Let the tinkering continue.....

**Greg Roles** · 31-08-2009, 10:34 PM

Good point, but you can always do a twin section over the axle. The difference for a full system in 3 then 3.5 is stuff all, so why not go nuts? You want as little restriction after a turbo as possible afterall, and there's plenty of room in the MKV, but I'll check the rear part tomorrow when I have a closer look at today's surgery.

I could always put another 2.5 in, and make the twin tips real..hmmm. Would weigh a lot I guess.

**gldgti** · 31-08-2009, 10:41 PM

yeah, i understand - you figure 3-3.5" price difference is negligible (materials wise) but it seems to end with whats practical with our small cars

- unlike a commodore where you can fit whatever you like under it :-P

anyway, just my 2c. if i were you, and could easily do a 3" or 3.5", i reckon i would

**oracle1** · 31-08-2009, 10:52 PM

I read an interesting post in a forum about a guy who changed his exhaust from 2.5 to 3 on a six cylinder turbo diesel cruiser who dynoed before and after, the difference was negligible some 2-3 horsepower with a large increase in the drone in the cabin. The 4.2 compared to 1.9 is more than double in capacity, and most 4x4 are running 2.5 to 3 inch systems, I wonder if any more than 3 is really going to give much gain in horsepower, I surmise it might help with the temps in the exhaust.

**gldgti** · 01-09-2009, 08:27 AM

Originally Posted by oracle1

I surmise it might help with the temps in the exhaust.

possible i suppose - in order to do so, it would have to realise a greater pressure drop on exiting the turbine.... but theres a trade off with velocity there. if it slows down too much, the static pressure increases... might even be harder to move the exhaust out... i dunno. i think a CFD model would sort it out... why didnt i do that for a final year project ???

there ya go - anyone reading this doing 2nd or 3rd year mech eng degree - for your FYP in 4th year, how about doing a "computational fluid dynamics analysis of a the flow efficiencies of a constant mass flow of exhaust gasses through variable cross sectional area pipelines?"

**Greg Roles** · 01-09-2009, 08:51 AM

Lots of variables there.

Firstly, all but the most modern 4x4's put out really pathetic HP. 3L and 4L TD's 4x4's from the 90's pump out figures around 90-120hp at the wheels, from what I've seen on several dyno printouts. You need to remember that until very recently diesel engines were indeed the weezy items everyone considers them to be. With a lot of tweaking you can get them motivated, but the current modern hi-po oilers are a very different beast. Far better flows, design, and big turbo pressure stock, makes for a lot more exhaust flowing out the other side. 3" may indeed be enough, but if VW saw fit to use a 2.5" mandrel system stock, upping the boost, adding other powerups etc is going to require more than the stock exhaust. Even just ditching the DPF may increase the flows enough, time will tell.

Add to that I aim to be putting a lot more into it as well, so it's all part of a cunning plan. More in, more out, the same old formula.

An increase in cross sectional diameter will obviously lower the resonant frequency, and that's where the correct choice of resonator comes in. Most drone occurs in the 50-500Hz range, and given the specifics of the diesel engine, you can't just wack on a big jap or petrol resonator and expect to get it right. Add to that that a lot of resonators are pretty much just mufflers and you see how easy it would be to get wrong.

If and when I go silly, it'll be with a specifically chosen diesel resonator, and even then it'll probably take some experimentation to get it right. Which for me is the fun bit!

I'm still sticking to the basic truth that you can't have a low enough restriction after a turbo, and back pressure doesn't help in any way whatsoever apart from lowering noise.

**gldgti** · 01-09-2009, 11:12 AM

Originally Posted by cogdoc

Lots of variables there.

Firstly, all but the most modern 4x4's put out really pathetic HP. 3L and 4L TD's 4x4's from the 90's pump out figures around 90-120hp at the wheels, from what I've seen on several dyno printouts. You need to remember that until very recently diesel engines were indeed the weezy items everyone considers them to be. With a lot of tweaking you can get them motivated, but the current modern hi-po oilers are a very different beast. Far better flows, design, and big turbo pressure stock, makes for a lot more exhaust flowing out the other side. 3" may indeed be enough, but if VW saw fit to use a 2.5" mandrel system stock, upping the boost, adding other powerups etc is going to require more than the stock exhaust. Even just ditching the DPF may increase the flows enough, time will tell.

Add to that I aim to be putting a lot more into it as well, so it's all part of a cunning plan. More in, more out, the same old formula.

An increase in cross sectional diameter will obviously lower the resonant frequency, and that's where the correct choice of resonator comes in. Most drone occurs in the 50-500Hz range, and given the specifics of the diesel engine, you can't just wack on a big jap or petrol resonator and expect to get it right. Add to that that a lot of resonators are pretty much just mufflers and you see how easy it would be to get wrong.

If and when I go silly, it'll be with a specifically chosen diesel resonator, and even then it'll probably take some experimentation to get it right. Which for me is the fun bit!

I'm still sticking to the basic truth that you can't have a low enough restriction after a turbo, and back pressure doesn't help in any way whatsoever apart from lowering noise.

your dead right ofcourse... (infact i find the term backpressure silly, given that it gives the wrong impression about the physics of what it is attempting to describe - a pressure wave scavenging effect). i think in terms of best flow, you want some kind of smoothly increasing in diameter conical outlet from the turbine exducer bore, out to the final diamter so as not to create sudden accelerations in the fluid media.... kind of like the old annular pipe restrictor vs venturi with same minimum diamter - the venturi can have a flow efficiency of above 90% where the annular restrictor can might only be as good as 60% for the same diameter ratio. obviously, the larger the ratio of small diameter to large, the more difficult it becomes to design the transition for high efficiency. this is what i was getting at befor - essentially the turbine to exhaust pipe transition is the latter half of the annular restrictor or venturi scenario - we're just ignoring the inlet side and concentrating on the outlet side. hence, the larger the difference between the turbine outlet and the exhaust pipe, the harder it is to make that transition flow efficient

phew.

having said all that - its difficult to know just what the ideal transitoin from the turbine outlet is.... and it expensive to get exhaust shops to do work INSIDE the turbine housing behind the turbine wheel

**Greg Roles** · 01-09-2009, 12:47 PM

Hmmm, very good points, and I agree a cone expanding out from the turbo to the final size would be ideal. Conversly, if you could taper the tailpipe down in relation to the cooling of the exhaust gas, I reckon there'd be no need for any mufflers at all. I kinda reckon a two stroke motor bike exhaust, that is expanding out to a volume, then tapering back down is the ideal pipe design, not so radical of course.

Anyhoo, I need to RETRACT MY FORMER STATEMENT ON THE MUFFLER DELETE. It's nicely louder now, especially at 1800-2000rpm, where the turbo gets on song, and at larger throttle efforts. Normal driving and at idle, you'd never know it was different, but on accel, held and decel at about 2500 and especially 1900ish, it's nicely more burbly, and it has a real extractor / induction noise to it now. I like it a lot. With the windows up, the noise kind of balances out the POD inlet roar, and is about as loud as my diesel clatter, in that you can hear it all, but not obtrusively so. With the windows down, it's beautiful, and I'd adore it to be like it is at 2000rpm all over. Makes it sound like a "real" engine!!

To me it now sounds "sporty" at certain rev / throttle ranges, and a full bore blast from standing sounds a bit better now. Probably going to be too loud once the DPF goes!

**Greg Roles** · 01-09-2009, 12:56 PM

Originally Posted by gldgti

on the exhaust size - what rout does the exhaust take with respect to the rear suspension on the mk5? on mk1-4, the rear axel beam kinda restricts the largest you can go to 3" (if your VERY GOOD at making exhausts) and 2.5" for normal people. just wondering if you'll be able to fit 3.5" in there?

P-l-e-n-t-y of room. This is a nice exhaust, Miltek I believe.