smaller exaust pipe for mpg

[Christ]

Originally Posted by stonebreaker

You are trying to argue that forcing a gas through a smaller pipe is somehow easier than forcing it through a large pipe.

If that's the case, then why do turbo cars, whose turbos operate off of the delta P before and after the turbine, have huge exhausts? Even more to the point, turbo diesel trucks, which operate in the rpm range we are discussing, come with 3 and 4 inch exhausts from the factory.

I'm not arguing that forcing a gas through a smaller pipe is easier at all, I'm arguing that the expansion of that gas will create a stacking effect as it cools, since density is proportionate to temperature. If the diameter of the pipe isn't small enough to maintain velocity, the exhaust pulse will slow as the gas expands and cools. Cooler gasses are necessarily more dense, and thus, could cause flow reversion if another pulse were to meet it. This is described as a "Stack Effect", which increases back pressure.
There are more forces involved than simple pressure and velocity.


Reread your question, then answer it for yourself.

Nevermind, I'll do it for you: Turbo compounding. It's a feed back loop, which means that the engine can have greater than 100% volumetric efficiency.

Since the intake is pressurized, there is a higher density of gas, which has more potential for expansion, and will require a larger volume of space to occupy for a high deltaP/low cF. The increased volume works in increasing levels with the turbo charger to create compounding, which continuously increases the mass of intake gasses under compression, to an extent determined by parameters outside the range of this discussion.

Take those same turbo engines and remove the turbo, keeping the same pipe, and tell me what happens to BSFC at low RPM.

The larger flow volume requires a larger pipe to maintain a higher deltaP with lower friction. I thought I already covered this?

You were right about bernoulli, that was my mistake. It's conservation of mass we're after here.

[stonebreaker]

No, you like nearly everyone else, is forgetting half of Bernoulli's equation. The drop in pressure perpendicular to the flow, which is the part everyone remembers, is made up by an increase in dynamic pressure - everyone forgets this part. Go back to your wiki article and re-read it. Note that for a given streamline, the actual equation is dynamic pressure + static pressure = a constant. A CONSTANT. So if there is a drop in static pressure, it must be balanced by an increase in dynamic pressure. And it's the dynamic pressure that causes backpressure - if I need to force X amount of gas through a pipe in a given amount of time, I have to force it through using a given pressure (no pressure differential, no flow). If I want to increase the velocity of gas going through the pipe, the only way to do that is to increase the pressure driving it.

You made a mistake about 10 posts back and now you're sticking to it rather than re-examining your premise. This was fun for a while but now you're just being stubborn. I'll talk to you tomorrow - some of us have work.

[Christ]

Originally Posted by stonebreaker

No, you like nearly everyone else, is forgetting half of Bernoulli's equation. The drop in pressure perpendicular to the flow, which is the part everyone remembers, is made up by an increase in dynamic pressure - everyone forgets this part. Go back to your wiki article and re-read it. Note that for a given streamline, the actual equation is dynamic pressure + static pressure = a constant. A CONSTANT. So if there is a drop in static pressure, it must be balanced by an increase in dynamic pressure. And it's the dynamic pressure that causes backpressure - if I need to force X amount of gas through a pipe in a given amount of time, I have to force it through using a given pressure (no pressure differential, no flow). If I want to increase the velocity of gas going through the pipe, the only way to do that is to increase the pressure driving it.

You made a mistake about 10 posts back and now you're sticking to it rather than re-examining your premise. This was fun for a while but now you're just being stubborn. I'll talk to you tomorrow - some of us have work.

I just agreed with you about bernoulli.

What exactly is the "mistake" I made? If you're referring to bernoulli and the flow equation, read the first line again.

PS - I'm doing my work while chatting with you. It's a nicety for me.

Keep in mind, that you can increase the pressure driving a gas through a pipe by simply allowing it's expansion to drive the increase in pressure.

Like I said, there's more to it than simple pressure and velocity. Since the flow is compressible, and is not adiabatic (it loses heat and expands), keeping a lower pipe diameter will increase the flow's velocity due to expansion until the expansion is complete. If the pipe is longer than what is necessary to exhaust the flow after complete expansion, the flow will stack and pressure will build behind it. The larger the pipe diameter, the faster the exhaust gasses will expand to fill the pipe, and the lower it's velocity, since more expansion is perpendicular to flow, and less is parallel. Lower velocity has less potential energy, and thus, will create less pressure drop across the valve.

If you have some piece of information that refutes this, please post it for review.

To keep things clear, I have never suggested that a smaller pipe is universally the answer. I have been suggesting this whole time that pipe diameter should be based on mass of flow, and that larger is not universally better for a given application.

It appears that you're arguing from a performance standpoint primarily, which is likely why you're refusing to see my stance on the subject. When one isn't looking for more potential power than is actually necessary to do the work required, it changes many of the variables in determining the proper size of pipe required for maximum efficiency.

For the third time, there is alot more to the model than force (pressure) and velocity.

So before you start thinking that I'm arguing with you simply to argue, It needs to be mentioned that I'm not saying that you're wrong at all. I just don't agree that the method you've used for determining what works best in your situation necessarily applies to what is necessary for the best BSFC at lower RPM.

To take your suggestion to extremes, the best possible configuration is always the larger diameter pipe, which can quickly get out of hand. Ask anyone who's ever over-ported a head.

[nemesis]

Turbocharged gas engine will need a different exhaust than N/A gas engine. Turbocharged engine basically want the biggest exhaust you can get. N/A cars is more complicated, a lot of people always think of flow, the thing is, velocity is where it's at. The best exhaust for a street car, whether it's a high hp car or high fe car will need proper exhaust, too small of the exhaust is better than too big.

[stonebreaker]

Originally Posted by Christ

I just agreed with you about bernoulli.

What exactly is the "mistake" I made? If you're referring to bernoulli and the flow equation, read the first line again.

PS - I'm doing my work while chatting with you. It's a nicety for me.

Keep in mind, that you can increase the pressure driving a gas through a pipe by simply allowing it's expansion to drive the increase in pressure.

Like I said, there's more to it than simple pressure and velocity. Since the flow is compressible, and is not adiabatic (it loses heat and expands), keeping a lower pipe diameter will increase the flow's velocity due to expansion until the expansion is complete. If the pipe is longer than what is necessary to exhaust the flow after complete expansion, the flow will stack and pressure will build behind it. The larger the pipe diameter, the faster the exhaust gasses will expand to fill the pipe, and the lower it's velocity, since more expansion is perpendicular to flow, and less is parallel. Lower velocity has less potential energy, and thus, will create less pressure drop across the valve.

If you have some piece of information that refutes this, please post it for review.

Sure, it's called the ideal gas law: pV=nRT. If temp drops or volume increases, then pressure must drop. Thus you are refuted by the laws of thermodynamics.

Originally Posted by Christ

To keep things clear, I have never suggested that a smaller pipe is universally the answer. I have been suggesting this whole time that pipe diameter should be based on mass of flow, and that larger is not universally better for a given application.

It appears that you're arguing from a performance standpoint primarily, which is likely why you're refusing to see my stance on the subject. When one isn't looking for more potential power than is actually necessary to do the work required, it changes many of the variables in determining the proper size of pipe required for maximum efficiency.

For the third time, there is alot more to the model than force (pressure) and velocity.

So before you start thinking that I'm arguing with you simply to argue, It needs to be mentioned that I'm not saying that you're wrong at all. I just don't agree that the method you've used for determining what works best in your situation necessarily applies to what is necessary for the best BSFC at lower RPM.

To take your suggestion to extremes, the best possible configuration is always the larger diameter pipe, which can quickly get out of hand. Ask anyone who's ever over-ported a head.

Head ports have nothing to do with cylindrical pipe flow - their shape is too complex to be directly comparable. However, just for S&G's, I'll point out that the typical gen I smallblock had 170cc intake ports and 67cc exhaust ports. The LS1 had 200cc intakes and 70cc exhausts. Now the LS3 has 257cc intakes and 86cc exhausts, gets better mileage, makes more power, and produces less pollution (12% less hydrocarbons and 40% less NOx) than the LS1.

Oh, and if we take YOUR suggestion to the extreme, since we seem to be going to extremes, when the exhaust hits the essentially infinite diameter at the end of the tailpipe, it should just stop and stack up, since it will expand and cool waaaaay more than it will in a larger pipe.

[Christ]

Originally Posted by stonebreaker

Sure, it's called the ideal gas law: pV=nRT. If temp drops or volume increases, then pressure must drop. Thus you are refuted by the laws of thermodynamics.

Head ports have nothing to do with cylindrical pipe flow - their shape is too complex to be directly comparable. However, just for S&G's, I'll point out that the typical gen I smallblock had 170cc intake ports and 67cc exhaust ports. The LS1 had 200cc intakes and 70cc exhausts. Now the LS3 has 257cc intakes and 86cc exhausts, gets better mileage, makes more power, and produces less pollution (12% less hydrocarbons and 40% less NOx) than the LS1.

Oh, and if we take YOUR suggestion to the extreme, since we seem to be going to extremes, when the exhaust hits the essentially infinite diameter at the end of the tailpipe, it should just stop and stack up, since it will expand and cool waaaaay more than it will in a larger pipe.

Thanks for making my point.

[stonebreaker]

Originally Posted by Christ

Thanks for making my point.

Oh, so you're reversing your position now?

[Christ]

Originally Posted by stonebreaker

Oh, so you're reversing your position now?

No, you made the secondary point, that you're introducing ridiculous suggestions.

I haven't reversed my position, and because of infinite expansion, the mass would also become infinitely dispersed, which would then not stack up and create back pressure. Inside a vessel in which the flow could not expand infinitely, it would have to maintain a positive flow of direction toward the path of least resistance (simple fluid behavior), and since expansion occurs toward the path of least resistance, the fluid would expand in all directions until it met a barrier, being the wall of the tubing and the exhaust flow coming against it from the backside. At that point, it would use the rest of it's internal energy to expand toward the exit while flowing in that general direction as well.

The less energy the gas has to spend expanding to fill the vessel that it's been introduced to, the more energy it can spend flowing toward the path of least resistance, which is open air. Expansion is NOT what you want, flow velocity is what you want.

IMO, you're still just arguing circles so that you can seem like you're right.

This discussion isn't going anywhere, so I'm not going to continue it.

[gone-ot]

...just as there's an optimum gas velocity (~300 fps) in headers for making HP, there's gonna be some minimum gas velocity (??? fps) in headers for making FE.

...in the first place (HP), the velocity has to be high enough to get out of the pipe with minimum slowdown...while in the second place (FE), the velocity needs to be just fast enough to get out of the pipe without impeding the 'next' cylinder's discharge.

[mwebb]

not only do you want the column of gases in the exhaust to not be "impeding the flow of the next puff .
but you want the column to be accelerating away from the engine so that it pulls a very slight vacuum or at least reduced pressure to
assist the next puff so the the combustion chamber is emptied
more completely

in the automotive repair world -
it is thought that the speed of the pulses / puffs through the exhaust column is equal to the speed of sound .

so when measuring the exhaust pressure / vacuum pulse train
at the tailpipe

there is a delay = to the time it takes for the speed of sound to get through the length of the exhaust that must be factored in
when syncing the pressure / vacuum pulse train an event in the engine
like spark ,
when searching for misfire


except for the long line misfire this 4 cylinder engine is
putting out a uniform series of pressure / vacuum pulses at the load during the capture - many systems do not have clear delineated pressure / vacuum pulses
and instead they have just hash
like on any V6 engine with single exhaust and UNequal exhaust pipe lengths leading from the exhaust manifolds to the Y pipe where everything comes together

i SWAG the the Uniform pulses in the pressure / vacuum of the column of exhaust
is extremely critical for optimum exhaust efficiency