[Throttle Crazy]

Actually "resistance to flow" would be considered the cause and back pressure would
be the result. Take for example a garden hose with no nozzle on the end, when it is turned on it will flow at line pressure. When you put the nozzle on the end of the hose you have just caused a resistance to flow and there for created a back pressure in the hose. Make sense? Your exhaust works the same way. If you have a resistance to flow exhaust then you will cause back pressure in the system prior to the point of that resistiction. After the restriction you will have a low pressure. If you think about how an a/c system works this principal also applies. The compressor outputs a high pressure gas which travels to the condensor. The condensor condenses the gas to a high pressure liquid. Then that high pressure liquid flows through a restriction known as the orfice tube where it is converted to a low pressure liquid on the other side of the resitriction. From there it travels through the evaporator where it is converted to a low pressure gas and sucked back into the the compressor again. The restriction is what creates that high and low pressure with the help of the compressor.

[Vettezuki]

Originally Posted by Throttle Crazy

Actually "resistance to flow" would be considered the cause and back pressure would
be the result. Take for example a garden hose with no nozzle on the end, when it is turned on it will flow at line pressure. When you put the nozzle on the end of the hose you have just caused a resistance to flow and there for created a back pressure in the hose. Make sense? Your exhaust works the same way. If you have a resistance to flow exhaust then you will cause back pressure in the system prior to the point of that resistiction. After the restriction you will have a low pressure. If you think about how an a/c system works this principal also applies. The compressor outputs a high pressure gas which travels to the condensor. The condensor condenses the gas to a high pressure liquid. Then that high pressure liquid flows through a restriction known as the orfice tube where it is converted to a low pressure liquid on the other side of the resitriction. From there it travels through the evaporator where it is converted to a low pressure gas and sucked back into the the compressor again. The restriction is what creates that high and low pressure with the help of the compressor.

This is definitely how I was thinking about it.

[BRUTAL64]

Originally Posted by Vettezuki

This is definitely how I was thinking about it.

So do you still think any "back pressure" is good for exhaust?
Yes, it's good for AC, but we are talking exhaust.

I still believe BACK PRESSURE is the wrong term -- Kinda like Harmonic Balancer.
It's really Harmonic Dampener. But for the sake of discussion I'll use the term Back Pressure.

So, again, wasn't your question something about "Back Pressure" being good for some engines?

[Throttle Crazy]

Here is my final answer. Back pressure should remain as close to zero as possible while maintaining the highest velosity of exhaust flow. It depends on where your engine will benifit from the power the most. If your engine is a low rpm engine then you need smaller exhaust diameter to maintain high velocity in the mid range. If you put to large of exhaust you slow down the velocity in that range. If you have a motor that makes big power at 7,500 then you need large exhaust to maintain that close to zero back pressure and maintain higher velocity of air flow in the exhaust. The "bigger the better" is not the key to a tuned exhaust. SO THE ANSWER SIMPLIFIED. YES, BUT AS CLOSE TO ZERO AS POSSIBLE IN THE RPM RANGE YOUR ENGINE WILL BENIFIT THE MOST!!!

[Vettezuki]

Originally Posted by Throttle Crazy

Here is my final answer. Back pressure should remain as close to zero as possible while maintaining the highest velosity of exhaust flow. It depends on where your engine will benifit from the power the most. If your engine is a low rpm engine then you need smaller exhaust diameter to maintain high velocity in the mid range. If you put to large of exhaust you slow down the velocity in that range. If you have a motor that makes big power at 7,500 then you need large exhaust to maintain that close to zero back pressure and maintain higher velocity of air flow in the exhaust. The "bigger the better" is not the key to a tuned exhaust. SO THE ANSWER SIMPLIFIED. YES, BUT AS CLOSE TO ZERO AS POSSIBLE IN THE RPM RANGE YOUR ENGINE WILL BENIFIT THE MOST!!!

This I understand.

BRUTAL,
It seems back pressure is non desirable in any case. The big diameter exhaust problem typical on small import engines (when guys just put on big exhaust) is not because of reduced back pressure, but because of reduced exhaust velocity. It think you said more or less the same thing referring to scavenging.

TC,
If the intersection of low BP (close to zero as possible) and high exhuast velocity in typical operating range is most desirable, then why do most OEM engines have that low amount of BP at cruise/off idle? I guess that's just the most optimum position they can engineer for the total system, but it has nothing to do with "utilizing" back pressure.

My guess that back pressure in small amounts may be desirable because it may increase cylinder pressure and therefore torque seems to be wrong. There is never a benefit to back pressure (talking just about the efficiency of a NA engine). BP is just a consequence of other design elements in the system, it is not intentionally designed into the system.

[BRUTAL64]

Originally Posted by Throttle Crazy

Here is my final answer. Back pressure should remain as close to zero as possible while maintaining the highest velosity of exhaust flow. It depends on where your engine will benifit from the power the most. If your engine is a low rpm engine then you need smaller exhaust diameter to maintain high velocity in the mid range. If you put to large of exhaust you slow down the velocity in that range. If you have a motor that makes big power at 7,500 then you need large exhaust to maintain that close to zero back pressure and maintain higher velocity of air flow in the exhaust. The "bigger the better" is not the key to a tuned exhaust. SO THE ANSWER SIMPLIFIED. YES, BUT AS CLOSE TO ZERO AS POSSIBLE IN THE RPM RANGE YOUR ENGINE WILL BENIFIT THE MOST!!!

You don't have to convince me, I already know the answer.


It's Vettezuki that needs to know.

[BRUTAL64]

Originally Posted by Vettezuki

This I understand.

BRUTAL,
It seems back pressure is non desirable in any case. The big diameter exhaust problem typical on small import engines (when guys just put on big exhaust) is not because of reduced back pressure, but because of reduced exhaust velocity. It think you said more or less the same thing referring to scavenging.

TC,
If the intersection of low BP (close to zero as possible) and high exhuast velocity in typical operating range is most desirable, then why do most OEM engines have that low amount of BP at cruise/off idle? I guess that's just the most optimum position they can engineer for the total system, but it has nothing to do with "utilizing" back pressure.

My guess that back pressure in small amounts may be desirable because it may increase cylinder pressure and therefore torque seems to be wrong. There is never a benefit to back pressure (talking just about the efficiency of a NA engine). BP is just a consequence of other design elements in the system, it is not intentionally designed into the system.

Scavenging is the adjustable part of the issue. You want no Back Pressure and you use scavenging to pull the exhaust gas out of the cylinder. The amount of scavenging is adjusted from a little to a lot depending on the flow of the intake charge and exhaust port. You adjust the scavenging ( like adding a 30 inch pipe to lenghten the collector to give more mid range torque) to fit the engine.

I don't like the term Back Pressure.

Throttle Crazy--"Actually "resistance to flow" would be considered the cause and back pressure would be the result"---This is as close to correct as we are going to get.


You want no "Back Pressure".

[Throttle Crazy]

Originally Posted by Vettezuki

This I understand.

BRUTAL,
It seems back pressure is non desirable in any case. The big diameter exhaust problem typical on small import engines (when guys just put on big exhaust) is not because of reduced back pressure, but because of reduced exhaust velocity. It think you said more or less the same thing referring to scavenging.

TC,
If the intersection of low BP (close to zero as possible) and high exhuast velocity in typical operating range is most desirable, then why do most OEM engines have that low amount of BP at cruise/off idle? I guess that's just the most optimum position they can engineer for the total system, but it has nothing to do with "utilizing" back pressure.

My guess that back pressure in small amounts may be desirable because it may increase cylinder pressure and therefore torque seems to be wrong. There is never a benefit to back pressure (talking just about the efficiency of a NA engine). BP is just a consequence of other design elements in the system, it is not intentionally designed into the system.

Bingo!! Back pressure is used to determine if there is a restriction in the exhaust system. Also bp is a result of emissions equipment and exhaust tuning. If you have an oem engine that makes good torque numbers at 2,200 and good hp up to 5,800 the exhaust needs to be tuned to create a balance. If the exhaust is to large then you loose the velocity at low rpm. If the exhaust is to small you have a "resistance to flow" which creates back pressure in the high rpm.

[Vettezuki]

Originally Posted by BRUTAL64

Scavenging is the adjustable part of the issue. You want no Back Pressure and you use scavenging to pull the exhaust gas out of the cylinder. The amount of scavenging is adjusted from a little to a lot depending on the flow of the intake charge and exhaust port. You adjust the scavenging ( like adding a 30 inch pipe to lenghten the collector to give more mid range torque) to fit the engine.

I don't like the term Back Pressure.

Throttle Crazy--"Actually "resistance to flow" would be considered the cause and back pressure would be the result"---This is as close to correct as we are going to get.


You want no "Back Pressure".

Crazy ass tangent.

We know that some (super fancy race) engines utilize variable length intake runners. Most modern super bikes use this technology. Has there ever been an engine with variable length exhaust?

[BRUTAL64]

Originally Posted by Vettezuki

Crazy ass tangent.

We know that some (super fancy race) engines utilize variable length intake runners. Most modern super bikes use this technology. Has there ever been an engine with variable length exhaust?

The only ones I've ever seen is the 180 degree exhaust systems. But I think that was to due to having to run header tubes all over and under the engine.