Grady..little experiment is done..

[bigchillcar]

grady,
going back to our discussion of selecting an optimum cruise rpm to maximize engine cooling, i tried the idea of running in 4th gear on the highway instead of 5th gear in the heat of the day (my stock 2.7 doesn't have an aux oil cooler).

I used a route I drive often of one hour at 75 mph and the temp today was 90 degrees (a/c turned on, but sucking as usual). Normally, in 5th gear i'm seeing 3,400 rpm at this speed and temps of 220-230. I tried the trip in 4th gear with engine rpm's at 4350 for 75 mph. after about 20 minutes, my oil temp had climbed up to 240, so i abandoned the test at that point, shifted into 5th and after about 10-15 minutes, temp dropped back to 230 where it remained the rest of the drive.

We talked about switching my fan to the turbo fan (245 mm) and pulley (i would need a double pulley) with a 1.82:1 ratio, as opposed to my current 1.30:1 version. My hunch is that had I the turbo fan today, your idea would have worked. In fact, i'd bet that were i to have a ratio of 'less than the 1.30:1' that there is a specific ratio where there would have been no difference between the 4th and 5th gear cruise temps..BUT with the higher ratio you suggest, which would be moving much more air, the difference between the two gears would become significant. i'm betting that your theory is dependent upon that fan ratio..the faster a fan can turn (as in the 1.82:1 version), the more air moved across the heads, the more cooling impact it would have on the oil temp as a consequence. it just doesn't work with a ratio of only 1.30:1..fan needs to turn much faster for 4th gear to improve engine oil cooling. would you agree? i bet there's an exact ratio out there where 4th gear and 5th gear at 75 mph would result in equal oil temps were you to graph them..and it would be somewhere between 1:1 and 1.30:1..

bottom line: i need the faster fan or an aux oil cooler or both. oh..and $$$
ryan

[Bobboloo]

There are other things you might consider as well to control your temperatures.

You might try synthetic oil. Higher octane. Dare I say it, Cool collar. Also, is all your ducting in good shape. i.e. no holes in any hoses or missing covers for spark plug holes? Maybe go to block off plates and disconnect the heater hoses.

Combining several inexpensive things might get you by for now considering that you are borderline.

[Grady Clay]

Very interesting.

What that tells me is the heat being put into the oil went up faster than the increase in cooling from the greater air flow. I wonder why.

I can think of two different possibilities. First the heat being generated increased. The other is that heat that was being dissipated somewhere else ended up in the oil.

The increase in heat generated can come from several sources. The most obvious is the increase in number of combustion cycles per unit time. More fire = more heat even if the load is less for each. Did you feel is if you were having to apply more or less throttle at the higher revs of 4th gear compared to 5th gear? I wonder what the change in fuel consumption is?

Another reason could be an increase in “pumping losses” inside the crankcase. That is from moving all the air/oil mixture around the crankshaft on the back side of the pistons. The engine is also having to provide more power to the fan, cams, oil pump, etc. There is an increase in friction between the rings and cylinder walls. The net result is more heat being generated and is dissipated by the increase in temperature.

Another possibility is the piston squirters weren’t open at 3400 rpm and were at 4350 rpm. Porsche specs them open at 4-5 bar (42-56 psi) oil pressure. The oil supposedly reduces piston temperature by “approximately 50C (122F.)” That heat goes from being dissipated by the cylinder (which we aren’t measuring) to being carried by the oil (which we are measuring.) The result might be moving the heat dissipation to an area that we are measuring. What are the oil pressures at these temperatures and rpm?

I am puzzled by this outcome. Does anyone have light to shed on this?



Let’s calculate the increase in air flow from the fan. I’m assuming the air flow (Af) is proportional (k) to the square of the fan speed (w).

Af = k * (w)E2

Therefore the change in air flow is equal to the square of the change in fan speed. Note that if we had included the fan ratio, it will drop out in the equation below just like the proportionality constant (k) does.

Af2/Af1 = (w2)E2/(w1)E2

(4350)E2/(3400)E2 = 1.89x10E7/1.16x10E7 = 1.64

Therefore you increased the air flow by 64% by going from 3400 rpm to 4350 rpm. That is a fairly substantial increase in air flow. For the oil temperature to have increased there must have been an even greater increase in heat produced or change in the heat flow.

For comparison let’s look at the change in air flow at constant engine rpm with the change in fan ratios.

Af2/Af1 = (w2)E2/(w1)E2

(3400 * 1.82)E2/(3400 * 1.3)E2 = (6188)E2/(4420)E2 = 1.96

Therefore you increase air flow at a given rpm by 96% by changing fan ratios. There is the same percent change at every rpm. The penalty is that it uses proportionally more power.

Best,
Grady

[cstreit]

Another possibility is the piston squirters weren’t open at 3400 rpm and were at 4350 rpm. Porsche specs them open at 4-5 bar (42-56 psi) oil pressure.

Interesting. Makes we wonder if a higher viscosity synthetic like Mobil-1 15/50 would create more pressure at high temperature and allow the squirters to open at 3400 RPM. (assuming they may not be now)

[fireant911]

Ryan,
I tried this too and pretty much had the same results - the oil temperature was certainly no cooler at higher rpm's with a light load. Maybe this is only applicable for track events where higher demands are placed on the engine because it just did not reduce my oil temperature on the highway - it caused it to creep up.

I still believe that each car is different in the way it generates and displaces heat and what may work for one will not guarantee similar results on another identical car given the same ambient circumstances. I was a passenger in another Pelican's car during track time and I was amazed at how quickly his oil temperature dropped during his cool down loop. He just essentially removed the added load by slowing down and drove at normal rpms (3,000 rpms) and his temperatures quickly returned to its normal range. He was running an Euro SC engine. The only way that I could closely replicate that quick of a reduction would be to immediately park the car, open the engine lid, and place two box fans directed at the engine - and my car normally runs cool (elephant racing finned oil lines, 28-tube cooler, and Mobil-1).

Last weekend I removed the passenger fog light based on recommendations from this board. I sealed off the bottom area from my valence to the 28-tube cooler and saw some very impressive reductions in temperatures - 93 degrees in Alabama with 185 degree max oil temp.

[patkeefe]

Originally posted by Grady Clay
Very interesting.


I can think of two different possibilities. First the heat being generated increased. The other is that heat that was being dissipated somewhere else ended up in the oil.

The increase in heat generated can come from several sources. The most obvious is the increase in number of combustion cycles per unit time. More fire = more heat even if the load is less for each. Did you feel is if you were having to apply more or less throttle at the higher revs of 4th gear compared to 5th gear? I wonder what the change in fuel consumption is?

Best,
Grady

I believe Grady is correct in the more combustion cycles per unit time. The more fuel you burn, the more heat you must dissipate. Imagine an over the top case of never using 4th or 5th gear. How would your fuel consumption be? What would the temps be like? The idea of the higer volume fan has merits, but likely can't keep up with the heat generated by the combustion process.

Somewhere I have quantative numbers on heat dissipation from cylinders...how much is due to air cooling, oil, etc. I'll try to find them and post.

Grady's air flow calcs are correct, but we really need to see the fan curve for the fan to see if we are overloading the wheel at higher RPM. Our interest is really heat rejection, which can be simply stated as the product 1.08 x CFM x delta T, in BTU/hr. Temp of the inlet air subtracted fron the temp of the air after exiting the engine is the delta T. 1.08 would apply in Little Rock, but not in the Rockies. We actually can calculate the heat needed to reject from the engine from the amount of fuel burned, efficiency of the engine, etc.

If someone can find the fan curve, we can really start to narrow this down. But, Ryan, you are doing an excellent job of field lab work here.

[Elombard]

Another thing, I noticed a dramatic rise (20+ deg) in oil temp when I went from a gutted two in two out to a normal 2 in 1 out. If you can stand the noise put on an open muff for a while and see what that does.

I wonder if you have a small top oil leak that drips thorugh the on engine oil cooler or cylinder fins and its gummed up reducing air flow. I cant remember the details on your car.

[klaucke]

I can confirm Ryan's data, in that w/ my car with a similar set-up (74 2.7 w/o external oil cooling), 4th gear on the highway yeilds no reduction in oil temps, but rather an increase.

However, I'm installing oil lines and a trombone cooler, in order to keep my temps lower. I hit 230ish where I stayed on a 75 degree night, cruising on the highway in 5th. I feel this is just too hot for a mag case 2.7 if I want any longevity out of the motor, not to mention the oil pressure gets pretty low with oil temps around 230 even with 20w50. I think I can get the components for about 700 bucks, and its well worth it when you consider what can be gained. I'll post a thread about the install when the components come in and I'm healthy enough to install them.

[Early_S_Man]

Ryan,

I don't believe air flow is the factor at work in your elevated temps ... your cylinder fins and heads probably need to be cleared [Shopvac to remove the debris after compressed air nozzle loosens] of leaf/twig debris and degreased of any oil/dirt coating. Removing the alternator and fan assembly, as well as the heater duct plates on both sides gives access for cleaning.

Your engine should have been very near its' torque peak at 4350 rpm, thus thermal BTU output shouldn't have been higher than at 3400 rpm due to better thermodynamic economy/efficiency at the slightly higher rpm level! Heat transfer from the fins is the problem, IMHO, and anything limiting airflow and any coating on the fins are two factors that can cause higher temps in the summer with A/C on ...

You may want to check the length of the two oil bypass piston springs to see if they have weakened materially from the 70.0 mm new length spec. I have seen springs showing only 68.0 - 69.0 mm length after years of west Texas heat, and oil pressure was improved after replacement with new 70.0 mm long springs!

Also, the 993 oil resrictors to the cam tower spray bars would be good experimental upgrade to try at this time.

[island911]

Originally posted by patkeefe
I believe Grady is correct in the more combustion cycles per unit time. The more fuel you burn, the more heat you must dissipate. Imagine an over the top case of never using 4th or 5th gear. How would your fuel consumption be? What would the temps be like? The idea of the higer volume fan has merits, but likely can't keep up with the heat generated by the combustion process.
. . .

If someone can find the fan curve, we can really start to narrow this down. But, Ryan, you are doing an excellent job of field lab work here.

Spot on.

fwiw, I remember sending Wil Ferch a fan-curve plot of some sort. I think it was a performance curve of as mounted . .rather than a strict pressure/volume fan curve . ..It was in German, though. Maybe Wil has decipherd it, or has found another.

. .. you really do need the #'s as mounted, rather than a typical pressure/volume fan curve. ..to take a stab at mathimatically finding the sweet-spot. Emperical is always good too.

. . .oh, yeah; like Warren says; clean the thing. (we can't have dirt screwing up the theoretical.

[pwd72s]

Warren made a good point. Could mice be involved? We've all seen those horrid pics of mouse nest debris clogging air flow...

[bigchillcar]

oh, yeah; like Warren says; clean the thing. (we can't have dirt screwing up the theoretical.

lol..easy killers..i know i'm arkansas, fellas..but i assure you that i wear shoes, have running water and there's more than 'one book in the library here!' (clinton library ) okay..i'll make surrrre i don;t have rodent nests, leaves, etc.

couple of things..we know for sure that the fan is required, right?? break the belt and the thing will over heat post haste, so the role of airflow plays a major part in controlling head temps and oil temps. i would like to repeat this test with the 1.82 ratio fan..i suspect that the temps wouldn't have creeped up..may have even creeped down like grady expected with that much greater airflow across the heads. what about a 5-bladed fan under the same circumstances? what's the ratio now? certainly less than the 1.30:1..i'd like to know what temps my engine would see under these conditions. if anyone is willing to send a 5-bladed version, i'm happy to swap it out for the sake of a test. of course if anyone wants to send the turbo fan, i'm willing to do same (i'll return it in october )..

just looking at the situation intuitively, in the absence of any airflow (broken belt) or a 'mega-fan' (blowing even greater than the 1.82:1 ratio..you would expect vast differences in temps to result. i agree that more heat was being generated through combustion in 4th gear than my fan speed was able to compensate for as it could in 5th gear. yes, grady..it did feel like i was giving the car a little more gas than in 5th, although not a lot more..but that could be a mis-perception on my part due to my built-in expectations that i should, but wouldn't fuel flow necessarily have to be greater in 4th in the same way that i must pedal faster in lower gears on a bicycle to keep up with higher-geared bikes?
ryan

[patkeefe]

'one book in the library here!' (clinton library ) okay

Remember, Tuesday is Ladies Night at the Clinton Library.

[bigchillcar]

Remember, Tuesday is Ladies Night at the Clinton Library.

only tuesday? pat..where you been?
ryan

[patkeefe]

http://www.engr.colostate.edu/~allan/heat_trans/page1/page1.html

Why scientists wear pocket protectors and rarely comb their hair.. Above link is a good treatment of the heat transfer, albeit the example is primarily a water cooled unit. It does demonstrate the complexity of the problem, and why experimental data such as what Ryan is performing is very important

There will be a quiz on Monday.

Pat

[bigchillcar]

btw, ryan spent 7 years as a research associate before he became a pilot and writer okay, so it was molecular biology-related

pat..it's the weekend..i gotta prepare for a monday quiz?? i better get to the clinton library..hehe..
ryan

[patkeefe]

I could tell you paid attention in lab class. You need some temp sensors for your experiments. I may have some lying around.

[Early_S_Man]

Ryan,

I wasn't suggesting a rodent nest ... that would definitely cause temp spikes in the 250°F++ range while using A/C at 75 mph!

Summer temps with A/C use bring out the marginal performace apects of every component in the cooling system, and just a very small amount of bermuda grass lawn clippings on the cylinder fins or cylinder head air passages can cause a significant temp rise in the summertime, but wouldn't even show up in winter driving!

I'm not sure that the torque curve of an internal combustion engine with five speeds is analogous to human performance on a bicycle ... even with 21-speed gearing!

[bigchillcar]

warren,
making an analogy between having to pedal harder in the lower gears of a bike in order to keep pace with bikers in higher gears..in the same sense that i would think a gasoline engine would need to perhaps burn more fuel, rev faster in 4th gear to maintain same speed of same car in 5th gear? i'm not sure either, but thought i'd throw it out to try and illustrate what my understanding of the dynamics are. i'm no engineer so attempts at analogies from me are the best i can do.
ryan

[Grady Clay]

Warren’s point is right on. The maintenance items need to be addressed. Every metal surface contributes to the heat dissipation. Anything other than perfectly clean inhibits the heat transfer to the cooling air.

A part like the oil pressure regulating spring could reduce the oil circulation and consequently the heat rejection by the engine oil cooler. If the engine thermostat doesn’t force ALL the oil flow through the cooler at high temperatures, the effectiveness is reduced.

The oil restrictors to the cams would be an interesting experiment. I think I understand why they were fitted to 911 engines but I don’t like that solution. Another thread sometime.

Pat, do you have comparative cylinder data for Nikasil vs. Alusil? I was interested to find out that Porsche tapered the cylinders on the latest air-cooled engines.

Chris, this also plays into the long running oil discussions and the multiple functions of the oil.

This recent thread also has some good information: “Engine Temps and Summer DEs” .


This started as Ryan’s getting his A/C to work best. “hot?! double insult..” Part of that is not killing the engine with heat. The solutions are at hand; more mass air flow, greater delta T while lowering the high temperature, better heat transfer to the air with large, clean radiators, improved heat transfer in the oil system, less heat generation, and much more. The engineering “art” is being most cost effective.

Best,
Grady