I was wondering if after a couple of track laps, would it be better to keep the engine running and have the cooling run at idle, or is it better to stop the engine ?

I would think that keeping the engine running would be better as it allows the coolant to circulate instead of stopping wherever it is, especially when very hot.

I am asking this in the context of very hot summer days where the outside temp is around 90F.

I'm thinking that you're better off letting it cool off while running. The car has a thermostat on the fan, so if you shut it down while it is still "hot", it is likely that the fan on the rear deck will come on to cool the engine. That certainly isn't going to cool the engine as well as coolant cycling through the engine.

That being said, I have a Turbo, and have sat in my driveway after a spirited ride home from work, and let it idle for 2 - 3 minutes. I then shut off the car, and finished listening to a story on the radio. As I was getting out of the car, the fan came on. So, the operable question is "How long do you need to let the car idle to properly cool it before you shut it down?" In my situation, 3 minutes wasn't enough, and I wasn't on the track. On a hot day, after track usage, you might find that it would take more than 5 minutes. BTW, if you park the car, and let it idle for a while, and after turning it off, the rear deck fan comes on, there is nothing wrong with restarting the car to get the coolant circulating to cool the engine.

I'm not a mechanic, this is just my experience.

larry

I dont think running the motor serves any purpose...a few laps? 20 minutes? your car will get warm but not overheat...you do a cool down lap at the checkered flag running at 30-50% speed...by the time you get into the paddock area shut it down and DONT SET THE E'Brake!

no I wouldn't run the motor - maybe open the rear deck...MAYBE (if you think it "looks fast") - keep in mind during the prototype phase of making the car Porsche punished the car...and it survived (presumably) they drove them in the coldest and hottest climates and thrashed them to collect data. you are not being too hard on it with a DE track day.... spend more time worrying about tire pressures and reading the wear marks so you know rather to adjust up or down - that will keep your mind off the issue at hand

After the cool down lap, which helps the brakes too, I let it idle for about a minute to let the oil settle down. This helps avert the mosquito fog effect on restart if the oil is too full. Then I open the lid to let the hot air out. That also reminds me to check the oil before the next session.

Usually I notice the temperature gauge goes UP when the engine sits and is just idling. Hard drive or not.

That's about the time all those fans start kicking in. Had the rear deck lid fan kick on the other day after a punishing drive in 95 F degrees summer heat. Didn't worry one bit about it.

Need airflow over the radiators to cool it down more quickly. If you wanted to get it to cool down faster the better thing would be to take an extra lap around or outside on the highway for a few minutes in 6th gear on a flat road going a steady 75mph.

If there is no combustion = no reason for it to continue "heating up"... it can only get cooler from there, I agree with vange.

Another viewpoint.

At the end of a 20 min DE session, I do a cooldown lap with my fan on full and hot setting to cool the engine down. When I pit I idle the car with the fan blowing full hot and I shut it down after the temp gauge goes down to normal. I do not have a engine fan. Shutting off the car immediately is only asking for issues.

Another analogy.
Try running in warm/hot weather then stopping immediately. Thermally you'll feel that you are overheating and sweat heavily.
Now try running in warm/hot weather and slowly cool down...

IMO, there is no reason to leave the car running. Just pull off and park. And don't lift the engine lid like you see some of the 993 folks doing.

If the car is running hot ON the track, grab a 3rd radiator.

-td

Air blasting thru the radiators and underside of motor will do more to cool the motor than letting the car sit stationary idling. If anything you may notice getting in the wind will cool a car that is hot from idling over a long peroiod.

On the track at speed and relatively high rpm, you have very good flow thru the radiators removing heat most efficiently and the high R's give max oil pressure and flow as well as max coolant flow thru the whole system.

A stock motor should not run too hot under these conditions

Just heat isn't the specific of a cool down. Valves rings and pistons are a good thing to let cool down with the engine running. Main thing is the valves as far as I'm concerned. They take the breath of the fire and get pretty hot like being worked in a bellows. Just shutting down going to cold can warp. Sure the exhaust is hot and might hold the heat to them for a while but if they are red hot I don't think its good to just shut down. I always figure let them idle for a while. Rings and vlaves are our friends

Give the car a cool down lap to help all the hot parts shed some heat.

In the 'pits' let the engine idle a few minutes. Let the radiator fans come on then shut off at least once.

The engine builds up a considerable heat load from hard running and while there is plenty of margin you want to give the engine every chance to last a long time and letting the engine shed some heat, letting the temp gradients even out, is one way.

Sincerely,

Macster.

Just because it's water cooled doesn't mean there isn't high temps back there waiting to escape. I always lift the lid on my 996 and did so with my cup car too. You can feel the heat radiate up and out.....

Also, air-cooled motors, especially 'built' ones, throw out hotter exhaust gases then water cooled motors. To compensate, I offset my position from all cars in front of me especially during a cool down lap to get as much clean air as possible thru the radiators....

Since it's not a race, why not just back off a couple of laps before the cool down laps?

My engine temp doesn't go any higher driving it hard at track then it does driving it normal on the street.

I run it for a min or 2 after the cool down lap. Mostly because it seems to smoke less the next time I start it up. Usually never lift the lid either but lifting the lid will cool the engine quicker.
I am not sure if idling after a good hot run is good for the cats.

The converters will be fine. They like hot. In fact here in CA to pass smog the usual technique is to drive the car a good distance (30 miles or so) to ensure the readiness monitors are complete and the engine/drivetrain and especially the converters are good and hot.

When I take my cars in I pick a hot day and after doing the above I don't shut off the engine until after the smog test is over with.

The test involves running the engine at 1500 rpms and then again at 2500 rpms (while the car is stationary, although the Boxster goes on the rollers while the Turbo does not).

The Boxster has been through smog test/checks now may 4 or 5 times and the Turbo at least once and neither have complained, nor have the cars' converters.

My bigger worry is as I touched upon in an earlier post is the heat load the engine (and other components) can accumulate -- under high loads the exhaust valves can run nearly red hot, for instance -- and a cool down lap and some idle time are a good way to give these components time to shed their heat loads, to let the temp gradients even out.

The exhaust valves mainly shed heat by sitting on their valve seats so at idle they see little combustion temperature and spend the most time on their seats.

Sincerely,

Macster.

The engine compartment fan on all 996s blow fresh air downward, I believe this is to keep hot air away from the plastic components all over the top of the motor - thus I never open the lid. With a proper cool down lap, I seldom see the fan come on after the engine's shut down. Conversely I've seen many people open their rear engine lids on 996s with the engine fan going for hours, though with the lid open it obviously will have a negligible effect.

It seems like there are many different opinions on this one. I prefer doing a cool down lap which for me implies low throttle, as little brake use as possible and low rpms in a higher gear as i believe that cooling the wheel bearings, brakes, axles, gearbox etc are just as -if not more important than just cooling the engine. I don't generally idle the car any longer than it takes to get back to my spot however I would NEVER pull in and park a car without a cool down lap.