Project [Rotary] FD RX-7: Part 5 - Cooling Oil and Generating Spark

by M-P Spierer

Whether it's high RPM, high power density, or for some, high failure rate, the word "high" is regularly used to describe various attributes of the rotary engine.  Today we are fighting against the rotary engine's high inclination for dangerous oil temperatures and ignition break-up. This requires significant upgrades to both the oil cooling system and the ignition system...and by upgrades, we of course mean complete redesign.

For those who are familiar with FD RX-7's and the very few factory options/trim models available already know one of the primary reasons why the R1 model is more desirable than the Base model. For those who don't already know, no it isn't the factory front lip, rear spoiler, or suede seats, it's the fact that they came equipped with dual oil coolers. All RX-7's had an external oil cooler behind the driver's side duct in the front bumper, but only the R1 model had a cooler behind the ducts on both sides of the car.

This may not seem like a big deal to some people because most piston engine cars don't have to take oil cooling quite as seriously, especially for street cars. Many performance oriented cars (Nissan 350Z, Subaru WRX STi, Honda S2000, Acura NSX, etc) have a water cooled oil cooler that is sandwiched between the oil filter and the block. This uses hot engine coolant to cool the oil and while it does work because water temperatures are usually cooler than oil temperatures, it is not as effective as an external air-to-oil heat exchanger.

One common and inexpensive modification for base model RX-7's like this is to simply add a second oil cooler from an R1 model and call it a day. That, however, isn't going to cut it for a 400+ wheel horsepower RX-7 that will see heavy track use and time attack sessions. In this car we need dual aftermarket oil coolers designed with racing in mind.


Enter the Earl's 19-row oil cooler. This is one of two coolers that we are installing on Project RX-7 and it should greatly increase the cooling capacity and efficiency over the OEM system. The Earl's coolers not only have eight more cooling passages (rows) than the OEM coolers, but they also have a higher fin-density between each row. The cooler also features -10AN inlets and outlets that won't restrict the oil flow required by the 13B-REW rotary engine and will accomodate the use of high-quality braided hose and aluminum hose ends.

The dual 19-row aluminum oil coolers from Earl's Performance Plumbing that we are utilizing to cool the oil in Project RX-7 are a substantial upgrade over the factory system. The combination of extra cooling passages and a higher fin density increases heat exchanger efficiency by increasing the total amount of heat transfer surface area. Surface area is very important because it is one of the three primary variables used to quantify heat rejection. The only real downside to increasing surface area through increasing fin density is that you also increase the static pressure required for airflow to pass through the core. This is generally only a problem at low speeds and can be remedied with proper (sealed) ducting. 

One common misconception in oil cooling is that you want the oil to be as cool as possible. This is simply not the case. Engines are designed for a certain temperature range of hot oil for proper lubrication, flow, and oil life. In order to make sure we get up to this temperature in a timely manner it is important to install a thermostat that keeps oil from being cooled by the oil coolers until up to operating temperature. For this we again turned to Earl's Performance Plumbing.


The Earl's oil thermostat is replacing the old aftermarket thermostat that was previously installed on the RX-7. Like the old thermostat, the Earl's thermostat begins to open at 160-degrees Fahrenheit and is fully open by 180-degrees. The difference is that the Earl's thermostat uses -10AN fittings via -10AN ORB inlet/outlet ports rather than smaller 1/2" NPT ports. The Earl's thermostat also has three 1/8" NPT ports for pressure transducers or temperature sensors and two thru-holes for mounting; both of which were missing from the old thermostat.

Because we love making random technical drawings in MS Paint so much, here is a basic diagram showing how the Earl's oil thermostat works. The upper image shows the thermostat in the closed state when the oil termperature is below 160F. In this state, the oil completely bypasses the coolers and is returned to the engine. Once the oil reaches 160F, the thermostat will begin to open and some percentage of oil will go through the coolers while the rest is bypassed. The lower image shows the thermostat after the oil reaches a 180F where it is completely open. At this point all oil flow will pass through the coolers for maximum cooling.
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Dan DeRosia
Dan DeRosialink
Wednesday, April 29, 2015 3:22 AM
Yay, a rotary update! Good use on the printed bits for prototyping fitment, and I find it slightly funny that you just happen to be using the same red color I am. Really do like the attention to detail on this whole build.
Wednesday, April 29, 2015 5:20 AM
Ahhhhh 3D printing. Soon to replace cardboard for part mockup. Though if it had been me, I would have acetone dipped and then painted the 3D printed part and just used it (with a couple design tweaks of course to account for it being plastic). For small brackets that don't see a lot of load, 3D printed parts are pretty sweet. We got large 3D printers at college in my Senior year and we used a lot of 3D printed parts on our SAE car, including our intake manifold. If you design around the material and printing properties, it's surprising what you can make with them. I'd like to get a 3D printer of my own and try wrapping the parts in carbon fiber. If done right, it should yield some super light, but strong parts.

I am also a huge fan of that lightweight A/N hose. Not only is it lighter and less prone to damaging other parts, but it is super easy to make hoses out of. It used to take me an hour to fab up fuel lines for our SAE cars with steel hose. With the nylon hose, it took about 15 minutes and no stainless steel splinters in the fingers either!

Nice seeing this RX-7 back on MIQ. I've wanted an FC for a long time, so I'll be paying close attention to this build to learn more about rotary builds. Looking forward to more!
Wednesday, April 29, 2015 5:39 AM
Could you have ran tubing instead of hose between the thermostat and the coolers?
Wednesday, April 29, 2015 6:04 AM
@Dan Derosia | Thanks. I use IC3D 1.75mm filament for ABS.

@8695Beaters | You will definitely see a few 3D printed parts as final parts on this project. The thermostat mounting bracket was not going to be strong or fatigue-resistant enough out of ABS. Nylon might have worked, but would have been very flexible.

Also, no joke about ease of assembly when using nylon braided hose. I cut it with an x-acto knife which is a million times easier and cleaner than using an abrasive cut-off wheel on stainless hose.

@smoketreegarage | Could you run tubing? Yes, but there are big negatives to doing so in my eyes. It would require huge tubing (-10AN is 5/8" ID) which would be very hard to source (and likely expensive) and it would be expensive/difficult to form (special bender and flaring tool).
Wednesday, April 29, 2015 6:40 AM
Awesome build. I used 5/8" seamless thin wall stainless tubing for part of my oil cooler plumbing. You can get it at Grainger for like $7/foot. I bought an inexpensive hand bender and flare tool that worked well. It is a massive pain in the ass to work with though and the bend radius is pretty big.
Wednesday, April 29, 2015 7:24 AM
How do you feel about those stacked coils being mounted horizontally? Seems like a fairly long moment arm every time you hit a bump or a curb.
Wednesday, April 29, 2015 7:47 AM
I was wondering what this car has been up to ;)

Supercharged, it would be easy enough to add a L-shape bracket going from the central mounting bolt the coils are on to the top mounting bolts of the base plate to add some support if deemed necessary later.
Wednesday, April 29, 2015 8:11 AM
@ED9man | That is a lot cheaper than I would have expected for 5/8" stainless tubing. Was it annealed?

@Supercharged111 | This was a big concern of mine initially when I saw other people implementing similar designs and was the driving reason I used high grade bolts threaded through the backside of the mounting bracket instead of bolting from the front side. The result is actually much more stiff than I ever imagined it would be.

If this becomes an issue (which is extremely doubtful at this point), there are several other mounting options that could fix it including the one that spdracerut mentioned. The others options are generally more complex or take up more space, so I decided to keep it simple.
Wednesday, April 29, 2015 8:36 AM
Not sure if it was annealed, it was 304 grade, .035" wall. It was easy enough to bend and flare by hand without any kinking or cracking. I used it based on the recommendation of my plumbing supplier.
Wednesday, April 29, 2015 12:28 PM
I certainly would have mounted the lines behind the bumper rebar. I know the coolers themselves are already in a danger zone , but why add more risk of severing a line?
Dan DeRosia
Dan DeRosialink
Wednesday, April 29, 2015 5:07 PM
Hm, thinking of 3d printing, you just reminded me that I was going to post something on the forum.

Raw engine bay stuff... I just can't bring myself to trust thermoplastics. Especially not with an intake manifold. I know that theoretically it should be doable maybe, but porosity and softening, plus with how damn hot everything gets near rotaries... yeah, pass for me.
Thursday, April 30, 2015 1:01 PM
@SM_Clay72 | This is a good point and something I will consider guarding in the future. For right now I think if I hit something hard enough to severe the line during a time attack or open track session then I have bigger things to worry about.

@Dan DeRosia | I agree with trusting standard 3D printing materials in an engine bay (ABS is sketchy and PLA is outright stupid). Nylon on the other hand is a perfectly good engineering plastic that I don't have any concern using in an engine bay for the right applications (maybe for something like ducts for the new oil cooler setup ;) ).
Dan DeRosia
Dan DeRosialink
Thursday, April 30, 2015 2:54 PM
I was thinking in terms of printing female molds for ducts, then coating the inside with mold release, filling it with expanding foam, then using the foam as a core for lost-foam carbon layups. But printing nylon is much much simpler, isn't it.

Hmm. Ideas.
Saturday, May 02, 2015 4:11 PM
@ Supercharged111 and Dan DeRosia

The way the coils are mounted is extremely common in marine rigging. I help maintain vessels used and thoroughly flogged by federal law enforcement They stack mount 3-4 control boxes at once, and the coils are much smaller than the control boxes. I guarantee that these boats see much harder hits from wave action on a much more regular basis than this car will see. After 4200 run hours we have yet to see a failure.
Saturday, May 02, 2015 4:17 PM
errrr I should have headed that last one "@ Supercharged111 and M-P"...doh!
Sunday, May 03, 2015 9:04 AM
What temperature was it hitting before the upgrade?
Monday, May 04, 2015 2:49 PM
Dont use Prolite 350 hose for fuel. It permeates fuel vapor like crazy.
Tuesday, May 05, 2015 7:54 AM
@j_tso | I don't have any before data due to the old configuration of the car being extremely different overall without a oil temperature gauge. This setup is on the low end of the RX-7 cooling spectrum. There is a chance that temperatures will still be too high and I will have to upgrade to 25-row coolers. We'll see...

@cartechs | We are not using the Pro Lite 350 hose for fuel on this car (see parts 2 and 3 of this project for more details on the fuel system), but most braided hoses will permeate fuel vapors to some extent unless they use PTFE instead of rubber. No experience with this specific hose and fuel, so I can't comment directly about that.
Thursday, June 18, 2015 7:57 PM
I hope you guys don't mind, but I'll be stealing a lot of your ideas. Amazing work so far!
Friday, June 19, 2015 8:06 AM
@thedgtl | Don't mind at all, steal away. Thanks for the compliment.
Friday, June 10, 2016 7:37 AM
I have to ask since I'm curious and about to start working on my own engine again, what do you do to prime the oil system prior to starting up the car? With those new coolers and new piping you'll have a lot of air to purge out. Also how do you know that the coolers are free of debris and don't have any contaminates in them? Do they come sealed?
Friday, June 10, 2016 7:58 AM
@riwanika, priming the oil system is a two step process for me, but only first the first time starting a new system like this.

Step 1 is to prime the engine and lines up to the thermostat. Most people pull the fuse that powers the ECU so that it can't command injector or ignition (known as the EGI Main fuse the FD RX-7). This should still allow power to the starter, then crank it for a while to purge the system of air.

Step 2 is to disconnect both lines at the oil cooler side of the thermostat and put oil into one of the hoses until oil comes out the other. Personally to do this I use a big oil injector thing that I also use to fill transmissions and differentials. Then just reconnect everything and the system should be bled well enough. One thing to note though...this only really works if your oil cooler ports are facing up.

For the debris, I flush them in the parts washer, blast them with air, and let them hang dry. They do come sealed and should be clean, but better safe then sorry. Generally speaking though, oil coolers should be positioned before the oil filter so that any contaminates that do exist will be caught by the filter before entering the engine.
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