Any ideas on a decent exhaust backbox?

Malt_Vinegar · 18 Sep 2006 at 15:04

MrSix said:
Not that I think it's "good", but is there any car that can't be made to look "better" or "acceptable" by simply adding wheel spacers, dished wheels, bolt on arches and some smoothing?

Im willing to bet my danglies to a Barndance that its had more than that

Hate said:
ram air kind of works at like 90mph, the 1l engine would have given up by then thou

Yer, he needs a couple of delta screamers wedged in the pipe

lol

Malt_Vinegar · 18 Sep 2006 at 15:05

Double!!
Post!!

MrSix · 18 Sep 2006 at 15:05

Malt_Vinegar said:
That setup sound pretty passive to me

I would go for Re-routed Induction, perhaps Ducted Induction??

Its definatly not forced!!

"Forced induction is a term used to describe internal combustion engines that are not naturally aspirated. Instead, a gas compressor is added to the air intake, thereby increasing the quantity of air or fuel-air mixture available for combustion. "

Fair one.

I referred to it "forced" because it's no longer as restrictive as the standard ducting and the air is "forced" up the ducting as you gain speed.

However I shall now call it "re-ducted-and-routed-slightly-different-positioned-less-restricted-and-better-flow induction".

penski · 18 Sep 2006 at 15:13

Hate said:
ram air kind of works at like 90mph, the 1l engine would have given up by then thou

WRONG.

The Ram Air Myth by Dave Rodabaugh said:
The Ram Air Myth is the most mythical of them all. It differs from the other myths, in that the other myths are misinterpretations of physical phenomena, whereas ram air simply does not exist.

MYTH: Use of a scoop on the front of the vehicle to collect intake air, or provide “ram air” can raise engine performance.

TRUTH: At automobile velocities, there is no ram air effect.

SIMPLE EXPLANATION

The "Truth" statement says it all. How much simpler can it be? The Ram Air effect is a total myth because it simply does not exist. “But Pontiac uses it on the Trans Am, and they know more than you do.” To those who offer this, tsk tsk. Careful reading of Pontiac’s statements on the matter reveal that the HP increase of the WS6 package are a result of a less restrictive intake, and a freer-flowing exhaust, NOT any ram air effect.

So why does Pontiac use Ram Air? Easy! To make people buy their cars! And they are quite effective with this strategy.

DEEPER EXPLANATION

Of all of the applied sciences, fluid mechanics is among the most difficult for many people to comprehend. It is a relatively youthful applied science as well, meaning that it has not had two or three centuries of work to mature into an applied science on par, with say, chemical combustion. To make matters worse, it is mathematically defined almost entirely by experimentally-determined mathematics.

This last point is the true differentiator between those who only understand concepts, and those who can quantify what they are discussing. Truly, quantification is the real skill of the engineer. It is one thing to speak about qualitative issues (the “what” of the physical sciences); it is entirely another to quantify them (the “how much” and “to what extent” of the same). In grade school, students are first taught about “closed form mathematics” and then that these mathematics are typical of scientific expression. A good example of this is Newton’s famed “law of action and reaction”, the mathematical expression of which is a succinct F=MA. So straightforward. So simple. Three variables in perfectly-defined harmony. Given any two of them, the third is easy to nail down.

Unfortunately, a vast, vast majority of the mathematics used in engineering are NOT closed form. Instead, they are multi-variable correlations valid only for a narrow set of circumstances. Deviate from those narrow circumstances, and a new expression must be experimentally derived. Fluid mechanics is almost entirely defined by these experimentally-determined expressions, further muddying an applied science not well understood.

And if there ever were an applied science for which common sense is wholly inappropriate, it is fluid mechanics. Virtually nothing obeys the “common sense” rules of observation, explaining why those who believe in ram air have extreme difficulty in believing that is simply does not exist.

The Deeper Explanation begins with a basic explanation of engine principles. Air and fuel must be combusted at a specific ratio, namely, 14.7 parts air to 1 part fuel (this is a chemical ratio). Stuffing more fuel into the cylinders without increasing the amount of air they also swallow will get no gain whatsoever. So the hot rodder’s adage “more air = more power” is proven correct. Figure out a way to stuff more air into the cylinder at any given RPM and throttle setting, and you can burn more fuel. Since burning fuel is what makes power, more air truly does create more power.

The amount of air which is inducted into a cylinder is a function of the air’s density. As the air flows through the intake tract, it loses pressure, and as the pressure decreases, so does the air’s density. (Denisty is mass divided by volume. Since cylinders are a fixed volume, increasing the density will also increase the mass of the air in the cylinder.) There are two ways to increase the pressure and density of the air inducted into the cylinders:

- Decrease the pressure drop from the throttle plate to the cylinders

- Increase the starting pressure at the throttle plate.

Ram air is an attempt to do the second. Under normal circumstances, the air at the throttle plate is at atmospheric pressure, and this pressure drops until the air reaches the cylinders. Ram air would start the process at some pressure higher than atmospheric, and even though the drop is the same, the cylinder pressure is higher because of the increase at the start.

Just how would this increase in pressure at the throttle plate occur? The oft-wrong “common sense” says, “If a scoop is placed in the airstream flowing around the vehicle, the velocity of the air ‘rams’ the air into the scoop, thus increasing the pressure.”

Why is this incorrect? There are two types of pressure: static and dynamic. Placing of one’s hand in front of a fan, or out of a moving car’s window, clearly exerts a force on the hand as the air diverts its path to flow around it. Most people would say “See? This is a clear indication that ram air works. Clearly there is pressure from the velocity of the air.” Well, this is correct, but only to a point. This is an example of dynamic pressure, or the force any moving fluid exerts upon obstacles in its path as the gas is diverted around the obstacle.

What an engine needs is static pressure. This is the pressure the same fluid exerts on any vessel containing it at rest. For those who were physics/chemistry geeks, it is the pressure caused by the force of the molecules bouncing off of the walls of the container. The key to understanding the difference between static and dynamic pressure lies in the velocity of the gas. Dynamic pressure is only a momentum effect due to the bulk motion of the fluid around an obstacle. Static pressure is an intrinsic property of a gas or fluid just because the molecules of the fluid are moving around. Any fluid which is moving can have BOTH dynamic and static pressure, but a fluid at rest only has static pressure.

The point of ram air would be to increase the static pressure, which would correspond to an increase in the in-cylinder air density, and of course, more air. Superchargers and turbochargers do what the mythical ram air purports to do. A supercharger trades the power of the belt and uses it to compress the air in the intake tract. This energy trade-off results in an increase in intake air pressure, more air in the cylinders, more fuel burned, and more power. A turbocharger trades the power of the hot gases and uses it to compress the air in the intake. The overall effect is the same – an increase in intake static pressure.

For ram air to work, it would have to trade the energy of the air’s velocity (as the vehicle moves through the air) for an increase in static pressure (since static pressure is a part of a gas’s internal energy, we see this is TRULY a trade in kinetic energy for an increase in internal energy). Now for the true reasons why ram air is a myth:

- The way for air velocity to be traded for an increase in static pressure is to actually SLOW IT DOWN in a nozzle of some sort. This is easily the MOST counterintuitive part of fluid mechanics for most people. The “common sense” mind says “In order to increase the pressure of the intake, the velocity of the air needs to be increased, just as increasing the speed of a fan exerts more force upon the hand.” Not only does this confuse dynamic with static pressure, but is also misses the point, which is to trade the kinetic energy of the gas for an increase in internal energy. How can this trade occur if the kinetic energy of the gas is increased? It cannot, and in fact, the only way to trade it is to use the velocity of the gas to compress itself – by slowing it down.

- Below about Mach 0.5 (or about half the speed of sound), air is considered “incompressible”. That is, even if the correct nozzle is selected, and the air is slowed down (the official term is “stagnated”) there will be zero trade. No kinetic energy will be traded in as work capable of compressing the air. The reasons for this are not discussed here; the reader may consult any reputable fluid mechanics textbook for confirmation of this fact. In plain English, a car is just too slow for ram air to work.

Still not enough evidence? Here is a little test. For ram air to work, the nozzle must be of a specific shape. The “Holley Scoop” for the Fiero is the wrong shape, by the way. The fact that it has no net shape at all immediately means it cannot effect any kind of energy trade off, so it cannot possibly create ram air. This is also true for the hood scoops on the Pontiac Firebird WS6 package as well, by the way.

What shape must it be? There are two kinds of nozzles. Pick one:

- Converging. This nozzle gets smaller as the air flows through it. It has a smaller exit than entrance. If the nozzle were a cone, the fat end is where the air would enter, and the narrow end is where it would exit.

- Diverging. This nozzle is opposite the other; it gets bigger as the air flows through it. With a larger exit than entrance, the narrow end of the cone is where the air would enter, and the fat end is where it would exit.

So, which is it?

Without hesitation, most of the “common sense” crowd will answer “Converging.” BZZZZT! Thank you for playing anyway! We have some lovely parting gifts for you! Bill, tell ‘em what they’ve won….

The answer is “divergent”. Yes, the nozzle would have to shaped so that the skinny end is pointed into the air stream, and the fat end connects to the throttle plate. How can this be right? Remember, to increase the static pressure of the intake air (which is the true “ram air” effect), the kinetic energy of the air must be traded to compress the air. This is done by slowing the air down, or stagnating it, and the only way to do this is with a diverging nozzle. Ah, but since air is incompressible at automobile speeds, it doesn’t matter any way.

Conclusion

Ram air is a myth because it does not exist, for the following reasons:

- Air is incompressible at any automobile speed., meaning that the kinetic energy of the air cannot be used to compress the air and raise the static pressure.

- The “ram air” nozzles commonly employed on automobiles tend to be the wrong shape. A divergent nozzle is required for ram air. Straight-profile scoops cannot provide a ram air effect.

Select one of the two types of intakes, warm air, or cold air. Beyond that its just about looks.

*n

penski · 18 Sep 2006 at 15:14

MrSix said:
I referred to it "forced" because it's no longer as restrictive as the standard ducting and the air is "forced" up the ducting as you gain speed.

Wrong. See my above post

*n

Hate · 18 Sep 2006 at 15:25

penski said:
stuff

copy n paste skillz

Its more that (with muscle cars with chargers stuck on the top) air is actually directed into the intake and will probably have less chance to heat up that leeetle bit.

Its like blowing a deskfan in your face i guess, still the same amount of air.

MrSix · 18 Sep 2006 at 15:28

penski said:
Wrong. See my above post

*n

I don't see how it's wrong even after reading your "I CAN COPY AND PASTE" post.

The standard route involved going at 2 90 degree angles and fighting its way behind the left hand headlight.

The new route involves a smooth curve up to the airbox which has no obstructions.

Tell me "o' wise one" how that isn't less restricted air flow than before.

*6

Hate · 18 Sep 2006 at 15:31

its less restricted, but you can't get a pressure over that of atmospheric

OpenToSuggestions · 18 Sep 2006 at 15:33

try without the top of the air box on and with the cold air feed unpluged, as even with no filter the air box and piping are way to restrictive. just have a quick go down the road without the top of the air box on. you just tried it without the filter that won't do much. only have a go down the road as a filter is there for a reason and won't do engine anygood if you permanently remove it!!, if youve done what is said to try correctly, i garentee you will notice a real roar when you floor it

this is what a air cone should sound like

penski · 18 Sep 2006 at 15:34

Hate said:
copy n paste skillz

If I think something's worth posting, I'll post it

It's an interesting read; puts a good argument across in layman's terms. Unfortunately, as with most useful things I post, nobody will probably read it.

Its more that (with muscle cars with chargers stuck on the top) air is actually directed into the intake and will probably have less chance to heat up that leeetle bit.

See?

As the article says - until the car is going mach 0.5, there is no 'direction' of air into the inlet. Besides, cowl induction is much more efficient.

*n

penski · 18 Sep 2006 at 15:40

MrSix said:
I don't see how it's wrong even after reading your "I CAN COPY AND PASTE" post.

The standard route involved going at 2 90 degree angles and fighting its way behind the left hand headlight.

The new route involves a smooth curve up to the airbox which has no obstructions.

Tell me "o' wise one" how that isn't less restricted air flow than before.

*6

Oh put your handbag down.

The air isn't 'forced' up the ducting. You were wrong. Accept it.

The increase you are seeing is due to COLD AIR INDUCTION.

Not 'forced induction' or 'less restriction'.

You are taking a direct inlet from a flow of cold air. Cold air is denser than warm air, ergo it carries more oxygen for a given volume and it means the engine can use more fuel to make a bigger bang.

I'll need to dig out the theory work but if I recall correctly, a drop of ten degrees results in a corresponding 2% increase.

*n

Malt_Vinegar · 18 Sep 2006 at 16:09

penski said:
factual stuff

I think thats a pretty conclusive reply

alexakasloth · 18 Sep 2006 at 16:34

eidolon said:
people will point and laugh.

and kick you in the shins...

MrSix · 18 Sep 2006 at 17:11

penski said:
Oh put your handbag down.

I understand what your saying and I did admit before that I was wrong (forced air induction post) but I thought you were dissagreeing with my statement about how the airflow was now less restricted.

I respect your knowledge if im being serious - I just like to see you getting passionate.

ScoobyDoo69 · 18 Sep 2006 at 17:15

Violent-J said:
Lo ladies I'm after a nice sounding backbox for my car.

Requirements:
Backbox only, preferably cheap (ish), can't afford full system.
Got to be loud, I want it really loud infact so you can hear it coming from 4 roads away kinda thing.
A feature I would like is for it to 'pop' kind of thing. After my mate let me drive his 1.8 mk2 golf with full system exhaust the other day I'm absolutely intent on getting a loud exhaust, his was lovely it had a really deep sound while under acceleration and then when I took my foot off the pedal completely from a high rpm as the car decelerated and the revs declined, it would kind of "pop pop de pop pop" rumble all the way down. As my mates was a full system I'm not sure if I could get this noise from just a backbox but it's worth asking.

Preferably nothing over 140 quid. I would order it and then get it fitted at a local garage probably.

I've read forums and seen some suggestions to people asking the same question but I really would value some first hand experience from the people of OcUK Motors. Thanks in advance.

So let me get this right... You want a big loud exhaust, popping and banging away, coming from what looks like a bog standard micra? :eek:

good luck with that, atleast I'll be able to recognise you out and about in Birmingham then

hittman · 18 Sep 2006 at 17:34

eidolon said:
People will point and laugh.

I would. lol! Even my granny would. lol

timbob · 18 Sep 2006 at 18:52

Nice post Penski, very interesting read

silversurfer · 18 Sep 2006 at 19:03

ajgoodfellow said:
Good plan but get something like a Clio 172/182 instead of the 206 GTI

I read the latest micra described as handling better then the clio and a few other cars in its class recently, the standard clio anyway.

Ramair f t w btw

ajgoodfellow · 18 Sep 2006 at 19:08

silversurfer said:
I read the latest micra described as handling better then the clio and a few other cars in its class recently, the standard clio anyway.

Ramair f t w btw

Yeah but could you honestly drive something looking like that?

silversurfer · 18 Sep 2006 at 19:20

I wouldnt choose to buy one really but I do drive my mums and the chassis feels ok to me. Suspension is overlong and slack but its lowered on the top model.
I dont like the looks obviously

penski said:
I'll need to dig out the theory work but if I recall correctly, a drop of ten degrees results in a corresponding 2% increase.

*n

Dyno din adjusts for such things, I cant find the forumula though

SAE is the american equalivant

The SAE J1349 relative horsepower calculation shows how air density alters the power output of a properly tuned engine. For example, at 30 deg C, 1020 mb barometer reading, 14 deg C dewpoint and 1520 m altitude, the engine only produces about 81.1% of the rated horsepower.

For the SAE J1349 relative horsepower calculations, the standard reference conditions are: Air temp 77 deg F (25 deg C), 29.235 Inches- Hg (990 mb) actual pressure and 0% relative humidity.

http://wahiduddin.net/calc/calc_hp_dp_metric.htm

Some rough figures show a rough 10% difference between performance on a cold and hot day which would vary a Micras bhp from 74bhp to 81bhp. A CAI wont exceed this imo and so is largely not worth the bother for a Micra, on a 400bhp car however it could mean a possible difference of 41bhp.

Code:

relative power 109.8% 23 Jan  07:45  0.9c   81bhp  439bhp

relative power  99.6%  4 July 16:30 30.8c   74bhp  398bhp

http://www.wunderground.com/weather...EOFG1&day=18&year=2006&month=9&graphspan=year

So these figures show a difference of about 3.4% difference in bhp for every 10c you can drop the intake temp.

For the best power you want a cold day with high pressure which is unusual as they are normally inversely proportional, 10% is an extreme.
The 2%/10c mentioned is probably closer to what happens most days