Oil temp vs. cylinder head temp question, one low and other high Options

[Bill Shaw]

Just put a new VDO cylinder head temp gauge in during the off-season (after getting scared that the oil temp gauge was off, though the car runs like it's new). After driving it around for a couple of days I have the following results (with Mobil 1, 15W50, in a '74 2 liter):

Oil temp on factory dash gauge - barely reaches the "T" after full warm up and no matter what rpm I'm at.
Cylinder head temp - around 300 driving around town (never gets beyond 325, even in traffic, such as it is in Kalamazoo); around 400 on the freeway @ 3200 in fifth at 75-80 mph

I thought the oil temp and cyl. head temp would be roughly equivalent, at least as far as I can figure from the past threads. The big difference has me stumped. Anybody have any explanation? Am I going to hear any expensive noises soon? What numbers should I be looking for?


(IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)

[Jake Raby]

These can vary and are NOT dependant upon each other! Thats why I recommend a gauge to monitor each- nice to see you have one of each.

Here are my tolerances for temps:
Oil=
180-225 is normal
225-235 Very warm and will thermally breakdown oil in 1500 miles (dino oil)
235+ HOT- Will kill idle oil pressure and erode rod bearings and break down oil in 1K miles

Heads=
275-325 VERY good for flat running with short bursts to 350+ while pulling a hill. About the only time you'll see these temps is with a non stock, more efficient engine. Stockers with injection seldom run this cool.

325-350 good for flat running withstock D Jet with spikes to up to 375 while pulling a hill.

350-375 is warm for flat running and may indicate some tuning issues, bad engine configuration or a cooling system thats missing pieces, etc.

If your engine ever reaches 400 degrees while flat running- Its' hot and will probably go to over 425 climbing a hill. Stock valve seats drop out at about 425 so remember that. A good set of heads with modern seat material will take up to 475 sustained.

As soon as your heads go over 400F you'll need a valve adjustment, as the extra heat will more than likely stretch some valves and could move some seats.

So, your 400F head temp is WAY high and shows that you need some tuning before you kill your heads.

Remember that:
Oil temp is directly impacted by RPM. RPM = friction, friction = heat and it's the oils job to remove that heat. Oil temps are slow to resond to changes due to all the oil having to attain temp before seeing differences.

Head temp is directly related to load and tuning. It responds quickly to change and can vary by 200 degrees in just one mile of normal driving. The faster you go the more the aerodynamic wall forces the engine to work, especially in higher gears. In upper gears engine RPM is less while load is greater and this builds head temps. I often call 5th gear in a 914 "The head killer" because so many people lug the engine at a low RPM while only going 60 MPH. This increases load terriably while killing fan speed and cooling system efficiency. Thats what cracks all those pricey 2 liter heads!

With that being said I'll say that my DTM in the 914 on a 170HP engine with 10:1 CR has NEVER seen head temps over 325 degrees! With the stock cooler removed and only a full flow external oil cooler the oil temp has never hit 205 degrees yet after 3500 miles of driving.

Stock engines work hard with heavy throttle positions because they don't make much power for their displacement. Due to this the hottest temps I have ever seen have came from stock engines, especially those out of tune.

Now for the crazy part- My 3 liter TIV with 235 HP runs head temps of 300 running flat @ 80 MPH and has spiked to 350 once! Oil temp with a very small external Mesa cooler thats really considered inadequate for the huge engine has never been over 230!

Sorry for the long post, but after spending about 3 solid years testing cooling systems and etc I have been consumed by the numbers and their sources...