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Discussion Starter #1
I was gonna buy the DC SPort TCS Exhaust, but then i read that it produces a lot of backpressure. Now I always thought that Backpressure was bad. But i guess maybe not. Does anyone know what backpressure is good for ? and is this exhaust any good?
 

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back pressure is bad
its more important that the flow of the exhaust is a good match to the engine...too little diameter will choke flow and too large will lead to poor/slow flow.
 

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Ah man not this back pressure crap again

Simply put back pressure is bad for high revving low displacement engines, the purpose of the exhaust system is to expel spent gasses freeing up room for a new intake charge, back pressure prevents this
 

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Hmm, I've learnt a little since we last had this discussion, and I feel not everything that has to be said has been said. So Ich, forgive me if we travel a well trodden path again.

From where I see it, there are three properties of an exhaust that determine it's performance, they are:

Back Pressure
Exhaust Velocity
Overall flow

Okay, we all know (or should know) how a cam works. In a non-variable cam shaft, the lift and duration are always the same. When expressed in degrees. Let's just say that the cams open the exhaust valve for X degrees. At 1000 rpm, the exhaust valve is open for Y milliseconds. At 2000rpm Y/2 milliseconds. at 3000rpm Y/3 milliseconds. You get the point. In any case, the duration in TIME (not degrees) that the exhaust valve is open shortens as the RPM goes up.

Now, if we design the cam, so that the duration is short, then at low RPM there is enough time for exhaust to leave the cylinder, but not enough for the fresh air and fuel to literally come in one valve and out the other. However, as the RPM goes up, the duration in time of the valve opening is getting very short. There isn't enough time to let all the exhaust out, and so the cylinder is still half full of hot exhaust. Not good for getting high end power.

If you reverse the situation so that you have long duration in degrees, then at high RPM, you still have enough time to vent all the exhaust. That's great. When are at a low RPM however, you vent the exhaust, but there's so much time left open, that even fresh air and unburnt fuel gets vented too. This is not good, especially in a forced induction engine. You lose a lot of boost that way.

This is where back pressure and exhaust velocity come in to it.

If you have long duration cams so that unburnt fuel and air leak out, then having some back pressure will stop this. By putting positive pressure on the valve, it means that less unburnt fuel and air will leak at low RPMS. However, it can mean that at high rpm, not all exhaust is vented. We have that same trade off as we do with cams.

Back pressure can be caused by two things: Restriction in the airflow, and also pure momentum of the exhaust charge. We'll discuss this more in a second.

High exhaust velocity on the other hand creates the opposite effect. If you have a high momentum charge heading towards the back of the car, then you are left with vacuum where the charge just came from. That means at high rpm, exhaust gas is not just pushed out of the cylinder - it's sucked out by the vacuum left by the last charge that just shot down the pipe. This creates for a great situation where you can get lots of high end power.

High exhaust velocity is created by having thin pipes, that have little or no restriction.

Overall flow, is hard to explain, but it's tied in with exhaust velocity. There is no point having a high velocity exhaust, if the overall flow can't get rid of the exhaust as fast as you're producing it. If you used an exhaust the size of Macdonald's drinking straw. Yes, it's blowing hard, but you're now creating back pressure, not a vacuum.

A 2" mandrel bent pipe and a 2.5" crush bend pipe might have the same overall flow (can get rid of the same amount of exhaust at a fixed pressure) but the 2" mandrel bent pipe, will get rid of it with a higher exhaust velocity. It's kind of like saying, do you want a garden hose turned on fast, or a fire hose turned on slowly. They both pump the same amount of water, but in different ways.

After all that lecturing, what does this mean in real life?

Well, in an ideal situation, you want your cams and exhaust to be working in harmony, so that at low RPM, no unburnt fuel and air leaks out. You want either the valves to close in time, or the backpressure to stop the leak. At the same time, at high RPM, you want the valves opened long enough for the exhaust to be expelled, or the vacuum created by the exhaust to pull as much exhaust out as possible.

I believe the best way to do this, is to have aggressive cams, and a thin straight through exhaust. Where bending has to be done, a mandrel bent should be used. When the car is at low RPM, the exhaust has very little momentum and so a condition of backpressure helps keep the unburnt fuel in the cylinder. When the engine is at high RPM and creating a lot of momentum, the vacuum helps suck the burnt exhaust out.

It also helps to have the exhaust stay hot. That way, it doesn't cool and reduce momentum.

In my S2000, I replaced the 2.25" reverse flow mufflers with 2.5" inch straight throughs, but I kept the 2.25" piping from header to "T" junction. I didn't lose or gain in the low end (because the back pressure was still there) but from 4000rpm, I gained about 6kw till redline at 9000rpm.

I've heard of plenty of people who have done 2.5" inch cat back exhausts on the s2000 who have gained absolutely nothing from the exhausts, so it supports my theory that a high exhaust velocity with a low back pressure is good for high reving engines.

Anyway, for a naturally aspirated engine, I recommend keeping the exhaust as straight as possible, and thin. (2 to 2.25". No bigger)
 

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The Backpressure Myth as eloquently stated by Larry at Endyn on the Backpressure issue:

You want zero backpressure instead of "some" backpressure, as you may sometimes hear on the street.

Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 2-5 psi backpressure. "Bolt-ons only" engine packages, in the past, used exhausts with some backpressure, since there is this incorrect belief that having a little backpressure prevents the fresh air/fuel from shooting into the header at cam overlap (when both the opening intake valve & the closing exhaust valve are simultaneously, partially open). The backpressure supposedly "pushed" the fresh air/fuel back into the combustion chamber rather than having it go into the header. This shooting of fresh air/fuel from the intake manifold and intake port into the header cannot happen at cam overlap, since the pressure inside the header is already much higher than on the intake side , even when there is zero backpressure.

In reality, having more backpressure reduces the difference between the higher pressure in the head's exhaust port and lower pressure in the header and cat. You need this difference in pressure going from the head to the exhaust system or "pressure gradient" to keep the exhaust flow speed or energy at a high level. Having some backpressure during cam overlap and the exhaust stroke means that the exhaust gas must now push against something and therefore, this backwards force slows exhaust gas down.

This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to backwards flow or "reversion", where the exhaust gas travels backwards into the combustion chamber and dilutes the fresh intake charge at cam overlap. At the very least, it slows exhaust flow velocity or energy and prevents the creation of a vacuum for scavenging.

So please ignore the obsolete "you should have at least some backpressure" sales pitch. It's all about the creating high exhaust flow velocity/speed or energy leaving the exhaust port, in order for the header-cat-exhaust SYSTEM to do it's job properly (i.e. remove all the burnt exhaust gases and help pull in fresh intake charge by scavenging at cam overlap) and make power for you.
 

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Stock backpressure is around 16 psi in a GSR
Eh? You're saying that stock backpressure is HIGHER than most turbo chargers? In that case, exhaust will be pushed back into the cylinder, and pushing air/fuel mix back out of the intake valve, which as everyone knows, in a naturally aspirated car, has a slight vacuum.

since the pressure inside the header is already much higher than on the intake side , even when there is zero backpressure.
I think this is where some confusion is. I'm assuming zero backpressure = 1 atmo. If the pressure in the header is greater than 1 atmo (1 bar, 14.7 psi, whatever) then some of that force has to be pushing back on the valve. Law of fluid dynamics. If the exhaust is pushing back on the valve, then to me, that's back pressure.

Can you clearly define what you mean by backpressure?

EDIT: Oh, last question. Sorry.

If, as you say, no leakage can every occur because header pressure is always higher than than intake manifold pressure, why don't we design cams with MASSIVE overlap all the time? There's nothing to lose by doing so.

Why is it in reality, when you have large overlap, that idling is poor and low end power is poor?

Why do F1 cars which rev to 18,000rpm and have large overlap have to have a rolling start with their engine already at high rpm?

EDIT 2: Just read through my post again. It might seem like I'm supporting the backpressure myth - only to a certain degree. Lots of backpressure is bad - it does cause reversion. However, a vacuum at all times is bad too - it causes air/fuel leak.

At 1 atmo, some leakage will still occur, not due to fluid dynamics (equalising pressure) but from momentum of the intake charge. Therefore a small amount of back pressure to counteract the momentum will be beneficial.
 

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Discussion Starter #8
:confused: :confused: :confused:

Ummmm, so if i'm building a all motor high reving engine w/ a 11k redline, strictly for drag purposes, would i be better off w/ some backpressure, or none at all ??
 

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SPEED54 said:
:confused: :confused: :confused:

Ummmm, so if i'm building a all motor high reving engine w/ a 11k redline, strictly for drag purposes, would i be better off w/ some backpressure, or none at all ??
None at all. You want a vacuum in your headers at all times to assist in exhaust scavenging.

A very small amount of backpressure is only good for daily drivers who spend a lot of their time between idle and 3000rpm.
 

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I guess at some point I need to make a sticky on back pressure.

Anyway exhaust system tech is one area that most all people get confused and for good resin exhaust systems are very complicated and there are many things at play when dealing with them.
When dealing with back pressure it gets even more complicated in my last post “see above” I say that” Simply put back pressure is bad for high revving low displacement engines
Now why?
Well the easiest way I can think to put it is, small displacement engines use high RPM to make torque “HP + RPM = torque” now at high RPM you need to expel the spent gas as fast as you can and refill the cylinders with a fresh intake charge just as fast.
So in order to remove the spent gas you need a really free flow “no back pressure” and at the same time with no back pressure you are creating a vacuum that in return helps to fill the cylinders with a fresh intake charge.

But by doing this you undermined your low end torque. exhaust systems are a give and take system and there is no best of both worlds “its high end or low end” so knowing that our little engines make all the power in the mid to top end I think is safe to go the no back pressure route that is if your looking for a REAL performance car.

You must also realize that in building a performance car there are tradeoffs when doing so, comfort, drivability, for the sake of speed.
 

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damn informative shit but any recomendations as far as exhaust systems/header combinations/cat or test pipe?
 

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i'm not taking credit for this post... it was found on www.automotiveforums.com from a member named texan...

and i quote...

"A lot of people have different thoughts on backpressure, and often confuse it with Velocity and Delta Pressure...
I will now post a colaboration of posts from Purehonda.com

"THE MYTH OF BACKPRESSURE"

…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say:

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.

It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace. "

-here's a reply to the above post-

"I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks.

There is one thing I'd like to add to texan's work:
Exhaust Scavenging
In essence, this is the opposite of the exhaust reversion that texan describes.

Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.

Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.

Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.

Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.

Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.

There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall effiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
You want low backpressure and high positive delta pressure. "

i think that covers it all...
 

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Backpressure is only useful in 2-stroke engines because without backpressure, their air/fuel mixture would shoot out of the combustion chamber since they don't have valves. 4-stroke engines have valves and therefore need no backpressure. A vaccum is the goal.
 

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Sunder said:
I didn't lose or gain in the low end (because the back pressure was still there) but from 4000rpm, I gained about 6kw till redline at 9000rpm.
6kW? that is way too much power to gain from just exhaust mods. That comes out to about 34hp. Please explain.
 

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BlackEG_Si said:
6kW? that is way too much power to gain from just exhaust mods. That comes out to about 34hp. Please explain.
Not sure what maths you're using.

My conversion formula says to multiply by 1.34. that's about 8hp.
 

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Sunder said:
Not sure what maths you're using.

My conversion formula says to multiply by 1.34. that's about 8hp.
Sorry! I don't know what math I was using either. I had it in my head that 1hp=176W. Where that came from, I have no idea.
 

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BlackEG_Si said:
6kW? that is way too much power to gain from just exhaust mods. That comes out to about 34hp. Please explain.
It’s more like 8HP like Sunder said but just to let you know if you treat the exhaust lit a full system and upgrade it like one the gains will be more. For example

Head work “the exhaust actually starts here with the exhaust valves and runners
Header “SMSP can net you 15HP or more”
Cat “better flowing cat that is”
Piping “less restrictive less bends and higher velocity”
Muffler “don’t know why I put this here it does nothing for performance

I can see 30HP or more from doing all this
 

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ich_folge said:
if you treat the exhaust lit a full system and upgrade it like one the gains will be more. For example

Head work “the exhaust actually starts here with the exhaust valves and runners
Header “SMSP can net you 15HP or more”
Cat “better flowing cat that is”
Piping “less restrictive less bends and higher velocity”
Muffler “don’t know why I put this here it does nothing for performance

I can see 30HP or more from doing all this
I totally agree. Too bad most people don't treat the whole system.
 

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a little off subject but a 2 stroke can run without back pressure, not great not super powerful but the can run. exhaust left, design and tuning are super important in a 2 stroke to get the tq/hp you want out of it. on street car or race cars i have no use for it. thats why they make cutouts.
 
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