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Radiator cap pressure rating

Does anyone have the radiator cap pressure rating handy for a 944 na?
Old 04-12-2016, 10:16 AM
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Early/late use a different cap.

NOTE: the radiator cap should be a KPA 150 or 21 Pounds--the expensive one for '87 and later cars.

The cheap KPA 110 or 15 pounds cap is often bought or given by the parts counter person because it "fits" and "should be OK-maybe" NOT!

(copied)

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Old 04-12-2016, 10:32 AM
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For a 85.5 na car?

How about an 87S car?

Where is the spec? Workshop mamual?
Old 04-12-2016, 10:39 AM
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only functional difference should be the coolant boiling point, right?
Old 04-12-2016, 10:57 AM
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Quote:
Originally Posted by djnolan View Post
For a 85.5 na car?

How about an 87S car?

Where is the spec? Workshop mamual?
The spec is in the parts manual... different part numbers for early cars than for later (series 2 and 924S) cars.
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Old 04-12-2016, 01:40 PM
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Our sponsor has the info:



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Old 04-12-2016, 03:10 PM
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The only thing I could find on the pressure rating was in the 88 workshop manual and it was for 1.0 +- .1 KPA. I couldn't find any other years info.

The Behr caps I currently run have a 150 stamped on them.

Our host doesn't provide a pressure rating for their caps.

I'm confused...
Old 04-12-2016, 03:50 PM
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FWIW - The early 2.5 liter cars used a 1.0 (15psi) cap. The later 2.5-3.0 used a 1.50 (21psi) cap.

I have an 88 924S and am using the early 1.0 cap for several reasons.

1. Am using a 160 degree thermostat with a cooler fan switch, so don't need the higher pressure cap to prevent coolant boiling.

2. With the lower pressure, the water pump seals, and water cooler seals have less stress and should last longer.

3. With the cooler engine temps there is less expansion of the aluminum block and head therefore less tension increase on the timing and balance belts. They are under less heat and dynamic stress therefore should last longer.

4. Underhood temps are less therefore less deterioration of wiring harnesses and rubber parts, i.e.. seals, hoses, rubber fittings, zip ties. power steering fluid, hoses and rack seasl etc.

5. Also, even the factory galvanizing on the metal parts is staying fresh and not changing color.

So this has worked for me for the last 15 years.
Old 04-12-2016, 10:02 PM
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This seems like a fairly sensible de-tune for an old cruiser like my 85.5. My trouble is the oil thins out and the pressure gets low after a run on the highway. My thoughts is higher cooling system pressure could transfer more heat from the block to the radiator but I am already using the 150 cap. I should have used the colder thermostat when I changed it recently.
Old 04-13-2016, 03:14 AM
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Quote:
Originally Posted by ltfisher View Post
FWIW - The early 2.5 liter cars used a 1.0 (15psi) cap. The later 2.5-3.0 used a 1.50 (21psi) cap.

I have an 88 924S and am using the early 1.0 cap for several reasons.

1. Am using a 160 degree thermostat with a cooler fan switch, so don't need the higher pressure cap to prevent coolant boiling.

2. With the lower pressure, the water pump seals, and water cooler seals have less stress and should last longer.

3. With the cooler engine temps there is less expansion of the aluminum block and head therefore less tension increase on the timing and balance belts. They are under less heat and dynamic stress therefore should last longer.

4. Underhood temps are less therefore less deterioration of wiring harnesses and rubber parts, i.e.. seals, hoses, rubber fittings, zip ties. power steering fluid, hoses and rack seasl etc.

5. Also, even the factory galvanizing on the metal parts is staying fresh and not changing color.

So this has worked for me for the last 15 years.
Thanks for the detailed explanation. The AZ metropolitan summer temps are brutal.

I also use the 160 degree thermostat with a cooler fan switch.

I do have the 21 psi cap on the expansion tank on both cars.

I found this for v2rocket. I do use Water Wetter for an ounce of additional insurance.

Beating the Heat: Advantage of a High Pressure Radiator Cap | Tuner University

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Old 04-13-2016, 06:55 AM
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One thing I remember doing on this car was changing the exhaust header from tubular to cast iron I wonder if this is making it run hotter. The old tubular header was cracked
Old 04-13-2016, 11:11 AM
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Do you have a lot of flying insects in VA? If your car is running hotter than it used to maybe the front of your A/C condenser or radiator is partially blocked?
Old 04-13-2016, 02:46 PM
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I wish it were something I could take care of with a garden hose. But even when the temps are in the 50's the oil thins out.
Old 04-13-2016, 03:11 PM
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In the other thread you said you had 10W30...
Your oil is too thin, to be in a 944 in non-winter weather...
Change out for 20W50...watch your oil pressure rise.
Old 04-13-2016, 03:57 PM
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For the record my 88 924S (mfg date Nov 1987) has a Behr cap with 150 on it. This is the original cap from the factory, I've owned this car since new.
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Old 04-14-2016, 12:57 PM
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I know this is an old thread but why are there different caps? What's different about the configuration of early/late necessitating different pressures? We put a 16psi on whilst chasing issues. Still chasing, going back to original 21psi cap. Just curious. We're running an '83 engine in an '87 924s and have erratic coolant behavior, levels moving around, overflowing, Car will drive all day and not get hot but levels are weird and she dumps on shut down. Also see some bubbling in expansion tank that has me worried about HG failure. We have bleed and bleed and even rented a pressurizer to force air out. Will continue to chase air whilst going back to original higher psi cap. Am planning on gravity and time bleeds between runs and see what happens.
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Old 03-07-2018, 08:42 AM
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there are NO differences between the early and late model NA cooling systems.
rad is the same, water pumps the same, blocks the same, exp tank the same, most hoses are the same (except the lower water pump hose longer on late models).

higher pressure only means higher boiling point and reduces likelihood of "micro bubbles" in hot spots in the engine.

bubbling in the tank from a purged system means HG, sorry...unless somehow you have a "backwards leak", under pressure the leak seals water in but as car cools (water contracts) it might be sucking air IN somewhere?
Old 03-07-2018, 09:22 AM
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Well a head gasket failure wouldn’t be the end of the world we’ve done everything to this car so far will just be another adventure but I suspected that earlier and ran a bunch of tests all said good so I’m gonna keep resisting that diagnosis
Old 03-07-2018, 09:27 AM
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I will add that when attempting to pressurize the coolant tank I found several leaks at various clamps, this was at about 15 psi. I got 'em all. Now going to try running the 21psi cap and that might induce more failure points. Spring is coming. C'mon little Porsche.
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Old 03-07-2018, 10:17 AM
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if you get a spare cyl. head ahead of time and have it freshened at a machinist (new valve seals and a deck cut at minimum, guides if high mileage or just worn), you can get your old head off, clean block, apply new gasket+fresh head and get the whole thing together in about 6 hours.
Old 03-07-2018, 10:31 AM
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