2nd Gen Overheating

[Scooter]

I have FINALLY sorted my car's overheating issues and I thought I would post the story for anyone who might run into this and be looking for a solution.

I started with a stock radiator and stock cooling system. I also made the mistake of assuming that if the factory gauge wasn't reading high that the car wasn't overheating. I found out this wasn't the case after ruining an engine.

Lesson one: The factory gauge doesn't read high until the car is way too hot. Second, it only reads water, and the oil temps can be very high also. So buy aftermarket water and oil temp gauges. Installing them required buying a sender block for the oil temp that mounts under the oil filiter (Racing Beat, I believe) and drilling out an already existing partial hole in the water pump housing to mount the water temp sender.

Lesson two: The water temp issue will only solve so much. I bought a huge Mazda Comp radiator and did the thermostat housing block-off and switched to water and water wetter and installed a big electric fan. (I did all this in stages.) All of this was fine, but it didn't fix it really at all. I would still do these things, (especially the radiator) but don't expect miracles.

Lesson three: The factory setup for ducting sucks. I blocked off all of the extra space around the radiator, including in front of the radiator where those factory plastic pieces are, and made an aluminum sheet that went from the top of the oil cooler to the bottom of the radiator. Then I made some big holes in the factory drip pan so that hot air coming off of the oil cooler would go under the radiator instead of right into it. I also made a straight piece that went from the middle of the air entry hole in the bumper cover to the top of the oil cooler. That way some air would go directly into the oil cooler instead of over it. I made the bottom of the drip pan hang down a bit in front to catch more air for the cooler as well. This helped a lot. Still not fixed, however. But I would do this again.

Lesson four: Get a second factory oil cooler!! This was the thing that finally fixed it. I bought one off of eBay for $50 that thankfully doesn't leak. I then followed a forum member's advice and got swivel T fittings and oil lines from Aeroquip. The second cooler mounted in front of the first one only flipped left to right. Then the T fittings went into the first cooler and allowed me to run lines to the second cooler. I ran the coolers in parallel, not series. So in to in, not out to in. I also added a new oil pressure regulator from Racing Beat that kept the oil pressure even higher than stock even after adding a second cooler.

Now it runs so cool that I'm going to have to put a working thermostat back in. (I'm using a gutted one now.) I also run Mobil 1. It takes about 7 quarts to fill it up.

89 ITS RX-7

[benspeed]

Good story - heat is the enemy of these motors big time. My cooling setup is from ISC and uses a Fluidine oil & water heat exchanger in addition to a custom aluminum radiator and a big aftermarket oil cooler. Water is usually 190 - oil 220. Some say the exchanger robs a few HP.

Anybody agree that an exchanger robs power?

Cheers,

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BenSpeed
#33 ITS RX7
BigSpeed Racing
NNJR BoD - Treasurer
[email protected]
NNJR

[Scooter]

Originally posted by benspeed:
My cooling setup is from ISC and uses a Fluidine oil & water heat exchanger in addition to a custom aluminum radiator and a big aftermarket oil cooler. Water is usually 190 - oil 220. Some say the exchanger robs a few HP.


I didn't know about this setup when until after I had invested in the Mazda Comp radiator. Which is really expensive. I think this ISC setup is $1,000. I probably have about that in my setup, plus a lot of work.

I think if you got a Griffin radiator you could do the whole setup for a little less than ISCs. But the ISC setup is plug-and-play which is a big plus in my book.

[gruff]

I have a stock cooling fan, stock fan shroud, stock oil cooler, stock thermostat, stock water pump with no bypass blockage mod. I never got around to reinstalling the aluminum undertray but it doesn't seem to matter. I do run the turbo oil pump. I run right at 200/200 water/oil temps. I am not convinced that one needs all of this expensive gear to run a 13B at race pace.

As far as the oil cooler robbing power, I really don't think so. The oil pump is going to be turning at a set speed for a set RPM anyway. It's the oil pump that robs the power. The 2nd gen comes with the oil cooler stock. The car is underweight anyway. You might as well keep the oil cooler.

My experience is that Mazda did a great job on this engine straight from the factory.

Later,
Gruff

[Scooter]

Originally posted by gruff:
I have a stock cooling fan, stock fan shroud, stock oil cooler, stock thermostat, stock water pump with no bypass blockage mod. I never got around to reinstalling the aluminum undertray but it doesn't seem to matter. I do run the turbo oil pump. I run right at 200/200 water/oil temps. I am not convinced that one needs all of this expensive gear to run a 13B at race pace.


I have also noticed that some cars just don't overheat. I ran into a guy running everything stock and his temps were right around 190/190 on an 80 degree day. I have no idea why this is. Which is part of the reason I didn't buy all of the stuff right up front. I heard several people say that you didn't need a second cooler.

Are you running the 89-up engine? (I am.) Maybe that's it? Fuel pressure regulator? (Although mine doesn't seem to run lean.)

Not that it matters now, but I'm curious as to why some overheat and some just don't.

[bldn10]

I have owned and raced 5 2nd gens over the last 12 years and not one ran 200/200 on even mild days. I've tried fans, heat exchangers, ducting, double coolers, Mazda Comp radiator, etc. Gruff, all I can say is that you must be ice racing, going slow, or you have hit on the holy grail and should exploit it for commercial gain. :-) Scooter, I wish you had posted re the Comp radiator before buying it - it is a beautifully constructed piece but I did not experience any improvement w/ it.

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Bill Denton
87/89 ITS RX-7
02 Audi TT225QC
95 Tahoe
Memphis

[Eagle7]

I curious where you measure your oil temp. I'm using the Racing Beat oil filter adapter to mount the oil temp sensor, but I'm considering moving it to the oil pan. Since the filter is directly after the cooler, my setup indicates the oil's coolest temp. I THINK Mazda talks about oil pan temps. Anybody know how much temp rise from the filter to the pan?

BTW, I've got a custom radiator from http://www.alumrad.com/ that includes an oil to water heat exchanger. Same idea as the Fluidine. Last year it was plumbed in series with the stock oil cooler (radiator first), and my temps ran 210 water and 205 oil. I'm going to try them in parallel this year.

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Marty Doane
ITS RX-7 #13
CenDiv WMR

[gruff]

I am running an 1986 6-port engine with the later high compression rotors.

As far as running slow, let's say I was running a 1:06 versus a 1:00. That represents a 10% difference. That's damn slow. If the heat output were an exact 10% decrease as well (it's not) then I would require only 10% less cooling. I guarantee that I am pressing just as hard on the pedal as everyone else coming down the straight. That's where the heat is being produced. What you guys are talking about is 200% more cooling. That doesn't account for the difference between my car and others.

Did any of you who did all these mods ever run a stock setup? Did you go straight to modified cooling setups? I can't deny your observations. I also can't deny my own. How do we reconcile them?

I run an adjustable fuel regulator at 40 psig. This is 10% over the stock setpoint of 36 psig. The idea was just to give a little bit of margin to be sure I didn't run lean and get into detonation. As far as I understand it, this doesn't really matter for wide open throttle. It would help for high load, low speed conditions that could cause ping. Lean is death for the rotary. So if others are not breaking apex seals we can assume that everyone's mixture is sufficiently rich.

I would like to say a quick word on ducting. For an air to water heat exchanger to work properly, the air must go SLOWLY enough through the radiator. I don't have any inlet side routing at all. As I said, the front aluminum tray is not installed. I do have the fan shroud installed. This will affect the rate of flow going through the radiator.

If flow is unducted you can also get a recirculation of airflow. In my system, all air must go through the shroud and through the mechanical fan. There is no recirculation. If you have unducted electric fans that do not capture 100% of the flow going through the radiator (i.e. surface mounted), then you can have airflow spilling out of the corners of the radiator (where the electric fan doesn't cover) and recirculating. Just because the car is moving forward through the air doesn't mean that the air goes only backward through radiator. Air will follow differential pressure. If the pressure in front of the radiator is lower than the pressure behind, then hot air from under the hood will circulate through front of the radiator. Now who here really knows what crazy airflows occur under the hood of our cars at speed? I'm not going to do CFD to find out. My point is that this represents a big difference between my car and others.

Every factory electric fan I have ever laid eyes on has a shroud. That is something to consider.

Consider running hot oil cooler air through the radiator. Certainly Mazda considered this and sized the radiator appropriately. Running a splitter to the seperate this one flow channel into two flow channels might not be a good idea. While you may have reduced the temperature of the air into the radiator, you might have reduced the area of the radiator that is seeing good air flow. Without data, one cannot know if this is better.

So, let's see if we can isolate the difference between my ice-mobile and the other heat-machines. First order of business is, Have you ever run the stock fan and shroud?

Always, always, always run a thermostat! The purpose is to regulate temperature. Your water pump is sized to provide the correct flow rate for removing heat from the engine with the minimal flow obstruction that the thermostat presents when it is wide open. If you can't get enough cooling, removing the thermostat is NOT the fix.

I'll address one topic right now as I am sure it will come up. I do not agree that a stock fan robs power at speed. Maybe in a dyno run where there is no airspeed, but that doesn't matter much to the practice of racing. The viscous clutch should be barely adding any torque load to the engine at high speed. Even if it did, one would have to determine that the stock fan robbed enough power to justify the added oil mass, added water mass, and added equipment mass, and oh yeah, the added budget impact.

My observation is that Mazda put an great deal of attention to detail into making this a real sports car. You can't disrespect the care that those Japanese engineers put into their design.

The engineer's mind works like this. How much cooling do I need? I need enough cooling to reject 100% of the waste heat during a 100% full rated power for an indefinite period of time to keep the temperature of the materials used in the engine within a safe level. Until you start making more than 100% rated power, you don't need more that 100% rated cooling. Now all of our engines are fundamentally stock in their performance. If your exhaust and your tuning get you 10% more power, you really only need to find 10% more heat rejection. Engineers, pessimistic as they are, always leave excess cooling capacity in their calculations. As such, one can probably get away with using up that excess cooling capacity in a race. My car works fine using all of these assumptions in the decision to NOT modify the cooling system.

BTW, I also have a MKIV Supra friend who swears by stock cooling having tried everything else to cool a rally Supra.

Later,
Gruff

[gruff]

On second look, I misunderstood Benspeed's comment on oil/water heat exchangers. I thought he was talking about deleting the stock oil cooler. Just the same, I don't think any oil heat exchanger of any type robs much power. It's the oil pump running up to max pressure that would determine the level of power robbing.

Later,
Gruff

[xr4racer]

Gruff, I have raced a 2nd gen for 4 years and I have 2 friends that also have raced them. In my experience with my car, the other 2 plus other 2nd gens I have come in contact with at races, I agree with the earlier post that you must be the luckiest person on earth. What I have seen is once you open up the exhaust with a header etc. Run a good rear gear (4.88 or 5.13), race nose to tail with other cars, the temps skyrocket. Do you have the stock radiator? Every car I have seen at an SCCA race or drivers school with the stock parts has overheated. We have attacked this in much the same way as earlier posted. I have a Ron Davis radiator and Setrab oil cooler along with ducting. I do not run a thermostat but I have a restrictor, I would never run a thermostat that could fail and cost me an engine,,if anyone does run a restrictor plug the hole just below the restrictor so that the water does not run in circles without going through the engine. At Mid Ohio shifting at 8000 following another 2nd gen (hotter exhaust) in 80 degree heat I Have run at 185 water and 205 oil. The other thing I have found that I feel has helped immensely is running distilled water and 10% Valvoline Super Coolant additive. You can buy The Valvoline at NAPA or at least order it there.

Matt #86 RX-7 MVR

[Scooter]

A couple of thoughts on Gruff's post:

I have seen one other RX-7 that didn't overheat when using the stock cooling setup. Like yours, it was an early engine with the later rotors. It also ran a fuel pressure regulator. (I don't have one because I understand the later cars run a little rich from the factory.) It would make sense that if my car was running lean it would also run hot. But I don't think that this is the case.

It also had the stock fan and shroud.

I really can't imagine the stock shroud matters much because there's no way even with a shroud that the stock radiator cools better than the twice-as-big Mazda Comp radiator. Also, and this may not be true, but I've been told that over a certain speed (like 30mph) a radiator fan doesn't do anything anyway. My old race car didn't even have one.

(Also I do have a thermostat housing in there, just gutted. For the proper restriction. I also blocked the housing hole, like I wrote before.)

Hmmm... the other thing that I noticed about the car that wasn't overheating was that he was much slower than I was. Maybe that car was making a lot less horsepower for some reason? Or maybe the internal oiling is much better on the early housings?

Anyway, the point of the original post was simply to help someone out who might have an overheating car. If your car's not overheating then count your blessings. Cuz it sux.

[Eagle7]

Originally posted by Eagle7:
.. I've got a custom radiator from http://www.alumrad.com/ that includes an oil to water heat exchanger. Same idea as the Fluidine. Last year it was plumbed in series with the stock oil cooler (radiator first), and my temps ran 210 water and 205 oil. I'm going to try them in parallel this year.
Update: I no longer recommend this setup. When the heat exchanger in the radiator fails, it's too difficult to know you have a problem before it does major damage. I lost an engine to this last year, and it tried to kill my fresh rebuild, but I caught it before firing it. Shame on the radiator shop that cleaned and pressure tested the radiator and called it good, and shame on me for not checking it myself.

I'm now working on the installation of the second factory cooler.

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Marty Doane
ITS RX-7 #13
CenDiv WMR