well ive read many people argue both ways that gasoline engines require or dont require back pressure.

now does anyone have any dyno runs to display engines w or without exhaust manafolds

 

It is not about needing back pressure, but needing resonance tuning of exhaust pressure waves. BTW, removing exhaust manifolds will probably show a loss of torque/power. Problem is confusing back pressure affects with resonance tuning effects.

 

Exactly Arnie.

Let the myth die.

A proper tuned length and diameter pipe with no back pressure
is what you want.

Take the same pipe and stick a catalytic converter on it...you
have back pressure and loss of power.

Change the length of the pipe:

short - high RPM

long - low RPM

If you understand audio, you can relate wave tuning ported sub boxes. Low frequencies are accentuated with long tuned ports, etc.

Matching pressure waves at RPM with exhaust length is critical for
power (similar to sound waves for audio tuning for a speaker).

YOu'll need some sort of manifold/exhuast runner to make power.

I wont even attempt to explain how that works, but if you were
to remove your exhaust system totally, you could not create
velocity and low pressure to help suck exhaust gas out of the
chambers and help scavenge new charge.

I wish I was an engineer

 

well the reason i ask is because the other day i was thinking about superchargers and turbos... and i was wondering why someone hadnt thought of a "charger" type thing that sucked the exhaust out.

 

Backpressure is defined as: a positive pressure that opposes exhaust gas flow. From that definition, it should be apparent that backpressure hurts output regardless of rpm. Further, backpressure is cumulative with increasing rpm, which is probably where the myth of backpressure's benefits got started. Since backpressure will have more effect as rpm's increase, it might be mistakenly taken that backpressure increases low rpm output. I have yet to see a dyno test where a decrease in backpressure hurt output at any rpm, but of course, while any decrease should net gains throughout the operating range, the cumulative effect means greater gains are likely as rpms increase.

Both the tuned-port in a speaker box and and an exhaust pipe have a frequency at which optimum flow(resonance) occurs. This is true of any pipe in fact, even square pipe. The volume of the pipe (cross-section x length) determines the resonant frequency.

 

For a speaker, we want to 'tune' the pipe to boost the air which is moved at a particular frequency. In this application we are only concerned with flow in one direction - away from the enclosure. Pick up a speaker design book from Radio Shack for the formulas to design these ports (and to design the enclosure needed to make the port effective). Basically, a larger cross-section or a longer pipe (both increase volume) will lower the resonant frequency of the pipe,... and vice-versa.

 

In an exhaust header we are concerned with optimized flow in both directions. We want to move the exhaust away from the engine as efficiently as possible, noting that the resonant frequency of the pipe will determine at which rpm optimum flow occurs. Additionally, we want to 'time' the scavenging pulse to arrive at an adjacent cylinder at a particular rpm. This is very important, because if the scavenging pulse enters the cylinder just before the intake valve opens, as it collapses and reverses it's direction away the cylinder it will exert a 'pulling' force on the incoming air (by now the intake valve is opening) which is up to 25 times greater than the downward movement of the cylinder.

So a properly designed exhaust header not only helps to move exhaust gases away from the cylinder but with the aid of the scavenging pulse it helps to pull the incoming air into the cylinder.

 

Not to make this an Audio thread, but...

How can you take something away and not replace it?

The air must be able to move back into the enclosure to fill the void, correct?

Just as with the reflections in the exhaust pipe, the pressure removed from the chamber must be replaced with something (hence
scavenging)
otherwise there would be a true vacuum.

The rate at which the pressure drops in the exhaust promoting flow will determine how much intake charge enters the cylinder...

True?

 

Kind of a funny anecdote:

In '99 I ordered (online) a set of SLP thirdgen headers. After I placed the order, I got a call from SLP. The man identified himself as John and said he was an engineer in the R&D department, and he was calling to check that I was ordering the right parts for my application. After confirming my choices, I decided to quiz John as to why the pipes were so mismatched. The longest pipes, by my calc's were too short to boost peak torque and the shortest pipes would provide a boost well above the peak hp rpm. John replied that pipe length differences of less than 12" were inconsequential. Now having learned a bit about exhaust gas flow, I knew this explanation was bunk. But I also knew that arguing with someone who has a degree would be futile. Especially arguing over the phone with someone who has a degree. I thanked John for calling and moved on.

Six month's later, SLP's 2000 catalog arrives proclaiming the virtues of their 'tuned-length' LS-1 headers. Now instead of advertising up to 15 hp gains (a la their thirdgen headers), they were touting up to 44 hp gains. Seeing the two types of headers side-by-side tells you why the LS-1 headers are so much more effective.

 

I didn't mean to imply that air only flows out of a tuned-port speaker enclosure, but rather that we are only concerned with outward flow which is tuned to produce an increase in sound-pressure at or near a target frequency. The air flowing back into the box is of less consequence as it does not have enough pressure to cause a significant increase in sound pressure. Further, the air moving into the enclosure is dampened by air already in the enclosure and, by the woofer cone moving forward to neutral or it's next excursion.

 

ok i was pretty sure that back pressure was a bad thing (nitrus on the other hand likes the compression does it not)

anywho back to my second question why doesnt anyone ever attach a belt or fan to the exhaust to force it out? seems like this would aid the upper end torque.

 

Keep in mind, that as the exhaust gases leave the cylinder they move past the exhaust valve at the speed of sound. That is why headers are noisier than manifolds: thin-wall steel tube doesn't dampen the sonic-boom as well as heavy-wall cast iron. You'd have to locate your evacuation system downstream of the collector, so that the scavenging pulse that trails the exhaust gas plug, could enter the collector, reverse and travel up an adjacent pipe. You don't really want to do anything that lessens the effectiveness of the scavenging pulse. So by the time you find a suitable place to locate this evacuation system, there would be little gain from it's reduction of backpressure vs a well-designed exhaust system.

Then there's the question of cost, complexity and reliability. Any time you add moving parts to a system, cost and complexity rise; and reliability falls.

 

By the time the exhaust gasses get to the tail pipe or even header outlet they are down to a couple hundred ft/sec. One often overlooked thing to do is place the outlets in an area of low pressure created by vehicle movement, or create one. At least don't put them in a high pressure area!

 

"Compression" is a totally different topic than "back pressure". Nitrous needs a large, efficient exhaust system, to handle the volume of gas produced by the extra combustion products and the high combustion temperatures.
There is already a huge positive displacement pump moving the exhaust gasses.... its called "the piston".

 

^ In addition to that, would air move better as "pushed" rather
than "pulled"?

The intake stroke has a difficult time filling total volume. Intake
valves are bigger to aid VE%

Taking a fan for instance and reversing the motor...then sitting
in front of it...I don't think you'd get the same effect, even if you
were sitting in a tube.