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Porsche TURBO Engine Cooling Issues

Two members are embarking on “Baby” TURBO engine programs. The power densities needed bring up a topic that I find in my thread search is really not discussed as perhaps it should.

I’m a lifelong air-cooled engine guy, 43 years. Starting as teenager in VWs then on to Porsches, add in 15 years of owning and piloting aircraft, now back to Porsches. This problem of properly cooling an air-cooled motor was researched at length before, during and shortly after WWII.

VW had serious head cooling issues in the late 60s which kept me working to pay for school by rebuilding VW motors in the evenings. Then it was common for #3 exhaust valve to be gone at 50K miles of interstate driving.

Porsche’s corporate secrets will probably remain corporate secrets however much of what they know can be glean from the development stages of their air-cooled product.

I’d like to start this thread as a clearing house of ideas, investigations and application.

I will begin (a discussion of the history of water vs. air-cooled motors):

“But as air quality awareness rose in the 1960s, and laws governing exhaust emissions were passed, unleaded gas replaced leaded gas and leaner fuel mixtures became the norm. These reductions in the cooling effects of both the lead and the formerly rich fuel mixture, led to overheating in the air-cooled engines.[citation needed] Valve failures and other engine damage was the result.[citation needed] Volkswagen responded by abandoning their (flat) horizontally opposed air-cooled engines,[citation needed] while Subaru took a different course and chose liquid-cooling for their (flat) engines.”

This brings up something I’ve learned about air-cooled engine powered aircraft. In operation, air-cooled aircraft engines are yes, air-cooled and oil-cooled (like Porsches) but also FUEL cooled (as are modern Jet engines). During a flight they effectively burn off the otherwise detrimental weight of their cooling system. What happens to our Porsche’s cooling when we optimize A/F and spark advance with modern electronics tuned on a dyno?

If you go to an airshow you will see WWII vintage aircraft leaving very visible exhaust trails (RICH mixture) at takeoff and later at full throttle flybys. I’ve heard it several times, “without fixing the stock WUR the engine runs pig rich in mid-range.” This maybe true, and it may be caused from wear, but just maybe Porsche did this for sufficient head cooling at mid range, high load operation?

Another consideration:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930093599_1993093599.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.go/19930093114_1993093114.pdf

Two old NACA documents. Lots of good reading but a highlight for me. 400 degrees cylinder head temperature (CHT) taken at the rear sparkplug flange is considered the limit for this high performance turbocharged, intercooled and supercharged air-cooled motor.

What is the limit and location for the air-cooled 911 motor? Surely Porsche works with a similar limit and location. This should be known to racers by instrumentation on their panels. Racers, can you fill in this critical piece of information to begin our discussion?
Old 08-02-2009, 09:33 AM
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Marty, without reading all of that I can tell you that some time ago I read a Porsche document on their engines that acutally used the words "fuel cooled".
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Old 08-02-2009, 10:28 AM
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There is a TON of info out there regarding this - Porsche explored the limits of head temps and thermal efficiency in boosted apps with the 935 cars, eventually going to water cooled heads which carried into the 962s. And yes, these track cars were always tuned on the rich side to aid in keeping head temps down - it's not fuel per se that cools the head, but it is the cooler combustion temp of the rich mixture.
Old 08-02-2009, 10:59 AM
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Excess fuel fed to a 4-cycle gasoline engine does not in itself cool the engine. What it does is provide for an excessively rich mixture that releases less energy, lowering cylinder peak pressures and temperatures, and produces less power. A secondary side effect is lowering of knock probability.

"What happens to our Porsche’s cooling when we optimize A/F and spark advance with modern electronics tuned on a dyno?" Cylinder peak pressure and temperature increase.

Don't have answer to the question, "What is the limit and location for the air-cooled 911 motor?"
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Old 08-02-2009, 11:48 AM
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Brian: can you find that document?

Kellcats521: yes, you are right, but we can't fix the problem with water cooling. Do you have any specifics from your 935 documents.

356-930: I interpret your reply as there is no issue. If that is true I respectfully disagree and support this with your own statement, "your cylinder pressure and temperature increase".

Your temperature and pressure may not induce knock or immediate mechanical failure but without knowing what the thermal limit is how do you know this "tune" isn't capable of exceeding the limits in an operating environment other than those established on the dyno? Water-cooled cars tell you - they boil over.
Old 08-02-2009, 12:47 PM
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Quote:
Originally Posted by copbait73 View Post
356-930: I interpret your reply as there is no issue. If that is true I respectfully disagree and support this with your own statement, "your cylinder pressure and temperature increase.”

Your temperature and pressure may not induce knock or immediate mechanical failure but without knowing what the thermal limit is how do you know this "tune" isn't capable of exceeding the limits in an operating environment other than those established on the dyno? Water-cooled cars tell you - they boil over.
You misinterpret.

You asked a simple question, "What happens to our Porsche’s cooling when we optimize A/F and spark advance with modern electronics tuned on a dyno?" I provided a simple and accurate reply, "Cylinder peak pressure and temperature increase." Optimizing A/F and spark advance to achieve maximum power do not alter Porsche’s cooling scheme, they simply overwhelm it with heat.

Porsche and all good dyno tuners maintain AFR and timing to conservative levels, i.e. less than optimum rich mixture and timing less than optimum to avoid both detonation and overwhelming cooling capacity of the engine’s heat exchanger’s; oil, oil cooling and air cooled heat sinks/fins.

A liquid cooled engine can damage itself long before the cooling liquid vaporizes. Whether air or liquid cooled, detonation will occur under high load if the AFR is lean to a critical ratio and/or timing is too far advanced.

Air is a poor heat extractor compared to a liquid. If one wants to "see" the two cooling mediums at work, heat a finned "air cooled" cylinder to 200F and blow on it with a fan for a minute, monitor its temperature throughout. Heat it again to 200F and drop it in a huge container of fast moving liquid. Monitor its temperature throughout. Liquid wins hands down, probably by a factor of 50. The ability of a liquid to cool is limited by the vapor pressure of the liquid, pressure in the liquid system, and the rate of the liquid container's heat exchanger (radiator) to extract and dissipate heat. As you note, if the ratio of liquid to air extraction is too low for the heat produced, extracted heat cannot be dissipated fast enough and the liquid vaporizes.

Turbos and supercharges are capable of supplying an engine with more air/fuel than it can handle. Perhaps this is the power density to which you refer? Without something (a dyno) to load an engine, monitor it with instrumentation, one cannot readily asses/tune it to safe peak power.

If, as you state, you cannot fix the problem with water, change the fuel to ethanol or methanol/alcohol. And if these fuels are not an alternative, look into water/methanol injection to supplement the intercooler.

The project sounds fun. Keep us posted.
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Last edited by 356-930; 08-02-2009 at 06:49 PM..
Old 08-02-2009, 02:54 PM
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Chris, thank you for your excellent detailed summary and yes we are on the same page.

Ok, lets try this on; has anyone seen a cylinder head temperature (CHT) gauge on a factory prepared car with air-cooled 911 based motor? Do you recall it's maximum allowed reading? or, have you read a factory statement to answer the question?

Marty
Old 08-02-2009, 04:44 PM
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Use 930 "straight blade fan", overdrive it 30%. Also, move the generator from fan and feed it off the crank with separate smaller pulley which is -10% geared (otherwise you will shred the belt). Keep AFR's around 12.5. Done
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Old 08-02-2009, 04:58 PM
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Quote:
Originally Posted by beepbeep View Post
Use 930 "straight blade fan", overdrive it 30%. Also, move the generator from fan and feed it off the crank with separate smaller pulley which is -10% geared (otherwise you will shred the belt). Keep AFR's around 12.5. Done
Goran:

Thanks, but no fair jumping to a solution. We need to understand and measure.

Have you seen or discussed CHT limits?

Last edited by copbait73; 08-02-2009 at 06:51 PM..
Old 08-02-2009, 05:04 PM
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Quote:
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Goran:

Thanks, but no fair jumping to a solution. We need understand and measure.

Have you seen or discussed CHT limits?
Come on you track guys. You must have CHT sensors and data to share.
If you haven't taken and looked at this data, you're driving with a blind eye to your engine.
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Old 08-02-2009, 06:41 PM
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Quote:
Originally Posted by 356-930 View Post
Come on you track guys. You must have CHT sensors and data to share.
If you haven't taken and looked at this data, you're driving with a blind eye to your engine.
Thank you Chris, I've done some Googling and found nothing useful.

Just about every water-cooled car has a gauge or light. The best the factory gives us is a alternator light that may or may not be telling us the fan stopped turning.
Old 08-02-2009, 06:58 PM
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Good info!

I wish I had an answer, but I'll bet most guys don't measure CHT's at the track unless it's a pro team with a larger budget. Most race cars I've seen don't have any thing.

When money permits, I'd like to go with Chris's suggestion of water/methanol injection to supplement the intercooler.
Old 08-02-2009, 07:18 PM
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Quote:
Originally Posted by copbait73 View Post
Goran:

Thanks, but no fair jumping to a solution. We need to understand and measure.

Have you seen or discussed CHT limits?

According to what I have read regarding alloy used in heads: heads shouldn't be subjected to more than 225 deg. C or they get soft. So that is probably a never-exceed limit. Chances are you would get into knock before that depending on ignition timing, CR etc.

But I don't think thermal management for small capacity aircooled 911 engines is radically different from stuff you have to do on big capacity engines. Bore is somewhat smaller but overall engine dimensions are the same. It wouldn't surprise me if smaller capacity engines are actually easier to call as there is more material in the heads compared to bigger ones.

So if your engine is not knock-limited (which means you can play back-forward with timing for given boost), I wouldn't let the heads go past 200 deg. C. (Un)fortunately, hotter heads mean higher probability of knock so I believe you have greater chance for running into knock than melting the heads.

As there are so many factors to take into equation here (boost, CR, twin plugs, IAT, octane, ignition timing) I would venture and say that overdriven fan is probably the easiest way of solving temperature problems if such exist. It introduces some other challenges itself (belt shredding due to generator inertia, failed generators etc.)
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Old 08-03-2009, 03:57 AM
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I have read/heard 450F is the working limit, which is consistent with Goren's data. But as those temps are approached, it's almost impossible to NOT have some pre-igniton, which will create hot spots. I've seen pictures of cooked heads where little droplets of aluminum where embedded in the pistons as the heads started to melt.

Last edited by kellcats521; 08-03-2009 at 04:39 AM..
Old 08-03-2009, 04:35 AM
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I do measure temperature on the lowest (closest to the cylinder) fin on my cylinder heads as a surrogate for coolant temperature for my EFI system. I measure it on the back side (closest to driver) of cylinder 3. I use an RTD epoxied directly to the fin.

I have never seen higher than 250.

I don't see what it has to do with maximum power limit for air cooled engines though. It is the internal surface temperature that limits power due to pre-ignition and structural concerns. The fin temp is something else entirely.

We use laser induced florescence and visible light tomography to identify "hot spots" inside a running engine now. I think 450 isn't a bad number to use. It depends on effective compression ratio, and mixture turbulence too. The hot spot is usually on the edge of the exhaust valve (see why we have sodium filled valves?). Obviously, this stuff has been raised to a high science in this, the 111th year of the Otto cycle.

The bulk gas temperature in the cylinder is not the same as the surface temperature either. A boundary layer forms close to the surface, and this actually limits heat transfer to the inner metal.

Usually, when you see melted metal in the combustion chamber, it is an effect of detonation. Detonation destroys the boundary layer, and allows heat fluxes an order of magnitude higher to the chamber walls. That will melt aluminum in a hurry.
Old 08-03-2009, 08:28 AM
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Thank you all. 450 degrees sounds right however I still find it odd this is so difficult to find and is still not directly attributed to a factory source (a corporate secret?).
Regarding location in my web searching I was reminded the 3.2 Motronic engine has a CHT sensor. I don’t remember it’s actual location. Can anyone remind us?
Old 08-03-2009, 10:47 AM
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Marty, Excellent discussion starting here.

The 3.2 motors have a CHT located on the forward side of #3. It mounts under the section of engine tin. Sal (Scarceller) has logged a lot of data from the 3.2 Motronic. He said that 320F is the peak under a high, hill-climbing load. I do not know what AFR the car was running to get them to this temp.

Wide Band O2 + data logger for 84-89 Carrera

I tried mounting a RTD on the fins of my heads. The problem was that the cooling fan would move so much air that I never read over 130F. Mine was located in the corner of #1 right behind the block-off plate. I wish I had had my heads drilled for a CHT insert.

I have the ability to log CHT with the EFI system. My psuedo-CHT measurement is only used as a warm-up indicator. I am more than willing to experiment if someone can suggest a better mounting location.
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Old 08-03-2009, 11:53 AM
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Hmm... I think something else might be wrong with your RTD JP , as the cooling air temp is also quite a bit higher than 130F under load. One has to make sure that the RTD used is compatible with the calibration in the ECU. I wonder if yours is.

It is not odd to find the maximum surface temperature atributed to a "factory source". The vaults are quite full with the secrets to the 200 mpg carburetor; we have no more room secrets ;-).

The reason is that the maximum surface temperature allowable is well below the autoignition temperature (around 530 F) due to the effect of the combustion process itself. The flame front starts at the plug and as it advances the pressure and temperature of the mixture in front of the advancing flame front increases. If the walls are hot enough, it will ignite and produce knock. Obviously this tendency is highly dependent on the combustion geometry, so an easy number for all engines does not exist.

Air-cooled engines have a problem here, since the hot spot is the exhaust valve, and it is a big one that is hard to cool; the fins are a long ways away. Check out a cross-section view of the Porsche cylinder head some time, and you will get an understanding of the problem.
Old 08-03-2009, 03:04 PM
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Quote:
Originally Posted by Speedy Squirrel View Post
The reason is that the maximum surface temperature allowable is well below the autoignition temperature (around 530 F) due to the effect of the combustion process itself. The flame front starts at the plug and as it advances the pressure and temperature of the mixture in front of the advancing flame front increases. If the walls are hot enough, it will ignite and produce knock. Obviously this tendency is highly dependent on the combustion geometry, so an easy number for all engines does not exist.

Air-cooled engines have a problem here, since the hot spot is the exhaust valve, and it is a big one that is hard to cool; the fins are a long ways away. Check out a cross-section view of the Porsche cylinder head some time, and you will get an understanding of the problem.
Yes yes yes but Porsche knows a location(s) they can place a sensor and based on controlled lab testing and racing of other 911/930 based engines, often to destruction, know when this particular engine is overheating or not. That's what we are after here.

Let's accept for investigation the sensor on the 3.2L motor is in one such location. Do we have any tuners like Steve Wong who map chips for high performance street and track use?
Old 08-03-2009, 03:47 PM
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Bottom line, keep the engine cool. Best mechanism? Water.

Does this mean a water jacket around the head? No. There are 911 racers in the SouthWest that use water misters behind the fan that blow misty air down around the heads, using the heat transfer capability to cool the engine in a open circuit (vs. closed circuit of a water cooled head), keeping temps way down. Also the use of alcohol and water in the intake tract keeps charge temps down, mitigating heat.

Even an air cooled motor can be water cooling assisted.
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Old 08-03-2009, 04:53 PM
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