Cooling is almost certainly going to be your #1 concern while flying. The rotary is capable of producing an enormous amount of power - far more than the Lycoming that the plans call for you to install. But power output is almost linearly related to heat generation, and unlike a pusher, the Cozy has no huge, fan-boosted front scoops with which to cool the engine.

Start by taking a moment to study the coolant flow paths for oil and water through the engine:

Fig. 1: RX-7 Oil Flow

Fig. 2: RX-7 Coolant Flow

A siphon tube draws oil from the pan up into the oil pump. That pumps it out of the front housing via a banjo bolt and through a hose into the oil cooler. From there it flows to the filter mount pad on the rear iron and through an oil filter. Two tubes flow down into the engine block from there. The first goes straight down to the bottom of the housing and into a pressure valve. This bypass-style valve sends excess oil back into the pan.

The other tube passes through the irons and housings and provides oil to the bearings at each step. As it reaches the front housing, it also has a passage into the eccentric shaft, which has nozzles that further lubricate the bearings. Finally, from the front iron, it has passages that feed to the oil metering pump, through a banjo bolt in the upper-left of the front iron that feeds to the turbos. The turbos return oil to ports in the lower-right front and rear of the engine, which are just channels back into the oil pan.

The modifications to this system that we will make include removing and blocking off the oil metering pump and its injection ports, moving the oil filter to an alternate location, and altering the turbo return plumbing to use a single return port.

The “pan” in the cooling system is the radiator. Cool coolant flows from the bottom of the radiator up into the bottom of the water pump. The water pump pushes coolant directly into the core, and also out through a fitting to the turbos. As it passes through the core, coolant flows backward through the housings along the left side of the engine through a series of passages in the housings. It circles around through the back of the rear iron, then returns to the front through similar passages in the right side of the engine. Heated coolant is then pumped out of the water pump and down into the radiator.

The water pump contains a thermostat that controls a bypass port. When the engine is cool, the flow bypasses the radiator entirely to allow the engine to heat up faster. The thermostat is programmed to begin opening at 177-182F and be fully open by 203F. If the thermostat fails your engine can overheat, so this guide calls for it to be removed completely.

The pressure cap/thermostat assembly also contains an outlet to the Air Separator Tank, or AST. This tank sits at the high side of the system, so air will naturally migrate to it. Coolant returns to the bottom of the radiator from the AST, and air (if all goes well) is expelled from the AST through a pressure relief valve to an overflow tank. In our case we replace this with an expansion tank and overflow bottle, which works basically the same way but with upgraded components.

Aside from having a different engine shape in the engine block for coolant flow, the rotary isn't much different from any other automotive engine. However, we do have two complications to allow for:

1. The rotary rejects 30% (some say more) of its heat through oil cooling, so the oil cooler isn't optional. In fact, it's a major design criteria to provide effective oil cooling.
2. The addition of a turbo means the addition of an intercooler, yet another radiator through which you must duct intake air. It also requires two more oil and two more water cooling lines in the system, although the stock 3rd-gen motor has provisions for these.
3. Without crankcase ventilation to release blow-by gases, the main oil seals will fail.

Regarding crankcase ventilation, we need it… but we don't want a full PCV system because of the failure potential it creates. A large tube from the top of the system goes to a ventilated catch can (some oil will collect here). The breather hole on the oil filler neck is the stock location, and is ideal for our use provided we cut down the neck so it fits under the cowl.

A helpful rotor-head has posted a frequently-quoted chart. I've edited it here for our use.

We also never want to exceed 205F oil temperature, as measured at the filter.

As you can see, we want our water temps to be in the 203F-226F range (or less). It's OK to run a bit hot as long as you're keeping up with it, such as to let it rise during take-off, then decrease as you climb to cooler air. But if you aren't able to manage that, you'll never get all the horsepower your new engine should give you. Don't bother porting or talking about intake manifold runner lengths until you address cooling!

Also, be careful of running your system too close to its theoretical maximum. Boiling your coolant is extremely dangerous. You will instantly lose a huge amount of cooling capacity (because the steam won't transfer heat), and you'll overflow your system, losing coolant out through the pressure cap/overflow vent. If you aren't in a position where you can immediately throttle back and cool your engine, it will probably eat itself.

The following modifications to a stock cooling system will increase its cooling efficiency:

• Use a higher-pressure cap, such as 16psi (Stant 11233).
• Reduce the antifreeze ratio in the coolant. The factory-recommended 70/30 mix of water/ethylene glycol (standard yellow-green coolant) raises the boiling point of the coolant about 8F (a 50/50 mix would raise it about 13F). But this comes at the cost of some efficiency - water cools better than antifreeze. Some racers go to 90/10, especially during the summer.
• Use distilled or “soft” (demineralized) water if you live in an area with hard water.
• Better ducting - force more air through the radiator. See my design notes below.
• Add heat shielding, particularly around the turbo/exhaust components, to prevent them from transferring heat into the rest of the engine bay.
• Use the “venturi effect” to improve cooling efficiency.
• Increase coolant line size. (But not above the stock size.) Again, see design comments below.
• Modifying coolant passages. This is an extreme step, one I chose not to do, but is common in racing applications. Pineapple, Racing Beat, and other vendors offer services whereby they will carve grooves in the cooling passages in your housings to improve heat transfer. It's expensive, and has to be done while you have the engine torn down, but is supposedly helpful.¹⁾

Before you begin, download this file: Oshkosh 2007 Cooling/Drag Forum (77Mb)|. This presentation, by Terry Schubert and Gary Hertzler, discusses cooling system design. It is a MUST-READ before you design your own system.

I designed my system on three basic principles:

• Model the base system on John Slade's proven configuration.
• Incorporate the design changes he would have made were he to rebuild his system.
• Incorporate the principles discussed in the above document.

John's cooling setup uses only the plans NACA scoop for cooling. It cools very well in most flight profiles, with only some areas that you have to be careful of. This is close to ideal: it means he has achieved a balance between cooling capacity and drag. His setup brings the plans NACA scoop into an expansion plenum²⁾. The plenum provides air to a water radiator, a pair of oil coolers, and the intercooler. Small take-off ducts provide intake air and blast tubes to the ignition coils and turbo heat shield.

Fig. 3: Plenum, Viewed from NACA

Fig. 4: Plenum, Viewed from Above

John is relatively pleased with the performance of his system and sees no need to change it. However, if he had to start from scratch, he said that he would use a double-pass radiator and a larger oil cooler. Photos of the products I chose are below:

Fig. 5: Dual-pass radiator

Fig. 6: One of a pair of oil coolers

The radiator is a Northern Radiator 16x31 double-pass radiator. This unit JUST BARELY fits below the CozyGirrrls' engine mount with a bit of grinding in one spot. If you feel like getting a radiator custom-made, a 15” wide unit would have been a perfect fit… Ed Klepeis from TechWelding is particularly recommended by everybody I've spoken to.

There are two advantages to a double-pass radiator. First, it is more efficient than a standard unit, and can improve your cooling capacity a bit. That was John's reason for wanting one. Second, both the inlet and outlet ports are on the same side. This lets you use shorter (less expensive and easier to route) hoses to connect it to the water pump.

Space around the NACA is tight, and I couldn't find a single oil cooler that did what I wanted it to - so I installed two, side by side. The plumbing runs via AN-10 fittings and hoses from the oil pump outlet on the front cover to an oil filter mounted on the firewall, then through the radiators (in series) and finally back to the engine via a bypass block on the top of the rear iron.

Fig. 7: Oil filter bypass block

Fig. 8: Oil filter mounting bracket

Fig. 9: New oil cooling flow diagram

Cut two aluminum 3”x1” U-channels just wider than the radiator (around 16”). Place four isolation dampers on the corners of the front and rear tubes of the engine mount³⁾ and using hot-melt glue or string, temporarily attach the two U-channels to the front and rear of the engine mount. Leave about 1/8” clearance between each U-channel and the engine mount.

Fig. 10: Isolators and U-channel

Fig. 11: Side view

Using wire, temporarily hang the radiator such that it sits flush against the feet of the vibration isolators, and with at least 1/4” clearance (1/2” would be better - just remember, this will affect your cowl line!) back from the aileron control tube. It's a good idea to temporarily place a radiator hose on one of the fittings to be sure you have enough clearance with the hoses installed.

Fig. 10: Oil/intercooler layout

Fig. 11: Side view, clearance not yet set

Now mark where the U-channels should be installed on the radiator. They should be attached through the side rails of the radiator with four 3/8”-16 x 4” bolts that extend from the U-channel all the way down through the side rails of the radiator. Cut 3/8” ID aluminum tubing to fit between the U-channel and radiator, and between the top and bottom lips of the side rails. These will act as spacers to prevent these items from bending as the bolts are tightened. The bottom of each bolt should be finished with a large-face washer and a lock-nut.

Now re-assemble the structure and hang it again. Verify the locations of the U-channels, then mark where holes should be drilled for the feet of the vibration isolators. Remove the components, drill these holes, then drill lightening holes in the center of the top face of each U-channel. You can remove the bulk of this material, but do not lighten the sides, and leave at least 1” of material near the vibration isolators.⁴⁾

Start by selecting an appropriate location on your firewall for the remote-mount oil filter bracket. This should be attached with screws or bolts. I chose to use bolts, but I wanted mine in the lower cowl area, where the nuts would be hard to reach under the spar. To address this, I riveted nut plates to a small piece of aluminum, and flox+BID-taped that in place.

Next, install a male/male AN-10 to 1/2” NPT fitting in one inlet port, and an oil pressure sender in the other. Install another fitting in one outlet port, and an oil temperature sender in the other. Your choice of fittings may vary slightly; I chose to use steel fittings for better flow to help make up for the choice of right-angle versions to make my oil line routing easier.

Prepare the oil cooler by installing appropriate AN-10 fittings, if necessary. Then arrange a SOLID mount for it. Remember that the oil cooler will be heavier by a few pounds once it's full of oil, and it must be absolutely stable. I installed a support beam made from two lengths of aluminum U-channel (which is quite rigid) between an aluminum plate in my left wing root and holes that I drilled/tapped in my engine mount. My oil cooler is further supported by the inlet duct that leads to it.

Now install the remote-oil-filter plate on the engine. Don't forget to lubricate the O-rings with vaseline or similar. Apply Permatex sealant to the oil filter stand on the engine and the bottom of the adapter plate.

Frankly, I wasn't that satisfied with the quality of this part. $65 is a lot to pay for a chunk of aluminum drilled with five holes and two O-rings. And mine came with metal shavings inside it - if I hadn't inspected it closely, I might have destroyed my engine! If you have a drill press and some extra aluminum⁵⁾ you could probably make your own in an hour. The drawing below shows you what needs to be made.

Next, plug the oil pressure switch hole just below the top of the pedestal. Mazdatrix sells a 1/8” NPT plug for this for $4.55, or you can get the same part from McMaster for pennies. OEM parts are expensive! This is a great spot to get oil for the PSRU, so don't plug it permanently. Just seal it well enough that you can do a system pressure test after the rest of the system is assembled.

Finally, install an oil filter temporarily on the filter bracket, or cover it with a latex glove to keep out dust/bugs/contaminants. Do the same for any other ports you haven't finished plumbing in the system.

If you've never worked with braided hoses and fittings before, watch this video. It'll show you how to attach the hose ends without mangling your fingers.

Install an M18-1.5 to AN-10 fitting in the front cover's oil feed port. This is a critical line, so make sure the fitting is tight - but don't overdo it! Remember that the fitting and front cover are both made of aluminum, and can strip easily. Also, it's easy to misinterpret the FSM here. It calls for “70-95 in. lbf” for tightening torque - that's INCH pounds. That means 6-8 ft. lbs. - which is not a lot!

Don't forget the copper crush washer, either. Its purpose here is to provide a gasket under the head of the banjo bolt, and it will do the same thing under a straight fitting like this one.⁶⁾ It's perfectly acceptable to drill out the washer if necessary for the new fitting - it shouldn't need much!

Plumb a hose from the front cover oil outlet⁷⁾ to one side of the oil cooler. Plumb the other side of the oil cooler to the inlet of the oil filter. Then plumb the outlet of the oil filter to the inlet of the remote oil filter mounting block on the rear of the engine.

A smaller hose should be plumbed from the top oil fitting on the front iron to supply oil to the turbo. The turbo drain line can drop straight into the stock front turbo drain line. A hose What size? fits perfectly over the end of the stock drain tube.

Need to also describe PSRU oil plumbing.

Make sure all fittings are tight, and support all hoses at various points along their lengths with stainless wire ties, brackets, or other techniques.

Finally, install an oil filter temporarily on the filter bracket, or cover it with a latex glove to keep out dust/bugs/contaminants. Do the same for any other ports you haven't finished plumbing in the system.

The CozyGirrrls' engine mount is made of round steel tubing, which is bent at an angle as it forms the front and back edges of the mount. This bend makes the bottom edge of the mount a perfect surface against which a flat bar can rest firmly, if you are clever about how you mount it.

Cut a 36” length of 2”W x 1”H aluminum U-channel exactly in half. While you're cutting, you may as well also cut spacers for the radiator brackets - these should be cut from 0.384” ID aluminum tubing to a length that JUST fits in between the radiator mounting rails.

Hold or clamp the U-channel to the bottom of the rear engine mount tube, forward enough that it almost touches the vertical engine mount bolt tubes (leave a bit of clearance to prevent rattling.) Then slip a U-bolt over each bent end of the engine mount tube. Mark and drill holes in the U-channel where the U-bolt needs to pass through.

Make sure you put the U-bolts in the bent/angled sides of the engine mount support tube, not in the middle. This will prevent the U-channel/U-bolts from slipping side to side, or twisting. Then repeat this process for the front side of the engine mount.

Next, use foam blocks or wire to support the radiator underneath the engine mount. The radiator should be angled upward about 10 degrees with the Cozy sitting level. At this angle, the front edge should be about 2” behind the firewall⁸⁾, the top face of the radiator should almost touch the U-channel on the bottom of the front engine mount tube, and the top face of the radiator should be about 4” below the U-channel on the bottom of the rear engine mount tube.

To get this fit, you will need to mark and grind away the outside edges of the radiator where the support brackets interfere with the engine mount bolt tubes. This can be done without damaging the radiator if you're careful. If you study the radiator closely you'll see that the outside edge does not contain coolant tubes - it's just a pair of support brackets. A Dremel with a fiber cutting wheel is one option for cutting out this section. I found it easiest to cut through the outside support bracket, then use needlenose pliers to remove the unnecessary inner fins one at a time, to avoid damaging the coolant tubes.

This installation method allows you to adjust the angle of the radiator, but not the fore-aft clearance. Double check your clearance against your aileron control tubes! Then mark and drill aligned 3/8” holes in the aluminum U-channel and radiator support rails. The radiator is installed via short front bolts and long rear bolts down through the two U-channels, with a washer and standard nut (use Blue Loctite here) underneath to provide a firm mount for the bolt in the U-channel. The radiator can be moved up or down on these bolts until you have the angle you want, and then fixed permanently with standard nuts and washers on top of the radiator (use Blue Loctite again), aluminum spacer tubes between the mounting rails, and finally washers and lock-nuts on the bottoms of the bolts.