turbo-overspeed?

[Pedro_The_Swift]

rather than hijack Aaron's thread on exhaust sizes,,
http://www.aulro.com/afvb/technical-chatter/58809-td5-3-inch-exhaust.html


I copied the relevant bit to here,,,,

"from TD5Alive.com

Why didn't we make a 3" system? A 3 inch bore system would not be appropriate for Td5, since the turbo outlet is only 2 1/4 inches in diameter. Age old (Proven) tuning theory states that the optimal turbo downpipe should be 10% larger in diameter than the turbo casting outlet port. This is precicely why land rover chose 2 1/2", as it's a perfect match to the turbo. Using 2 1/2" pipe maintains a rapid flow of exhaust gasses to get them out quickly! Using 3" pipe would reduce the velocity of exhaust gasses, their fore retaining excess heat at the turbo, and of course the fragile exhaust manifold casting. Another factor is the reduction of back pressure you'd get from a 3" exhaust, as the wide open space of the exhaust takes more gas to fill it, and hence push out the tailpipe. The result could poentially cause turbo overspeed, which can seriously decrease the lifespan of your turbo!"

Just a couple of questions,,,

1. If the larger pipe SLOWS the gas down, how can a lack of pressure and slower gas overspeed the turbo?
2. Why is overspeeding a bad thing?

[Dougal]

A 3" pipe with a nice transition would solve the first problem they're talking about.

As for the second, for a turbo to over-speed, it has to either massively over-boost or try to supply far more air than the makers intended. Given you've got a fixed size (2.5L every 2 revs) pump in the middle, supplying more air isn't possible without huge boost.

But fit a TD5 turbo to a 12L engine and you could overspeed it to death without big boost..

[isuzurover]

What Dougal said.

Sure - a bigger pipe will mean lower gas velocity, however that isn't the key issue. You want to minimise pressure drop through the exhaust, which will increase flow, and decrease any backpressure on the turbo.

[rick130]

A 3" pipe with a nice transition would solve the first problem they're talking about.

<snip>..

Which is what I did on the Tdi system by using 2.5" OD tube and swaged it up to 3" at a 15* angle right after the flange off the turbo dump. I also relived the ID of the dump to match the ID of the 2.5" tube and flange. (the OE ID was approx 58mm)
IIRC, anything greater than about 15* angle taper results in flow separation and turbulence.

To keep heat in the dump pipe and velocity up all you need to do is HPC coat it or lag it with header wrap.

My maximum EGT's dropped by well over 25* when I went to a 3" exhaust, so I reckon TD5Alive.com's 'theory' about retained heat and overspeeding turbos has holes in it big enough you could drive through.

[Pedro_The_Swift]

Thanks Rick,,

though a turbo that overboosts just by putting a bigger pipe on is cheap horsepower;)

The gentleman that loaned up a wheel at Donald was parked up due to a slight turbo problem,,
The alloy plenum chamber had blown open due to "excessive boost",, he said he was HOPING FOR around 36 pound and after seeing the chamber I have no reason to doubt it!!:eek2::oops2::burnrubber:

[Blknight.aus]

Actually all td5 alives theory work is correct.

If you happen to be talking about turbos with both waste gates and blow off valves on the inlet manifold which, since we run diesel engines and not petrol headed rice burning rev monkey engines, we dont have blow off valves.

The problems arise in some configurations because without the back pressure to limit the flow rate out of the turbo and wastegate it will spool up as hard as it can and if the wastegate open its effectively blowing out to atmosphere or worse back into the inlet tract before the turbo so theres nothing to stop it from spinning until it sheds a wheel or eats a bearing. Since we have closed inlet tracts after the turbo (unless you blow something apart) the compressed air on the out side of the compressor brakes the turbo...