smaller exaust pipe for mpg

[stonebreaker]

Originally Posted by Christ

Once again, the pipe doesn't need to be 18 feet to benefit from scavenging. That's the "ideal" length for a given engine size at WOT flow, assuming (I believe) 100% VE.

WOT or not, it doesn't matter. The length of the pipe is determined by rpm. Pipe diameter is determined by engine displacement.

Originally Posted by Christ

In reality, that wouldn't be happening most of the time, and considerations could be made to take advantage of pulse scavenging, although not in the most ideal sense.

Harmonics are a different beast altogether, and can be taken advantage of on increasingly smaller amounts after primary harmonics, but can still be usable by length/cross section tuning.

No, you can't. 2000 rpm is too slow to make use of pulse scavenging in a streetable car.

[bgd73]

I learned this from a mechanic.

4 cyls have no overlap, exhaust has to be precise for homogeny. That is many times a locale, the common fuel, turbo, no turbo, carbed, injected, low compression, high compression, big volt ignition, low volt, single fire , double fire, and how much air from idle to full throttle?

and then you know what I learned..the same exact exhaust can be swapped through all engines, because there is no two cylinders firing at the same time.

in the 8 cylinder world, and beyond, there is 90 degree overlaps, then in the 10 cyl it is 72 degrees, the 12 it is 60.. if you are running deep, there is reasons, very rare, to have what look like stove pipes, like a diesel...

other than that, it is trial and error. even ignition strength alters what is needed. The length of the exhaust sytem may call for something else..my own is 1781cc, with a 2.25 axle back, on a carbed engine...sounds bizarre, until mentioning 14.6 feet to get to the back..the little pipes it could use, are bad because of it.
and that brings how much heat is sustained, are pipes covered? going by a spinning driveshaft drops temps..fwd does not have to worry of this..etc etc etc etc

there is a precision after all.

[stonebreaker]

Originally Posted by Christ

The myth of exhaust backpressure [Archive] - My Pro Street

This pretty much has it.

You need to read it again.

[Christ]

Originally Posted by stonebreaker

WOT or not, it doesn't matter. The length of the pipe is determined by rpm. Pipe diameter is determined by engine displacement.



No, you can't. 2000 rpm is too slow to make use of pulse scavenging in a streetable car.

The pipe length is not solely determined by RPM. The length primarily is used in tuning harmonics, while cross section has the largest effect on DeltaP, or pressure drop across a field.

The WOT point was made to convey the fact that the calculator was based on assumed figures, including WOT. In reality, at 20% throttle, 1.0L is not even close to 1.0L. The pipe could be much smaller, increasing deltaP, without having much impact on friction, to a given extent.

The idea is to get the fastest flow with the least friction for a given RPM. This means that there is always a compromise between diameter and length. At any length, pulsed flow can be used to scavenge flow on another cylinder, since as the flow expands and speeds up, it's relative pressure is reduced, which creates a sort of vacuum at another opening, drawing against the other opening's exhaust valve.

Harmonics tuning creates a wave-driven effect in which exhaust gasses can be forced out of the pipe at a higher speed by pressure reflection, which also reduces pressure, increasing deltaP (desirable).

[stonebreaker]

Originally Posted by Christ

The pipe length is not solely determined by RPM. The length primarily is used in tuning harmonics, while cross section has the largest effect on DeltaP, or pressure drop across a field.

The WOT point was made to convey the fact that the calculator was based on assumed figures, including WOT. In reality, at 20% throttle, 1.0L is not even close to 1.0L. The pipe could be much smaller, increasing deltaP, without having much impact on friction, to a given extent.

The idea is to get the fastest flow with the least friction for a given RPM. This means that there is always a compromise between diameter and length. At any length, pulsed flow can be used to scavenge flow on another cylinder, since as the flow expands and speeds up, it's relative pressure is reduced, which creates a sort of vacuum at another opening, drawing against the other opening's exhaust valve.

Harmonics tuning creates a wave-driven effect in which exhaust gasses can be forced out of the pipe at a higher speed by pressure reflection, which also reduces pressure, increasing deltaP (desirable).

No. That's another myth, caused by people misinterpreting Bernoulli's principle. I suggest you go back to the calculator site and read at least one of the books he used to program his calculator. I used Smith when I built the headers on my impala.

[Christ]

I suggest you actually read the posts on that forum.

Originally Posted by from that link

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

If pressure isn't changing with a change in velocity, how do you explain how carburetors work? The venturi is specifically designed to accelerate the flow of the less dense fluid, further decreasing it's internal pressure, to draw fuel into it's stream. This is common knowledge. If the book you read refutes this, I don't want to even look at it.

Originally Posted by wiki "bernoulli's principle" (Yeah, I know, it's wiki... blah blah)

The carburetor used in many reciprocating engines contains a venturi to create a region of low pressure to draw fuel into the carburetor and mix it thoroughly with the incoming air. The low pressure in the throat of a venturi can be explained by Bernoulli's principle; in the narrow throat, the air is moving at its fastest speed and therefore it is at its lowest pressure.

This would also explain a decrease in pressure of an exhaust flow with an increase in velocity, which supports the idea of increasing deltaP, which pretty much makes my whole argument.

[Christ]

This calculator is a little more effective when determining exhaust length/diameter, as well as intake length/diameter for a specific tuning characteristic.

Many more inputs, as well.

Pipe Sizing Calculator

[stonebreaker]

Bernoulli's principle only covers pressure PERPENDICULAR to the direction of flow, thus carbs still work.

[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.

[redyaris]

I am informed by my engineering friends and family that the general rule for exhaust tubes is that they should have the same cross sectional area as the port at max valve lift. As I understand it for most of the exhaust cycle the port area varies with the valve lift, and changes with respect to time. Therefore for most of the exhaust strock the major empedament to flow is the exhaust port. so that at low rpm there is relatively lots of time for the gases to get out, even if the tube is smaller diameter. I agree with stone bracker that a larger tube diamerter is best, however the more important issue is how much differance will it make to a FE engine at low rpm? not much, unless the tubes are cosiderably smaller than the port area at max lift. If you really want to know what is best you will have to do a lot of testing to find out. Most "high performance" stuff is to maximize toque or power at a specified rpm so that the vehical will accelerate faster... what we are looking for is minimal fuel consumption. what we share with the HP folks is a desire to reduce pumping loses and to improve over all engine efficiency, so that we can burn less fuel, fast acceleration uses lots of fuel!