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#1 |
Rotary Fan in Training
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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..) Last edited by scotty305; 03-05-2008 at 12:59 AM.. |
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#2 |
Rotary Fan in Training
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7. Optimum Temps
65C (150F): too cold. According to service manual, EGR valve is non-operational below 150F, "to improve drivability when cold." 82C (180F): getting warm. Thermostat begins to open, circulating coolant through the radiator. Some coolant is still bypassing the radiator. 95C (203F): fully warm. Thermostat is fully open, not bypassing the radiator at all. 100C (212F): boiling point of pure water at atmospheric pressure. 105C (221F): getting hot. Stock ECU will activate fans to cool the car down. Fan speed will be low, or medium (if A/C is already on). 108C (226F): hot. Stock 93-95 coolant thermoswitch activates, changing fan speeds from low>> medium (or med>>high if A/C is already on) (switching to an FC thermoswitch will change this temp to 203F) 115C (240F): getting dangerous. OEM temp gauge begins to rise. 117C (243F): dangerous. boiling point of pure water with 13psi pressure cap. 121C (250F): too hot. OEM temp gauge will point to white line. Boiling point of pure water with 16psi pressure cap. 124C (256F): way too hot. Boiling point of pure water with 19psi pressure cap. Boiling point of 50/50 coolant mix with 13psi pressure cap. 127C (260F): way too hot. OEM temp gauge will point to red line. 133C (273F): way too hot. Boiling point of pure water with at 30psi (2.0bar) pressure cap. This is what Carlos Iglesias is using on his track car. Many race cars use even higher pressures: I've heard rumors that F1 teams run their coolant (pure water for efficiency) at 50 psi or higher (boiling point would be over 290F). FYI, a 70/30 mix of water/ethylene glycol (standard yellow-green coolant) will raise the boiling point by about 8 F compared to pure water, and a 50/50 mix will raise the boiling point by about 13F. A Mazda competition preparation manual, dated 1980, gives the following recommendations. Note that this is for a non-turbo 12A motor in a racing environment: Warm the engine up at an idle speed of 2000-2500 RPM, until the oil temp reaches 160F (70C). Recommended coolant temperature (outlet side): 160-195F (70-90C). Max coolant temperature: 205F (95C). Recommended oil temperature (oil pan): 195-230F (90-110C). Max oil temperature: 250F (120C). 8. Coolant Flow Water Pump exit port pushes cold coolant directly into driver's side of the engine block (near intake ports), heated coolant returns to WP from the passenger side of the engine (after passing exhaust ports). If the thermostat is closed, a bypass port is open and coolant goes back through the water pump again without flowing through the radiator or AST. As the OEM thermostat opens, it closes the bypass port. Hot flow exiting the waterpump flows through the open t-stat, out the filler neck to upper rad hose, through the radiator where it is cooled. The water pump sucks coolant from the lower radiator hose. Warning: rumor has it that 'parts-store' brand thermostats don't close this bypass. Regardless of thermostat position, the throttle body coolant line takes coolant from the passenger side of the rear iron, and is pulled back into the waterpump housing inlet. Regardless of thermostat position, the heater core is fed by a large coolant line below the oil filter, and is pulled back into the waterpump housing inlet. It always has substantial flow, and always bypasses the radiator, decreasing cooling capacity. Turning the heater on, and running the cabin fans at full speed will dissipate some heat. Turbo cooling starts at the return port for the wp, goes to turbos, returns at wp inlet pipe. Not much pressure drop, so not much flow, and flow reverses at shutdown, making t-stat housing gauges jump, but this is very local volume of hot coolant thermally siphoning through turbos. AST is functional when thermostat opens, and flow goes across top of filler neck and drops to 10mm exit port. Then comes in a 10mm port to ast mid body, and lower hose feeds this flow to lower rad tank, then to wp inlet. To limit the amount of coolant bypassing the radiator, a .086" pill is intregrated to the lower nipple of the stock AST. Coolant expands as it heats up, and pushes air and coolant through AST pressure-releif cap to overflow tank. (description taken from KevinK2 , http://www.rx7club.com/showthread.php?t=465083) With AST installed: With AST removed and FC filler cap installed: 9. Links Maintenance: Coolant Flush How-To's: http://www.fd3s.net/cooling_system_flush.html http://www.robrobinette.com/flush.htm Coolant Burping Tool/ Radiator Cap Funnel http://www.rx7club.com/showthread.php?t=342309 Part Numbers for replacement Coolant Lines: http://www.rx7club.com/showthread.php?t=331644 Replacing Turbo Coolant Lines: http://www.robrobinette.com/turbo_lines.htm Modifications: Stock Water Temp Gauge Linearization: http://www.rx7club.com/showthread.php?t=252334 AST Elimination http://www.robrobinette.com/coolant_tank.htm http://www.fd3s.net/coolant_tank_elimination.html Fan Mod: http://www.fd3s.net/fan_mod.html http://rx7.voodoobox.net/howto/fanswitch/fanswitch.html http://www.rx7club.com/showthread.php?t=462760 << electrical schematic of relay circuit, with description Upgraded Water Pump http://www.rx8club.com/showthread.php?t=116423 Electric Water Pump: www.meziere.com << Meziere sells electric pumps and adapters for the 13B http://www.rx7club.com/showthread.php?t=414533 <<example here 10. Misc / Unfinished Business It would be nice to get some before/after temperature data for various mods, using the following test: -Let the car sit at idle, measuring time from 180F >> 210F (stock ECU will activate the fans at 210F when the parking lights are on). -Fans will turn on, then measure time from 210>> fans turnoff (180F). Many people swear by Evans coolant. I haven't tried it. The advantage is it doesn't boil and doesn't require the cooling system to be pressurized. The disadvantage is it's expensive and doesn't transfer heat as efficiently as water. I'd like to discuss coolant-seal failures further. I've begun gathering photos from engine tear-downs and it seems like the failures occur in the same general vicinity. (figures 5d-9) Thanks for reading, -scott- Last edited by scotty305; 03-05-2008 at 01:22 AM.. |
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#3 | ||
Rotary Fanatic
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Excellent post!
here to add: a diagram i made myself of coolant flow both the factory and without the AST system. Please enjoy!
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Last edited by Cp1; 03-05-2008 at 01:06 AM.. |
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#4 |
Rotary Fan in Training
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Done. I hope there's not a time-limit on editing posts, I'm planning to go back and add to this eventually, probably copy some content so there aren't so many links to other websites (can we say rx7club.com here? can we say rotarycarclub.com on rx7club.com? the internet is to big lately, I'm so confused!! Are we still OK with nopistons.com? Great engine building section over there, someone needs to bring Lynn Hanover to this site)
Last edited by scotty305; 03-05-2008 at 01:27 AM.. |
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#5 |
The Judge
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Last fall I installed the Koyo "N" pass radiator and it works very well with my FMIC.
My previous all AL rad was 1.75" thick and single pass. This one is 2.25" thick and triple pass. It does the job very well. Mazmart now offers an improve FD water pump for low $300s. It has an improved impellor blade that does not cavitate. http://www.mazmart.com/ under performance parts. Their site sucks though and you would have to call them to order. Last edited by cewrx7r1; 03-14-2008 at 01:44 PM.. |
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#6 |
600rwhp club member.
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I really need to make some changes. I could not make a run thru the dragon with out over heating! High driving and all is fine. Runs a little hot but its ok. But under hard driving it does not take long. I am running a APexi "thick" fmic and a base KOYO.
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#7 |
Rotary Fan in Training
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I found these values after Googling for a bit when I was changing my anti freeze a few weeks ago. Most antifreeze testers won't tell you the percentages. They tell you the unpressurized boil and freeze points which had me scratching my head a bit until I found the values.
Coolant/Water Boil point Freeze point Ratio 40/60 126°C -23°C 50/50 129°C -37°C 60/40 132°C -52°C 70/30 136°C -64°C Someone please correct if these values are not valid. |
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#8 | |
Avid Installer
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#10 |
Still not fast enough!
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Absolute legends, great help for my problem! When i switch engine off, a lot of water is dumping out of the ast overflow pipe, but mine seems to be broken as it dumps straight to the floor! Thinking the pipe has never been complete since i bought the car!
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#12 |
Guest
Posts: n/a
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WOW.. nice write up.
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