smaller exaust pipe for mpg

[nemesis]

Auto Exhaust Science Good read about exhaust.

[stonebreaker]

Reducing the diameter of your exhaust pipe will do nothing for your torque at low rpm. That's a myth.

The myth got started because back in the carburetor days, adding headers and a performance exhaust to a car leaned it out, causing a loss of torque at low rpm's. Thus the myth that if increasing the size of your exhaust lowers low end torque, then decreasing the size of your exhaust must then increase torque, right?

The real solution to the issue of lowered torque with a big exhaust was due to the headers actually doing their job and lowering backpressure, which leaned out the air/fuel mix. Once you re-jetted the carb to account for the headers, you INCREASED torque all the way across the rpm band.

With modern fuel-injected cars, the computer will compensate for the reduced backpressure automatically so there will be no loss of torque.

About the only thing you could do that would hurt your torque output would be to reduce the diameter of your exhaust system. It will increase your backpressure, causing your engine to work harder to pump the exhaust out the back.

[Frank Lee]

One thing that has stopped me from pursuing this further is not wanting to have the system "neck down" as IMHO that would be bad. Does that mean the ports in the heads are too big? Exhaust valves too? Probably.

[moonmonkey]

i think the valves are a completly different animal frank, because at that point the other cylinders dont affect its individual flow as much as when all the pulses mix together at the collector, im sure you are correct that changing all the paramiters would be best ,i just keep thinking that by changing the exaust it could make an improvment, because at low rpm's not that much air would be tring to get past the valves ,, but i have only modified v-8 carburated cars so i will defer to the more modern builders advice,i have seen big changes in v-8 cars depending on header primary diameter/length, and collector diameter/length, some of those cars highly modified, some stock,,but all 302+ cubes

[Christ]

Originally Posted by stonebreaker

Reducing the diameter of your exhaust pipe will do nothing for your torque at low rpm. That's a myth.

The myth got started because back in the carburetor days, adding headers and a performance exhaust to a car leaned it out, causing a loss of torque at low rpm's. Thus the myth that if increasing the size of your exhaust lowers low end torque, then decreasing the size of your exhaust must then increase torque, right?

The real solution to the issue of lowered torque with a big exhaust was due to the headers actually doing their job and lowering backpressure, which leaned out the air/fuel mix. Once you re-jetted the carb to account for the headers, you INCREASED torque all the way across the rpm band.

With modern fuel-injected cars, the computer will compensate for the reduced backpressure automatically so there will be no loss of torque.

About the only thing you could do that would hurt your torque output would be to reduce the diameter of your exhaust system. It will increase your backpressure, causing your engine to work harder to pump the exhaust out the back.

That's not entirely true, but for the most part, it is.

Reducing the diameter of the pipe can allow for less turbulent flow through the tube if the volume of flowing gasses isn't enough to "fill" the tube at it's current temperature/density.

There's alot more to it than that simple phrase, but basically, exhaust tubing follows the same set of rules as an external flow, except with a much thicker boundary layer attached to the walls of the tube.

In other words, you'll get more bottom end if you have a properly sized pipe, as opposed to an obnoxiously large one.

Regarding carbs, once you retuned the carb, you were adding more fuel to the new amount of airflow to acheive more power across the board, which still means that you're burning more fuel at a specific RPM than you were. If the amount of fuel you're burning extra is less than the VE increase, you get a net gain in efficiency. If it's not, you're either breaking even or worse for efficiency, even though you could still be making more power than you were before.

Our purpose isn't normally to increase low-end torque above what's already there, it's to make the engine more efficient at creating it. That means that anything which requires additional fuel, with few exceptions, is going the wrong way for our intentions. Most vehicles already have plenty of torque to do what they need to do, so adding more isn't really going to help, unless it's a byproduct of increased efficiency.

[stonebreaker]

Originally Posted by Christ

That's not entirely true, but for the most part, it is.

Reducing the diameter of the pipe can allow for less turbulent flow through the tube if the volume of flowing gasses isn't enough to "fill" the tube at it's current temperature/density.

How do you figure that? The Reynolds number calculation for a pipe is

R = QD/vA

Q = volumetric flow rate
D = diameter of the pipe
v = viscosity
A = cross-sectional area of the pipe

Since the volume of flow and the viscosity don't change, the only variables are the diameter and the area; and for a circular cross section, the area increases as a function of the square of the radius * pi; so the Reynolds number is always going to be lower for the larger diameter pipe.

[Christ]

IF there isn't enough fluid to fill a vessel, it will not flow in a linear pattern.

[stonebreaker]

Originally Posted by Christ

Regarding carbs, once you retuned the carb, you were adding more fuel to the new amount of airflow to acheive more power across the board, which still means that you're burning more fuel at a specific RPM than you were. If the amount of fuel you're burning extra is less than the VE increase, you get a net gain in efficiency. If it's not, you're either breaking even or worse for efficiency, even though you could still be making more power than you were before.

Our purpose isn't normally to increase low-end torque above what's already there, it's to make the engine more efficient at creating it. That means that anything which requires additional fuel, with few exceptions, is going the wrong way for our intentions. Most vehicles already have plenty of torque to do what they need to do, so adding more isn't really going to help, unless it's a byproduct of increased efficiency.

Increasing low end torque output means you can run a smaller amount of throttle, increasing your vacuum at cruise, and thus net an overall reduction in fuel usage.

There are also modern cams to consider, as well as egr function. The net benefit of running an efficient exhaust at cruising speeds is to reduce the amount of energy required to push the exhaust out the tailpipe.

I think for hypermiling purposes the only thing you can do that would even possibly have a noticeable effect would be an aftermarket muffler, since the stocker is nearly always a POS.

You can see in this pic that the exhaust coming into the stock muffler has to make a right angle turn to get to the exit, and this is actually a pretty good stock muffler:


On the other hand, a good performance muffler has a straight-through design that is a lot less restrictive:


The reason stock mufflers don't look like this is because performance mufflers won't pass the noise requirements the govt. places on the OEM's.

[stonebreaker]

Originally Posted by Christ

IF there isn't enough fluid to fill a vessel, it will not flow in a linear pattern.

Since we're talking about a gas, it will always expand to fill the available space.

[Christ]

Originally Posted by stonebreaker

Since we're talking about a gas, it will always expand to fill the available space.

And as it expands, it cools, and as it cools, it slows, and as it slows, it creates a stacking effect, increasing pumping losses in the engine.

For mufflers, PVC is acoustically inert, and with properly placed dimples, could be tuned so that sonic frequencies cancel themselves by reflection without compromising flow. Of course, nobody wants a plastic muffler, cuz "it could melt".