How to cool down your tt after a hard run?
07-01-2009, 11:04 AM
#16
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This is an amusing thread. I'll bet the real turbo experts are getting a good laugh out of this one.
Kevin, how about commenting on where the damaging heat comes from and how long it takes for the turbo shaft to reach it's idle speed.
Kevin, how about commenting on where the damaging heat comes from and how long it takes for the turbo shaft to reach it's idle speed.
07-01-2009, 03:06 PM
#17
Race Director
Ok. Good to know. I normally let engine idle for a moment or two before shut off....
Yes.
The rpm of the turbos is a function of engine rpm. If you are driving at say 2500 rpm under no boost, the turbos are at that time spinning at rpm's substantially higher than their idle rpm. If you motor along easily (no boost) at 3500 rpm, the turbos are spinning at an even higher rpm. Once the engine reaches idle rpm the turbos do not immediately decelerate to their idle rpm...they are very free spinning and decelerate only by friction. The heat created by this friction is reduced by the oil in the turbos...but this oil is not provided once the engine is shut down. So the higher the "motoring around rpm" the higher the turbo rpm...and the longer it takes the turbos to spin down to their idle rpm once the engine reaches idle. If the turbo is deprived of oil during this spin down period, the heat generated can cause damage to the turbos.
The rpm of the turbos is a function of engine rpm. If you are driving at say 2500 rpm under no boost, the turbos are at that time spinning at rpm's substantially higher than their idle rpm. If you motor along easily (no boost) at 3500 rpm, the turbos are spinning at an even higher rpm. Once the engine reaches idle rpm the turbos do not immediately decelerate to their idle rpm...they are very free spinning and decelerate only by friction. The heat created by this friction is reduced by the oil in the turbos...but this oil is not provided once the engine is shut down. So the higher the "motoring around rpm" the higher the turbo rpm...and the longer it takes the turbos to spin down to their idle rpm once the engine reaches idle. If the turbo is deprived of oil during this spin down period, the heat generated can cause damage to the turbos.
From now on I'll make a 2 minute idle the standard before shutting engine off to give turbos chance to spin down.
Sincerely,
Macster.
07-01-2009, 07:39 PM
#18
Yes.
Once the engine reaches idle rpm the turbos do not immediately decelerate to their idle rpm...they are very free spinning and decelerate only by friction. The heat created by this friction is reduced by the oil in the turbos...but this oil is not provided once the engine is shut down. So the higher the "motoring around rpm" the higher the turbo rpm...and the longer it takes the turbos to spin down to their idle rpm once the engine reaches idle.
Once the engine reaches idle rpm the turbos do not immediately decelerate to their idle rpm...they are very free spinning and decelerate only by friction. The heat created by this friction is reduced by the oil in the turbos...but this oil is not provided once the engine is shut down. So the higher the "motoring around rpm" the higher the turbo rpm...and the longer it takes the turbos to spin down to their idle rpm once the engine reaches idle.
What a crok o' you know what!!
It's all about cooling the turbos after hard running.
After blasting around at full throttle for several or more minutes the turbos are smoking hot. Shutting them down at this point will probably lead to metal warpage or worse. They will spin down to idle speed within 10 seconds (or so) max. Motoring around at low power levels is a perfectly fine way to begin turbo cooling.
Waiting for the turbos to SLOW down...
Now I've heard everything..
Phil
07-01-2009, 08:07 PM
#19
Race Director
Well, which is it? Cooldown or spin down? I've only owned...
What a crok o' you know what!!
It's all about cooling the turbos after hard running.
After blasting around at full throttle for several or more minutes the turbos are smoking hot. Shutting them down at this point will probably lead to metal warpage or worse. They will spin down to idle speed within 10 seconds (or so) max. Motoring around at low power levels is a perfectly fine way to begin turbo cooling.
Waiting for the turbos to SLOW down...
Now I've heard everything..
Phil
It's all about cooling the turbos after hard running.
After blasting around at full throttle for several or more minutes the turbos are smoking hot. Shutting them down at this point will probably lead to metal warpage or worse. They will spin down to idle speed within 10 seconds (or so) max. Motoring around at low power levels is a perfectly fine way to begin turbo cooling.
Waiting for the turbos to SLOW down...
Now I've heard everything..
Phil
However, engine made plenty of boost even at relatively low rpms -- diesel you know -- max torque was around 1900 rpms -- so I assumed turbo always hot.
Sold car on with over 120K miles and turbo as good as day I bought the car. (Of course, followed a schedule of 5000 mile oil/filter changes, used quality synthetic motor oil approved for compression ignition engines to give turbo and engine all the help I could.)
While I know turbos can reach rpms -- in some cases but probably not in the case of the 996 Turbo -- approaching 100K I find it hard to believe it would take no more than a few seconds after exhaust gas flow reduced for turbos to spin down to near idle speed.
It would take longer of course for the heat load the turbo assembly had built up to be shed.
Sincerely,
Macster.
07-01-2009, 08:41 PM
#20
Why have manufacturers and race teams been devising systems to combat the dreaded "turbo lag".
Because as soon as you're off the gas the turbo is slowing quickly, and within seconds, if you get back on the gas, there will be a lag time while the turbos spool back up to speed.
Turbo lag not as big a problem now as in the 930 days I'd presume?
Ever drive a 930? Big time turbo lag!
The turbos are driven by the exhaust gasses passing through. When the exhaust flow slows to idle it will also slow the turbos to the driving speed of the gasses.
Phil
Because as soon as you're off the gas the turbo is slowing quickly, and within seconds, if you get back on the gas, there will be a lag time while the turbos spool back up to speed.
Turbo lag not as big a problem now as in the 930 days I'd presume?
Ever drive a 930? Big time turbo lag!
The turbos are driven by the exhaust gasses passing through. When the exhaust flow slows to idle it will also slow the turbos to the driving speed of the gasses.
Phil
07-02-2009, 12:53 PM
#21
Three Wheelin'
As for the brakes, I have heard that the best thing to do given the "heat" of the pads is to roll the car a few inches, 2-3 times after a lap, so that the rotors don't warp in one spot fom the crazy heat eminating from the pads. Who cares about the parking brake, as you can just leave the car in gear once its shut, and no PB needed
07-10-2009, 08:06 PM
#22
Rennlist Member
Of course you don't HAVE to use the parking brake after a run but I've seen people do it as an instructor. There is a bunch of heat that is trapped on the other side of the rotor. If you don't want to worry about it.................don't. The slow running I mention is just as close to idle as you can get but keeping the air running through the radiators so you have active flow over the various heat exchangers. I am in Southern California and it can get desert hot here, a little air flow never hurt nothin'. Again I'm only an autocrosser (intermittent nowadays) but you blast through a run and then sit in line with your run group while temperatures climb so that by the time you pull off in SoCal your temp gauge is on the higher side. Slow (real slow) running keeps an air flow over things and brings temperatures down before parking.
07-10-2009, 08:11 PM
#23
Rennlist Member
P.S. forgot to add, per all the advice I've been given I also don't shut the motor off the minute after I pull in............you do give it a minute or two with a stationary idle.
07-11-2009, 06:36 PM
#24
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After track is everything very hot and idea of cooling down 2 minutes is basically nothing. To cool down even a cup of coffee takes more than two minutes, so the "spin down" idea is more believable... But the other hand, "turbo lag" shows that there is nothing to spin down...
So lets hope that some expert will help us...
So lets hope that some expert will help us...
07-11-2009, 07:00 PM
#25
Race Director
Coffee cup example doesn't apply. As long as engine running water pump circulating coolant through radiators and oil cooler and through engine. If this system can remove the tremendous amount of heat an engine can and does generate at full chat it can certainly pull the heat out of an idling engine in short order. 2 minutes of idling should do it, though honestly better to take a cool down lap to help remove heat from not only engine, but drivetrain and brakes.
Hopping off of freeway one is mainly faced with a hot engine and possibly turbos.
Sincerely,
Macster.
Hopping off of freeway one is mainly faced with a hot engine and possibly turbos.
Sincerely,
Macster.
07-11-2009, 07:07 PM
#26
Race Director
Brakes are hot and parking brake hardware included. But there's no real mass there and worry about warping brakes using parking brakes I think overblown. More likely what will happen is some one will come in and not set brake for fear of some warping and in a hurry to get out and talk over the last run forget to put car in gear. Not a good end to a track day to see one's car rolling off down pit lane road...
One concern about parking car with hot brakes is leaving hot pads in contact with hot rotors. This can result not in warping but the hot pad leaving some pad material on the rotor that upon next usage will cause unequal friction and uneven pulsing braking action. This is often blamed on warping but it is not warping. A check of rotor with dial indicator will not find any run out and a check of rotor thickness will not find any measurable variation in rotor thickness.
Sincerely,
Macster.
One concern about parking car with hot brakes is leaving hot pads in contact with hot rotors. This can result not in warping but the hot pad leaving some pad material on the rotor that upon next usage will cause unequal friction and uneven pulsing braking action. This is often blamed on warping but it is not warping. A check of rotor with dial indicator will not find any run out and a check of rotor thickness will not find any measurable variation in rotor thickness.
Sincerely,
Macster.
07-11-2009, 07:34 PM
#27
RL Community Team
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Since the parking brake does not use the rotors or main pads, there is little harm (if any) in setting the parking brake with hot brakes. The rotors and brakes will essentially cool just as rapidly with the parking brake set.
07-12-2009, 04:48 AM
#28
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There is explanation: http://en.wikipedia.org/wiki/Turbocharger
Go down and find chapters "Reliabilty" and "Turbo Lag".
Shortly --- cool-down 10 seconds is enough for most cases, after race 2 minutes is enough to keep oil running through turbo.
Go down and find chapters "Reliabilty" and "Turbo Lag".
Shortly --- cool-down 10 seconds is enough for most cases, after race 2 minutes is enough to keep oil running through turbo.
07-12-2009, 05:00 PM
#30
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Per Mark's post #16, if you drive your car and shut the engine off, you will imprint the turbine wheel with a hot shutdown. Oil will silkscreen a spot on the journal surface. Please realize that THESE turbochargers on these engines are NOT water cooled. We also have a sump pump that is driven off the intermediate shaft that needs time to pull the HOT oil that is being used to cool the turbochargers down. This oil needs to migrate back into the engines scavenge pump.
The heat is built up from the headers to the turbine housings. It take time to drop these temps. If you shut the engine off the heat wicks directly thru the turbine wheel and in the bearing housing. There is no watercooling to cool the center section. Coking starts to occur when temps are cooking the oil over 550 degrees.
The best advice is to spend "some" time idling your engine before you shut the engine off.
The heat is built up from the headers to the turbine housings. It take time to drop these temps. If you shut the engine off the heat wicks directly thru the turbine wheel and in the bearing housing. There is no watercooling to cool the center section. Coking starts to occur when temps are cooking the oil over 550 degrees.
The best advice is to spend "some" time idling your engine before you shut the engine off.
