Backpreasure myth

[projectmx]

Ran across this on the miata forum... thought it would be helpful in our discussion on exhaust.. maybe add to the 4 cyl. FAQ?

http://forum.miata.net/ubb/ultimatebb.php?ubb=get_topic;f=1;t=027972

[Custommx3]

An internal combustion engine does requires some back presure, If anyone needs a proof I will scan a page of one of of my mechanical engineering books on internal combustion engines as proof.

<small>[ October 26, 2004, 10:56 AM: Message edited by: Custommx3 ]</small>

[projectmx]

so the miata forum discussion is completely incorrect as is the article they posted?

[jumboplay]

Originally posted by Custommx3:
An internal combustion engine does requires some back presure, If anyone needs a proof I will scan a page of one of of my mechanical engineering books on internal combustion engines as proof.

I agree. Everyone says this or that, but just take off your muffler and drive your car. There's no way that the car is faster that way! A specific amount of backpressure is required and an engine certainly doesn't run better/faster without...

[projectmx]

seems like removing that much of the exhaust would also have huge affect (adverse) on exhause velocity and the 02 sensor.. hmm loss of power?

[Gro Harlem]

Those ppl on miata.net are retards. That one guy didn't even read the article he posted. It talks about why you do need some backpressure and ways to get power out of your exhaust w/explanations.

[MX3-Freak]

Ok, I'm going to have to go out on a limb here. Remember, I'm not saying you are wrong, but rather I just want to know how your answer is more accurate then what logic dictates.

Logic says: It takes energy to move gases.
Logic says: The more pressure to push those gasses in one direction (back pressure) the more energy it will require to move them in the other direction (the piston moving upward on an exhauast stroke). This almost ties into the law of interia: the more mass/energy (remember, mass and energy are niether created nor destroyed, but always remain constant and can be changed from one another) the more energy it requires to change that objects direction.

Now, in perspective of motors. Since the back pressure is creating an "energy source" pushing the gas back into the cylinder, the piston must have more energy/inertia then the pressure of the gas. Where does it get this energy from? Its the same energy that spins your wheels and runs your ac. Thus, by relieving backpressure, less energy is required to move those gases, and then more energy can go to your wheels. Or so says my logic.

Please, point out the flaws; I'd like to know the correct answer and why.

[What]

I'd be interested in seeing the simplification from the text book. I have some ME type texts on internal combustion engines and they usually cover too much about every other part to go into good specific details about the neccessary theory involved with exhaust design.

I do agree that an exhaust with higher overall pressure can produce more power where required than an exhaust with lower pressure although the point is not to create backpressure. You are trying to propoagate pressure/sound waves and people should be paying attention to much much more than just tring the biggest exhaust they can find. Turbocharged vehicles add a different level to the theory, but there are still good reasons for/against many of the manifold types out there.

An excerpt from "Scientific desgin of exhaust and intake systems".. both authors are ME, one Bachelor in science, PHD...etc, etc.. Good sources and a good book.

"The average pressure is largely that against which the exhaust discharged is pushed by the piston on the exhaust stroke - a simple mechanical operation. At the end of the stroke, on piston reversal through teh overlap period is the critical instant. At this point the back-pressure at that particular port may be in excess of the mean pressure in teh system. In this case the bmp of the engine will be reduced proportionately. On the other hand, the port pressure may be less than the mean or even under atmospheric due ot the good pipe extraction characteristics and wave motion of the gases and in this event the power output could be better than that of an engine showing a lower average exhaust pressure"

[wytbishop]

Though you may not know it, the exhaust gas leaving your engine is actually a fluid. Thus it abides the laws of fluid dynamics.

If you force a fluid out of an orifice, straight into atmosphere, it goes from a state of high pressure (and thus energy) to a state of low pressure and energy very quickly and loses velocity and generally becomes turbulant (which is bad). If you control the exit of the fluid carefully you can help it to maintain a laminar flow (the opposite of tubulant). When the fluid is laminar and has high energy, it will evacuate the cylinder quickly and, as stated in the other thread, create a bit of vacuum outside the exhaust port, pulling future exhaust pulses out of the engine (this is good).

Backpressure is a non issue...and a misnomer. Having too small an exhaust would be restrictive, and though the flow would be controlled, it would lose velocity due to the restriction. This is opposition, not back preessure as it is commonly referred to.

Scavenging is a function of the inertia/momentum of the exiting gases.

Think in terms of efficient flow. Having too large an exhaust system, or none at all, robs the exiting gas of momentum and kills the scavenging effect. Having an exhaust pipe of the correct size for the engine in question is more efficient than having a pipe too big for the volume of gas being expelled by keeping the gas moving quickly. This is why a 1.6L or 1.8L N/A engine does not benefit from a 3" exhaust. It loses too much energy too quickly and does not scavenge well.

After the turbo in an engine of that sort is different. The exhaust gas has in theory given up most of it's energy already to the turbine so scavenging here will not be very effective anyway. You want a very large exhaust after the turbo to allow exhaust to leave the turbine with little or no opposition.

Having said all that, I am not an expert on the application of this branch of physics. I am a designer of machines but I think my understanding of the issue is pretty strong.

[Shady469]

I understand the theories and can also tell of my first hand experience. I put 2.5" exhaust on the car (SOHC) right before i did my B6T swap. The car didn't go anywhere without revving the heck out of it. It actually performed alot better with the silencer in... I'm talking like 10 times better.

[What]

Very good write up... it's nice to see some brains coming out of the wood-work here. All the wave tuning theories come in from the fact that it is not a steady state situation. And that lack of steady state is what neccessitates the need for 'scaveging'.

The exhaust valves begin to open before BDC after combustion at a really high pressure... initial high pressure to low pressure sets up a wave that continues to help draw out exhaust until a little before half way up the pistons exhaust stroke. At this point it should be around it's lowest pressure. From here another compression wave on a poorly scavenged engine will go back above atmoshpheric and back to high pressure. This is at a time near the end of the exhaust stoke where you really need to doing all you can to help get the dirty air out because the intake valves are just about to or beginning to open and piston speed is low to none and the only thing to help your dirty air leave the cylinders now and draw fresh air in from the intake is the kinetic energy imparted from the initial wave. With the improperly sized exhaust system and positive pressure in the system, the dirty air is going to want to go back towards the intaketo fill it with all the junk that it just took out. In addition to the rest of the negative effects,the dirty charge air in the cylinder for the next combustion stroke will also mean less fresh air to combust and less power.

Ideally, a properly sized (length and diameter) exhaust system will not have as low of a negative pressure after the intial drop about half way up, and instead of going to a positive pressure near the end of the stroke, where it can force air into the intake, it will stay at a negative pressure and help draw fresh air in.

This is also something that starts to get into cam design and getting valves open quickly (helps set up initial pressure wave), but that starts another topic and starts to be for all sorts of other wonderful anologies to go through. I think that what should be taken from the thread is:

scavenging does exist...
poorly designed exhaust systems will kill it and it's benefits. Practically, if guys have shown that 'x' size works wonderfully... use it and don't go crazy unless you have some kind of reason to.

(*edit* where I had low wasn't as clear in paragraph. Trying to explain it as it goes through stroke... easier with a graph vs. talk mind you)

<small>[ October 27, 2004, 12:23 AM: Message edited by: What ]</small>

[manowar821]

I totally understand, but why are open headers so great?
Assuming you don't have to worry about sound level laws, like on a drag strip or something I meen.

I do like the explaination though. Thanks a lot, some people needed it.

[Gro Harlem]

unless your car self-adjusts its fuel delivery, you need SOME backpressure. Our cars do not have dynamic fuel delivery, the pressure is pretty much constant via the vaccuum operated soleniod and FPR.

Some debate that having zero backpressure will burn up your exhaust valves as well since they would be cooling much faster than if they did have backpressure (make sense?). But this is a debate I've read time and time again and I've yet to see anyone prove it or disprove it.

[BuGS]

The only engine i know that does better with free-flow exhaust (N/A) are rotary engines. My friend took is exhaust totally off and man did it sound kewl. It was also running faster. It was lucky he had already had 2 engine fires or he probably would have has a third. His wheel cover was glowing red

[LongJohn]

I got into this thread late. wytbishop and What hit the nail on the head with scavenging.

I think scavenging, and the tuning around the impulses in the airflow is also what makes a VICS useful since it doesn't really add to the total airflow.

Actually it would be the reverse of scavenging since the goal is to increase the pressure at the intake valves where scavenging reduces the pressure at the exhaust valves... What is this called at the intake side?