CHT vs Oil Temp

[Amplus]

Is there a need for monitoring Oil Temps if you are already watching CHT? I've got a LM1 with Aux Box and it comes with a thermo couple for monitoring either CHT or EGT.

I guess my question is does oil temp tell a different story than cylinder head temp or are they both just giving you a feel to if your engine is overheating? Is one a better indicator or more accurate?

I plan on monitoring AFR, #3 CHT, Oil pressure, throttle position and rpm with the aux box. It is capable of recording one more 0-.5V signal.

[Jimmy111]

They both mean different things. You are not overheating untill your oil is too hot. You can have 400degree head temps and 180 degree oil. This shows a fuel problem. You can have 350 head temp and 250 oil. This is a airflow or oil flow problem. Get it?

[Eaallred]

I overheated my camper bus motor and killed the motor without getting oil temps over 180.

Get the heads hot, and it don't matter what the oil temp is/was, your day is done. Mine was, and it happened on my honeymoon trip to Yellowstone. Great memory for me and the wife to cherish for years to come. Gee, and I thought I was doing myself a favor installing a killer oil cooling system on that thing to save me any and all headaches.

IMHO, head temps are critical. As long as the oil temp doesn't get hotter than the oil itself can handle, then oil temps don't matter.
_________________
Eric Allred

You have to remember something: Everybody pities the weak; Jealousy you have to earn.

[Jimmy111]

Yes eric. But that is not the motor overheating. You had a problem with something else. Your oil will get scorching hot from flowing thru the heads when they are too hot. Unless you had a really huge amoung of oil cooling. Im kinda suprised that you did not notice that something was wrong. Was it a type 1 or type 4?

[Eaallred]

[SkrapMetal]

I almost cooked my stock 1600 due to dirty fan shroud, dirty cylinder tin, dirty cylinders. While my heads reached 450F or so, my oil was about 190F. Run oil thats too thick with a huge pump, and your oil will probably get too hot while your head temps can remain normal. If oil and head temps are related, it's a very very distant relation. Sort of like a 5th cousin... or something.
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oo9less since 2007
-72 Ghia-

[zorrro]

I think I saw this in some old Gene Berg literature. He did a discussion on his opinion of gauges. For the most part the quality of the automotive gauges and sending units is pretty poor compared to "real" test instrumentation and for this reason he didn't care for gauges. However this part of the discussion I remember so well I'll quote it. "If you want a gauge that's really worth something, get a cylinder head temperature gauge." The reasoning was that cylinder temperature changes can happen rapidly and allow you time to react before they come critical. Oil takes time to heat-up and time to cool down. It may too late to save an engine once you notice your oil is hot...........Z

I have a great deal of respect for the wealth of knowledge Gene Berg has contributed to air cooled VW performance and I apologize if I've misquoted the man. I do think I've correctly summarized the intent of the discussion.
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'68 Fastback. 1776cc, W-100 cam, 34-ICT, balanced c/w crankshaft. 70mi round trip daily driven. Never restored.
1970 beetle. (WIP) new heater channels and various body sheet metal. Owned since I was 12, sold when I was 38 bought back when I was 46! Most likely on its way to be a street/class 11 machine…

[Amplus]

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FWIW, several years ago when Eric Allred was McGyvering on his Bus, he drove around for a while with NO OIL COOLER AT ALL (not even the stock one in the shroud). And it did not run hot oil temps unless he was over 50mph or so. Crazy.

He knows a lot about oil, oil temps, and cooling, so listen to him.

John
Aircooled.Net Inc.
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[Jimmy111]

The cylinder head temp is mainly a factor of fuel load and mixture. The oil temp is a mainly a factor of bearing pressure and cylinder head temp. Cylinder temp contributes to this by the oil flow thru the head. On tests I did, the oilflow was much more important than the airllow when the motor was removed from the car. Many people who have had flaps and a thermostat in their shroud have seen that when the thermostat fails closed the car can still be driven around untill the oil gets too hot. This is a factor of the oil cooling the head.
The oil gains most of its heat thru the compression of the oil film thru the bearings. This heat rise of the oil can be as much as 80 degrees on a 1600 at a 45hp load at 3800 RPM.

Both are important. They just have different causes.

I dont know why eric fried his heads. Possibly the altitude change?

[[email protected]]

Head frying can happen climbing the "hills" we have around here. The oil temp will be fine but CHT thru the roof.

I suspect he didn't have a CHT gauge. We've all been there, usually some blow up leads to an increase in our knowledge.

I consider oil temp to be proportional to RPM, CHT to load. You can free rev a motor on a stand with no load and overheat the oil if you want to prove this to yourself. Especially with thick ass oil and oversized pumps. Most people just stare at me when I tell them their oil temps would drop if they'd use a thinner oil. OK, don't believe me? Use some Valvoline 60W and really cool it off.

John
Aircooled.Net Inc.
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[nikita]

John, There you go again trying to inject facts and logic into a discussion.

Here is my take on this. CHT can change in seconds, where oil temps take several minutes to move significantly. Oil temp and pressure relates primarily to the bottom-end health of your engine where parts are pressure lubed, CHT and EGT the top end where combustion takes place. Ive repaired/replaced many valves and heads and never had to replace a crank in stock engines.

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I could be wrong but I think I heard that the Berg's cam break in machine gets oil temps around 170F, and that is with NO COMBUSTION, just the valvetrain!

John
Aircooled.Net Inc.
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[Jimmy111]

There are big hills around where I live. But I havent ever seen temps over 400.. The cylinders and heads are cooled mainly by the exhaust and incoming F/A mix. The blower is there mainly to provide circulation because it is in an enclosed space. The fan requires a lot of HP to overcone the pressure differances between the incomming negitive air pressure and the presurized area under the car. The freer flowing the exhaust and the higher the vaccum, the more cooling of the heads and cylinders that takes place.

I have 1915's and 2332's on mining equipment that is completly unshrouded. They run at about 400 CHT and they are jetted a little rich but I have little problems with them. I just use 72 plate oil coolers with fans and have little problems.

I always thought that VW should have designed a draw thru cooling system that would pull the air from Below the car and exhaust it out the back. Take advantage of the 2 psi or so under the car and reduce the energy load on the motor.

I know that busses are much more prone to overheating the heads. I had 2 busses and never had CHT guages so I couldnt tell you the temps... But I never fried the heads either.

Just my experiences and views. I really need to look into the overheating problems with busses.

John. I have a good study on bearing oil clearances and heat generation if you are interested in it. It is not VW but it is good.

[Eaallred]

Here's my take on it from what i've experienced.

Head temps are related to engine load. That's about it.

Oil temps are slightly effected by head temps, but the majority seems to be effected by engine rpms. Heavy valvetrain/springs seems to increase this heat over light stock components. My theory there is the flat tappet cam and lifter setup has the oil taking a lot of the friction/heat abuse. The more load on this (via lifter to cam pressure, and an increase in rpms) the higher the oil temps get.

Oh yea, John, the 'magic' number was about 53mph. As long as I kept it under that speed, the oil temps stayed under 220 in that 90 degree weather.

One more bit of useless blabbering from me. My drag motor running E85 runs cool as a cucumber on the cylinder heads. But just idling (to get the head temps up before I run) with those K-800 springs and big heavy stainless valves, my oil temp will get right up close to 200 degrees even before I make a pass down the track. When I ran gasoline, the heads warmed up so fast oil temp was never an issue, lol.
_________________
Eric Allred

You have to remember something: Everybody pities the weak; Jealousy you have to earn.

[Jake Raby]

[1432]

If cylinder head / cylinder wall temps have no little or no relationship to oil temps, think about why our basic water cooled cars need no added oil cooler to maintain oil temps no higher and usually lower than their corresponding water based coolants.

[Jake Raby]

[1432]

Jake,
I guess you just don't see the basic relationship I'm pointing out. the one that's been relied on for over 100years by the automobile manufacturers

[Jimmy111]