Adding front oil cooler, in series or parallel?

[Charles Freeborn]

Originally Posted by Dave at Pelican Parts

I believe that it is possible to wind up with less cooling in some circumstances when you add a cooler in parallel. If, for instance, the second cooler flows a lot more oil than the first, but does not cool as much (likely in part because the oil isn't in it very long) most of the oil will go through the new cooler. And you would wind up with less cooling.

That is the theoretical reason for Walt's caveats about "two similar coolers", I believe.

Of course, practice can and often does trump theory, so.... You can give it a try and let us know how it works!!

--DD

I'm going with this. Unless one cooler has smaller fittings / more constrictive construction, you'll get better cooling in series.

[Walt Fricke]

Yes, that's the reason for the caveat. A 935 style front end has a large center opening. Large enough for two 964 oil coolers side by side. That is the situation where you'd run them in parallel.



But the usual aftermarket front valances, whether they incorporate a bumper or, as in this one, are only under the bumper, doesn't have a large enough center opening, nor is there room enough, to put side by side coolers in. If you can get the cooler forward some into the spoiler opening, you get adequate flow out the back for good cooling.



In a modified car you can cut out some of the front tub sheet metal to provide a good path for the cooling air to exit. Like up through an opening in the hood.

The fact is that stock SCs have enough cooling from a front and center cooler of reasonable size and design that they don't need the stock cooler. But why not use it as the second cooler - you will get some cooling from it, and your best cooler will have the best operating condition and do most of the work. If you keep the stock external thermostat you won't be overcooling the oil.

[3rd_gear_Ted]

A 964 turbo fender style nose piece has more space behind it than an IROC nose.
I have now over 3" of space behind my 72 row front mounted cooler on an 80 SC tub.
This is more than twice the space I was able to get with an IROC nose.

I just experienced my first hot event since the 3.6 L conversion at the last POC event in Fontucky (95 degrees @ AAA Speedway). She & I were real happy with the oil temps and my new helmet blower/cooler worked really nice, had a blast

[txhokie4life]

Originally Posted by bpu699

Thinking way back to physics class and fluid dynamics...

Flow is proportional to resistance, I think. So if the resistance in one oil cooler were twice that of the other, it would get 1/2 the flow...but it would still get flow...

By having 2 coolers in parallel, you are dropping downstream resistance, which might actually increase total flow to the coolers...therefore increasing total cooling.

Sounds like putting them in series is tried and true. But I do wonder if doing it in parallel would work better...

In series, oil that has already been cooled off by one cooler, now goes into the next cooler. The difference in oil temp to ambient temp is lower, so the second cooler "cools" much less...

Most cars have their coolers set up in parallel. Think of a bugatti, with its 8 coolers or what have you. They are not in a row, rather, in parallel. Same with the Ferraris: 2-4 coolers, one to two on each side, in parallel. NOT in series.

Same with radiators in pick up trucks. The water coolers are in parallel, not in series...

I suspect that in the 911's added oil coolers are added in series, because its easier... not necessarily "better."

Do realize that there is a time constant to performing the heat exchange.
So if the flow was infinitely fast (no resistance) the oil would spend no time in the cooler and the oil would return as hot as it entered.

Mike

[bpu699]

Originally Posted by txhokie4life

Do realize that there is a time constant to performing the heat exchange.
So if the flow was infinitely fast (no resistance) the oil would spend no time in the cooler and the oil would return as hot as it entered.

Mike

Right... so adding a cooler in parallel decreases the pressure to each cooler to half, thereby decreasing velocity to 1/2.

IE. When done in parallel, the oil goes through each cooler more slowly... In series, velocity is constant regardless of how many coolers you have.

But, it should be a wash either way...

In series, lets say oil spends 4 seconds in cooler #1, and 4 seconds in cooler number 2 at speed X. IE. 8 seconds total cooling time.

In parallel, oil is traveling at x/2...ie, half speed, and spend 8 seconds in each cooler. BUT, only the oil only goes through 1 cooler, rather than 2. So, the heat exchange time in a cooler is the same...

The only "trick" with parallel coolers is that you need to make sure most of the oil isn't simply going to one cooler, and not the other. For example, a 90/10 split would render one cooler pretty much useless.

But a 40/60, 50/50, or even a 30/70 split would cause much greater cooling than one lone radiator.

[cstreit]

Originally Posted by bpu699

Thinking way back to physics class and fluid dynamics...

Flow is proportional to resistance, I think. So if the resistance in one oil cooler were twice that of the other, it would get 1/2 the flow...but it would still get flow...[snip]

In series, oil that has already been cooled off by one cooler, now goes into the next cooler. The difference in oil temp to ambient temp is lower, so the second cooler "cools" much less...

Hmm... Good point. I didn't take fluid dynamics. I agree that the temp differential issue you mention would lend itself to parallel coolers being better.

[KTL]

Makes sense about the second cooler cooling less, assuming the first cooler does its job well. But in this case we're under the impression that the first cooler isn't handling it's business and thus the need for another one.

Another thing to consider is dwell time. While the second cooler may not be as efficient from a temperature differential standpoint, that being ambient temp vs. reduced hot oil temp, the cooler oil is going to pass thru the cooler less quickly and therefore it's dwell time in the cooler provides more time for the heat to be extracted?

I think the series arrangement is the way to go from a plumbing simplicity standpoint.

[bpu699]

Originally Posted by KTL

Makes sense about the second cooler cooling less, assuming the first cooler does its job well. But in this case we're under the impression that the first cooler isn't handling it's business and thus the need for another one.

Another thing to consider is dwell time. While the second cooler may not be as efficient from a temperature differential standpoint, that being ambient temp vs. reduced hot oil temp, the cooler oil is going to pass thru the cooler less quickly and therefore it's dwell time in the cooler provides more time for the heat to be extracted?

I think the series arrangement is the way to go from a plumbing simplicity standpoint.

I think, from my now exhaustive internet search, that series is preffered for simplicity and relative effectiveness...

I guess the only concern left is about back pressure for the scavenge pump...

I see tons of folks using the mazda coolers, but there is no way that thing flows a lot of oil...

Lots of folks are increasing the inlet/outlet size, and I plan to do the same going to AN12... But that doesn't necessary address what the internal flow might be...

Gotta look more into that...

Someone mentioned that the right rear fender thermostat also has a pressure overflow in it? Never heard of that before. IE. If the line is crushed, it bypasses flow to the front fender cooler? Anyone know if this is true?

[KTL]

I don't know how any cooler flows a lot of oil. Looking at the innards of a Mocal cooler, the oil is being pushed thru a bunch of tiny plate openings!



The original Porsche external t-stat does have a pressure bypass function in it. That's why the thing has two threaded caps on it. One cap is for the cavity with the temperature regulator and the other is for the pressure bypass. There was a recent thread about the temperature function

How External Oil Thermostat Works - Illustration

Grady Clay (RIP fine sir) always mentioned that the pressure bypass in the factory t-stat was a really important feature. It has to do with the oil pump scavenging the cold oil which creates major pressure in the system, including the cooler even though the external t-stat is in the cold position

Here's just a few I found. Do an advanced search with the name Grady Clay and search terms like "thermostat" or "cooler" or "bypass" or ..........

Large leak from front oil cooler

Oil Cooling System Advice

Grady was a huge wealth of knowledge and usually when he chimed in it prompted a lot of good discussion.

[bpu699]

Thanks for the info, appreciate it.

Bo

[KTL]

No problem dude.

You going to Blackhawk for Octoberfest? I can't make it but there will be some good guys there camped by the pavilion next to the big mound. cstreit will be there, MotoSook (#550 light blue spec mangina), dzik (#347 red '87 Carrera), StefanS (#187 silver '87 Carrera) and a few others I believe (Ray and Brian?). Good bunch of dudes to talk shop with.

[bpu699]

Originally Posted by KTL

No problem dude.

You going to Blackhawk for Octoberfest? I can't make it but there will be some good guys there camped by the pavilion next to the big mound. cstreit will be there, MotoSook (#550 light blue spec mangina), dzik (#347 red '87 Carrera), StefanS (#187 silver '87 Carrera) and a few others I believe (Ray and Brian?). Good bunch of dudes to talk shop with.

Would love to, but car is apart as roll bar and race seats going in... Never enough time. Still finishing the major on the f-car... Hopefully done this weekend.

Next years though, I plan to go nuts and track a couple times a month, mostly Blackhawk and roadamerica...

I met several folks with the pca at Blackhawk this year, out there three times. Jody was one of the instructors, and I think Chuck...

Hopefully we get to meet soon.

Bo

[stownsen914]

A couple things to keep in mind:
1. Oil coolers have a pressure drop due to internal resistance. If you run in series, this is additive (double the pressure drop for two equivalent coolers). If you run in parallel, I think you would get better flow since each cooler would only have to manage half the oil flow (again assuming equivalent coolers). Good points made about using two non-equivalent coolers - one will likely get more flow than the other.
2. Earlier in the thread, someone made a good point about the flow through some of the small plates in Mocal and some other brands of coolers. It's good to keep in mind that a larger Mocal type cooler with, say, 50 plates will flow substantially more oil than a cooler with half that number, since the plates become limiting factor in the smaller coolers with fewer plates. They key thing is adding up the cross sectional area of one plate, times the number of plates. Keeping in mind that the plates are more restrictive than oil lines, so you want that added area to be something rather larger than the cross section of your oil hose size. (And of course the fitting sizes in the cooler are important as well. Use at least -12, and make sure the internal passages are that large.)

Scott

[bpu699]

Originally Posted by stownsen914

A couple things to keep in mind:
1. Oil coolers have a pressure drop due to internal resistance. If you run in series, this is additive (double the pressure drop for two equivalent coolers). If you run in parallel, I think you would get better flow since each cooler would only have to manage half the oil flow (again assuming equivalent coolers). Good points made about using two non-equivalent coolers - one will likely get more flow than the other.
2. Earlier in the thread, someone made a good point about the flow through some of the small plates in Mocal and some other brands of coolers. It's good to keep in mind that a larger Mocal type cooler with, say, 50 plates will flow substantially more oil than a cooler with half that number, since the plates become limiting factor in the smaller coolers with fewer plates. They key thing is adding up the cross sectional area of one plate, times the number of plates. Keeping in mind that the plates are more restrictive than oil lines, so you want that added area to be something rather larger than the cross section of your oil hose size. (And of course the fitting sizes in the cooler are important as well. Use at least -12, and make sure the internal passages are that large.)

Scott

See the other thread on the mazda oil cooler too...

The flow to the oil cooler is around 1 gallon every 2 seconds? There Is no way thats going to flow through the Porsche cooler, then an AN12 line, then a mazda oil cooler, and then back... not at 1 gallon every 2 seconds at least...

That adds another twist. Perhaps flow through serial coolers is simpler to plumb and somewhat better in general. But doesn't that assume all the oil goes through both coolers? How much actually gets diverted by the pressure bypass valve to the oil tank?

Would then oil coolers in parallel be better, at least at high flow rates?

I wish I had more time to tinker...

[Walt Fricke]

Here are a couple of dimensions to consider when pondering how large hoses, pipes, and fittings might need to be for external oil cooling systems.

The 964 uses a single oil cooler, located up front in a fender (isn't it there?) with good air flow. It works so well that Porsche replaced the engine mounted oil cooler and replaced it with a second (it is the second, isn't it?) oil filter (which they had first done on their racing engines back when). Here are the fittings which connect the oil line hoses (which link pipes to cooler).



The ID of the fitting is 0.580"/14.732mm (more or less). The seal which connects the scavenge side of the 964 pump to the case (and thence to the external lines) is 116.3mm.

The ID of -12 hose is 0.750". The ID of a -12 fitting is 0.609"/15.469mm (again, more or less). The ID of the standard Porsche long oil pipe to and from front coolers is 0.865"/21.971mm. The ID of the barbed fitting (found on the lines which have a hard line and a rubber line)is 0.710"18.034mm. The ID of the hemispherical male end fitting on the steel line from the scavenge output fitting on the case is 0.793"/20.142mm. That steel line itself is 0.750"/19.05mm.



From this one can see that Porsche didn't obsess about line inner diameters, and that -12 stuff should be adequate.

I wondered if there was some easy way to estimate the effect of line length, line ID, and the effects of fittings which either reduced the ID of the line, or increased it. It seems all these can be calculated, but not easily unless you deal with this kind of thing regularly (like an engineer calculating flow/pressure drop). There are different factors, for instance for how the ID of a system is decreased or increased. A thin plate with a smaller hole in the middle has one effect, while a long tapered change of dimension has another. Some losses (or changes) can be recovered - I think most of the change in flow/pressure through a nice tapered restriction is recovered. Obviously the effects of lengthening a path (which increase friction, so to speak) cannot be recovered.

But while I'd suppose an engine cooling system designer for a car company would be expected to do these calculations, along with figuring how well the radiator would work under various conditions, it seems to me that for home brew improvements to the 911 cooling system for track purposes (or the street in hot temperatures) this is rather over the top.

The disadvantages of oversizing things include the added cost. Dash 20 stuff is rather more expensive. Packaging issues to the front cooler (the fittings are bulkier) are there if connecting to the cooler is constrained - as it is in attaching a cooler set in an opening in the center of the front valance with a stock tub to the rear. And running the -12 lines forward under the car is not as neat and protected as running the stock brass alloy (or whatever) lines is, so this is exacerbated when you use -16 line.

[winders]

Originally Posted by Walt Fricke

The disadvantages of oversizing things include the added cost. Dash 20 stuff is rather more expensive. Packaging issues to the front cooler (the fittings are bulkier) are there if connecting to the cooler is constrained - as it is in attaching a cooler set in an opening in the center of the front valance with a stock tub to the rear. And running the -12 lines forward under the car is not as neat and protected as running the stock brass alloy (or whatever) lines is, so this is exacerbated when you use -16 line.

I really don't think -12 is big enough. I run -16 lines all through my car. I run to 964 oil coolers welded together at the front of my car. My oil lines would never run under or down the side of the car. They run through the cockpit where they are safest.

[Walt Fricke]

What we both have is opinions.

My track car came with -10 lines up to a large front cooler, with -12 lines back from the front oil tank. It was a roller, so I don't know just what motor it had, though it might well have been a 2.4 as it was raced in SCCA in th3 1980s - maybe GT3?

I thought that might be "too small," so when I was upgrading things from 2.5L to 2.7L I upped it to -12 and -16 back.

And the stock 911s use a slightly larger tube ID, but with restrictions here and there which are smaller than -12.

What I suspect none of us have done is to instrument things and quantify them. I think it is obvious that the smaller lines are perfectly adequate for cooling purposes - they certainly have been for me, even the -10 up setup I first used because it was there.

Just when I was feeling smug about my -16 return line from the oil tank, I saw a track prepared car on a much larger budget than mine with -16 lines up, and -20 back! At some point with the return (suction) line you may have to insert a coiled spring to avoid the potential of its collapsing.

So we are left with the question of whether the pump has to work harder with smaller lines (despite restrictions here or there for more stock based systems), and if the additional parasitic losses occasioned by this affect engine power enough to worry about.

Anyone have any dyno data on that?

Running the oil lines through the cockpit is the way to go on a race car (I like to run them through the heater tubes to keep them out of the way and separated), but probably not right for a street or dual purpose car - they are pretty hot, and a passenger seat is going to get in the way.

[bpu699]

Well, I will add some more info on my little journey...

Hooking up AN16 is looking to be difficult. I am working with Len from this site (makes custom lines) to see how difficult it would be, and how much it would cost, to use AN 16 lines...

The issue coming up, is that there are no available M30 female to An16 adaptors made. Len has found a way to have one machined, so will look into that. The rest of the AN adaptors are pretty much stock...

For folks that have used AN16 line, and left the stock wheel well radiator, how did you do it?

Not sure if this project is viable, if one uses an $85 radiator and then has to use hundreds of dollars of lines/fittings/machining/etc...

May end up going with An12, as that's pretty much all that's available unless you start custom making bits...

Will keep folks posted as I get more info. Len, by the way, has been very generous with his time and knowledge.

Bo

[3rd_gear_Ted]

My $.02 worth;
When you are talking about fluid heat exchange, its all about managing the Feet Per Second (FPS) flow rate through the oil cooler.

A -12 line creates a higher FPS flow rate into and through the oil cooler than a -16 line by a factor of 2.

The slower the FPS into and through the oil cooler the greater possible rejection rate of BTU's to the air flow occurring through the oil cooler.

As stated by others -12 will work and work well, BUT, the system is then pedaling twice as fast as it needs to.

BTW, I found all this while reading a "Cameron Hydraulic Data" reference book at a Holiday Inn.

[Walt Fricke]

Ted's FPS consideration argues for using coolers in parallel.