[OLNOLAN]

Originally Posted by 001mustang

that sounds right.

how about a typical street situation where exhaust side is over sized compared to intake side because no hole in hood to put proper high flow intake?

IMHO, If you have to add exhaust restriction to make your engine run right, you've got the wrong engine or you put together the wrong package(mismatch of components) or you just don't have the power in your combo to satisfy you.

I think that OLRED gave a good explanation on exhaust velocity and scavenging. If you think of the engine as a air pump, low inlet restriction, low exhaust restriction makes for greater air velocity through the pump.

Its all about a compromise to get the most out of a muffled exhaust system to me. The most power to be had is in open exhaust headers as in NO restriction.

Okay, That's four cents. I'll back off and open the floor.olnolan

[ericnova72]

I agree some here are confusing small pipe size for velocity reasons and backpressure. Small pipe size with velocity may be just what some combinations need ....but backpressure is different and is always undesirable.

There is no way that backpressure in an exhaust system is ever enough pressure to "hold" the burning charge in the cylinder longer...and that is the statement I am saying is completely wrong. Cylinder pressure when the exhaust valve opens is at least 20 times any backpressure reading... short of shoving a potato up the pipe to completely close it off, and then the engine won't run.

If you are thinking to use backpressure to prevent overscavenging the cylinder at valve overlap all you are doing is putting a band-aid on the fact you have the wrong cam grind in the engine.

[ericnova72]

Originally Posted by OLNOLAN

IMHO, If you have to add exhaust restriction to make your engine run right, you've got the wrong engine or you put together the wrong package(mismatch of components) or you just don't have the power in your combo to satisfy you.

I think that OLRED gave a good explanation on exhaust velocity and scavenging. If you think of the engine as a air pump, low inlet restriction, low exhaust restriction makes for greater air velocity through the pump.

Its all about a compromise to get the most out of a muffled exhaust system to me. The most power to be had is in open exhaust headers as in NO restriction.

Okay, That's four cents. I'll back off and open the floor.olnolan

I agree, with both you and OldRed.

[ericnova72]

Originally Posted by oldred

Yes exhaust is very critical on a two stroke and back pressure is indeed necessary but a four stroke engine is a whole 'nother animal! By the time the exhaust valve starts to open cylinder pressure has already peaked, or at least the power stroke is in a declining situation, (this is not a statement of fact but rather they way I understand it, right or wrong) and by that time it is exhaust scavenging that should be desirable. It would seem to me that at the instant the valve starts to open clearing the chamber of exhaust gases would be the main concern and that is what has me puzzled by all this, how can holding the gases in the cylinder at this point contribute to power?



True but it has little or nothing to do with back pressure either, place a set of short NON RESTRICTIVE pipes on the ports and it will run just fine. There is a variety of reasons an engine will hardly run in that condition but I don't think back pressure would be an issue one way or the other in that situation.

YES, EXACTLY.

[ericnova72]

Originally Posted by oldred

I have read and re-read these posts and I still can not see the mechanics involved in exhaust backpressure holding a cylinder charge until combustion is complete.

Same here...it just isn't possible, basic Laws of Physics

[oldred]

I think maybe some might be assuming that a smaller pipe just automatically means increased back pressure but that is simply not true. If an engine is performing poorly at low RPM (loss of torque) and too large exhaust pipes/header tubes are the culprit then smaller pipes may solve the problem but it is not because they add back pressure, at least not in the improved performance RPM range. Swapping to smaller pipes/header tubes in this instance could actually LOWER back pressure at the exhaust port in the lower RPM range because of the increased exhaust gas velocity thus a power gain due to lower back pressure and better scavenging of exhaust gases from the cylinder, that is until RPM rises to the point that the pipes are then too small to handle the larger flow of exhaust gas which then causes back pressure and a resultant power drop or down turn of the torque curve. That is why smaller pipes may be a blessing at low RPM but a curse as RPM increases, smaller pipes do not always mean back pressure increase, at least at lower RPM, and indeed any power gains achieved by swapping to smaller pipes will be due to lower exhaust port pressures and not increased back pressure.



If the pipes are too big and not enough back pressure actually did cause power loss then why would it not make sense to just partially plug the tail pipe to solve the problem? This would certainly cause back pressure and back pressure could even be controlled but I just don't see how simply creating back pressure in the exhaust could improve power in any RPM range.

[001mustang]

decreasing pipe diameter increases pressure drop in a steady state flowing pipe.

quickly closing valve at entrance of flowing pipe can cause a temporary low pressure region just down stream of closed valve.

this temporary low pressure effect increases w/ gas velocity and mass.

[oldred]

Originally Posted by 001mustang

decreasing pipe diameter increases pressure drop in a steady state flowing pipe.

quickly closing valve at entrance of flowing pipe can cause a temporary low pressure region just down stream of closed valve.

this temporary low pressure effect increases w/ gas velocity and mass.

True and what I have been saying is an over-simplification of the question of large vs smaller tubes but then simply stating that an engine NEEDS some back pressure to run properly is a huge over-simplification too. You are certainly correct and these exhaust pulses make a great deal of difference, header tubes/collectors/exhaust pipes in a well designed system will use that phenomenon to lower pressure at the exhaust port at the right time to extract as much burned gas as possible. Everything I have ever read or been told about performance exhaust systems is meant to extract exhaust from the cylinder and I just can't understand the reasoning behind using back pressure to hold it in the cylinder. I may be wrong but IMO some are confusing the reasons for increased lower RPM power increases from smaller tubes by thinking that the smaller tubes always add back pressure when in fact at the right RPM/engine conditions the smaller tubes can actually lower pressure vs larger tubes at the exhaust port. Again IMO it is the lower exhaust port pressure from the increased exhaust velocity in the smaller tubes that result in the lower RPM power/torque increase and not back pressure holding the burned gases in the cylinder longer.

[001mustang]

Originally Posted by oldred

I may be wrong but IMO some are confusing the reasons for increased lower RPM power increases from smaller tubes by thinking that the smaller tubes always add back pressure when in fact at the right RPM/engine conditions the smaller tubes can actually lower pressure vs larger tubes at the exhaust port. Again IMO it is the lower exhaust port pressure from the increased exhaust velocity in the smaller tubes that result in the lower RPM power/torque increase and not back pressure holding the burned gases in the cylinder longer.

Sounds logical.

I like the "at the exhaust port" bit.

on the whole, smaller pipe will produce more pressure drop.

"at the exhaust port" could potentially see moments of reduced pressure w/ smaller pipe.

if you reduce the 4000F cylinder residence time of an incompletely combusted mix your gonna emit more hydro carbons out your tail pipe.

i still think exhaust pipe pressure can affect cyl residence time; don't know how that affects power.

lots of variables to consider.

[oldred]

Originally Posted by 001mustang

i still think exhaust pipe pressure can affect cyl residence time; don't know how that affects power.

No doubt about it, it surely does but the question has been how does using back pressure to hold exhaust back in the cylinder after the exhaust valve starts to open going to provide an increase in power? An exhaust pulse that provides reduced pressure at the port as the valves starts to open would seem to be quite desirable however. Of course there is far more at work here than just a simple small pipe providing an increase in exhaust pipe velocity from one cylinder, a properly tuned system is going to have different cylinders realizing a pressure drop from exhaust flow from other cylinders in a real situation. This is where header tube length and collectors come into play however the end result is always it seems to reduce pressure at the port and I have never heard of exhaust tuning theory that intentionally CREATED back pressure (except for two stroke engines), indeed everything I know of has been to reduce pressure at the port when the valve opens. While back pressure can certainly affect cylinder residence time of the burned gases I still can't understand how using back pressure to accomplish this after the exhaust valve starts to open could possibly increase power, DECREASE power maybe but increase it?



I think there has been some high tech systems using exhaust pulse to control or prevent over scavenging at certain engine situations but this would be to provide less of a pressure drop and not actual back pressure in the exhaust system, certainly not a situation where it could be said that "some back pressure is necessary".