mr2peak GRM+ Memberand Dork
6/20/22 11:18 a.m.

My FD RX7 has dual air oil coolers, but I sometimes suffer from oil temps spiking way above the coolant temp.

I was thinking of tying the system together by using an air to water core for the oil. Connect it to the oil after the oil hits the oil coolers, and after the coolant passes through the radiator. Or plumbed before it reaches the radiator/coolers? I don't think it would need to be large to help balance out the temps a bit. I would definitely still be keeping the dual 19 row oil coolers on the car, possibly changing those for 25 row units.

Which way is the better way to plumb the system? Any serious flaws in this idea?

gearheadE30 Dork
6/20/22 11:49 a.m.

Most modern motorcycles and many production cars and trucks are set up this way, usually either with a lube cooler module mounted to the block (coolant is water jacket temperature) or a heat exchanger in one of the radiator end tanks. I've seen them on both the inlet and outlet end of the radiator; at the end of the day  it doesn't matter much. Extreme examples (usually trucks from what I've seen) often run the engine oil through an air-to-oil exchanger and then through one of the radiator end tanks. This ensures that you don't overcool the oil but gives you that extra cooling capacity. Any of these setups are good for cooling oil, but also great for warming the oil up at light load/low duty cycles as cold oil isn't good either.

In your case, I'd run them through the stock coolers, then through a water-to-oil exchanger, and then back to the engine and keep an eye on pressure drop because that's a lot of coolers.

I'm not as familiar with rotary stuff, but a few cars I built had oil temperature issues because I was running a bit too much advance in some conditions. Not enough to hear knock, but backing a degree or two off in key areas lowered oil temps by around 40 F (I was spiking pan temperature close to 300 F on the track).

wvumtnbkr GRM+ Memberand PowerDork
6/20/22 11:50 a.m.

That is very car dependent!  


Water is a much higher temp  versus air, but has a much higher capacity to transfer heat.  

The other issue is that you are gonna put much more heat into the radiator.  This can cause higher coolant temps as well.


That being said, a rotary uses the oil to cool the engine predominately.   Like 66% of the cooling is from oil (or at least that is what I have heard and it makes sense based on where the oil is versus where the coolant is).


I would tread very carefully to not accidentally cause a coolant temp issue...

rslifkin UberDork
6/20/22 12:05 p.m.

As long as you have enough radiator capacity to handle the extra heat load, I consider water/oil heat exchangers to typically be a better solution.  Unless the engine runs particularly low coolant temps it lets you avoid needing a thermostat on the oil cooler and it also helps get the oil up to temp faster.  

fidelity101 UberDork
6/20/22 12:35 p.m.


get an oil cooler with a fan, setup a simple relay that closes a circuit (relay ground) at a specific temperature (if you don't run a standalone to control an extra fan)


big fan of their products and do well on the rally car this would simplify the routing of that twin system but you need to fabricate a mount and etc, well worth it. 


Like the posts above mention it depends what your current radiator setup is on that car, wankels are sensitive to high coolant/oil temps so spending good money here goes a long way. 

RXBeetle Reader
6/20/22 12:54 p.m.

I'm running a '09 Ram 2500 transmission water to oil cooler on the turbo 13B in my RX8. It's just a big brazed plate cooler and can be had for less than $100. I do have a large radiator for the additional heat load and a 170F thermostat. The fittings are -8AN on the oil side so hookup is pretty straight forward. My water side is plumbed to the heater core supply barb and back the the bottom rad line, same as the heater core. I have my heater core plumbed separately though so the flow isn't restricted. I just hooked an oil temp sensor in my remote filter block downstream of the cooler and it shows ~10F above coolant temps. I haven't had an opportunity to really push it hard on track or anything though. 

infernosg Reader
6/20/22 2:00 p.m.

More details are needed I assume we're talking about a motorsports environment, and not daily driving. What temperatures are you seeing, specifically? I'm not a good enough driver to push my car at 10/10ths all the time but I still see maximum water temperatures around 200F and oil temperatures around 225F. Some will say the latter is too high, but I never thought so. Maybe they're right because I did have to pull and teardown my engine a month ago for lost compression (flattened corner seal springs). However, multiple used oil analyses and directly inspecting the bearings shows no evidence of contact/breakdown so I don't think that was a cause. I always thought the dual FD coolers were more than adequate for motorsports. How's your ducting? Still all there?

YoursTruly New Reader
6/20/22 2:34 p.m.

Most modern Hondas and many Mazdas have had this built in from the factory. I have the OEM one on my Miata.

I would be willing to bet you can find one from a Miata or Honda that will fit since they share a thread size and OD for the oil filter (where it mounts).

wspohn SuperDork
6/21/22 11:22 a.m.

I saw one installation using a Mazda rotary where they had a temp switch that operated a mister that sprayed water (or possibly water/alcohol mix, I don't recall) onto the oil cooler  for extra cooling. No idea how big the coolant reservoir was or how long it lasted but I would guess for a 1/2 hour race at least.

j_tso HalfDork
6/21/22 11:51 a.m.

I imagine there's an upgrade for FDs that involve bigger coolers. It would be simpler than adding more plumbing.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
6/21/22 1:11 p.m.

'83-85 12As had water oil coolers.  They had a reputation for cooking the dowel O rings because of how hot the oil gets.

Mazda got the coolant from the hottest part of the engine, great for the heater core but not good for cooling oil.  Ideally the oil will never get over 180F, you would need coolant temps in the 150-160F range for that...

I always wanted to drill and tap the water pump housing and take coolant for the oil cooler from there, which should be the coolest coolant in the system.

chaparral Dork
6/21/22 1:16 p.m.

I would recommend putting it on the inlet side of the radiator. That will increase the water temperature going to the radiator, so it will transfer more heat away, without heating the water going to the engine. 

codrus (Forum Supporter)
codrus (Forum Supporter) GRM+ Memberand PowerDork
6/21/22 3:55 p.m.
j_tso said:

I imagine there's an upgrade for FDs that involve bigger coolers. It would be simpler than adding more plumbing.

I agree.  The last thing the FD needs is more rube goldberg plumbing. :)


clshore Reader
6/21/22 8:04 p.m.

I plan to place a finned power steering cooler into my engine oil sump.
I will run coolant through it.

About $40 off of eBay

I was originally looking at heater cores, but the PS coolers looked more robust.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
6/21/22 8:37 p.m.

In reply to clshore :

That will help, but even in a rotary with its 40% heat rejection via oil, the oil pump itself puts a lot of heat into the oil.  This is why I like the shallower pumps, and why Mazda engineered a dual inlet pump layout for the FD - less heat gets put into the oil if there is less turbulence in the pump.

The Mazda oil system is kinda neat and optimized for cooling the oil.  Aside from an emergency bypass, all oil from the pump goes through the cooler(s) first.  The regulator is in the engine after the oil is cooled but before the main oil passages (before the filter, even) so excess oil gets dumped back in the pan for a second chance at going through the cooler before it goes through the engine.  Almost every other engine has the regulator built into the pump, you see, so any inline oil cooler will only cool the oil going into the oiling system.


They also have "reverse flow cooling" by design - the hot parts get coolant first.  It is a really good design for a racing engine moreso than a street engine.

mr2peak GRM+ Memberand Dork
6/25/22 1:18 p.m.

It's a single turbo, V Mount, already upgraded the dual coolers. Just looking for a way to handle some of the oil temp spikes I can get when I'm up in the mountains. Oil temp rises much faster than the water temp, trying to buy myself a little more time before the oil overheats.

I didn't realize this was a common system on newer cars, that lends more credibility to the design.

It will be ~$600 all in to upgrade from the 19 row coolers on the car now, to 25 row coolers. Just exploring my other options in that price range

kb58 SuperDork
6/25/22 2:32 p.m.

I went through much the same during development of my car, Midlana. It's mid-engine, and running oil lines all the way to the front of the car was less than optimal, never mind not having space for a proper oil cooling radiator, so I looked into heat exchangers. I ended up with a Mocal unit and couldn't be happier. Coolant temperature comes up faster than the oil, so in that situation, it helps the oil warm up faster. Once up to near-equal temperatures, the coolant moderates the oil temperature from rising higher. I placed the heat exchanger in the hose leading to the radiator. My thinking is that it's the best place to dump extra heat into the system, so only the radiator sees it, not the engine. I can drive the car as hard as I can and oil temp never gets above about 210 F, and coolant stays at around 90 C, though that's more about coolant flow and radiator efficiency.

mr2peak GRM+ Memberand Dork
7/3/22 1:50 a.m.

Tie it in before the coolers and radiator, or after the coolers and radiator?

What's the concensus here?

Canton Racing makes a nice small one, jsut enought to tie it together without taking up too much space

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