Exhausts , GTI - Lack of.

[LINCOLNSHIRESLUPOGTI]

if someone just made one that looked just like the standard one (well maybe with slightly bigger twin pipes and in a better finnish) with a little more noise and a bit of a performance gain itd be a big seller

I have twin 3.5inch centre that fits my standard bumper just right. It polished stainless steel and quite loud. Not ****ty noise though, it rumbles at idle and is deep until 6500rpm than it turns abit higher pitched.

Not sure on power loss/increase to be fair but car does feel better with it on.

Quality is very good, its been on 20000miles trouble free and was done by Exhausts UK in Sheffield.

[Jabo]

QUOTE(Ryn)

The added back pressure and fewer restrictions in the pipe work can only be a good thing.

I am not intending to have a go you Ryn but can you explain to me (or anybody else for that matter) why 'added' back pressure helps?

P.S. Isn't it contradictive 'added back pressure and fewer restrictions'? To get any presure in any part of a exhaust system there must be restrictions to create it. The lest restrictive possible exhaust is each exhaust valve discharging directly into atmosphere, where' the pressure in such a system?

Dragsters have no air filtration whatsoever and ehxaust systems (top fuel) to get max power with principle behind it being NO RESTRICTIONS to gases flow. Internal combustion engine is one smelly air augumented air pump and we all know that any pump will hve the best volumetric efficiency with the least level of restrictivness on inake and discharge.

PLEASE if I am wrong here tell me since I'd hate to go on thinking about the above as being right if it's not...

[Crusoe]

Jabo, you'd be right about the top fuel dragsters but they don't run a TDi engine like in your car and a straight to air outlet from your engine would loose you a lot of power.

For a turbo engine exhausts for dummys class 101 style="vertical-align:middle" emoid=";)" border="0" alt="wink.gif" />

in designing the exhaust you've got quite a few variables to consider, reduction of back pressure is a factor but you also have to consider the controlled management of exhaust gas boundary layer, tyring to avoid exhaust diameter changes/steps and minimizing of exhaust gas turbulence and friction.

In a perfect world (that is, one without the sound police, emissions, routing of the pipes and other regulations), an exhaust system for a turbocharged engine would be a short, `horn-shaped` exhaust pipe that extended outward from turbine discharge. Think of a horn speaker or a megaphone, that's the shape that will make maximum horsepower, not a straight pipe!

The ideal turbo exhaust system would smoothly taper in the other direction- from smaller diameter (the same size of the turbine discharge outlet) to larger diameter over a short pipe length. The length of the exhaust system would be short enough to minimize exhaust gas friction while long enough to incorporate a suitable `horn-shape` with a diameter `rate-of`-change aggressive enough to minimize exhaust `back-pressure` while gradual enough to keep from inducing boundary layer disruption (you want it to expand lowering the pressure but also to stay laminar and not detatch form the surface where the seperation would cause the flow to become turbulent increasing pressure, energy dissipation and adding friction)

here endeth the lesson style="vertical-align:middle" emoid=";)" border="0" alt="wink.gif" />

[Crusoe]

I'll add a P.S. for the NA guys.

Normally aspirated engines as most of you should know, use exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here style="vertical-align:middle" emoid=":)" border="0" alt="smile.gif" />

[Tigz™]

So how do you find the perfect exhaust size?

Trail and error, or is there a way of working it out?

[tdwindustries]

i read somewhere that it cant be worked out, but then again that would seem silly for F1 / WRC who must spend a fair bit on research

[Snoopy]

Can't be worked out isn't the same as can't be optimised.

[tdwindustries]

but the question was "can it be worked out"

i am in a bad mood today. style="vertical-align:middle" emoid=":(" border="0" alt="sad.gif" />

[Snoopy]

I know, but what I meant is, just because it can't be worked out, doesn't mean that F1/WRC teams haven't got it pretty much perfect by means of trial and error.

[Crusoe]

Mostly down to experience i think though there are obvously things that you'd try and constraints with packaging that would limit the options. If you measure the emmisions you can see how clean the burn of the fuel has been and you can measure the exit velocity and see how much more free the exhaust is flowing. On a N/A car the usual standard fit exhaust will have welds and rough joins inside with some size changes at the manifolds etc. The more expensive exhausts use more expensive methods which reduce the problems, avoiding `crush-bent` tubes (use mandrel bends); avoid `tight-radius` turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are `non-perpendicular`); and there are lots of gains from a high flow sports cat.

For a turbo car i's a bit different there are some equations you can run easily and genreally the larger the pipe the better (hence the drain pipes on scoobys and skylines etc.)

simplified example for you to show how larger exhaust would help a turbo engine:

Say you have a turbo at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

So here, the turbine contributed 19.6 psig of backpressure to the total.

Now you max power it with a drain pipe to give a much lower backpressure. Same turbo, same boost, etc. You measure maybe 3 psig backpressure at the turbine discharge. In this case the engine then sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig

So in the end this engine saw a reduction in backpressure of 12.6 psig when you swapped exhausts. This reduction in backpressure is where all the engine's power gains would come from.

[Jabo]

Thanks Crusoe, great info, we should make a sticky out of these bits scattered around for futurre reference.

So, in our `non-perfect` world (TDi engine) the best solution would be to have the shrtest pipe possible (I was mad enough to consider sth like you desribed i.e. a short pipe with incresing diameter directly from turbo's waste discarging straight under the car but I suppose heat sheilds, noise and the like would make a real pig to do...?).

So, let's say turbo discharge gate diameter is ~2" (guessing here, never measured nor got hold of VNT15 specs yet) then get tube loudspeaker piece of appropriate lenght (how long would that be and how to determine this in practice I dunno yet style="vertical-align:middle" emoid=";)" border="0" alt="wink.gif" /> ) ending at 5" diameter (what's the biggest possible to fit pipe diameter can go under 1.4TDi? ) and discharging at the back or to the side.

Would that make a practical sense?

Crusoe, what you said about scavenging (I assume you mean a lower pressure created temporarily in coletor(s) of cylinder(s) which are in suction/combustion state) of exhaust gasses should also apply to `pre-turbo` part of TDi exhaust system, shouldn't it? I suppose that's why it is so important to choose right turbine for your turbo charger to a particular engine style="vertical-align:middle" emoid=":)" border="0" alt="smile.gif" />

About calculating, I could have done right now is that was for liquids style="vertical-align:middle" emoid=":lol:" border="0" alt="laugh.gif" /> style="vertical-align:middle" emoid=";)" border="0" alt="wink.gif" />

I do not feel comfy nuff with gaseous fluids to attempt any of this yet style="vertical-align:middle" emoid=":(" border="0" alt="sad.gif" />

All I know by now is that each collector pipe should be of exactly the same lenght and volume and as 'sweeping' as possible to avoid disruption of laminar flow (exhuast gases extraction for each cylinder is at the same pressure creating identical environment in each of the cylinders).

[Crusoe]

one question at a time style="vertical-align:middle" emoid=":)" border="0" alt="smile.gif" /> your right about the header pipes being the same length but in practice it's not very possible in a small engine bay with price constraints. if you use equal length turbo headers it will time the arrival of the pulses at the turbine equally and keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE (volumetric efficiency).

If you didn't have a wastegate to worry about which should be okay for your Tdi. So the optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter (as large as possible maybe 5" as short as possible so front exit) via a straight conical diffuser of `7-12`° angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Though you'll probably find that the new Tdi engines have the diffuser section cast right into the turbine housing (thougg i haven't looked on mine). A hyperbolic increase in diameter (like a trumpet snorkus (what a cool word lol) is theoretically ideal but you never see them and the improvement would probably be negligable over a straight cone.

[Crusoe]

my sencible posts are under rated lol, i just needed an excuse to use the word snorkus style="vertical-align:middle" emoid=":D" border="0" alt="biggrin.gif" />

[Tigz™]

Can this info be put in the "Info" section of the site please?

At the moment in time my degree in turbine engineering hasnt arrived yet, but Im surfe one day soon I'll understand at lease one part of it!

Great thread Crusoe.

[Jabo]

QUOTE(Crusoe)

my sencible posts are under rated lol, i just needed an excuse to use the word snorkus style="vertical-align:middle" emoid=":D" border="0" alt="biggrin.gif" />

style="vertical-align:middle" emoid=":lol:" border="0" alt="laugh.gif" /> Maybe I'll call my Loop 'The Snorkus' ?

8)

I think with hyperbolic geometry of the transition piece any gains are only quantifiable at much higher flow rates (jet propulsion etc). Imagine all the money spend on top notch CAD/CAM to manufacture one style="vertical-align:middle" emoid=":D" border="0" alt="biggrin.gif" />

Here's the picture with dimensions for your perverse pleasure

[DanGTI]

oh no....another post gone down the open university route