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Dougal and John know a hell of a lot more about this than I do, so this is pretty rudimentary.
Apparently turbo overspeed can happen, but you really have to push the turbine well outside it's normal operating envelope
From Garrett.
◊ The Choke Line is the right hand boundary of the compressor map. For Garrett maps, the choke line is typically defined by the point where the efficiency drops below 58%. In addition to the rapid drop of compressor efficiency past this point, the turbo speed will also be approaching or exceeding the allowable limit. If your actual or predicted operation is beyond this limit, a larger compressor is necessary.
◊ Turbo Speed Lines are lines of constant turbo speed. Turbo speed for points between these lines can be estimated by interpolation. As turbo speed increases, the pressure ratio increases and/or mass flow increases. As indicated above in the choke line description, the turbo speed lines are very close together at the far right edge of the map. Once a compressor is operating past the choke limit, turbo speed increases very quickly and a turbo over-speed condition is very likely.
Now whether that is possible with a small factory VNT
I'm guessing you'd need to substantially up the fuel map and install a short 3" pipe to see that happen, but I really don't know for certain
I'm with Steve, I reckon the rest of the reasons given against a big pipe are bollocks, except that I don't doubt for a second that a Tdci engine won't see any gains over the factory exhaust unless huge changes are made elsewhere too.
Most modern cars are very good in terms of inlet and exhaust optimisation, you really need to up the HP/torque potential to need a more efficient exhaust.
Anyway, FWIW I built a 3" system for my TDi years ago.
Yes, it's two generations older than the current engine, but it's still a direct injected turbo diesel and the stock system is pretty good. 60mm tube, all mandrel bends, straight through perforated tube main muffler and resonator.
Was it overkill ?
Yes. 2,3/4" would have been overkill too (but much more exxy) and 2.5" wasn't a whole lot bigger than stock. (but in hindsight probably the 'right' size)
I'm an ex-racer and in my mind back pressure was bad for a turbo.
F1's and Champ Cars used a short stub, so i was after minimal back pressure too
The first noticeable thing was a drop in EGT's at maximum fuel/load on my test hill. (stock pump settings)
This was repeatable over a number of weeks, and by the same amount each time, to the tune of 25*.
The second thing was an immediate lengthening of the torque/rev range.
Prior to the new exhaust 3rd gear peaked at around 70km/h, it just wouldn't rev past that. With the new system I could hit the governor in third, although it wasn't pleasant, (don't ask how fast) and 90km/h was and still is pretty easy, although it achieves nothing over changing at 80km/h and using the torque in 4th, it can be handy in some situations.
It'll also achieve an 18.04 second 0-100km/h time with 33.3" Mt's and weighing at close to 3 tonne (had it over a weighbridge a few months back) so not too shabby for an asthmatic Tdi with close to 300,000km on it.
The exhaust build was over 5 years ago and the turbo is still spinning, it hasn't flown to bits yet (touch wood)
Anyway, the point of all that was;
1. the bigger exhaust reduced backpressure and gave an improvement in response and pressure operating range on that engine.
2. the reduced backpressure also resulted in reduced EGT's which means reduced heat in the manifold/turbo housing. This gives more scope for increased fuel=more power within a safe operating range.
The downside ? (other than the aggro of doing it myself)
Too bloody loud.
It's hard to make quiet unless you introduce a chambered muffler. The minimal main muffler and decently sized resonator I have aren't enough.
Originally Posted by roverrescue
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