Disclaimer: I don't consider myself to be an expert in this subject, but I've spent some time researching and compiling info. Hopefully this thread proves helpful to a few RX-7 owners. My boss at work is fond of saying, "In order to finish first in the race, you must first finish the race."
1. Introduction
The 3rd Generation RX-7 was intended to be a “pure sports car.” Independent teams of engineers were assigned the task of reducing the weight of the car, and the performance and/or longevity of certain parts or subsystems was sometimes comprimised to reduce weight. In addition, the unique design of the rotary engine's combustion chamber generates more heat than conventional piston engines. For these and other reasons beyond the scope of this article, the RX-7 tends to run too hot, especially if the car is driven hard and/or modified for increased power. This can lead to overheating.
2. Measurements
To further exacerbate this problem, the RX-7's coolant temperature gauge is center-weighted and inaccurate. It will indicate 'medium' temperature when the engine is still cold, and the needle does not move during normal operation. By the time the needle begins to indicate the coolant is 'warm,' your motor has already reached dangerously high temperatures. It's recommended that every owner install an aftermarket coolant temp gauge, or perform the 'linearization' mod to the OEM temperature display. (see links section below)
3. Basics
The cooling system's purpose is to keep the engine at the desired operating temperature, and to prevent engine parts (the iron and aluminum rotor housings, as well as the rubber/silicone/teflon coolant seals) from getting so hot that they warp or fail. If you don't already understand how cooling systems work, visit HowStuff Works.com for a good explanation.
http://auto.howstuffworks.com/cooling-system.htm
For the purpose of this article, we'll define overheating as the point when the cooling system is no longer removing enough heat from the engine. The signs of overheating are an uncontrollable increase in coolant temperature, usually accompanied by boiling coolant.
4. History
The original cooling system design included a 1.3 bar (19 psi) pressure cap. This would allow the engine to run at higher temperatures without boiling coolant, but it put additional stress on the hoses and rubber seals. At this pressure, the original rubber hoses would fail prematurely, and many owners experienced engine fires due to coolant hose failures. In addition, if the coolant was lost completely the engine would often overheat before the driver took action.
A recall in 1994 replaced the following components: filler neck, filler neck cap, water pump bearing housing, water level sensor, upper radiator hose, thermostat gasket, all water hoses above the engine (throttle body lines, AST lines, possibly heater hoses). The AST cap was replaced with a lower-pressure 0.9 bar (13 psi) unit.
5. Maintenance
As with most things, preventative maintenance is a great place to start. A healthy cooling system is adequate for an unmodified or lightly-modified RX-7. Here are some basic things that will ensure the health of the coolant system:
The coolant should be flushed yearly, and replaced with the proper water/coolant ratio. Mazda recommends a 70% water / 30% coolant mix for warm climates. Due to the superior heat-transfer properties of water, a few racers recommend running as high as 90% water/ 10% coolant in hot climates. Be sure to use the proper amount of antifreeze in cold climates.
When refilling the system, be sure to fill slowly to prevent air pockets from forming. Using a Lisle funnel is recommended (see links section); it's also very important to check the coolant level regularly for the next 2-5 trips after a coolant flush, adding coolant as necessary.
Ensure that the filler and AST caps are sealing properly, and venting at the correct pressure. If your pressure caps aren't working properly, the coolant will boil at a lower temperature, which will allow the engine to overheat more easily. New pressure caps are $12-25 from the dealer, which is much cheaper fixing an overheated engine. It wouldn't hurt to buy new caps every 3-5 years.
Replace old coolant hoses every 5-10 years. If a coolant hose fails, the cooling system won't hold pressure, allowing the coolant to boil at a lower temperature, which will allow the engine to overheat. It's very common for hoses to develop small cracks or splits that are hard to see, so it's better to just replace old hoses than to wait for a catastrophic failure. Don't forget the coolant line that connects the AST to the overflow tank. If this line doesn't hold vacuum, coolant overflow won't be sucked back into system when the car cools down. The cooling system will slowly lose coolant (which decreases operating pressure and reduces the system boiling point, leading to overheating).
6. Modifications
If you modify your engine to make more power, it will generate more heat. The RX-7's cooling system is marginal compared to most other cars, so it's important that your cooling system is upgraded to handle the additonal heat generated by the motor. Don't forget that oil performs a large percentage of engine cooling in rotary engines, since there are no coolant passages for the rotors, rotor bearings or eccentric shaft. The only engine modification to the R1 and R2 models was an additional oil cooler, which suggests that the single stock oil cooler setup was inefficient for track use at stock power levels.
Upgrades, in order of cost: (it is assumed that 'free' mods cost time)
16psi cap (Stant 11233). Raises boiling point of coolant.
Drilled OEM thermostat. Reduces the amount of coolant bypassing the radiator, allowing more coolant to flow through the radiator.
Radiator ducting. Forces air to flow through the radiator, preventing 'spillage' at high speeds.
FC thermoswitch. Lowers the temperature at which the stock ECU activates the fans.
Fan mod. Similar to FC thermoswitch, but allows driver to control fan operation.
Removal of A/C and/or power steering. The removal of the air conditioning condenser will allow more airflow through the radiator.
Air Separator Tank (AST). OEM plastic tank is known to fail. Replace or eliminate.
Downpipe. The OEM precat generates a lot of heat, and its large size prevents hot air from escaping the engine bay. A downpipe will improve cooling system performance in addition to increasing HP.
Modified battery. Installing a miniature battery, or relocating a normally-sized battery, will improve airflow through the radiator.
Radiator. OEM radiator uses plastic endtanks, which are known known to split, crack or fail.
Upgraded Water Pump. This is a new product from a company called RE-Medy. Haven't heard how well it works on the FD, but it looks very nice.
Vented hood. Airflow to the radiator plays a huge role in cooling system efficiency. A vented hood will greatly improve airflow, allowing heated air to exit the cramped engine bay.
Upgraded oil coolers. Oil performs a large percentage of engine cooling in rotary engines; don't neglect it. The only engine modification to the R1 and R2 models was an additional oil cooler, which suggests that the single stock oil cooler setup was inefficient for track use at stock power levels.
Electric water pump. Eliminates cavitation experienced by OEM mechanical water pump at certain RPM's; requires custom fabrication and relocation of alternator. (see links section)
Teflon-coated coolant seals. Modification performed during an engine rebuild; replaces OEM rubber seals with high-temp teflon-coated silicone o-rings. (note: some builders complain that the poor fitment of teflon o-rings is not worth the additional heat they can tolerate)
Modified coolant passages. Modification performed to water jackets, carving grooves for increased heat transfer. A common trick among engine builders such as Racing Beat and Rotary Power; used for race engines. (Other builders might do this, such as Gotham, Pineapple, Rotary Aviation, etc..)
FD3S Cooling Thread
Linear Mode
