That's the point. There is no way, in the real world, you can guarantee that both coolers will have the same quality of airflow to prevent one from running cooler than the other.

Perhaps this is why Porsche routed their watercooled radiators in series.

With the thermostats still in them it really does not have to be that precise. If one runs hotter than the other (less airflow) it will open the thermostat more and increase the flow through the cooler (vs the internal bypass) to compensate. Plus we are talking simply about plumbing here, parallel vs series. The assumption has been made that the mounting and ducting of the two coolers is the same. I submit, and can prove that if that is the case parallel is always more heat rejection and less oil flow resistance (pressure drop) than series.

The simple fact that the flowrate is roughly HALF yet the entering oil temp is equal (instead of the outlet temp of the first one as the inlet temp the 2nd one thus less heat transfer in the 2nd cooler). This is the reason this will ALWAYS be more overall cooling capacity. You can't break the rules of heat transfer thermodynamics. Just like you cant break the rules of other physics...

Can someone touch base on this. I would like to hear thoughts on it. By welding them together, they both would be fully divided just like stock. basically cut the face of the endtanks off and slap them together. It would just make one thick cooler, then you only need to route ducting to one place ect ect.

That is exactly the problem. The oil will preferentially flow through the hotter cooler, since the cooler one will have more resistance to flow.

That's why I think to work 100%, a bizarre exit-side thermostat would need to be used that would vary the flow resistance, so the one with cooler exit-side oil would be allowed to flow more.

There's no question of that - both coolers would be starting with the same temperature differential vs. the airflow, whereas in series, the second cooler would not reject as much heat as the first one.

Yet when faced with marginal systems, people invariably find that reconfiguring to a series configuration will result in lower temps.

You are right Peejay.
Consider what happened here. The system is "size configured" to operate on one oil cooler. Flow of hot oil will take the route of least resistance (the hottest oil cooler). There is no reason for the oil to select the colder cooler.
Barry

The stock T-stat in the cooler just bypasses the core if the oil isn't hot enough. Both coolers in parallel will see the same temp oil coming in. The T- stat decides where the oil is going to go, not the oil. So the next question would be if the T-stat is effected more by the oil coming in or the oil going out. They go in from the bottom if I'm not mistaken, i wonder if flow direction could be an issue with its function.

This of course is based on the hypothesis that the bypass is a much higher pressure drop than the flow through the cooler meaning the one with better airflow will see less flow as it cools down.

I can see your point, in that case.

In a race application i still think that the oil is hot enough at the inlet of both coolers to open both thermostats all the way. I suppose it entirely depends on ducting.

I would still not pipe two FC coolers in series. The total pressure drop is too great. Get one much larger cooler with the right pressure drop, or if two coolers is what you want make sure the total pressure drop of both is less than the FC cooler (which is probably the case with the RX8 and FD dual cooler setups.

Agree. I always find the "problem" easier to visualize/explain if I make the analogy from the hydraulic setup to an electrical circuit.

Actually Mazda calls this thermostatic valve a bypass valve, in aircraft terminology it is called a vernatherm . It is open when cold which bypasses oil back out of the cooler. It closes at about 62ºC as the temps go up forcing oil through the cooler.
The parallel system would work well until the second cooler’s valve is totally closed. At that time the oil would favor the cooler with the least resistance. One would run low temps and one higher temps. As described by speedturn below.

Note the valve also opens at a differential pressure of 50 PSIΔ.

We need a valve that would control flow through the parallel coolers equally.
Barry

That's what I was envisioning - something in the return Y-block that would restrict flow from the hottest unit.

Or, one could monitor outlet temps on separate gauges and tweak the air ducting to suit.

At some point, one of the nice single coolers starts to look like a nicer option.

I have run both series and parallel OE oil coolers on my 12A powered IT RX7. As far as temps were concerned, both worked equally well - temps were around 200-210F for both setups regardless of ambient temps.

The parallel system delivered better oil pressure and oil pressure came up faster at start up. So parallel is what I will continue to use, going forward.

To be clear, my car will NOT cool it's engine oil with one OE cooler so two coolers are an absolute must on my car.

I plumbed mine using tee junctions at the oil pressure out and the oil pressure in ports on the engine. I used all -10 braided steel lines and AN fittings.

I remove the OE thermostats and install a machined aluminum dowel that blocks the bypass hole. The dowel is held in place by the thermostat cap. That way my coolers are always moving oil through the core and I never have to worry about temp differentials or a failed thermostat. No issues with getting the car up to temp....that happens by the first lap in most races!

We ran our parallel piped stock FC oil coolers for the first time this weekend. It was not hot out, mid 70s, rained for most of the sessions, but we did get a couple dry ones.

With a single stock oil cooler we saw temps flirting with 245deg coming BACK from the cooler when up behind another car.

With 2 coolers in parallel, reverse return plumbing arrangement, we never got over 180deg, even in traffic.

Good to hear! What is your ducting situation?

BTW - What I decided to do is to redo the ducting to make absolutely 100% sure that air cannot go around the cooler on its way to the radiator. All air going to the bottom half of the radiator has to go through the oil cooler first.

Ducting is a huge issue when you are dealing with multiple layers of coolers. You either want them as close as you can or seperated and with seperate discharge so you don't double up on the air pressure in front of the oil cooler if you can help it. The air will find the easy way out.

My top cooler is within a 1/4" of the radiator and my bottom one hangs down from the cross tube of the unibody. The lower cooler exhausts with a curved shroud from the tube down to the lower edge of the radiator. You really don't need a huge cross section of air opening to feed the radiator.

I've done three cars with similar set ups and never have a cooling problem.(EP level prep cars and a bridgeport)

I tried some ducting solutions first and only saw perhaps a 15 deg drop in temps. This included opening up the front bumper some and forcing the air through the cooler with ducts from the bumper to the cooler. Once 2 coolers were in its basically barely above tstat opening temp...

90+ deg out at the race this weekend. We couldn't even get the oil to go over 200...even in traffic....that is until a water seal blew, and the water (steam) temp spiked to 260. Then it went to a "high" of 205 deg LOL....oil temp comes up to temp very quickly with the stock thermostats still in place...

I am sold on parallel stock coolers...

I agree.
Most of the people reporting probelms running series plumbed systems seem to have been running second gen oil coolers. The FC coolers are a dual pass design. While slightly more efficient in terms of cooling, the pressure drop through a dual pass compared to single pass on the same core is 16 times higher. Having mounted oil pressure gauges before and after the oil cooler, I was seeing over 40 psi of oil pressure dropped through the stock coolers. Doubling this pressure drop by running them series mounted is much worse.
On factory series-plumbed coolers, the cores are always single pass. They are plumbed this way always for simplicity of plumbing, and to keep costs at a minimum.
To maximize cooling efficiency, you need to keep flow to its maximum value, which means keeping pressure drop to its minimum. This cannot be achieved with series plumbing.

The GSL/SE and SA oil coolers I have used are also double pass coolers. In fact every Mazda SA/FB - FC cooler I have ever seen was a double pass cooler.

I had a 12A wth a racing oil pump and racing regulators and I was able to run two GSL/SE coolers in series with minimal pressure drop. But with a standard pump I had to run two coolers in parrallel to maintain pressure. Both plumbing methods worked as far as temps were concerned. I wouldn't attempt to race a rotary without two coolers.