Cooling Turbo

[Trumpy]

Well explained Andrew.

Perhaps those that are unsure should go for a drive at night somewhere, and see how hard you have to drive to get it glowing.

I too come straight off a freeway into my suburb, and unless I have been "playing" on the way home, it rarely has an orange tinge to it.

Go for a spirited drive or a hill climb, however, and the glow stands out. Still only takes a minute of idle for the glow to disappear.

If your just driving normally, dont worry about it.

If you've been getting up it, idle it for a minute. Easy

[Miyagi]

didn't know that! I'll be a tad more gentle next time I have one of those 'sessions'... :-)

[mickq]

From what I understand about turbochargers, a turbo keeps spinning for quite a while after you shut the engine down. The reason it's advisable to idle the engine for about a minute is to reduce the speed at which the turbo is spinning to as slow as possible, before shutting the engine down. Shutting the engine down removes the oil supply to the turbo (sourced from the engines oil pump), so the turbo is spinning without a fresh oil supply, causing a risk of siezure. At idle after about a minute, the exhaust gasses propelling the turbo are at an absolute minimum therefore turbo revolutions are minimum also. The greater the speed at which the turbo is still spinning after engine shutdown the greater the risk is for turbo siezure......there is little chance of a turbo siezing, but its more wear & tear on the turbos bushes/bearings.

Its nothing directly to do with turbo spin speed.

Its all related to oil temperature. The same problem could occur even if you ensured the turbo was not spinning much when you killed the engine.

In most tubos, when you stop the engine, you kill all new oil being pumped to the turbo. So the oil that is in there at that point in time has the lovely role of sucking up lots of the heat from the often red hot turbo.

Once the oil reaches a certain temperature it breaks down. In extra bad situations this equals the oil turning to crappy gunk. (that's an official technical term....)

The crappy gunk can gather up and eventually effect the bearings in the turbo...the same parts that allow it to spin at up to 100,000 rpm. As you can imagine a part that sensitive and carefully balanced doest require too much before it is effected and doesnt do its job properly at those insane speeds. The result eventually is a seized turbo bearing. This can kill your turbo as it simply cant spin properly.

As you said, the turbo blades still spin for a min or two after the engine is shut down, so this is made even worse because the turbo for a min or two has its bearings lubricated by nothing more than crappy gunk.

This situation can happen in two ways:

1) You seriously, seriously hoon for a while, shut the turbo off, and the oil happens to fry so quickly and so badly it kills your bearings right away (this is quite uncommon on production cars and due to the low boost of the XR6T is very unlikely).

2) Over time the bearings slowly become effected, and after a few years are rooted. This is what I would expect will happen with the XR6T's if not taken care of.

Now the XR6T has 2 differences from most turbos:

a) the turbo puts out very little boost compared to what it could do, so isnt very stressed nor very hot compared to what it could be doing. Its not the same situation as you would find in, say, your normal WRX which is more highly strung.

b ) It has the thermo syphon that people mention. I imagine, using logic, this simply means that either gravity or heat cause the oil in the engine, after the engine has been shut off, to flow through the turbo and drain into the sump. So for a few mins their may be oil gurgling through the turbo and removing some of that nasty heat and stopping the oil from being staying still and being fried and rooting the turbo bearings.

Given all this, the answer is use your common sense:

- If you drive really hard, and continue to do so within 3 mins of stopping, then give your car a min or so at idle speed or else idle along whilst moving to allow the turbo to cool a little. (Suss out how long it takes by lifting the bonnet at night - preferably when you kill the engine you want the turbo to be glowing only a little....as much as it normally would when the car has not been driven hard and is just idling).

- If you are a normal driver, and really dont hoon much (ie dont frequently go high in the rev range under full throttle) then in general you can not bother unless you happen to be finishing a crazy drive, or if it is damn hot and you have been doing lots of hills. Most drivers fall into this category even though they might think they are serious drivers. This is why Ford dont mention letting the turbo cool in the manual - its simply not relevant to the majority of drivers of XR6T's. (Other Ford turbo cars have had manuals that do mention it)

Edited 19/12/03 02:17 PM by mickq

[Turbo6man]

mickq> the "thermosyphon" effect works with the coolant not the oil. It ensures that the oil heat sink problem that everyone has described does not occur. Even though the residue oil no longer circulates it does get a cooling effect from the coolant which does continue to circulate through the bearing housing so the problem of the oil breakdown and those nasty bits of "gunk" being deposited on the bearings is not likely to occur. At least not in a hurry. The idea to drive steady for the last couple of km or leave the engine idle down for just a minute before shutting down is a good practice.

[mickq]

mickq> the "thermosyphon" effect works with the coolant not the oil. It ensures that the oil heat sink problem that everyone has described does not occur. Even though the residue oil no longer circulates it does get a cooling effect from the coolant which does continue to circulate through the bearing housing so the problem of the oil breakdown and those nasty bits of "gunk" being deposited on the bearings is not likely to occur. At least not in a hurry. The idea to drive steady for the last couple of km or leave the engine idle down for just a minute before shutting down is a good practice.

I cant see how that works, given that as far as I have seen the particular turbo in the XR6T is oil cooled, not water (ethelene glycol) cooled.

As far as I have seen, there is no coolant in the turbo or its housings. Just a decent amount of oil flow that does the double job of lubricating while it also cools the turbo by taking heat with it.

I double checked: go to Garrett's website and you can see all the intake and output ports from the turbo and there are a total of 4 (excluding electrical and measurement ports): air intake and output, and oil input and output.

I still think in the XR6T the thermo syphon effect most likely is related to the oil, and its behaviour after the engine is turned off: flow is continued briefly as a result of gravity causing the oil to run through the turbo back to the sump or else temperature leading to a similar effect.

If you have any proof that the turbo is water cooled, Id love to see it - that would mean the Xr6T setup is fairly different from what I thought.

Edited 20/12/03 11:31 AM by mickq

[Turbo6man]

mickq>If the turbo used is a GT series (I think it's a GT45??), then according to Garrett's specs, ALL GT series turbos have a water cooled central housing.

Geez, even my old TO3 is water cooled, so I feel almost certain that a newer design would not be any less technologically advanced ...

Furthermore, I fail to see how oil can thermo syphon - yes it can gravity feed back to the sump but this is not thermo syphoning. I understand that the Ford manual describes the thermosyphon effect and I therefore assume it must be working with the coolant.

My son's ute will be home next week so I'll have a good look at the plumbing and take some pics for you if I can verify and identify the cooling lines ...

...sometime later... OK, I'm back after finding a turbotech site that says the GT45 has an optional air cooled housing. So it is possible it seems that water (coolant) is not used on the XR6T - so the Q I still have is "why do Ford and everyone else refer to thermosyphoning?"

[Antisocial]

I had a look under the bonnet to today.

Definitely water cooled.

2 pipes in 2 pipes out and 4 great big ones for the air, both hot and cold.

I don't think you have been able to get a non water cooled turbo car since 1984.

One wate pipe comes from the bottom of the thermostat housing, another from one of the heater pipes. The oil comes from a fitting near the oil pressure sensor on the passengers side of the block around to the turbo, and I assume drains out thru the bottom into the sump.

After I give the car a hard time, let it idle for a few minutes and turn it off. I can hear a sort of gurgle gurgle, then qiuet for a few secconds then gurgle gurgle. I assume this is the thermo syphon system passing water thru the turbo to keep it cool.

[Marcos]

From what I understand about turbochargers, a turbo keeps spinning for quite a while after you shut the engine down. The reason it's advisable to idle the engine for about a minute is to reduce the speed at which the turbo is spinning to as slow as possible, before shutting the engine down. Shutting the engine down removes the oil supply to the turbo (sourced from the engines oil pump), so the turbo is spinning without a fresh oil supply, causing a risk of siezure. At idle after about a minute, the exhaust gasses propelling the turbo are at an absolute minimum therefore turbo revolutions are minimum also. The greater the speed at which the turbo is still spinning after engine shutdown the greater the risk is for turbo siezure......there is little chance of a turbo siezing, but its more wear & tear on the turbos bushes/bearings.

Its nothing directly to do with turbo spin speed.

Its all related to oil temperature. The same problem could occur even if you ensured the turbo was not spinning much when you killed the engine.

If it had everything to do with oil temperature, what would 1 minute of idling do to oil temps? very little. It is relevant with an XR6 T, having a watercooled housing maybe reducing the turbos temp. before shutdown, but with older turbos that were not watercooled, a minute of idling would achieve a slow turbine speed before removing a fresh oil supply (shutting engine down). I dont think oil temps are affected in 1 minute of idling.... What do others think???

[God666]

It probably won't drop the oil temp in that short period but it will allow the turbo housing and tubine to cool considerably in that minute, particularly if you have been driving it like a bas*ard and things are getting a little cherry red.

Any temperature oil is going to immediately fry on something that hot so give the water cooling a chance to work for a minute and the exhaust gases a chance to flush some of the heat from the exhast side of the turbo housing. An engine at idle produces hot exhast gases but they are still way cooler than a glowing red turbo housing so they will flush away a fair bit of heat. It is all about temperature differential so it will help to normalise the temps.

[Turbo6man]

Thanks YCL - that is how I remembered it, but not having the car available to check I was starting to believe it was possible I was mistaken. It has happened once before :lol: