Duel downpipe .. a work of art!!

[Speedworks]

No problem Marco, me neither. The 7 community is small but we shpould help eachother.


I work in automotive accessories industry on a european level and I\'m quite familiar with high tech developments as we base our developments on those discoveries.


Just very simple...Hot air is dense so when it exits the exhaust manifold itw ill need to pas throught the exhaust pipe. Which is, due to its size, a restriction.
Exhaust gas speed is high because the hot air wants to escape and the engine is pushing it out. restriction will slow it down.

if you change the midpipe and catback with a 90 or 110mm catback you wil create so much space in the tubing that the gasses will lose speed and start floating around.

Just start your car with an OEM catback. You\'ll see fumes coming out with speedhold your hand at the tip and you\'ll feel pressure. Do this with a 110mm catback and you\'ll feel just a little wind passing.

When gasses stand still at the back of the car, they will slow down gasses more to the engine and create back pressure as well.

so to keep optimal speed you should do something like 80mm downpipe going to a 70mm midpipe and catback. preferably with a 70 mm tip as well

this will keep speed in the exhaust gasses and stil create a little backpressure to let the engine breath in some exhaust gasses for recirculation (like valve overlap with high performance piston engines).

Even more important is the pulse of the engine that it is putting out. The rotary has more pulses (combustions) per rotation, so exhaust gas speed is even more important. A pulse works best if you put it in a continious environment . So for the pulse you should take a 70mm pipe front to back.

Putting in the 80mm front pipe is a little compensation for the hot gasses to expand, but the rest must be a continious environment to ensure optimal flow.

So exhaust gasses are pulse driven by the engine. by using different diameters (76mm DP, 70mm midpipe and 100mm catback) you create an unstable pulse and variabnle exhaust speed which will make the gasses do odd things you could never imagine (ever seen a 3D simulation of that?)
Gasses being sucked in again and such...


for a oem turbo\'s or a small single I would recommend a 76 or 80 mm DP and a a full 70mm midpipe and catback. Free air after the exhaust tip should the creat your venturi effect.

So all those big ass, loud and over the edge catbacks with 110mm tips and 100mm tubing... hell no...Look at RE Amemiya and their dolphin tail tips, no big bores there...

it\'s all about size... but the biggest is not always the best :D/D

[marco]

thanks a lot for the post, it gave me a lot to think about! esp about the pressure pulses, i had always assumed that because of the turbine blades the pressure pulses would be really high frequency (i.e. the speed of the turbine rotation) but on further thought i guess you would get the base pressure variation due to the engine with the effects of the turbo overlaid onto that?!

i am still not with you on the speed issue though, surely this is to do with mass flow? assuming that density remains constant (so ignoring the effects of cooling) and assuming the exhaust gas isnt approaching mach 1, then we can assume that the volumetric flow is proportional to the mass flow. so in simple terms we have a set volume of gas to move each second. as we pass this set volume through different diameter pipes the speed will vary accordingly- if you have a pipe with a cross sectional area of 1m^2, and we need to move 1 cubic meter of gas per second, then the flow velocity is 1m/s, if the cross sectional area is halved then the velocity must be doubled to make up for it. however every time you change the diameter (going smaller to larger or vice versa) unfortunately the energy transfer isn\'t 100% and so you lose energy. if you lose too much energy the flow stagnates and just sits there. obviously in an exhaust this isn\'t going to happen and it will be forced along, but the turbo will see this as greater back pressure. (the turbo sees the work required to move the exhaust gases out as a back pressure). so ideally you want an exhaust with uniform cross section?

similarly if you take into account the viscosity of the exhaust gas, the velocity of the exhaust flow makes an effect- the higher the velocity of the gas the greater the energy loss to skin friction. now i will happily admit that the energy loss due to this is probably small compared to other issues, but still, it points to larger diameter exhausts being the way to go?

thing is, something about what your saying sounds correct, i just cant work out why!

Marco

[dhracer]

Just very simple...Hot air is dense so when it exits the exhaust manifold itw ill need to pas throught the exhaust pipe. Which is, due to its size, a restriction.

Nice post :Thumbs-up however hot air is not as dense as cold air, just thought that should be corrected

[MikeLMR]

looking at it another way

isn\'t it just because of the space restriction, two pipes are flatter than one large pipe, other downpipes are squashed in this area to clear so it could be a simple packaging reason. I doubt most of these products have any great science behind them, it would cost far too much to fully understand what is going on.

[Nik da Greek]

I still think that speed of gasflow is of more relevance than just volume, so long as there\'s enough volume to hold all the expelled gas. I take on board what Marco was saying as sound physics, but at the end of the day, there\'s no point in having a massive amount of space in the downpipe if all it does is let the gas float about doing nothing and going nowhere. I think that\'s kinda what Speedworks was getting at, but my puny brain gave up the ghost a bit! I\'ve now fortified it with some gin, and that seems to help!
 
I admit that most of what I know about performance engines comes from bikes and it doesn\'t necessarily translate, but all the real pro performance bike zorts (Yoshimura, Akrapovic, even Micron) have tapered headers to accelerate the gas, then a fat bit for the capacity to hold enough gas, then a thinner bit where the downpipes meet in a collector for the silencer. I also agree with Mike, in that a lot of manufacturers are doing what looks nice or what the trends dictate without much research into the science of it, and on the road it\'s probably academic, but people like Arrow or Akrapovic sell the same exhausts to trackdayers as they do to World Superbike teams, and believe me, they spend a LOT on R&D. If you\'re charging £700 for the silencer alone in the cutthroat bike market, it\'d better work! If they think you want tapered headers for gas speed, I think they may be right!

[Speedworks]

Race bikes do have a bigger mid section to create backpressure towards a non turbo engine. It creates more low end power for acceleration.

For those who ever drove Honda Camino 50cc bikes, they should know.

You had differnet brands of exhausts back then.

Proma: very big but short mid section-> lots ow low end power, limited top speed.
Simonini: thiner mid section but long and smooth _> less low end power but high speed

[Ricco_04]

If by having different lengths and diameter\'s of exhaust you can tune an engine, why then do drag cars/bikes use large diameters with virtually no length to them?

[dhracer]

If by having different lengths and diameter\'s of exhaust you can tune an engine, why then do drag cars/bikes use large diameters with virtually no length to them?

Just a guess but drag cars/bikes have virtually no noise regulations either :Gigglemost exhausts are a compromise.  Not forgetting that exhaust manufacturers also "tune" their exhausts to make a nice noise

[Ricco_04]

Yes true, but im on about making the power.  Regardless of noise, if they could make more power with a tuned system then surely they would.

As for the downpipes there cant be much peak difference.  Personallly I would go for the more normal single pipe, if for nothing more than because there is less metal.

I was always told that it\'s more critical to have a stepped exhaust because, again, I was told that the gas expands more as it journeys to the exit of the exhaust.  Hense why Apexi (and others) make stepped exhausts (Ie Apexi GT-Spec)

Oh, im not saying im right by the way, im also trying to understand it by asking questions

[Speedworks]

because they run such big turbo\'s any decent size of tubing will not compensate

[RobMatthews]

Ok well i\'ve finally got mine fitted:cool  and it sounds absolutly amazing!:D so quite just taking it easy and when its ticking over its not loud but very basey i suppose is the word!and then when you floor it,-well its just says more more more and its not a hearing killing noise which is exactly what i want:rollin , the car feels a lot more responsive and faster on acceleration to.Will get it on the rollers on sat hopefully and get the power figures and compare them to my last lot a couple of months ago(the exhaust is the only thing changed since then)
Oh and having two pipes running down the length of the car looks great to!:cool