The whole back pressure deal is a myth. All that tells you is that you installed a crappy exhaust system on your truck. It could be a number of things either being too big of tubing combined with stock exhaust manifolds or a combination of the two. While not even taking into consideration the changes that need to happen with the mapping for the air/fuel mixture. Back in the "old days" you could throw on headers a intake manifold re jet your carb and be on your way. You never heard about guys worrying about back pressure. Because you did everything at once to make sure your car/truck was running 100%. With computers on the new cars it has become difficult for the average guy to do the complete setup when changing the exhaust and intake to make sure the the air fuel mixture is running right as well. I would imagine with the Up Rev mod you would not have any concern with these problems.Clancy said:Zillions on threads on this here. Defintely do the x or h pipe, but be prepared to lose some bottom end. I know it makes no sense, but everybody here swears that these engines need backpressure...
Should be OK on the bottom end as long as the pipe sizeClancy said:.............................. but be prepared to lose some bottom end. I know it makes no sense, but everybody here swears that these engines need backpressure...
I totally agree. The whole system is basically and air pump. Getting more in and out is the name of the game. But just as you pointed out - making capacity to get it out without matching capacity getting it in does wonky things to the computer...JCollins28 said:The whole back pressure deal is a myth.
....
I have the Nismo system and it is true duals. I really didn't care for the tone, so I had a H pipe added. I like it much better. As for performance, I couldn't tell anything. I thinks it's all a matter of preference.daptech said:Should be OK on the bottom end as long as the pipe size
is not too large. It seems like 2.0-2.25 pipe is becoming the
preference for Titans. The Nismo cat-back is a true
dual system.
orso said:all vehicles need some backpressure because the backpressure helps with the scavaging effect in the cylinder heads. opening the exhaust up does increase hp and tq but to much will hurt the effeciency of the motor and result in lost power due to the cylinder not getting all the spent gases out of the cylinder and replacing it with a fresh charge of air and fuel. it will still have dirty air in the cylinder. so unless you are going forced induction you should be careful of how free flowing you make your exhaust.
You may want a little static pressure to help evacuate the exhaust gases, backpressure is the wrong term.JCollins28 said:Well I respectfully disagree. The only way to get back pressure is to restrict the exhaust. By modding the exhaust you always want the best free flowing system you can get period. No one builds there system with intend to add back pressure.
Nice just a few notes I bet any of you could go to any putcar/trucknamehere[/url] forum you will find a thread title XYZ car/truck needs back pressure.Papa_D said:You may want a little static pressure to help evacuate the exhaust gases, backpressure is the wrong term.
Here's an interesting article I saw, Miata exhaust anyone, lol.
Techno’s
“Know your car” Series #4
Back pressure, Exhaust velocity and scavenging
As an avid reader of Miatanet.com’s Forum section, it is quite intriguing to see just how misunderstood the need, or otherwise, is for backpressure in the exhaust system. There are comments that MX-5s need backpressure and those who see it as a bad thing. Often there is no real understanding of what backpressure is or of its consequences.
OK, so here is Backpressure 101.
The purpose of the car’s exhaust system is to evacuate gases from the combustion chamber quickly and efficiently. The exhaust gasses do not flow in a smooth stream. Because the gasses are vented at each opening of the exhaust valves there is a pulse of gasses from each cylinder. Just put you hand near the exhaust tip and you will feel the pulses. In a MX-5 engine there are four pulses per cycle (except if it’s John Pitt’s supercharged V8 then there are eight really big pulses per cycle).
The exhaust gasses produce a positive flow in the exhaust pipe. Backpressure can be likened to resistance to the positive flow of the exhaust stream. Taken to its extreme backpressure can lead to a reversal (albeit momentarily) of the exhaust stream.
Is Bigger Better or is Faster Best?
When contemplating a modified exhaust system there are those who want the biggest diameter pipe that can be had. Their idea must be that fatter pipes are more effective at venting than narrower pipes. This sounds reasonable but it is not quite correct. Sure wider pipes have greater volume and higher flow capacity, but that is just half of the story. Capacity is one consideration but gas velocity is the other factor.
An experienced exhaust designer knows that the best exhaust is one that balances flow capacity with velocity. A given volume/time of gasses will travel faster through a 2" pipe than the same volume of gas passing through a 3" pipe. So when taken to its extremes we can see that a too narrow pipe will create backpressure (restrictions to positive flow) problems and a too wide pipe will cause a very slow flow with no backpressure.
The optimum is where the fastest velocity is achieved with the least constriction possible.
This situation will arise when the pipe is wide enough so that there is the least level of positive backpressure possible whilst achieving the highest exhaust gas velocity.
The faster the exhaust gas pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The scavenge effect can be visualised by imagining the high-pressure pulse with a trailing low-pressure area behind. The faster the high-pressure pulse moves the stronger the draw on the low-pressure gasses and the gasses behind that. The scavenge action is like (but not exactly) suction on the gasses behind.
The greater the clearance burned fuel from the combustion chamber the less diluted the incoming air/fuel mix is. Scavenging can also aid intake on overlapping valves (where the exhaust and inlet valves are open at the same time) by drawing in the intake. These are good things to happen.
So instead of going for the widest pipe possible we should be looking for the combination of the narrowest pipe that produces the least backpressure possible. In this scenario we achieve the least restriction on positive flow and the highest gas travel speed.
Exhaust pipe diameters are best suited to a particular RPM range. If we used a constant RPM engine this would be easy to specify. But a variable RPM engine will mean that not one size suits all. It is possible to vary the size of exhaust volumes according to rpm but it is very expensive (Ferrari has done it). The optimum gas flows (volume and speed) are required at the RPM range that you want your power band to be located. For a given engine configuration a small pipe diameter will produce higher exhaust velocities at a low RPM (good) but create unacceptably high amounts (bad) of backpressure at high rpm. If you had a car with a low RPM power band (2,000-3,000 RPM) you would want a narrower pipe than if your power band is located at 5,000-7,000 RPM.
Urban Myth Number 42: "MX-5s need backpressure"
It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realises that he has lost power right through the power band. The connection is made in his throbbing brain…. put on 3" pipe = loss of backpressure = loss of power. Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.
If Max had chosen a 2 1/4" pipe he would have achieved better performance in the mid- to high-RPM power band. You need the combination of the least positive (close to zero) backpressure possible with the highest gas velocity achievable to create performance. The diameter of the pipe (and smoothness of internal finish and bends) will strongly influence if your exhaust change is going to create performance or lose power.
As a general rule, a normally aspirated MX-5 will get better high RPM performance with a 2 1/4" exhaust system (2 1/2" or above is just too wide to retain exhaust gas velocity for street driving). The general consensus is that a 2 1/4" system is for mid to high RPM petrol heads. Your mechanic should be able to advise you what exhaust system will best suit you driving style and needs.
Forced induction (turbo or supercharged) MX-5s perform better with the high volume pipes (2 1/2" to 3"), but that’s another story. The choice of a 4 into 2 into1 or a 4 into 1 header to exhaust set is yet another story.
Safe journey
Rob (Techno) Spargo
Mazda MKX-5 Club Victoria
Very true and more then likely lifted from a after market performance perspective, but this is why OEM's have to keep the lobe separation wide and duration as short as possible, keeping "leakage" to a minimum. This is necessary to reduce hydrocarbon emissions and optimize fuel economy. The "holding" of fresh charge into the cylinder is MANAGED, by VALVE timing; the closing of the exhaust valve. Not by a bouncing-back-wave in the exhaust system. If you want to stop the fresh charge from exiting the cylinder through the exhaust valve...you close the exhaust valve.Scavenging can also aid intake on overlapping valves (where the exhaust and inlet valves are open at the same time) by drawing in the intake. These are good things to happen.
Thanks for the additional insight and hopefully some people will take this to heart when changing their exhaust systems and headers.JCollins28 said:Nice just a few notes I bet any of you could go to any putcar/trucknamehere[/url] forum you will find a thread title XYZ car/truck needs back pressure.
Very true and more then likely lifted from a after market performance perspective, but this is why OEM's have to keep the lobe separation wide and duration as short as possible, keeping "leakage" to a minimum. This is necessary to reduce hydrocarbon emissions and optimize fuel economy. The "holding" of fresh charge into the cylinder is MANAGED, by VALVE timing; the closing of the exhaust valve. Not by a bouncing-back-wave in the exhaust system. If you want to stop the fresh charge from exiting the cylinder through the exhaust valve...you close the exhaust valve.
During the valve overlap period, some fresh air and fuel DOES escape out the exhaust, and that is desirable**. The desired result is to "wash" ALL of the old exhaust from the cylinder, and the valve overlap period is when this is accomplished. Any remaining exhaust gas in the cylinder at this time will dilute the fresh charge, making it weaker.
The desired behavior is for the exhaust to use the pulses to create a draw, or "negative" pressure on the back side of the exhaust valve as a result of VELOCITY and the momentum of the exhaust gasses leaving the cylinder. This "negative pressure" helps to draw the fresh charge into the cylinder during the valve overlap period achieving a clean cylinder w/a fresh, undiluted charge. This is why exhaust (headers) that are "too big" hurt low end power; because at low RPM, the exhaust gas doesn't have enough speed (and momentum) to create that draw on the chamber.
"Tuning" is not the same as "back pressure". "Back pressure" is the result of an out right restriction and is NEVER a desirable feature.
If I were putting dual exhaust on my truck, I would go with a 2.25" pipe and an X pipe given what I have heard right now and if nobody can prove something else performs better. Good luck and happy modding.07titanbullys said:I am getting true duals because of sound with 2.50 inch pipe. i am trying do decide on either an X or H pipe. Definetly getting one or the other. Is there any difference in sound or performance between the two?
I do not spreak for anyone but my actual experience with true duals in my QC Titan 4x4.07titanbullys said:I am getting true duals because of sound with 2.50 inch pipe. i am trying do decide on either an X or H pipe. Definetly getting one or the other. Is there any difference in sound or performance between the two?
