AGC

I got another question regarding the 993TT. I have never owned a car with turbo before. I have heard that cars with a turbo should not be shut off immediately after use, one should wait a few minuted prior to shutting the car off to cool the turbo. My question is this true and why is this done. What should I do prior to shutting the car off and when should it be done after each use or only on long drives. I sometimes drive the car over to a friend house, which is about 5 minutes away, should I let it cool and for how long.
Thank you

Kevin

Argyle;

With our air cooled engines, you need to have a cool down drive before you enter your drive way, and wait at least 2 minutes to cool your turbo's.. This is done to prevent the heat soaking that occurs, which is heat from the turbine side transfering to the bearing housing which smokes the shaft and bearings. It will literally turn the shaft blue. If you do not take the extended time you will shorten the life of your turbo's..

vjd3

Having owned a Turbo ... the problem occurs when the heat from the turbo cokes the oil. Allowing the car to idle for a minute or two will let the heat die down a little bit. The turbos get super-hot when they are used. I think it's less of an issue when you're using synthetic oil, and it really only needs to be done if you've been driving the car hard and on the boost a lot. If you're just driving normally through traffic for five minutes, I wouldn't worry about it; the car won't even be warmed up at that point.

Vic
95 C4
87 930 (once upon a time)

Flying Finn

One of main reasons is also that if your revs are high when you shut the engine (don't to that 'blip the throttle' shut down) turbo keep spnninf (fast!) after the engine has already shut down, which obviously is not good...

This is because turbo(s) turn a lot faster than the engine.

ScottMellor

I believe the conventional wisdom on this oft mentioned topic is this:
Don't turn the engine off after running your best ever lap time at the track. A cool down lap or two off the boost should do the trick.
On the street, a few minutes of low rpm driving through your neighborhood to your garage is more than sufficient to let your turbo's cool down, and just sitting idling in your garage is pretty much unnecessary. As you know from observing your boost gauge, the turbos aren't even getting spooled up at low rpm, so you are cooling down on your drive from the freeway to your house IF you're not red-lining every shift.

Greg Fishman

If I am not mistaken the turbos spin all the time. It is when they are under boost that they generate a lot of extra heat.
Mark Dalen posted a good explanation of why you need to let the engine idle of a bit after you run the car. From what I remember it is basically to keep the oil from coking, which leaves tiny bits of metal that will over time wear your turbos out prematurely.
Greg

SundayDriver

[quote]Originally posted by Greg Fishman:
<strong>If I am not mistaken the turbos spin all the time. It is when they are under boost that they generate a lot of extra heat.
Mark Dalen posted a good explanation of why you need to let the engine idle of a bit after you run the car. From what I remember it is basically to keep the oil from coking, which leaves tiny bits of metal that will over time wear your turbos out prematurely.
Greg</strong><hr></blockquote>

Correct. Under boost, the turbos are red hot, with oil flowing through them. If you immediately shut down the engine, you would have a huge heat soak in the turbos which would cook the oil, causing coking. A couple of minutes of cool down after hard runs is needed to allow cool oil to flow and for the overall turbo temps to come down. Not nearly the problem with synthetic oils as it was in the days of dino oil.

This is just not an issue for street driving as you can almost never get in a situation where you are at hard boost followed immediately by a shutdown, unless prompted by the Highway Patrol (and they aren't going to listen to you try to explain the need to cool your turbos). As Scott said, the easy drive into a neighborhood/etc is ample to cool the turbos.

ScottMellor

I didn't mean to imply that they are not spinning at all at low engine rpm, but they are not generating boost and heat.

A.J. - 95 993

For those who are really in a hurry, or want to be careful to the nth degree, there are turbo timers. Those will allow the engine to run for a specified amount of time, usually from 30 sec to 2 min after turning off the key, exiting the vechicle and locking it. I have never used one, but considered one on an older RX-7 I used to own. If I was in a hurry to get to work or a movie (causing the turbos to glow red hot <img src="graemlins/c.gif" border="0" alt="[ouch]" /> ), it was difficult to sit in the parking lot long enough to give the turbos enough time to cool. Has anyone used one of these timers on a 911 Turbo? Are they worth using? Seems like they cost $100 or so.

<img src="graemlins/offtopic.gif" border="0" alt="[offtopic]" /> Incidentally, I had problems with the exhaust side seals on that RX-7 Turbo. When they had it apart for repair, they found out that the escaping gases caused heat concentrations which cracked the maifold and turbo (ouch). The intake seals were bad, too. 10K miles later, I suspected the seals were letting go again, so I got rid of the car. Years later I met someone at work who had the same problems with his RX-7, same model. He had 2 turbos fail, and finally realized that the oil line that feeds the turbos was what was getting coked with oil. The lines weren't replaced with the turbo. The next turbo, incidentally, didn't last long. Root cause - the oil line had a support bracket that was mounted to the exhaust manifold. A design, in essence, that was guaranteed to coke oil. He looked on a friend's Thunderbird Turbo Coupe (Mazda/Ford partnership?), and it had the same setup. I never verified this design with my own eyes, but found it very interesting.

Jeff 993TT

Just remember that idling your car won't necessarily cool it down very much. That's becuase there is no air flowing through the oil cooler(s).

If you get a turbo timer, you should probably install a oil cooler fan switch also. I've noticed that idling my car with the switch turned ON very rapidly reduces the oil temperature. The amount of heat being blown out near the passengar side is incredible.

Jeff

KC993

I did have a turbo timer installed when I had my 300ZX tt. It would calculate how long the cool down time should be and shut the engine off; normally around 30 to 45 seconds...But for the life of me at the time I can not wired it to work with the factory alarm system, so I end up sitting in the car waiting...

Rocket

To put a scale on the kind of heat we are talking about, temperature in the cylinder at WOT (wide open throttle) is probably in the order of 2000 degrees C. EGT (exhaust gas temperature) after the turbo when driven consistently hard are probably around 900/950 degrees C so the turbo and impeller can heat up to somewhere in between these figures. It is not difficult to comprehend the benefits of letting the turbo spin at low rpm for a short while after vigourous driving, but the EGT (exhaust gas temperature) which gives an indication of the temperature of the turbo falls quite rapidly within 30 seconds or so to 300/400 degrees and won't drop much below that unless you switch off the engine.

Also as pointed out the engine oil tends to heat up at idle speeds without air passing over the engine and oil cooler so leaving the car for a considerable period doesn't help a great deal in the overall cooling down scheme of things. 60-120 seconds is probably adequate even after hard driving.

Rocket

P.S my experience comes from owning a number of turbo cars running programmable ECUs and EGT gauges mounted in the exhaust to aid tuning.

SundayDriver

[quote]Originally posted by Rocket:
<strong>To put a scale on the kind of heat we are talking about, temperature in the cylinder at WOT (wide open throttle) is probably in the order of 2000 degrees C. EGT (exhaust gas temperature) after the turbo when driven consistently hard are probably around 900/950 degrees C so the turbo and impeller can heat up to somewhere in between these figures. It is not difficult to comprehend the benefits of letting the turbo spin at low rpm for a short while after vigourous driving, but the EGT (exhaust gas temperature) which gives an indication of the temperature of the turbo falls quite rapidly within 30 seconds or so to 300/400 degrees and won't drop much below that unless you switch off the engine.

Rocket

P.S my experience comes from owning a number of turbo cars running programmable ECUs and EGT gauges mounted in the exhaust to aid tuning.</strong><hr></blockquote>

Either your engines did not last long or you need to take a better look at the temps you claim. You are claiming temps in excess of the melting point of steel, aluminum alloys, etc and well beyond the temp at which turbo impeller blades would get very, very soft and flatten out.

PramTT

I have a modified Mazda turbo with EGT gauge and under a lot of boost application the EGT (sensor right after the turbo) will indicate 1300-1350F (not C). Idle or stop and go traffic will show 500-600F. The EGT will go down pretty fast from 1300F to 6-700F within 1-2 minutes idle.

On another note, I have yet to see anybody succesfully install a turbo timer in a 993tt

Rocket

Mark.....I am, of course, referring to the EGT not the temperature of the metal. Perhaps it is an assumption to assume that the turbo parts would raise proportionally to these kind of temperatures, but it isn't going to be far off. Afterall it isn't unheard off to melt turbos.

I take it that your Centigrade is the same one over here i.e 1000C=1820 F.

I used a Mitsubishi Lancer EVO VI as a track car with an EGT gauge (mounted about 3 inches after the turbo), which saw temperatures of about 950 C all day. I ran the car for 12 months and 20,000 miles at this state of tune with no detrimental effects to the engine or turbo.The EGT was primarily used to tune the engine timing, fueling and boost as the narrow band Lambda sensor (oxygen sensor) became inaccurate above about 600C.

This picture (right gauge) shows EGT after just three blasts through the gears and EGT is straight up to 700 degrees C. Left gauge is boost (1.4 bar = approx 20 psi) middle lights top Lambda, bottom white lights Knock (pink/ping) lights.


Rocket