Is It Really Running as Hot as Indicated?

[Jeff Higgins]

I'm starting to suspect maybe not... Here's my story.

My car is a 1972 equipped with a high compression, twin plugged, MFI inducted 3.0 liter. It's been fairly well chronicled on this forum. I track this car regularly. I have been fighting, from day one, oil temperature problems on hot and humid days.

When first completed some 16-17 years ago, I started out with a single Elephant Racing "Widemouth" fender cooler and finned lines. This has always gotten me through our relatively cool Pacific Northwest track days, but had always been lacking when I find myself somewhere warmer, like SoCal.

So, I added a second "Widemouth" along with the ST style crossover lines. Honestly, it didn't make as big of a difference as I was hoping. It still, on the hottest and most humid days, runs "too damn hot". This last Wednesday found me at our local Pacific Raceways on a 90+ degree day with 70% humidity. It took no more than 10-12 minutes of lapping for oil temps to reach 250 degrees, so I pulled off early from every session.

I believe I'm getting plenty of air flow to these coolers, running 911 "R" style open horn grills. I run a fan on the right, no fan on the left, plus some holes in the bumper on the right left over from my single cooler days:





I have tested both thermostats, and I have tried two different sending units. Everything seems in order with those. I have not tried a different gauge, but it always reads "normally" - 180 to 200 - on the street, even on hot days, when driving as irresponsibly as I dare. Both coolers get hot, so both are seeing oil, no restrictions apparent.

So, here is where it gets a bit interesting. A buddy was running his SC, featuring just one Carrera cooler, no venting to it (not even that little scoop). His was running at a gauge indicated (he retrofitted to one with actual numbers) 220-230 degrees, so about 20-30 degrees cooler than my car, in spite of my additional cooling capacity. So, we broke out the little thermal heat gun, and checked numerous locations on both motors and cooling systems.

Those numbers were essentially identical between our two cars. The cases where the temp sender mounts, the sump plate, valve covers, oil tanks, oil coolers - pretty much the same between both cars. Yet my gauge shows 20-30 degrees hotter. Weird.

Thoughts?

[dannobee]

Verify temps with an IR gun at the case, coolers, and various pipes.

[fintstone]

If I had to guess, I would say you have a bad gauge or a mismatch between your gauge and sender. If you are running a gauge from a '72 and the typical 3-liter sender, that is certainly the case.

[Flat6pac]

Make sure you have matching gage and sender.
The 3.0 uses a chrome temp sender
The 72 uses a brass temp sender.
Bruce

[Tyson Schmidt]

Ditto the above. The difference between the senders and gauges is just about exactly 30 degrees too.

[Jeff Higgins]

Yup, that was one of the things I knew had to change when I first built this thing, the temp sender. It does have the correct early sender. I even stole the sender from my son's '68, which has the original 2.4 from this car in it, and tried that one a month or so ago. Same part number, as verified with both in my hand, so I didn't really expect any change, and didn't see one. So, I know the sending unit is correct, and functioning.

I'm no electrician, but I think I understand that the temp sender changes resistance as it heats up, which is how it communicates with the gauge. So, I'm wondering - what if the wiring in between is faulty and, as it heats up, my be adding to the "signal"?

[fintstone]

Nope. Not hot wire.

Since you are likely not the original owner of the engine or gauge, look at your sender again and see what the numbers on it say. Maybe show a photo. Also of your gauge.

If no mismatch, you likely have a bad sender or gauge. Borrow you son's gauge if his is identical and see what it read (or vice versa).

[rwest]

Thinking out loud here, could you ohm out the sender in some hot liquid measured with an accurate food thermometer and then when your gauge shows the same temp as your baseline, read the ohms on the installed sender and compare?

Another possibility that a co-worker has been having on a water cooled muscle car is that he’s flowing too much so the coolant isn’t getting enough time in the cooler.

[Jeff Higgins]

When I swapped out sending units, I verified that they were both the same unit and the correct unit for my car. Both read identically, so I assume both are functioning properly. Unfortunately, his gauge mounts completely differently, so it would be a bit of a hassle to swap those out. Maybe I'll find a buddy locally who can loan me an early gauge that would fit. That might be next.

So, here's a weird thought. Why would Porsche change the sending unit with the advent of the SC? Could it be the differing thermal properties of the new aluminum cases, replacing the mag cases?

[Tyson Schmidt]

Forgot to mention, if you use a late sender with an early gauge it will read 30 degrees low.

If you use an early sender with a late gauge it will read 30 degrees high.

Is it possible some swapped in a later gauge? They oil temp gauge section easily unscrews from the gauge housing and is easily swapped. Might have been changed at some point.

[Jeff Higgins]

Aren't later gauges readily identified by their lack of any numbers?

I'm the second owner of this car, having owned it now for over 20 years. It would be unlikely that the temp side of the gauge has been swapped out. I certainly haven't done so.

The gauge reads as expected under normal driving conditions. I can actually see when the thermostat to the front coolers opens, and when it closes. Temps will range from 180-ish to 200, 210-ish, going up and down as the thermostat opens and closes on hot days. Anything under about 85-90 degrees ambient and it never gets above about 180. This all leads me to believe that, in those oil temperature ranges, I'm seeing accurate readings.

On the track, on these hot and humid days, it will take ten to fifteen minutes to exceed about 220-230, but then it's "off to the races", with it hitting 250+ within minutes of doing that. I can almost watch the needle move once it exceeds about 220-230. It's pretty amazing how fast, once it gets "hot", it progresses to "too damn hot". It's certainly not a linear progression, time-wise.

Could I simply be overloading the thermal capacity of the old ST style coolers? After all, Porsche never ran anything larger than a 2.5 liter before they went to the center mount cooler for the 2.8 liter RSR motors. While the twin fender mounted coolers do have greater surface area than the single center mount, is air flow that severely compromised as to contribute to this? The last thing I want, aesthetically, is to have to add that center mount to my '72 "R"...

[fintstone]

Why guess when you can walk out to your car and see what gauge you have and what sender?

If no mismatch, probably either a bad gauge or sender...or both. Two Carrera coolers and hard lines should sure cool that down.

[Sboxin]

Jeff,

We went thru this issue with a 3.8L 964 race engine. There are some threads about
"air flow" . . . This may help you to improve your oil temps.

We also run near/at 250F - - but we are using 3 coolers (2 front fender, 1 center) and
large air flow sheet aluminum to make sure the air gets thru the fender coolers. I'll
look for our old photos of the system . . .

Another help is using the best oil you can buy - we use Amsoil Race 15-50 synthetic
and have a 15 quart system. Over 10 years for this engine we have only had worn
rod bearings (not crankshaft) and worn camshafts and rockers (mostly due to cold
starts). Rings and cylinders are great - no wear.

My son (the driver) often is pushing the engine to 6-7K rpm because we use shorter
gears for short tracks. . . sometimes he will short shift to cool the engine down on
longer tracks - but 5th is 0.96 . . . he mostly runs in 3rd and 4th gears.

Your engine probably has the engine mounted oil cooler (we do not)? This plus the
front two fender coolers should be enough for you - - BUT, with better air flow in the
front - that's where the Porsche center cooler can really help (we have a body mod
that allows the center air flow to exit under the car).

You have a beautiful '72 911 - - we had one back in the day that was Aubergine . . .

Wish you the best on your project,
Regards,
Roy T

[gled49]

I had a temp anomaly in my outlaw with front cooler on cold mornings. Temp would rise to 180* then drop 40* in a minute, thought it was thermo opening, etc. then temp would rise again slowly back to 180*. I wanted to change back to metric instrument, sent gauge to Palo Alto got it back and anomaly gone. I guess some gauges wear out. Gordon

[Sboxin]

Jeff,
This is a photo of our air flow duck Pass side . . .
I completely understand you not wanting to change the '72 911 appearance . . .
Regards,
Roy T

[Jeff Higgins]

Fint, I pulled the temp sender and verified the part number. 90164163200, which is correct for the year. I pulled the gauge but, unfortunately, no part number is visible. The only markings are "VDO Made in Germany" on both the temp and pressure gauges. I did note, encouragingly, that the green "witness" paint mark is still on the fasteners that hold both of them into the gauge housing, so it's apparent that neither has ever been removed. So I think I'm comfortable that we have a match.

Gordon, I was hoping someone might chime in with experience with a gauge malfunction. I need to swap gauges and eliminate that as a possibility. Unfortunately, with it cooling down up here in our Pacific Northwest, I won't get to test it at another really hot track day until next season.

Until then, Roy, I believe I have a little project to complete. My passenger side cooler is all sealed in all around with sheet metal and what looks like the same foam pipe insulation you guys used. All of the air that gets to it flows through it, it has to. The driver's side, however, I have not yet taken the time to seal in a similar fashion. Nice easy fun little winter project.

Before I added the second cooler, I used to run with the passenger side headlight removed, replaced by a wire mesh screen. I have cut most of the headlight bucket out behind it. That, and the holes in the bumper, were my earlier efforts at cooling with a single cooler. I figured once I completed my "R" tribute (my version of what it would have been had Porsche continued to make them into '72), opening up the horn grills as I did and adding the second cooler, that removing the headlight and/or sealing in the second cooler were just "gilding the lily". Looks like I'm going to have to rethink that. Pre - "R" conversion:



I'm running Brad Penn 20w 50, and have been for probably 15 years. It sees probably eight to ten track days per year, and has just rolled over 100,000 miles. I certainly have nothing to complain about with regards to longevity. The high temps, being an old Bruce Anderson disciple, worry me though. I'm firmly into his "too damn hot" range.

But, is it, really, in this day and age? My other interests run towards motorcycles. Old Ducatis and Harleys, with a Honda thumper for the woods. In each and every one of those arenas, we would be absolutely tickled pink if we could get them to run as "cool" as 250 degrees. Run an old air cooled Ducati really hard, and you'll soon see 280-290. The Honda will run all day in the desert at 300-ish. Yet no one ever seems to worry about it.

[Schulisco]

Is it only getting hot at humid days or also on others?

Do you know (and measured!) the ignition curves/settings on the car? (Too) Late ignition or defective advance of the dizzy can cause serious exhaust gas (and oil) temperatures...

Thomas

[targa72e]

You should be able to test and compare the temperatures seen with a IR gun to the gauge to get a good idea if it is reading correctly. The oil filter is at the coolest part of the oil circuit after it has gone thru all the coolers. Use the IR gun to read the temperature of the filter. The temperature sender being at the end of the main galley should have a temperature very close to the temperature at the filter. Keep in mind the temperature seen at the gauge is the coolest oil in the system (oil in tank should be the coolest).

john

[manbridge 74]

Originally Posted by Schulisco

Is it only getting hot at humid days or also on others?

Do you know (and measured!) the ignition curves/settings on the car? (Too) Late ignition or defective advance of the dizzy can cause serious exhaust gas (and oil) temperatures...

Thomas

I was going to mention this. A noticeable temp difference with different ignition timings on my MFI 2.7RS engine. I follow the factory spec fairly close at 36-38 BTDC at 6000.

I think the reason air cooled bikes get away with higher oil temps is cylinder head location that is in direct airflow vs 911 fan cooling. They also have less weight which lessens engine load.

Many other small items help with temp control. Sealed tin in engine compartment, full opening of engine mounted T-stat, fan pulley ratios, etc.

[rdimarco]

You can bench test the sender/gauge combo. Pull the sender, put it in boiling water or hot oil with a good thermometer to verify temps. If temps are really getting too hot, consider a leak-down test. My oil temps started getting too hot due to excessive blow-by.