Naturally Aspirated - Backpressure

[phenom86]

Here is some good reading, I just copied and pasted it here for everyone to read. Very informative specially for everyone who isn't going to force induce their car but wants to get as much power from it being naturally aspirated. Remember bolt on parts are good to get HP but to make everything work together, its BEST to get your car tunes to get OPTIMAL HP.

"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 important 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 achieve optimal 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; because 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 exhaust 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 un-burnable 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. "

Reversion: at the beginning of the intake stroke during cam overlap, exhaust 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 efficiency (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. "

Needing Backpressure - Myth or Reality?

The goal of any exhaust system is to efficiently remove burnt gases from the combustion chamber, prevent reversion at overlap, and by enhancing exhaust gas velocity leaving the chamber, create a vacuum to help draw or scavenge in more intake charge volume at cam overlap.

The key is maintaining exhaust gas velocity or energy as the gases leave the exhaust port when the exhaust valve opens.

So as the exhaust gas leaves the exhaust port in a 4 stroke engine , it creates a series of pressure waves travelling at the speed of sound that move towards the exhaust tip (or forwards) and then some reflects back. Like the water waves coming onto the beach, forward and back, forward and back. The main overall direction is forwards but there is some reflection back to the exhaust port (reversion).

Simple enough...everyone knows this. So what's new and groovy?

The problem is at cam overlap (when both the exhaust valve and intake valve are both partially open and when the pressure in the chamber is greater than in the intake port).

If a high pressure wave is reflecting back and arrives at the exhaust port at the wrong time (i.e. when burnt gases still need to leave), it blocks the flow out. You see these instances when a high pressure wave is reflected back at the wrong time as dips in the torque curve AT REGULAR INTERVALS (usually in the midrange rpms).

If a low pressure wave is reflecting back at the correct time at the exhaust port it actually helps pull burnt gases out of the chamber and also helps pull in more intake air/fuel at overlap. You see these favourable low pressure reflected waves occurring on your torque curve as small torque increases AT REGULAR INTERVALS.

Now here's the first bone of contention and a source of debate between exhaust makers.

1. Is a reflected high pressure wave always bad?

Most of the experienced people I speak to and read on the various boards say YES! You never want backpressure and you want it as low as possible for as long as possible. The low backpressure assists in maintaining that high exhaust gas velocity. They then design anti-reversion chambers and/or place steps (increases in diameter at various proprietary points along the length of the header) to prevent the reflected waves from travelling back to the head.

There are also some pretty smart people who believe slightly differently ...They believe that if you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion.

However, these people do NOT use the exhaust diameter as a way to create this backpressure. That would be too crude or less precise, since the backpressure would exist at all times and they only want this backpressure over the few crankshaft degrees when the exhaust valve is just about to close ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Using an exhaust just to have backpressure then is like cutting butter with a chain saw.

The people who agree with this will often tell you that combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So some intake flow into the chamber can get pushed out the closing exhaust valve by the higher combustion chamber pressures.

So all you guys that say backpressure is a good thing...I don't believe so...not at all crankshaft degrees which is what you get with a restrictive diameter exhaust. You don't want to have too big a diameter (actually it's cross-sectional area) that will slow or kill velocity or energy. But no backpressure most (99%) of the time is good.

2. How do we get low pressure waves and high pressure wave to arrive at the correct time?

The conventional way to get the exhaust gas harmonic to do this dance of low pressure to pull in more intake charge and high pressure to prevent bleeding off all at the right time is by changing the tube layout on the header: using lengths, diameters, collectors with various merge angles. But these are limited to one harmonic or exhaust gas speed.

So some Japanese engineers at Yamaha (figures, it's always some genius engineer at some bike manufacturer that comes up with these wild ideas) thought: "What if you have an exhaust throttle valve (located in the header collector or at the entrance to the secondary tubes in the first merge collector) that could control the pressure wave behaviour?".

The throttle valve angle would vary as the speed of the exhaust gases changed to control the reflected waves. In an 11,000 rpm bike, the valve opens progressively as the rpms climb as the tubes are "in step" with the engine harmonics and less reflected waves occur but at around 7000 rpm, the valve is closed down to 40-60% of wide open when the harmonic is "out of step" with the engine and at 8500 rpm the exhaust throttle valve is progressively opened. How much to change the throttle angle is based on crankshaft angle input or ignition signal input to an ECU with then controls the throttle valve angle knowing the harmonics of the engine.

you keep reading "you need velocity", but nobody ever quantifies how much, in what conditions, where in the system, or how much is too much. To me, this is no different than saying "bigger is better". A lot of velocity will hurt power because pressure losses increase exponentially with velocity. That's the whole idea behind restrictor plates in racing. The air velocity is very high through them, but they certainly aren't helping power output. Most people seem to attribute a power loss from a large exhaust to a reduction in velocity. However, they never really investigate the cause. In all of the cases I've seen, the larger exhaust made the engine run lean, and that was the true cause. Correcting the mixture strength back to LBT typically restores the lost power plus a little extra.

It's sort of both. When you increase the exhaust size after the collector/cat/whatever, you're decreasing backpressure, and allowing the engine to induct more fresh charge during overlap. When that happens and you don't compensate by adding more fuel, the engine runs lean. If you run significantly leaner than LBT, you'll lose more power than you'll gain from the VE increase. The leaner the mixture is from LBT, the slower the burn becomes, and the more timing you'll need for MBT.

There is definitely a point of diminishing returns. As long as you have a good diffuser for your collector and you tune the engine, I don't know if there is a point at which a larger exhaust will hurt you for power output. However, the exhaust will still be heavier, so overall performance can still drop. It will also be louder.

Basic physics would suggest that the momentum of the exhaust gasses in a smaller exhaust would help to induce more vacuum at the exhaust port, but based on the testing I've seen, it's more complicated than that.

Now that I think about it more...

The only reason that the velocity is higher with smaller tubing is because the smaller cross-section restricts flow more, which means more backpressure is developed. When more backpressure is developed, there is a larger pressure gradient to the atmosphere, so velocity has to increase accordingly. The advantage comes in the form of timing. When the velocity increases like that, a delay is added into the system. Air speed takes a little more time to develop, but due to its momentum and the inherent increase in the pressure differential, the air speed will drop back to "normal" at a later crank angle. If you can take advantage of this phenomenon with your valve events, then it can be useful (namely, in the case of exhaust primaries and intake runners). If you can't, then all you're doing is increasing backpressure and decreasing charge purity. Unless you're spending the time to try and optimize the system, you're most likely going to be stuck with just more backpressure.

MBT ("Mean Best Torque") is the timing advance needed to produce best torque, and LBT ("Lean Best Torque") is the mixture strength needed to produce best torque.

To add to this...

If your collector is crap, like most of the generic stuff, the first ## inches of the exhaust tubing effectively becomes part of the collector. The collector should act sort of like a one-way valve, so momentum effects become more relevant.

Also, if you're running a cat, just put something big after it. The honeycomb will pretty much kill any sort of gains you could get from resonance or inertial effects.

Source: Backpressure- - Automotive Forums .com Car Chat

[Guff]

Originally Posted by phenom86

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.

I think this is supposed to be inches Hg. Pressure is measured in Inches of Mercury.

[Sportsguy83]

Originally Posted by Guff

I think this is supposed to be inches Hg. Pressure is measured in Inches of Mercury.

inH2O is also used for pressure. (think water column)

BUT, I believe you are probably right, as it is more traditional to use inHg.

[serialk11r]

Perhaps it should be noted that at high rpm on engines with no lift control there is usually no overlap, so the effect of exhaust tuning is a lot smaller because it can only influence scavenging via slightly dropping the pressure or increasing the pressure of the residual exhaust gas.

[BigFatFlip]

Originally Posted by Guff

I think this is supposed to be inches Hg. Pressure is measured in Inches of Mercury.

I've seen it measured in both, just a different standard I believe. (ie degrees C vs. degrees F).

[Guff]

Originally Posted by BigFatFlip

I've seen it measured in both, just a different standard I believe. (ie degrees C vs. degrees F).

Yeah, I know. I just have never seen it used in this context.

[Dimman]

Originally Posted by Guff

Yeah, I know. I just have never seen it used in this context.

In automotive stuff in the US, cfm is almost always referenced against water. Heads will be something like 280 cfm @ 28"WC (water column), basically at 1psi. Some flowbenches use 10"WC (Superflow, I think), so you have to do your math to compare between them.


This is a better exhaust article, but still incomplete. For example, the wave returns 'AT REGULAR INTERVALS' as it is in the text is actually not correct. The return timing effects at low rpm and high rpm are different because the wave changes.

[phenom86]

Originally Posted by Dimman

In automotive stuff in the US, cfm is almost always referenced against water. Heads will be something like 280 cfm @ 28"WC (water column), basically at 1psi. Some flowbenches use 10"WC (Superflow, I think), so you have to do your math to compare between them.


This is a better exhaust article, but still incomplete. For example, the wave returns 'AT REGULAR INTERVALS' as it is in the text is actually not correct. The return timing effects at low rpm and high rpm are different because the wave changes.

I'll add what you think is needed..

[Mitch]

Originally Posted by Sportsguy83

inH2O is also used for pressure. (think water column)

BUT, I believe you are probably right, as it is more traditional to use inHg.

The standard conventions are inches of water and mm of mercury.

[Dimman]

Another thing that isn't covered about velocity in the pipe is that it is also related to pressure differential. So it is also affected by exhaust cam timing. If it opens before BDC, and there is still residual pressure (say 30psia) in the cylinder, and the pipe is at 17psia, you get more velocity than what is typically explained by the piston's speed/motion 'pushing' the gas out through the port. So what wave tuning for the reflected negative wave can do is drop local pressure at the valve and increase velocity without needing a smaller pipe.

[Gardus@Supersprint]

Looks like a very good description of the exhaust workings, I'd like to add a few things.

About headers:

1st and foremost, on NA engines like the GT86, and on midly Supercharged cars (so we're not talking on drag racing SC engines with huge overlaps) the headers and the 1st cat have the main influence in the power delivery
This is why I'm always skeptic when companies claim big power increases on catback or even axleback systems.
This is obvious, the primary pipes (and secondary in case of 4-2-1 manifolds) collectors are the primary source of returing wave pressure to the exhaust ports, and the second is the cat.
The diameter and routing of these pipes are also very important for flow, as well as the efficiancy of the cat, that can be a heavy restriction on the flow.

It's also important to avoid steps between the exhaust port and the pipes.
If you want bigger primaries either increase the ports size or make stepped primaries.

A T junction (instead of a Y) between two primaries is not a good thing but is actually less important than most people think. The ones you have in the stock GT86 header joins primaries coming from exhaust valves that opens not in sequence but at opposite times of a cycle, and these junction are close to the exhast ports where the waves are very defined.

It's more important that the secondary pipes collector don't have a steep angle, and it's even more important in 4 into 1 collectors. Two or four gas flow must enter at the same time, the compressible nature of the gas means the arrival of the wave pressure is not like "single persons" arriving at the different times, which means they all can pass by the same door, but it's more a flow of many people, with the density of them changing at regular intervals. While you don't have the 4 flows arriving all with the highest density "of people", they cannot pass between a door large enough just for a single flow, and you don't want the flows to collide with each other...

Talking about 4 into 1: in this type of headers is very important that all the primary pipes are equal length, otherwise you seriously risk to have the highest quantity of gas caused by the waves arrive at the same time at the collector.
This is the reason why many OEM headers are 4 in 2 in 1, they are cheaper to make as they can be stamped instead of being tubular, they occupy less room in the engine bay and collectors can be at a steeper angle.

The precence of tight bends in the primary pipes doesn't have much of an effect as the drop in flow is much less important than the effect of the pressure waves.
After the collector the eventual drop in flow (brought by a tight bend, a restriction, a chamber-type silencer etc) is to be avoided to keep the gas velocity high.


Talking about tuning the pressure waves to improve the scavenging effect, you can do this for a single rpm point, and less so on multiples, but on a road engine is quite hard to obtain huge improvements.
An interesting idea is to use this system on pure hybrids, where the engine works at a constant speed!
As already said many engineers have tried various methods with exhaust valves, with multiple "tuned" wave reflectors on the line etc.
They can be effective but they're just one of the factors.

On the intake instead variable lenght systems work very well!
It's obviously almost impossible to make a continously variable lenght exhaust system, so you can make a continously variable flow one, or you can make a bypass valve that shorten the exhaust, but that is for different reasons...



Less talk about DIAMETERS
Some say that bigger the better, if you can keep the correct A/F ratio etc.
Then you fit a 3,5" system on a 200 hp car and it lose power...why?
Many people talk about insuffient backpressure. This, why it is a easy to comprehend argument, is quite improper.

The two main factors are:

-Sudden variation of the diameter: you usually see this when you have flanges, and the pipe before the connection is 2", and the one after that is 2"5 or more. You can imagine what happens when the mostly linear flow enter the bigger pipes: it gets turbolent, you get drops in pressure, and the speed of the gas suffers. Also you introduce unwanted returing pressure waves.
This is the worst when you have it at the exhaust ports.
If you don't have these steps but all the diameter variation are gentle slopes, there's the second factor:

-Inertia: the gas column inside the exhaust has weight, and with weight you get inertia. To get this column moving you spend energy, especially when you drasticaly increase the speed in sudden acceleration, opening the tap!
This energy comes from the gas explosions in the chamber, that you want to be trasferred to the wheels...
You can easily understand that a bigger column (a bigger bore exhaust) weights more...but it's not a direct proportion!
As you increase the diameter the surface of the exhaust increases 3,14 times more. You have hear exchange between the hot exhaust gas and the normal air around. The exhaust gas get colder, more dense, heavier.
Also, to expand in a large volume the exhaust loose heat.
As it gest colder (and heavier) it slows down and it takes more time to get our of the exhaust, and the more time it stay inside the more it gest colder...

This is the reason why you get extremely short exhausts on race cars
This way you can have large bores without any issue!
On road cars you must reach compromises: the exhaust must exits from the back (usually), you need silencers that take space, the packaging of the car is complicated (suspension parts, braces and other components are in the way), and the design is important too.

The bypass valve on some cars works well in this way as on low rpm when the car bust me quiet and performance are less important you can have a longer route, and you have also drops in flow brought by silencers.
In the upper range, when you want the best performance, you open the bypass valve, shortening the route and bypassing the silencers, keeping as much energy as possible

[Turdinator]

Some good info here guys. Subscribed

[SillySaxon]

This thread needs to be stickied

[Captain Insano]

Whoa... nice resurrection.