About a year ago, I was driving thru 100 degree (F) temps at about 80 mph for an hour or so. With the AC on, and fans off, coolant temps stayed around 200 (F). When I turned on the fans, temps dropped about 5 degrees. I did this test several times on that trip with the same results.

I have a stock hood, and no special ducting to remove heat from the engine bay.

 

One of the things I've noticed with the FD's stock engine compartment is how there's a ton of stuff crammed right behind the radiator: stock airbox and associated plumbing, IC/duct, stock battery tray. I'm sure turning the fans on high helps with airflow past all those components.

When I went to an M2 airbox (slimmer than stock) and Jeff's 51R battery tray (smaller) with a Greddy SMIC, it opened up a lot of space behind the radiator. I did notice slightly lower cruising coolant temps in warmer weather and quicker recovery after extended boosting, but no exact numbers, as I only have a linearized stock temp gauge at the moment.

 

To change the direction alittle, so at 85 to 90 mph or higher, the 99 spec front end would NOT help with cooling due to the radiator at that speed being the main hinderance and the majority of the air would spill out of the mouth opening and around the car instead of going through it?

Tim

 

Sorry Tim, guess it got a little off topic there...

The radiator isn't really the main hindrance. It's the ability to create more of a pressure difference between the two sides of the radiator. The '99-spec front end would probably help, especially with the lower lip to act as a sort of extension for the radiator opening. Since the opening is larger on the '99-spec, you'll have more volume to produce more positive pressure in front of the radiator. That said, you're still going to get airflow stall and "spillover" out the front, because unless you have more negative pressure behind it, the same effect will occur-- it will just happen a little later. But I do think it will help.

I bought a '99-spec front end partially for that reason, but haven't installed it yet.

 

Running the fans will help move more air from behind the radiator, and out of the engine compartment. This will reduce the "stall" you are referring to.

 

In the driving conditions Tim describes, my temps never pass 89 (take your guess as to whether I mean C or F ) without the fans coming on. My a/c system is removed entirely, so the fluidyne is exposed to the incoming air.

 

As Tim mentioned, he has the Mazda Speed replica hood from Attila. The same as mine. It's design helps to pull air out of the engine compartment.

 

You are new here and I understand your confusion but those of us with a bit of experience knew exactly what Mr. Benton was referencing with his PFC. It is a given when talking about a PFC that it is celsius.

Goodfellas was spot on.

 

I thought those were to help flow thru the radiator...so the air couldn't take the less resistive path around the sides of the radiator???

 

It will reduce it, but not by very much. In order to transfer the additional heat that's generated from working the engine harder as your speed increases, you're going to need a lot more flow through the radiator than what can be created by just the fans themselves.

 

Tim, if air spillage is truly your problem, then a different front bumper might help, especially if it has a deeper air dam or a splitter of some sort. In fact, adding a splitter (or R1 or 99 spec lip) to the OEM front bumper might help as well.

The plastic covers on the bumper opening might be acting as a crude diffuser section, which will be more and more helpful as speeds increase. A diffuser will decrease the air velocity and increase the pressure, forcing air to flow through the radiator rather than around it.

Here's a crude MSpaint diagram:


It's explained in pages 555-559 of Race Car Vehicle Dynamics, by Milliken & Milliken. ( ISBN: 1-56091-526-9 ). You should be able to find it on Amazon.com




Adam C , I've noticed the same situation with my fans at high speeds. My engine bay is mostly stock (stock rad, airbox, hood, IC. Aftermarket PC680 mini battery). Turning on the A/C (which activates the cooling fans) will reduce coolant temps at 80mph, assuming ambient temps are below 90-100F. When it's hotter than 90F ambient, the heat generated by the A/C will cause coolant temps to increase.


Recently, I installed a vented hood (or more accurately, bought a used OEM hood and cut some big ugly vents in it). Coolant temps don't rise as quickly as they used to, especially when idling in traffic. The fans don't seem to make as much of an impact at high speeds. Using the tape-and-yarn method, I've concluded that air is being forced (or sucked) out of the vents at high speeds, which is what I was hoping for. My vents are two big ugly holes towards the rear of the hood, and the area of the holes is about the same as the area of the front bumper opening.


Summary: prevent air spillage using a splitter and/or diffuser. Vented hood helps a lot, even on stock setup.

-s-

 

You both give Mazda too much credit on this one. Have you really looked at the stock Mazda radiator? It is crudely sealed on both sides with minimal gaps.

But if you look directly behind and below the corners pieces, there are large openings where air may leak out below the car. The corner pieces may reflect the air away from here more towards the center of the rad. It is a cheaper and poorer fix.

That is why many of use remove the corners and then fix what Mazda didn't.
Our results is a larger opening with less leakage.

 

In my "test" I was able to consistently reduce coolant temps 5 degrees by running the fans. This was in 100 degree weather, at 80 mph. I did the test a few times to confirm the results. I don't know if you consider 5 degrees to be very much. I do know that I "liked" seeing my coolant temps 5 degrees (F) lower .

 

Tim, it's possible the main (4 blade) fan isn't working properly. I had to replace mine because it was not spinning freely. I purchased another one from Phil and luckily he brought all the spare fans he had because of the 3 he had, only 1 was good. We opened up the ones that were not working and found the inner magnets had broken loose and gound into a dust in one and one had a melted coil. I determined these fan motors were the 4 blade ones that sits under the battery and the one that runs the most. I think the other fan (5 blade) only comes on when the A/C is on and when they are on high. You might want to remove your radiator fan shroud and test both fans to see if they indeed spin properly.

 

I agree with everything but this part. Due to the relay & wiring configuration, when one fan runs they will both run. It's not possible for the ECU to run them separately unless you've modified the relay wiring.

I would bet that the difference in fan failure is due to leaking battery acid, differences in the 4- vs 5-blade design, or the different load on the fans due to airflow obstructions such as the IC or battery.

-s-

 

This is turning into a pretty damn good thread

 

Are you sure? This is how the electric fans work in nearly every car I've seen with A/C. One fan runs to cool the system and the second one comes on for high demand or when the A/C is switched on.

 

No, I'm not implying that your method to drop cruising temps is a bad idea, or that a minor drop in coolant temps is useless. I only meant that depending on the fans to increase airflow through the radiator is a pretty weak exercise. Once the engine encounters more load, the heat production will increase dramatically, quickly overpowering the airflow the fans can generate.

 

Yes. If you look at the wiring diagrams in the FSM, the fan motors are wired in parallel. The fan motors are three-speed units, and switching the A/C on increases the fan speed by a step, depending on the coolant temps and electrical load.

 

Your just saying that since I bought your previously owned black RX7 strut bar that is going to look great on the car. It's going to be so cool looking on the car that my temps will drop 3 to 4 degrees F just by adding it

Tim

 

This happened to me once. I bled the crap out of the cooling system and it went away.

 

When I had my FD and I had to drive ~45 miles to work, 90% of which being highway...

I would cruise at anywhere from 75-85 to speeding between 85-95...

My temperature gauge which read in "F"...never rose above 190F, and just like every other FD owner, and car in general, the temperatures essentially would lower at a lower speed.

Ex. 55mph cruising with no traffic = ~160-170F readings.

All of this with a near stock FD. Pettit AST and Greddy 3" DP if anything, that relates to this topic.

I agree with the earlier post regarding: The faster you go, although you have more coming in, essentially, there is also more air swirling around under that hood of yours...and that air is hot...

My n00b answer to this would be to either raise the hood (the drift kiddies these days use washers)...or, purchase a heat-extractor style'd hood.

Scoot is always good too. I <3 that hood. :P

Yeah.

 

Here's another few things to consider:

1.) The OEM thermostat starts to to open at ~ 180F (82C), and is not fully opened until 204F (95C). In my opinion, coolant temps below 95C aren't a very accurate indicator of your cooling system's ability to dissipate heat, because some the coolant is bypassing the radiator.


2.) If your airflow is depending on the stock fan control setup (stock ECU and thermoswitch), your cooling fans will wait until 230-235F (110-112C) to turn on HI. If you've got an Apex'i PFC and commander you can adjust the fans' turn-on temps.


3.)Another thing to consider is the oil cooling. Your oil cooler could be saturated, due to the high ambient temps or due to a lack of airflow. I keep hearing that oil cooling plays a HUGE role in engine temps. I've heard numbers from 30-70% of total engine cooling. Is anyone monitoring oil temps with the stock oil cooler(s)? I haven't really searched for data on this, but I haven't been looking much either.


-s-

 

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/extreme-radiator-ducting-damian-style-%2Apictures%2A-455724/

Theres a guy who made ducting for his rad and tracked his car the next day.

 

Your gauge is inaccurate. The t-stat will keep the minimum temps at 180F or so. Running below 176F on the stock ecu will cause the double throttle plates to remain closed.

I did extensively. Try a search using my username for older posts on it. Going from single stock cooler to dual 25-row coolers lowered my oil temps 35-40F.