backpressure

[wiredlittleman]

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...

[GreyGoose006]

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.

[curtis73]

Originally Posted by wiredlittleman

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.

[KiwiBacon]

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.

[Neutrino]

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.

[GreyGoose006]

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?

[534BC]

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,

[GreyGoose006]

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.

[Neutrino]

Originally Posted by GreyGoose006

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?

[curtis73]

Originally Posted by GreyGoose006

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.

[Neutrino]

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.

[KiwiBacon]

Originally Posted by Neutrino

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.

[Neutrino]

Originally Posted by KiwiBacon

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.

[UncleBob]

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.

[UncleBob]

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.