Looking for feedback on radiator ducting

I'm planning my Winter improvements for the 2021 track season. After getting my RX7 running in 2017 then finally back on track in late 2019 I've learned some things about the car. I've already solved my high IAT issues with a proper airbox and my water temperature seems okay. It pegs itself around 200F even when ambient temperatures are 95-100F. Now I'm looking at oil temperature. In the same ambient conditions I'll see 220F+. I don't think this is the end of the world but it's higher than I expected with my setup. Since I'm not racing I'll usually take a "cool down" lap or two, which would bring temperatures down to ~205F.

For those unfamiliar the the FC RX7 has an oil cooler and radiator in series that are both slightly angled toward the front of the car. I've left the heat exchangers in the stock location but tried to improve the ducting. This is what the front of my car looks like:

The inlet is split all the way to the oil cooler. This is what the backside looks like out of the car without the radiator and oil cooler:

And with the oil cooler:

The inlet area to both coolers is ~25-33% of the frontal area, which seems to be the general rule of thumb. The flow area increases as you get nearer to the coolers, which should promote turbulence and slow down the airflow for better heat transfer. I'm trying to find things that would net me a 10-20F drop in oil temperature. The only thing I've done since my last trackday was add a vertical support to the split:

My thought is since the split is slightly angled downward the aero load could be pushing it down slightly closing off the inlet to the oil cooler.

Some other ideas I have:

1. Cut out part of the lower bumper cover. I wonder if that lower curved piece is acting like an air dam for the oil cooler inlet. I figure I can cut out 1-1.5" to give the air a straighter shot but it won't look as nice.

2. Cut out the front frame rail brace. This is the round bar that goes through the duct. I'll need to keep the ends to support the oil cooler. This won't help oil temperatures but will open up space to the radiator, which is always good. I don't think this will affect chassis rigidity because the front subframe and bumper support are still there and this is out in front of the front axle.

3. Continue the split past the oil cooler all the way to the radiator. I'm wondering if I'm getting a high pressure zone between the radiator and oil cooler that's inhibiting flow through the oil cooler.

4. Similar to (3) but run the split after the oil cooler at a downward angle to the bottom of the radiator then cut out the bottom of the duct. This would exhaust the hot air from the oil cooler under the car. This has a benefit of not running that hot air through the bottom of the radiator but I've never seen a heat exchanger setup that's vented under the car. I assume this has some kind of negative aerodynamic impact since it'll be slightly raising the air pressure under the car.

5. Duct the backside of the radiator out the hood. I already have a vented hood so the option is there. I would need to go with a much smaller fan shroud and would have to have a weird cutout for the upper radiator hose.

Are there any obvious issues with my current setup or ideas I'm missing?

It looks like the bumper cover is shrouding about half of the inlet to the oil cooler.  
cut it out - you can make it look nice, and it's so low that no one will ever notice. 
 

more [properly placed] hood vents to help evacuate the engine compartment 

and 220 isn't terribly high.  If it keeps going you'll run into issues but I understand that oil can run at 250* with no problem.   I'd verify that but you might not have much of an issue. 

Unless there's a reason rotaries need colder oil than normal, 220 isn't concerning at all.  With dino oil, I'd start to worry around 240.  With synth, 260 - 270 is the point of concern in my mind. 

So, the first thing I'll note is that your inlet area is further forward than you're thinking it is.  I see it as approximately the green rectangle below, which is constricted to the two red squares by the bumper shape and the metal at the corners of the OEM:

So, from my view, you're probably not increasing the duct area (and thus slowing the speed) as much as you think you are.  
 

My understanding of the system, longitudinally is that it looks something like this crude drawing:

if so, then there's a number of things I'm thinking about.  First, yes, you probably want to eliminate the rounded lip of the bumper opening in front of the oil cooler.  I think it, the placement of the ducting split, and the area between the oil cooler and the radiator are probably creating a stagnation bubble in that area.  I'd guess you're seeing oil temp benefits mainly from the increased oil system capacity, and not the cooler cooling.

Based on the info at hand, it's hard to consider the pros/cons of ducting the oil cooler below the radiator.  It would probably decrease front downforce, but sometimes you gotta do what you've gotta do.  But, you'd probably be better served by getting the oil cooler up against the front side of the radiator... if possible.... instead.

rslifkin said:

Unless there's a reason rotaries need colder oil than normal, 220 isn't concerning at all.  With dino oil, I'd start to worry around 240.  With synth, 260 - 270 is the point of concern in my mind. 

220 is basically "just up to operating temp" for oil. Most cars will run 210-220 just cruising on the highway. Any sythetic oil you'd bother running in a track car engine is good to at least 300, and at this point there's no real reason to be running conventional oil in an engine you care about. On track in my cayman I back off around 250 but realistically could go to at least 270 without having any issues.

dps214 said:

220 is basically "just up to operating temp" for oil. Most cars will run 210-220 just cruising on the highway. Any sythetic oil you'd bother running in a track car engine is good to at least 300, and at this point there's no real reason to be running conventional oil in an engine you care about. On track in my cayman I back off around 250 but realistically could go to at least 270 without having any issues.

Yeah, a good synthetic will be fine at 300, but in a lot of engines, viscosity drop and/or baking seals to death starts to become a concern once you get that hot. 

I had a long discussion with the people at Ron Davis Radiators regarding a race car that was hard to keep cool. Among other things there was an emphasis on getting the heated air out of the engine bay. Given the obstacles and less then straight exit path the air can slow or stall and create problems.

Radiators need lower air pressure on the back side and higher on the front side. At slow speeds an engine cooling fan provides the lower pressure on the back side of the engine coolant radiator. When moving at speed you shouldn't need to have a cooling fan running.

The bigger the pressure difference the better (to a point I'm sure). The fabricated duct in the OP appears to allow air to flow over the top of the oil cooler into a larger space between the oil cooler (radiator) and then flow through the engine coolant radiator. This will greatly reduce the effectiveness of the oil cooler because the pressure difference fore/aft of the cooler won't be very great and as sleepy eluded to the turbulence may also reduce effectiveness of both radiators. The airflow follows the path of least resistance so air will flow over the cooler until a balance occurs where pressure + resistance through the cooler becomes equal with the pressure on the back side of the cooler with a larger volume of air (per Sq. in.) flowing over the cooler rather than through it. Trans and oil cooler manufacturers recommend placing the additional radiators very close to the engine coolant radiator (when being installed in front of the big radiator) and often include thin rubber spacers to separate them the recommended amount during installation.

Following the same train of thought, L5wolvesf post is along the same lines. You want the pressure in the engine compartment as low as possible compared to the pressure on the front of the engine coolant radiator. Sealing the radiator to the duct, sealing off the front of the engine compartment to reduce air coming into it around the radiator, tubing, wiring, etc. can help reduce pressure in the engine compartment along with well placed vents to allow air to escape by being "pulled" into fast moving lower pressure air moving over the hood and/or along the fenders.

EDIT: Hey sleepy, can you move this thread to the aero forum?

NOT A TA said:

EDIT: Hey sleepy, can you move this thread to the aero forum?

I could see it either place.  I'll wait for some feedback from the OP.

Mazda has always recommended lower oil temperatures compared to piston engines. I don't think it's about the oil breaking down, but more to keep the rotors cool since oil is sloshed around inside them.

Thanks for the responses, all. Wow, where to start...

First, oil temperatures with Mazda's rotary engines seem to be a little different. I've read that "too hot" can be anywhere from 210-250F depending on where the temperature is taken. Mazda Competition says 250F in the oil pan. Racing Beat says 210F but doesn't specify where that's taken. My sensor is just before the oil filter, after the oil cooler. It has to travel about 20" of 10 AN hose and then 10" up the rear of the engine between the oil cooler and the sensor. I think the hottest I saw this year was 224F.

This is a pretty good representation of the side view of my setup:

It's a bit like a converging-diverging nozzle except I know I'm not hitting Mn=1 in the throat. So velocity is first accelerating at the duct inlet then slows down approaching the coolers.

The general consensus seems to be I need to address the lower lip on the bumper cover and the open space between the oil cooler and radiator as well as do something after the radiator with the vented hood. The first is easy enough. For the second I'll probably just continue the split behind the oil cooler all the way to the radiator to enclose that area. I know this won't be as efficient as moving the oil cooler back but that would involve making new mounts and lines. Either way, this will significantly reduce the pressure immediately behind the oil cooler. Ducting the oil cooler under the car is just an idea to maximize flow through it while also giving as much unimpeded flow to the radiator as possible. I don't have a front airdam so it's not like there's a vacuum under the car so this will probably stay conceptual for now. As for the hood, it's already vented, so I'm not too worried about under hood pressure, but the vent isn't connected directly to the radiator shroud. Here's a pic showing the vented hood from the underside to give an idea of what I'm working with:

I've got many others if other angles are needed. It wouldn't be too hard to make something of of aluminum to duct the back of the radiator directly out the hood. The challenge will be avoiding the airbox and upper radiator hose and I'll likely need to go with a fan shroud that isn't so deep.

What about just drilling some holes thru the front frame brace instead of removing it?

Dumb question, but I can't remember the answer to it.. What's the temperature of the thermostat in the oil cooler?  I think it was a 210*?  I mean, you may just be bouncing around the stock thermostat temp at this point..

In reply to sergio :

That's an idea.I guess I don't think it'd make much of a difference. The effective flow area is always smaller than the actual area of the hole. Holes in thin, flat plates only flow something like 70% effective area due to inlet losses, pressure heads, etc. I don't know how effective they'd be in this situation since we're talking round holes in a hollow, round bar. In the end I'd probably end up removing so much material I might as well just cut the hole thing out anyway.

In reply to WonkoTheSane :

According to the FSM the "relief" temperature is 149F. A "full open" temperature isn't specified but I think it's around 160F. It's a bypass style thermostat so there's always some oil going through the cooler. I don't consider the car truly warmed up until oil temperature is >-170F. If I'm driving on the street I struggle to get my temperature above 150F when the ambient temperature is 50F or less.

The radiator thermostat starts to open at 180F and is fully open at 205F. Unless I'm on track or just sitting there idling it won't budge from the 179-181F range.

Since the diagram shows the oil and rad ducting separate you could exit the oil duct down under the car and the rad elsewhere. 

In reply to L5wolvesf :

That's kind of what I was thinking of originally. To use the same diagram with some additional marking...

My only concern was I found no examples of this being done in motorsports in my limiting searching. I don't know if this is some kind of big no-no that I'm missing.

rslifkin said:

Unless there's a reason rotaries need colder oil than normal, 220 isn't concerning at all.  With dino oil, I'd start to worry around 240.  With synth, 260 - 270 is the point of concern in my mind. 

Rotaries don't like hot oil.  You can feel power dropping off when the oil gets over 180F, I'm told because heat reflecting from the rotor face preheats the air charge and hurts VE.  Whatever it is, you can really feel the effect once you realize what is causing it.

I like the ducting idea.  I have a front mount cooler type FB, and air can easily bypass the oil cooler on its way to the radiator.  I made a simple duct/dam out of a piece of mudflap that I ziptied to the chassis crosstube above the oil cooler, fencing off the air gap between the oil cooler and radiator.  I can adjust it higher or lower to force more or less air through the cooler.  It helped tremendously.

So, beyond reducing the semi-circular lip on the lower side of the radiator opening, another thing that would help increase your efficiency would be adding seals anywhere the duct opens to allow something to pass through, and to keep air from going between the duct and the heat-exchanger.  Several of those places highlighted in red below:

also, around the radiator, too.

I tend to prefer a "staged" approach on these things.  So, my first suggestion is similar to Pete's, with a couple of tweaks:

It's not real clear from the duct picture,  it you probably want the oil cooler and the radiator at the same angle to the 'flow'.  And I reckon you probably want to keep the "duct area" constant... which would probably mean the fairing off the cross-beam would be angled more than above.

Additionally, no, I wouldn't suggest putting holes in it, or cutting it out... yet.  If you're going to add a flange to direct the flow from the oil cooler to the radiator... then it should be a simple matter to make it "two surfaced" to reduce the disturbance of the bar.  You're accelerating the flow enough, that I expect the bar won't be too much of a hindrance, so long as you soften the path around it, and again add seals so the air has to go up and around.

I think I've seen some setups that duct downish.  I think I've seen them referred to as "K" cooler setups?  I forget where that was.  The main drawback is adding the air under the engine will decrease front downforce.... especially if you have a splitter/undertray.  Less so, if you don't.

if you decide to do that, I suggest you move the oil cooler forward of the crossbar, and angle it back instead of forward.  This gives more room to reduce the angle off the crossbar to the bottom edge of the radiator.  Although, it might still be too far and there might be some stagnation at the bottom of the duct there.  But, pushing the duct out of the oil cooler down will help create a low pressure to pull air out of the oil cooler duct.

In reply to Pete. :

I always wondered why Mazda never sealed the top of the OEM oil cooler. There's literally nothing preventing air from just going it. That was why I added the split but for whatever reason I stopped it at the oil cooler, which now seems like such an obvious error.

In reply to sleepyhead the buffalo :

Excellent point about the seals. I don't have any good pictures but most of the edges you highlighted do have some kind of seal or trim on them. Around the circular cutouts and the entire back edge that mates with the radiator is lined with some generic rubber edge trim I got from McMaster. If you look closely at the bottom of my engine bay picture in the first post you can just kind of see seals on the sides. The rectangular cutouts for the sides of the oil cooler are pretty tight and I don't have anything there. I do still have some of the OEM foam seal left on the bottom of the oil cooler. It's definitely not air tight and I made sure to keep any gaps as small as possible but there is always room for improvement. Especially that small gap at the top of the radiator. I'll probably get some more closed cell foam like I have between the radiator and shroud. I like this idea of "fairings" before and after the frame bar. This is likely the direction I'm headed. Now I'm getting all excited to work on the car again but I still have to wait 1-2 weeks until the holidays are over...

What's most interesting to me is how this all shows just how much more severe racing is on cars. I don't really drive this car on the street except for tuning and going to the occasional show/meet. In near 100F ambient temperatures my water temp will stay at 180F and oil temp around 170F. That's with generally keeping engine speeds in the 3000-5000 RPM range (because rotary - my neighbors love me). After idling for an extended period of time my no-name Summit Racing 16" electric fan brings my water temp down from 205F to 185F in about a minute. Even on track, taking just one "cool down" lap where I'll stay in 3rd/4th gears instead of dropping down to 2nd is enough to bring oil temp from 220F+ to around 200F.

Sustained high RPM drives oil temperatures up significantly due to increased friction, etc. in an engine.  Track use has much higher average engine load as well (compared to almost anything you could do on the street), which pushes up both oil and coolant temps. 

One other thing.  I'll look for it when I get home, but there is a NACA (I think) video about diffusers and turbulence.  Basically, any diverging angle greater than 7 degrees devolves into turbulent flow rather than laminar flow.  So, if you have, say, a 35 degree diverging angle on the outlet of the duct, you would get better results with four blades inside to divide it up into five 7-degree sections.

rslifkin said:

Sustained high RPM drives oil temperatures up significantly due to increased friction, etc. in an engine.  Track use has much higher average engine load as well (compared to almost anything you could do on the street), which pushes up both oil and coolant temps. 

The oil is 30-40% of the cooling system in a rotary, so oil cooling is extremely important.  The insides of the rotors are cooled solely by oil, as well as most of the intermediate housing.

Pete. (l33t FS) said:

One other thing.  I'll look for it when I get home, but there is a NACA (I think) video about diffusers and turbulence.  Basically, any diverging angle greater than 7 degrees devolves into turbulent flow rather than laminar flow.  So, if you have, say, a 35 degree diverging angle on the outlet of the duct, you would get better results with four blades inside to divide it up into five 7-degree sections.

So, I'm not sure what technical paper that's from; but with 7deg being relatively small, I'm going to guess they were talking about the boundary layer being turbulent... and not what's frequently thought of as "turbulence"... akin to being "the kind of flow when I open my car's window at 60mph"

Yes, lower angles are generally better, and splitting up the angle of an exit can be helpful... although you're adding two surfaces of friction each time you add a 'blade'.  So, "it depends"... sometimes the flow/structure mean that the trade off is worth it for more blades in the exit.  Sometimes it's better to try and extend the exit ramp, and give it a radiused profile.  ymmv.   See also "Reynolds Number"

Infernosg 

do you have any "hood down" shots, so I can get an idea of your hood duct geometry?

infernosg said:

In reply to L5wolvesf :

That's kind of what I was thinking of originally. To use the same diagram with some additional marking...

My only concern was I found no examples of this being done in motorsports in my limiting searching. I don't know if this is some kind of big no-no that I'm missing.

All this would do is treat the exit air separately. Some race cars (like formula cars) have separate coolers. 

Re your diagram. where your oil cooler arrowhead is I would end the lower panel to dump out the heated air. It would allow the under car flow to pull out the heated air. But I don't now what structure(s) are there / in the way.

In reply to sleepyhead the buffalo :

This made me realize I didn't have any straight on or side shots of the hood so I had to run out to the garage to snap a couple.

The hood vent/radiator alignment isn't that great. The vent in the hood is a good 4" wider than the radiator and I think they're too close. The back of the vent is pretty much in-line with the front of the engine and the bottom of the radiator and the front of the vent is pretty much aligned with the front of the radiator. The "bottom" of a duct that would run between the radiator and the vent will be nearly vertical relative to the ground. That doesn't seem like it would be good for flow. An ideal setup would have the radiator pushed forward and laid down more than it already is.

In reply to sleepyhead the L5wolvesf :

What you're saying is what I'm trying to show, I think. The bottom of the duct would be open behind the oil cooler and continuing the split past the oil cooler to the bottom of the radiator would force the hot air from the coil cooler out the bottom. There's nothing in the way as this is all hanging out in front of the front sub frame so it's still forward of the sway bar, steering rack, etc. I don't run a front air dam yet but there should still be lower pressure under the car. My only concern with this arrange is if routing the air out this way would be sufficient to increase pressure under the car and cause some weird aerodynamic affects like lifting the front.