backpressure??

[redefine]

so i hear that for an engine its better to have a certain sized exhaust, and any bigger will cause a loss of "backpressure" and a loss of low end.

however, if you look at how an engine works, wouldnt the least backpressure possible be ideal because the backpressure would only stop exhaust gases from escaping the combustion chamber? in fact, other then weight, wouldnt the only reasons not to have a million inch exhaust be because its excessive for the gains, or that the change in size causes losses?

i get how exhaust pressure would help a turbo setup...but for NA, why is too big bad??

[aaronng]

Backpressure is the incorrect term that people use to describe this. You are right that less backpressure is ideal since there is less resistance to flow. However, when you go too big an exhaust diameter, then you lose exhaust gas velocity. Loss of this velocity means you lose the scavenging effect of the exhaust pulse and also lose the benefits of the tuned length exhaust manifold. All of this means you lose power. So you need to balance the exhaust diameter to the flow of exhaust gas. A stock D series has a smaller ideal exhaust diameter when compared to a stock B or H series engine.

[trism]

The statement that a NA engine needs backpressure is a myth,
The reason that most people arrive at the conclusion that they need back pressure is that the engine feels unresponsive in the lower rpm ranges after they go to an "open system" or delete the mid pipe/ muffler/cat/ resonator.And this is true, the engine will lose some torque in the mid range, but the reason it does this is not strictly to do with air speed (scavenge velocity) though it does have an effect.

A highly tuned engine relies heavily on "overlap scavenging" by the exhaust to get the best from the longer duration cams used.This scavenging occurs due to pressure waves that reflect up and down the exhaust system at varying velocities due to rpm.These pressure waves are created by the high pressure exhaust gas exiting the chambers and through the ports being forced though a change in the exhaust systems cross section. Either the end of a pipe or the restriction of a muffler or cat.

In a road car the pressure wave is reflected by the cat, in a race car it is the open end of the pipe.So by simply removing the cat what you are doing is dramatically lengthening the point at which the exhaust negative pressure wave is reflected back to the cylinders.It is this pressure wave that helps to pull the intake flow into the cylinder while BOTH intake and exhaust valves are open (overlap).So the engine see's a rather dramatic loss in pressure drop through the intake ports(suction).That is why the engine loses some torque in the mid range. Eventually as rpm rises the window of overlap decreases in relevance and the torque loss is recovered.

At this stage I know little of the Honda engines tuning requirements, though an engine is only an air pump so they are all similar. However, I would think that the goal of most would be to maximize power upwards of 5000rpm, if this is the case, it is all about minimzing restrictions, whilst being midful of the intake/ cam/exhaust tuning.
For the street... high flow cat is the go.
For the track.... play with different reducers/expanders and varying cross sections of pipe, the fun never ends!!!

[D R U]

Not as technical as the posts above, but think of it as blowing through a straw..

If the straws too small, its restrictive, and pressure is built up in your mouth as you force the air through..

If the straw is too big, the air is not restriced, but there's no pressure, so the air doesnt flow out, its needs to be forced out, requiring effort..

If the right size straw is used, pressure can be maintained, and the air flows out evenly and constantly, with less effort than either of big straw or the little straw..

[bennjamin]

consider this - an infinitum exhaust (ie nothing past the header collector) is almost total loss of throttle response and entire power band. no low mid top just fart noise.
But place an exhaust system on (any) and you will instantly gain.

[JasonGilholme]

I ran my car with just headers and i felt it performed alot better then when i put a cat back system on there.

Velocity is maintained up until it leaves the exhaust system anyway, so who cares if it leaves the exhaust system just after the header?? Why do you think race cars dump as soon as they can?? only reason exhaust "systems" were created was to reduce/stop noise.

When i ran my car with just headers it sounded terrible on idle, but sounded great at 8500. (to me anyway, i'm sure the neighbours didn't like it)

[RandyRhydah]

thanks for the info guys.. i always hear people talking about back pressure, but as soon as i mention race cars running very little exhaust and even the piston aeroplanes i fly have about a 20 cm pipe dumping off the turbo and thats it, they cant give me a reason... now i have some knowledge go to with my arguments :) good thread

[curtis265]

Wow that was an awesome read.

Time for some fluid mechanics revision, lol

Bernoulli time!
P + 1/2pV^2 + pgz = constant along streamline.

where P=pressure, p=density of fluid, V=velocity, z= height above ground

Neglecting the change in height,
we have P + 1/2pV^2 = a constant along the pipe.

As you can see, an increase in velocity would lead to a decrease in pressure, in order for bernoulli's equation to remain constant... If we want to keep pressure low, (and stop the engine from having to 'push' air out, then we need to increase the velocity, which can be done by adjusting the exhaust diameter.

Mass conservation:
Mass flowrate, M = pUA
where p= density, U = flow velocity, A = pipe diameter

From that, we want to optimise the mass flowrate which can be done by adjusting the pipe diameter.

Of course, the optimum would only be for a certain rev range, which is why mroe expensive cars have the exhausts that open and close pipes to create a flatter torque band

[aaronng]

Mass conservation:
Mass flowrate, M = pUA
where p= density, U = flow velocity, A = pipe diameter

The constraining factor will be M. M is limited by the amount of exhaust gas produced by the engine. Hence when you increase pipe diameter A until the point at which you reach the upper limit of M, then flow velocity U starts to decrease. When you reach the point where flow velocity starts to decrease, then as per Bernoulli's equation the pressure P will increase to maintain the constant. This is the point at which an exhaust that is too large will decrease your power/torque.

[trism]

another thing to think about is that as the exhaust gas cools down it loses velocity, so you want to keep as much heat in the exhaust as possible. I dont like the idea of heat wrapping, however, coatings such as jet-hot coatings on the inside and outside of headers/collectors keeps the heat in the exhaust, and keeps gas velocity up

[JohnL]

My understanding is that back-pressure is of itself never a good thing for helping to get the spent gasses out of the engine (which makes more room for fresh mixture and thus more power), but really helps to keep the exhaust quiet...

Shortening the pipe will tend to have the effect of moving the power band higher up the rpm range (because it decreases the time it takes for positive and negative pressure waves to pass up and down the pipe). So in some cases the theoretical power band may have been moved so far up the rpm range that the engine can't actually reach the sweet spot of the short exhaust, i.e. the engine just can't rev hard enough to take advantage of the characteristics of the short pipe, this being due to flow restrictions on the induction side.

This would give the impression that a short pipe kills power across the entire rpm range, when in reality it just moves the power band out of reach...

[redefine]

wow. nice replies guys. i knew there was a scavenging effect coming from the length of the headers, but i didnt realise that the same happened from reflections from the end of the exhaust/cat.


trism: wouldnt cooling of the exhaust gasses reduce the pressure anyway? thanks to the ideal gas law PV=mRT

[aaronng]

trism: wouldnt cooling of the exhaust gasses reduce the pressure anyway? thanks to the ideal gas law PV=mRT
In Bernoulli's equation, V is squared. So the pressure is reduced slightly, but the effect is much worse on the velocity (by a power of 2) and in turn increases the pressure. So the effect of cooling to reduce pressure is overcome by the effect of increasing pressure from slower velocity. Hence the advantage of keeping the exhaust hot.

[redefine]

thats right...forgot about the v^2. thanks

[curtis265]

Aaronng how on earth do you know everyting about everytihng? What's your day job might i ask?

[aaronng]

Aaronng how on earth do you know everyting about everytihng? What's your day job might i ask?

I'm a chemical engineer. Bernoulli's equation was drilled into my head when I was 16 years old....

[curtis265]

16 :O Wouldn't that have been year 11? that's pretty fricken nerdy bro! Well done though.

I've just learned it last year... and once again in second year fluid mechanics. I'm a civil engineer.

[aaronng]

16 :O Wouldn't that have been year 11? that's pretty fricken nerdy bro! Well done though.

I've just learned it last year... and once again in second year fluid mechanics. I'm a civil engineer.

Where I come from, 16 is year 10. Was forced to as it was the education syllabus, so couldn't escape lol.

Good thing though, since it made uni much easier. ehhehe.

[saikou27]

this would be a good sticky. im sick of reading this back pressure is good nonsense

[curtis265]

I absolutely love finding somemething interesting and somewhat useful to apply uni course material to.