Thread: turbo-overspeed?

rather than hijack Aaron's thread on exhaust sizes,,
TD5 3 inch exhaust.


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?

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...