Twinair - should i let turbo cool down?

[bilsland]

Was parking our Twinair today and idly remebered that on my fathers Saab 9-3 he always let the turbo cool down for a fdw moments when parking up before switching the engine off.

Do i need to do this on a TwinAir?

[Spitfire2]

It is worth doing so, but it's not so
Much about the turbo cooling down as slowing down.

If you turn off instantly after stopping the oil supply to the turbo stops instantly too. If its still whizzing away it will spin to a stop without the supply of lubricant which can wear its bearing faster.

So letting the car tick over after stopping for a short period allows the turbo to stop spinning before depriving it of oil.

[sawman]

is it not just enough to drive the last mile to your destination slower so that the engine is off boost. One of the problems with idling the engine after you stop is that airflow stops, whilst the engine is running and so generating heat.

[craigjm]

Spitfire2 said:

If you turn off instantly after stopping the oil supply to the turbo stops instantly too. If its still whizzing away it will spin to a stop without the supply of lubricant which can wear its bearing faster.

Do you know that for a fact with this engine or is that remembered from turbo cars of old? is it not possible that the engine designers have thought of that and actually you don't need to do this. When you consider how many turbo cars there are around these days I would be surprised if that was still the case.

[Nigel_O]

All journal-bearing turbos use a film of oil to support the turbo shaft. This film of oil is constantly replaced while the engine is running, which keeps the oil (and the shaft) cool(ish). When the engine is turned off, the oil pressure will remain for a while, but the oil flow will stop. At this point, it's pretty obvious that the slower the turbo is spooling, the gentler it will be on the oil film still in the turbo.

At some revs, on some turbos, turning the engine off whith a very hot, fast-spinning turbo can effectively crystalise the oil (imagine what oil would look like if you roasted it in a tin over a hot flame - it bakes solid)

It's always a good idea to let the turbo coast down before you turn the engine off when you come to a standstill. 20 - 30 seconds is usually enough (as a journal-bearing turbo slows down quite quickly when it's not being driven)

I seem to recall reading that some turbo cars are now using electrically-driven oil pumps to maintain a flow of oil to the turbo for a few seconds after the engine is killed - good idea, although a little complex.

The alternative is to splash out on a roller-bearing turbo - much more robust, but even then, its still a good idea not to kill the engine while the turbo's still spinning hard.

My last turbo survived for 140,000 miles, despite being used VERY hard at pressures of up to 2 bar. All down to using decent quality oil and being a little bit thoughtful at the very beginning and very end of each journey.

[arguti]

craigjm said:

Spitfire2 said:

If you turn off instantly after stopping the oil supply to the turbo stops instantly too. If its still whizzing away it will spin to a stop without the supply of lubricant which can wear its bearing faster.

Do you know that for a fact with this engine or is that remembered from turbo cars of old? is it not possible that the engine designers have thought of that and actually you don't need to do this. When you consider how many turbo cars there are around these days I would be surprised if that was still the case.

Unless there is an alternative method of keeping the turbo lubricated after switching off engine, it will accelerate wear.

Bear in mind that most modern cars are not built to last 20 years....I for one would love to see the premature failure rates of modern VW/BMW turbos and how long the manufacturers realistically expect them to last.

Then again, what is premature, our VW diesel turbo went with less than 40K on the clock and that was religiously cooled down after each journey.

The eco-trend of small displacement turbo and/or supercharged engines is there to drive down CO2 figures; I wonder how many of these types engines will last great mileages and beyond 5-6 years without major costs/repairs

[Spitfire2]

craigjm said:

Do you know that for a fact with this engine or is that remembered from turbo cars of old? is it not possible that the engine designers have thought of that and actually you don't need to do this. When you consider how many turbo cars there are around these days I would be surprised if that was still the case.

It's still the case.

[Nuttypaco]

Turbos need to be cooled down as they run at very high temp and without a supply of oil it will cause premature wear. The best thing to do is to drive the last few miles of your journey off boost. This will help cool the turbo and keep the supply of oil flowing, plus you have the added cooling of air flow. For serious turbo charged cars some owners fit a turbo timer which keeps the engine running until the turbo has cooled down, even after you turn off the ignition. There are two problems with turbo timers, one is it's illegal to leave a cars engine running while unattended and secondly it may invalidate your insurance because of reason one and the fact it can provide easier access to the ignition circuit for thieves.

[bilsland]

Thanks all - although some advice nt really possible (i cant replace the turbo or drive it off boost)

Sounds like its better to be safe than sorry.