tuned header designs intended to help scavange exhaust over a wider band of rpm (4 into 1 and tri-wye etc.) in open systems.
There are no exhaust systems for these engines that will come close to doing this at present time.
Using mufflers and pipes after the cats may have small effects on engine power but IMO are not cost effective, sound is not a consideration as is has no value for performance except for acoustic tuning within a total performance system.
Cam timing advance and retard generally has much more influence on these parameters to the rate of about 300 rpm up and down, than any exhaust change would have providing the exhaust was not so restrictive as to be considered a poor design.
Enter the 3 valve ( improved breathing) and variable timing of the new 5.4 engines that do this very function. The factory or aftermarket could not use exhaust to ever gain the power increases that has been attained by those methods.
On these engines, the exhaust design is far from being poor for the intended use of the engine.
Remember that the exhaust gas volume flow is the same as the intake but at an elevated temperature and speed so the out let pipeing need not be very much larger especially after the cats because cooling reduces volume unit flow capacity needs, where the pipe crossection can even stay the same size as stock without causing back pressure above that caused by the cats.
Thanks for the link but I have been dealing with performance for more than 50 years with everthing form Go-Karts to Sprint cars to stock cars.
Presently running a 550 hp 351w powered Sprinter and a K.B. super charged 5L street car.

 

Thanks guys, I agree totally, I had been hearing the same things on here and was even kind of starting to question myself whether I was wrong and maybe back pressure was needed. But as I stated in another post. If you look at you engine as though it is a big gas powered air pump then you will want your exhaust to be able to move more air than the engine can pump out at peak load. Back Pressure requires the engine to have to generate more PSI to push the air through the exhaust and for every PSI it will have to generate then that translates into a loss of CFM that you are unable to intake. You cannot intake more until you have removed the old.

I find it hard to believe that things have changed that much or are different in the late model Ford Engines than they have been in the past or on any other make. A combustion engine still performs the same basic functions to generate energy as it has since it's conception.

For those that feel they want backpressure I ask. Why would you want to have your exhaust putting excess pressure on you valve train let alone with heated spent air? I would think that would greatly reduce the life of your exhaust valve guides as well as prevent the spent air from the next detonation from being able to escape the cylinder. Thus preventing new fresh air and fuel from being taken in. Then when the valve guides do get weak from normal wear after 75,000 miles or so, and the exhaust air will begin to seep through the exhaust valves in to the cylinders during the intake stroke "increasingly so with the more back pressure that is applied" resulting in a much weaker explosion and a loss of HP and torque. Combined with the loss of compression due to valve guides no longer sealing up the combustion chamber properly. With this in mind then this just does not sound like a logical Performance set up to me. Personally the right logic to me is to get the heat and spent air out and away from the motor as quickly as possible and shove more fresh in, but that’s just me though.

 

you have to find a happy medium between too much back pressure and not enough.when you have no back pressure it all goes into the exhaust and slows downand just sits there.when you have the right amount of backpressure it helps scavenging and gets more exhaust out and keeps it hot keepin the pressure up and makes it move out quicker.its like the intake.the shortest air intake path is not always the best.the shorter the intake runners the more high end horsepower,and the longer the more low endtorque because the air has to travel a longer distance it speeds up and increases density.i don't know why this is so hard for people to understand.

 

Now worries, that link was just a circle track article from racers much like yourself. Hope you didn't take it as a flame or anything, that wasn't the intention at all

It's always good to hear from folks who have as much technicall knowledge as you have That's what makes these forums great.

 

Your cool.
I just get wound up with nonsense like fast way spreads about back pressure from someone who has never learned about the subject from anywhere but hearing it repeated over and over on these boards.
Does no one ever think that the EGR does the same thing that back pressure would do " introduce exhaust gas" back into the combustion chamber.
The EGR operation reduces power but is not seen during cruise when a lot of power is not called upon.
To make it work the fuel is cut back and ignition timing is advances that makes it less noticeable.

 

i am no expert on backpressure, front pressure or the sort, but how would back pressure in the exhaust effect the flow on the intake side? that doesnt mesh at all with the theory of the 4 stroke engine. the intake and exhaust valves are never open at the same time, and if they are the piston is at TDC of the exhaust stroke so there is not alotta space there for air from either side plus your vacuum would take a crap if this happened and rendering the egr valve to a useless expensive waste of money because the engine would be putting exhaust back into the power stroke naturally. on higher milage motors the scenario might be differnent cause of wear and tear (weak valves) but the intake and exhaust systems are totally different systems separated by the valves.
I'll probably get flamed for this but this is basic theory you can learn from a briggs and straton book

 

As I understand it, some valve overlap - intake & exhaust valves open at the same time - is needed to fully scavenge all exhaust gas from the combustion chamber. The exiting gas also helps draw the intake charge into the cylinder, I believe.

A little overlap gives the engine a smooth idle and good low speed torque, but at the expense of high rpm performance. A large amount of overlap allows excellent engine breathing at high rpm but causes a rough idle and poor performance at low rpm, hence the lumpy idle of a cammed V8.

We're talking about small overlap, not like the valves are completely open together for an extended period of time.

 

Pat, respectfully, you don't know enough about cam timing.
All the 5 L engines from back in the early 80s right up to the last ones, never had any valve overlap.
These 4.6/ 5.4 don't have any either.
Neither do the highly touted Corvette engine.
This is done to get the best overall min emmissions from the engine and to ensure smooth idle.
These type of cam profiles are also excellent super charger cams.
It is true as over lap is introduced and cam duration lengthened, performance is increased as well as moving it to a higher rpm.
The term over lap is applied to the valve events where the intake is just closeing and the exhaust is just opening.
The lift point this happens is determined by the spacing of the cam lobes called the displacement angle between the intake and exhaust for the same cylinder.
These engines have lobe angles in the 118* range +/-.
If the same cams were to be ground with less lobe seperation say 110* and not changing the profile, overlap could be made to exist, improve performance operation (without emmissions consideration) and change how the engine runs.
Given how these engines are used and the rpm limits, overlap is of no consideration but getting the exhaust to evacuate at a fast enough speed still pulls on a closed cylinder.
This is why altering the engineered exhaust system loses this function and is seen as a loss of low end torque.
So you should be able to see that this is not in any way a loss of back pressure that results in losing low end power.
This back pressure idea is a myth that just keeps on truckin.