backpressure

A guy told me that what alot of people dont know is that you can loose low end torque from a high flow exhaust on an older motor. now my question is that if this is true could you pick up more torque by making your exhaust more restrictive? at least low end torque...

i believe that your friend may be wrong.
one thing i know is that adding backpressure is bad and will only absorb power.
it takes a certain amount of power to push the exhaust thru the exhaust system. any extra restrictions are gonna cost you.

A guy told me that what alot of people dont know is that you can loose low end torque from a high flow exhaust on an older motor. now my question is that if this is true could you pick up more torque by making your exhaust more restrictive? at least low end torque...

Its a complicated topic. The answer is that the exhaust should be properly sized. Too big and you can lose low end torque, but too small is just too small. It has to do with velocity of the exhaust gasses. Fast moving exhaust is good for low RPM torque, but when you reach higher RPMs you're asking it to move more exhaust than it can and it chokes off HP. Conversely, a wide open exhaust kills low RPM velocity but lets the engine breathe at higher RPMs.

Your stock exhaust is probably slightly restrictive. You could install an aftermarket exhaust and pick up a few HP without killing torque, but bigger is not better. Anything above what you need and you will start noticing a loss of low RPM torque.

The stock exhaust is usually a trade off between a proper match and lower noise levels. So its already on the small side of properly sized. That is why increasing exhaust flow with an aftermarket exhaust usually increases overall power output with a little more noise.

Going smaller will be a mismatch unless you are changing other engine components to match the smaller exhaust. Everything needs to be matched. A smaller exhaust is only beneficial if the engine's RPM range is lowered and the torque peak is at a lower RPM.

So, you're both right in different ways.

It's simply the engine needs retuned to run with a less restrictive exhaust.

It will now have less retained exhaust in the cylinders and will probably have mixture issues at several points in the rev range.
With a retune the engine will perform better than it did with the smaller exhaust on it.

As far as power and torque are concerned, there is no such thing as not enough backpressure.
But noise and backpressure do have an intimite relationship.

if i may bring my 2 cents:

From my research I have come to the conclusion that the backpresure advice is a myth. What does matter is the energy and of course the directly related speed of the exhaust gases.

Best scenario: the gases will exit very fast and hot creating a low pressure area behind. This will create a scavenging effect for the remaining gases in the combustion chamber allowing for a cleaner burn next cycle.

Now the problem is a small exhaust will keep the gases hot and fast but will be restrictive and create unwanted backpressure. On the other hand a dumping the gases in a large exhaust will not encounter as much backpressure however once gases meet the larger enclosure will expand and lose energy and ofcourse speed.

Therefore a balance needs to be struck to reach the optimal compromise. This of course its further complicated by the fact that this compromise will be change based on RPM and load. So you end up with effects such a loss of low end tq when using large exhausts systems .

One thing one can do when switching to a larger piping is: have it ceramic coated and/or thermally warped. This will limit thermal dissipation compensating a bit for the energy loss during expansion that I've mentioned earlier.

wait. hold on.
how can having a large diameter exhaust lower torque?
i thought that once the exhaust was out of the engine, you wanted to get rid of it. does this mean that having straigh pipes poking out behind the the front wheel wells is NOT the best way to do it?

i just dont understand the physics of it i guess.
how can Too Big of an exhaust effect the torque of an engine?

Here's my take on it since no specific exhaust part was questioned and was kinda generic.

I think Curtis is exactly right for parts right up to the collector or turbine outlet. After that I agree with those who stated that it cannot be too big.

To rephrase: exhaust valves, seats, ports, headers,and presure pipes CAN be too big and lose some power , but after the scavenge and/or pressure drop the BIGGER is better.

How does that sound? brymmm, brummmbbrumm,

well on a turbo, between the turbo and the manifold should be 2-2.5".
after that, i dont see how it matters.

on an NA car, i dont see how the exhaust matters either.

well on a turbo, between the turbo and the manifold should be 2-2.5".
after that, i dont see how it matters.

on an NA car, i dont see how the exhaust matters either.


using general statements like that number you pulled out does not count for solid engineering that takes into consideration advanced physics regarding gas flow. Even small changes in the shape and or size of the pipes can create turbulences in an otherwise close to laminar flow.


Try looking researching a bit fluid dynamics and you might start to understand that things are not so simple. Unless you think it doesn't matter to the plasma flow in a fusion reactor if its shape is circular or more like a "D".

And yes on NA cars the exhaust matters a lot, did you not read my post or something?

wait. hold on.
how can having a large diameter exhaust lower torque?
how can Too Big of an exhaust effect the torque of an engine?

A big part of filling the cylinder is using exhaust scavenging. During the overlap time when both valves are open, the rushing exhaust gasses start to draw in some intake charge before the piston starts down the intake stroke. A properly sized exhaust maximizes velocity of the exiting gasses which helps draw more intake charge in.

If you have a torque peak at 2500 rpms and you put on a 5" exhaust, the velocity won't be there to help torque at 2500 rpms. It won't reach a velocity that is beneficial until 7000 rpms or so. Therefore, choosing an exhasut that is too big can lower torque production.

This overlap is the same feature that makes really hi-po cars have that lumpy idle. They have a lot of overlap which makes scavenging really good at high rpms, but at idle the extra duration causes exhaust reversion into the intake manifold. Whereas normally you get pretty smooth flow with a low-overlap cam, on a bigger cam the exhaust opens too early while there is still plenty of exhaust stroke. The exhaust reversion not only dilutes the intake charge at idle, but it makes intake manifold vacuum very jumpy and low which also makes the lumpy idle.

In the case of a turbo engine, everything after the turbo is waste, so in that case, bigger is better. Turbo cams have very little or no overlap, so scavenging isn't an issue, and after you've used the exhaust's energy in the turbine you want to get rid of it as efficiently as possible.

curtis is right on the money, this is why modern high tech engines use variable length manifolds (Ferrari seems quite fond of them) and variable timing and lift valves (as the ubiquitous vtec) you get the best of both world.

Same reason I'm a fan of variable geometry planes.

If i may add something on turbos however: the larger the pressure diff between the turbo manifold and exhaust the better. Faster moving gases in the exhaust should (in theory) create the same low pressure area behind the turbine which will increase the pressure differential benefiting turbo spool.

If i may add something on turbos however: the larger the pressure diff between the turbo manifold and exhaust the better. Faster moving gases in the exhaust should (in theory) create the same low pressure area behind the turbine which will increase the pressure differential benefiting turbo spool.

Nope.
The flow out of the turbo is close to steady state, the turbine has chopped up and spat out all the exhaust pulses as a steady flow.

To accelerate that flow to a higher speed requires an increase in backpressure.
Your optimum for low pressure is a diffuser cone from your turbine outlet to as big as needed to create close to zero exit loss.

Bernoulli has it all covered.

The only ways to create a low pressure zone is having something suck on the exhaust (boats can acheive that) or be right in behind an exiting pulse wave.

Nope.
The flow out of the turbo is close to steady state, the turbine has chopped up and spat out all the exhaust pulses as a steady flow.

To accelerate that flow to a higher speed requires an increase in backpressure.
Your optimum for low pressure is a diffuser cone from your turbine outlet to as big as needed to create close to zero exit loss.

Bernoulli has it all covered.

The only ways to create a low pressure zone is having something suck on the exhaust (boats can acheive that) or be right in behind an exiting pulse wave.


As you will notice my theory was based on the part I bold-ed in your post. But if you say and are not mistaken that the turbo smooths the wave then yes you are correct, it would not work.

NA engines are a much different topic than turbo'd engines.

Turbo'ed engines don't care a whole lot about what happens before the turbo. As long as it flows OK, doesn't make any sharp turns, and isn't way too big or way too small, it'll all work well, on boost it will make simular power.

After the turbo, you want as little pressure as possible. The optimal exhaust after the turbo is....none. No dump pipe, no nothing.

That doesn't mean, that attention to the header is worthless, of course, but its the classic bottle-neck question, does the current exhaust cause a reduction in hp due to the way its designed/sized, and if so, how much gain is there to be had there?

Now NA, there's also sorts of exhaust pulse tuning. Different primary lengths, diameters, and how they join. 4 into 1, 4 into 2 into 1, 4 into 2, all have strong believers that believe one has great advantages over another. Then you have balance tubes between some of the cylinders at key points. If you really want to see the extreme of this, you look at bikes. Aftermarket exhaust for motorcycles has some fierce passion in what works better.

Here's my belief on it. Its all about velocity. Smaller primaries equal higher velocity at lower RPM's. They will give you better low end scavenging at the cost of higher RPM power. This doesn't take much research to see. Many stock exhaust vs aftermarket exhaust....aftermarket is almost always larger primary pipes, always lose some bottom end power. This, IMO, is the reason why.

The theories behind the collectors, and even more complicated systems, such as Yamaha's EXUP (I don't remember what it stands for, but its a butterfly valve in the exhaust to change the exhaust back pressure at different conditions, controlled by a computer) will have varying effects at different RPM's. Much like different intake track lengths. There is a lot of variables and RPM range specific enhancement. But as far as primary diameters, its a pretty well believed belief that smaller is better for low end power.

Now if you really want to get into some of the cool stuff that has been developing, some of the super high output motorcycle engines have been making some rather drastic changes in engine configurations to acheive it. The biggest change is in the exhaust ports. They have shrunk. DRASTICALLY! You look at the exhaust ports for a ZX10R (160HP 1.0L engine) vs the older ZX11 engine (135HP 1.1L engine) and you'll notice that the exhaust ports on the ZX11 are a good 30% bigger than the ZX10R ports.

Interesting stuff. If you ever want to see what the most cutting edge engine developement is, look to bikes.

oh, I almost forgot to mention, no matter which system is being discussed, there is one thing for sure: back pressure will always reduce HP.

This goes back to the basics of an engine. An engine is an air pump. The more efficient the air pump, the more power its capable of. Back pressure reduces air flow, therefore its bad.

Now that doesn't mean that a motor with some back pressure will always make less power than a motor with none. As with everything, there's too many variables involved for such a statement. But a motor with back pressure is throwing away power if it was designed differently. That is a fact.

This goes back to the basics of an engine. An engine is an air pump. The more efficient the air pump, the more power its capable of. Back pressure reduces air flow, therefore its bad.

Not only does backpressure reduce airlflow, it directly sucks up an engines power in pumping loss.
Pumping loss = pressure * flowrate.

True, but lets not confuse power (measured in HP) with the original spirit of the question which concerns torque.

Its quite true that more flow equals more hp, but that is not necessarily true with torque. As a general guideline, exhaust velocities of about .35 mach are where things start getting restrictive. So (again, very general ideas here) if a person were to choose an exhaust that sustatained just under .35 mach at WOT at redline, then it would probably be the best torque production you could expect without a significant loss of HP. If you're going for all out HP, go bigger. If you want a quieter more torque-producing exhaust, choose something a little smaller that coincides with the torque peak.

True, but lets not confuse power (measured in HP) with the original spirit of the question which concerns torque.

Its quite true that more flow equals more hp, but that is not necessarily true with torque. As a general guideline, exhaust velocities of about .35 mach are where things start getting restrictive. So (again, very general ideas here) if a person were to choose an exhaust that sustatained just under .35 mach at WOT at redline, then it would probably be the best torque production you could expect without a significant loss of HP. If you're going for all out HP, go bigger. If you want a quieter more torque-producing exhaust, choose something a little smaller that coincides with the torque peak.

Torque and power are different ways to measure the same thing (engine output).
Anything which requires power (like pumping against exhaust backpressure) also requires torque.

So where the pumping loss is: Power = pressure * flowrate.
The torque loss is: Torque = (pressure*flowrate)/rotating speed.

Where the crossover lies between torque lost (to creating high speed exhaust pulses) and torque gained (through scavenging) is obviously the subject of much R&D engineering.

its a definition thing.

When most people say "that engine is torquie" what they really mean is, that engine produces good HP at low RPM's.

Low back pressure does not necessitate a "non-torquie" engine. Quite the contrary. A low back pressure system with properly sized exhaust/ports will greatly enhance the "torquie"ness of an engine....it very well could cause suffering in higher RPM's of course

Somebody told me if you didn't have any backpressure or little backpressure you'd burn the valves.

that is only true if you have extremely short exhaust. As in, exhaust that is a total length of 6 inches.

Back pressure isn't what holds the heat, its the metal that the exhaust is made of. If its of sufficient length (and it doesn't have to be very long, really) then it will prevent the exhaust valves from cooling off too quickly.

Torque and power are different ways to measure the same thing

Well... yes and no. Mostly the "no" part. :grinyes:

Engines produce torque. HP is a derivative of that torque. HP=TQxRPM/5250, so HP is completely dependent on torque production.

Consider this formula....

MKA=AGE x SEX

Where, MKA = Mary Kate and Ashley
AGE = 18
and SEX = when we can bang them legally.

MKA is producing torque, but they aren't making HP until age 18, so we can't make sex with them until a certain RPM... wait...... no, I mean sex with torque...

crap, I messed it up again.:screwy:

Somebody told me if you didn't have any backpressure or little backpressure you'd burn the valves.

Partially an old wive's tale. If you run an engine with very little exhaust (like open headers or no manifolds at all) you run the risk of cracking valves. After running an engine and getting the valves screaming hot, the lack of exhaust means ambient air is exposed to the valves. Screaming hot valves plus ambient air can mean cracked valves.

Mostly the "no" part....
Nice use of (MK+A)(O). It's going into my Calculus notes.

Oh man this is funnny: http://www.noiseoff.org/exhaust.shtml

that is only true if you have extremely short exhaust. As in, exhaust that is a total length of 6 inches.

Back pressure isn't what holds the heat, its the metal that the exhaust is made of. If its of sufficient length (and it doesn't have to be very long, really) then it will prevent the exhaust valves from cooling off too quickly.

since bikes are you area, maybe you can help answer this.

i read that bikers that fit open race pipes to their bikes often find that they desroy valves due to such low back pressure, so the exhaust isn't any shorter, but it is far more free-flowing. so what is it about the more free-flowing exhaust that destroys the valves?

Torque and power are different ways to measure the same thing (engine output).
Anything which requires power (like pumping against exhaust backpressure) also requires torque.


Ah...no. Torque is a measure of work, Power is a measure of work over time. Engines can produce massive torque while not making much power, and vice versa, they can produce massive power without much torque. Long stroke diesel vs formula 1 engine as examples.

Remember though, that power is just a mathematical toy, derived from torque. Torque is the only thing an engine actually produces.

since bikes are you area, maybe you can help answer this.

i read that bikers that fit open race pipes to their bikes often find that they desroy valves due to such low back pressure, so the exhaust isn't any shorter, but it is far more free-flowing. so what is it about the more free-flowing exhaust that destroys the valves?

I've yet to see such a situation, so I say BS

Ah...no. Torque is a measure of work, Power is a measure of work over time. Engines can produce massive torque while not making much power, and vice versa, they can produce massive power without much torque. Long stroke diesel vs formula 1 engine as examples.

Remember though, that power is just a mathematical toy, derived from torque. Torque is the only thing an engine actually produces.

torque is a really handy measurement in a plane of existance without time.

Since we don't exist in such a plane.....

Since we don't exist in such a plane.....


Is that a royal "we"?
Because I have blue telephone box in the garage that can get around a few time related issues.

Is that a royal "we"?
Because I have blue telephone box in the garage that can get around a few time related issues.

let me guess....to use this box requires hooking up electrodes to your scrodum sack...? :icon16:

Is that a royal "we"?
Because I have blue telephone box in the garage that can get around a few time related issues.

if you're reffering to Dr. Who, it wasn't a telephone box, it was a police call box.

British patriotism strikes again...

torque is a really handy measurement in a plane of existance without time.

Since we don't exist in such a plane.....

?? If i'm trying to turn a wrench on a stuck bolt but it's not going anywhere, im still exerting torque on the bolt.
Just the same, you can only measure the torque produced on a dyno, the horsepower is then derived (and only if the computer knows the speed of the engine!)

Actually many dynos measure HP and calculate TQ. Many chassis dynos measure the amount of time and speed at which the engine is capable of accelerating the dyno's brake which can be extrapolated to HP.

HP (although mathematically linked to torque) is its own thing. It can be felt if you've ever driven an older porsche. The 2.0L engine makes about 200 lb-ft at 3000 rpms and 200 hp at 6000 rpms. Trust me, the 200 hp puts you back in your seat more than the 200 lb-ft does. Although its a tough concept to grasp, HP is its own force, not just a mathematical derivative of torque.

?? If i'm trying to turn a wrench on a stuck bolt but it's not going anywhere, im still exerting torque on the bolt.
Just the same, you can only measure the torque produced on a dyno, the horsepower is then derived (and only if the computer knows the speed of the engine!)

a torque wrench isn't a good example. No work is being done with a torque wrench.

When work is wanted (moving something from A to B) time is a factor. Not much work gets done if you apply torque to object and it doesn't move with that amount of torque. Thats called infinity :licka:

Actually many dynos measure HP and calculate TQ. Many chassis dynos measure the amount of time and speed at which the engine is capable of accelerating the dyno's brake which can be extrapolated to HP.

HP (although mathematically linked to torque) is its own thing. It can be felt if you've ever driven an older porsche. The 2.0L engine makes about 200 lb-ft at 3000 rpms and 200 hp at 6000 rpms. Trust me, the 200 hp puts you back in your seat more than the 200 lb-ft does. Although its a tough concept to grasp, HP is its own force, not just a mathematical derivative of torque.

any vehicle that is high revving will show how true this is. Modern 600cc sport bikes only make 40 ft/lb's. On paper, they look quite pathetic. But at 15K RPM's, that 40 ft/lb's is making over 100hp. The bike is "gutless" (even though it has a fairly flat torque curve) until you get it over 10K RPM's.

a torque wrench isn't a good example. No work is being done with a torque wrench.

When work is wanted (moving something from A to B) time is a factor. Not much work gets done if you apply torque to object and it doesn't move with that amount of torque. Thats called infinity :licka:

Actually it is a good example…

It is a good example how torque is not equal to work.

Torque is more akin to force, even if it is not a force in the true sense. There is only work done when there is torque and revolutions. Power comes into play when there is torque and revolutions per unit time.

As well, if you move something from point A to B then you have done work. If you want to know how fast the work is done, then power is of concern.

Torque is more akin to force, even if it is not a force in the true sense. There is only work done when there is torque and revolutions. Power comes into play when there is torque and revolutions per unit time.

As well, if you move something from point A to B then you have done work. If you want to know how fast the work is done, then power is of concern.
If there is work, then there is time. You may or may not be concerned with the time factor, but its always there.

If you are not concerned about the time, then you aren't concerned with the power. They are directly related to each other. Since torque, when involved in a rotational device, is so easily increased or decreased through gear multiplication, there is no limitation to how much torque you create for such a device. You can easily make 1,000,000 ft/lb's of rear wheel torque on any vehicle, but it won't be going anywhere quickly.

As soon as something revolves, there is a time table if you so desire to use it for anything. Since work is usually time sensitive, its more than a minor data point.

Ah, i think we are agreeing with each other but in different ways... again.

Just the same, you can only measure the torque produced on a dyno, the horsepower is then derived (and only if the computer knows the speed of the engine!)

yah, I'm bored, I wanted to go back to this

No dyno knows what torque an engine is producing until you tell the computer what RPM the engine is at.

All dyno's involved measuring energy. For a drum inertia dyno, all the machine knows is the mass of the drum. When you turn the drum, it calculates the acceleration of the mass OVER TIME and calculates the amount of work being done.

In other words, it first finds the energy being applied to the drum, you could say that its deriving this from the rear wheel torque (torque to the drum shaft) and then finding the energy level, but it can't calculate that until it has the TIME factor.

So the order of calculations: First find the amount of energy, acceleration of mass over time, which is then converted to HP, and THEN...assuming you have an RPM lead to the engine, you can calculate the torque of the engine.

If you have no RPM pickup, then the engines torque is unknown. But the HP is.

Actually many dynos measure HP and calculate TQ. Many chassis dynos measure the amount of time and speed at which the engine is capable of accelerating the dyno's brake which can be extrapolated to HP.

HP (although mathematically linked to torque) is its own thing. It can be felt if you've ever driven an older porsche. The 2.0L engine makes about 200 lb-ft at 3000 rpms and 200 hp at 6000 rpms. Trust me, the 200 hp puts you back in your seat more than the 200 lb-ft does. Although its a tough concept to grasp, HP is its own force, not just a mathematical derivative of torque.

You have that backwards.

Dyno's measure the reaction torque and the rotating speed. Power is calculated from those two values.

Power (kilowatts) = torque (Nm) x rotating speed (radians per second).

It's only you backwards folk in america who need funny conversion factors to make up for imperial units.:iceslolan


The conversion from power to torque is equivalent to converting force to power.
Power (kilowatts) = Force (N) x Velocity (m/s)

Most people can comprehend force better than torque.



For example, a chair provides force (it holds things up) but produces no power as it's not moving.

If there is work, then there is time.

Technically speaking work is independent of time. Regardless of whether or not time is always present, when calculating work time is irrelevant.




Power (kilowatts) = torque (Nm) x rotating speed (radians per second).

It's only you backwards folk in america who need funny conversion factors to make up for imperial units.:iceslolan

One of the benefit of being a backwards American engineer is learning two system of units :crying:

I assuming the you are calling the following equation funny:

Power (Horspower) = torque (lbf-ft) x rotating speed (RPM) / 5252

For your equation to be correct it should actually be:

Power (kilowatts) = torque (Nm) x rotating speed (radians per second) / 1000

And to be fair I doubt tachometers oversea actually read rad/s, so:

Power (kilowatts) = torque (Nm) x rotating speed (RPM) / 9549

Technically speaking work is independent of time. Regardless of whether or not time is always present, when calculating work time is irrelevant.



explain please. The very definition of 1 HP is raising an object X distance within X time. Are you refering to a different unit of work?

explain please. The very definition of 1 HP is raising an object X distance within X time. Are you refering to a different unit of work?

Its a bit complex, since the common units like the watt, hp, and btu, originally were defined by time, they exist irrelevant of time.

You can do 100 watts of work over a period of 10 seconds or 10 years but it doesn't change the fact that 100 watts of work was done. What you're describing is watt-hours or watt-minutes.

The net result is that time has passed, but the amount of work done is existent regardless of time. Its basically a drag racers scenario. It takes 900 hp to move a 2000-lb car down the 1/4 mile in 9 seconds. The 900 hp exists regardless of the 9 seconds it took. That same 900 hp also means 5.6 seconds in the 1/8th mile, or 16 seconds in the mile, but we don't define the engine's output by its 1/4 mile time, we define it by its work done.

HP is measured in... well, HP. Not 33,000lb-ft/min. Once you take that 33,000lfm number and call it HP, you've defined the work done regardless of the time.

explain please. The very definition of 1 HP is raising an object X distance within X time. Are you refering to a different unit of work?

Horsepower is a unit of power, not work. The definition you provided is a correct definition of power.

The equivalent definition of work would be:

raising object X distance

The SI unit would be joules (J), and the U.S Customary unit would be inch-pound-force (in-lbf) or feet-pound-force (ft-lbf).

Note: the subtle difference in the equivalent units of torque which would be N-m, lbf-in, or lbf-ft, respectively.

just when you think you got it all figured out... :D

Thanks for the clarification

explain please. The very definition of 1 HP is raising an object X distance within X time. Are you refering to a different unit of work?

To simplify:

Work is getting it done (time doesn't matter).

Power = work/time.

Getting the same work done faster, takes more power.

Alastor caught me out with my power equation, should have been Watts, not kilowatts.:wink: