No, you are wrong!

Heat exchange takes time. In order to remove heat from coolant, it must stay in the radiator long enough to cool. You cannot simply run the coolant thru the radiator at some infinite speed, and expect it to lose enough heat to help cool the engine. If you run it too fast, it simply can't cool enough. Dubulup wade a good point about this earlier.

Kevin, you are incorrectly applying your absolute temperature approach. you need not convert degrees F to R because heat transfer is proportional to Delta T, the change in temperature, it doesn't matter from where on the temperature scale. the rate of heat transfer, assuming everything else constant (conductivity of aluminum and specific heat of water and such, which is not a good assumption, but for the sake of this clarification is just fine) the rate of heat rejection is EXACTLY the same with intake temps of 150F and exit temps of 120F as is it were 5000F and 4970F.

ryan

You underestimate the speed of energy transfer between water and aluminum. It takes very little time for water entering a radiator to transfer the majority of its energy difference (aka temperature delta). Infinite flow rate is not under discussion, but increasing the stock flow rate is NOT a bad thing even an increase of 10 times the stock flow rate would be helpful (if the flow thru the system remains smooth and isn't overwhelmed by turbulence).

In addition a higher flow rate has a benefit not mentioned so far, reducing the temperature of localized hot spots within the engine. If the coolant is not moving fast enough hot spots can boil the coolant, with water vapor now in the system the temperature spikes and is no longer linear causing damage. Increasing coolant flow rate smoothes out the temperature highs and lows within the engine's coolant flow path. It is important to remember you want the lowest average system temperature not the lowest radiator output temperature. If slower was better the stock water pump wouldn't be anywhere near caviation speed at 6,500rpm Mazda would have changed the pully sizes to spin it much slower. Instead they picked pully sizes to get maximum flow rate thru the rpm range they expected a sports car on the street to be hitting, caviation inefficency is largely ignored due to the small amount of time spent above 6,500 rpm on the street. Racing conditions are completely different and that is why Mazdaspeed recommends changing the pully sizes since the time spent above 6,500rpm is much larger/longer.

DamonB is correct when he says an increase in the mass flow rate helps the cooling system, this is a well understood engineering principle used in many applications not just internal combustion engines. Tens of thousands of engineers can't be wrong.

There also seems to be a misunderstanding of the purpose of a thermostat, the reason is not to limit the flow rate to increase heat transfer. Its purpose is to keep the engine within its optimal temperature range since running too cold is not optimal it limits the flow rate thru the radiator.

I'm wwwwrrrroong with explanation ... :(

Still re-read my example, you mis read it.

Any airflow has heat energy flow if it is above absolute zero. In my example the high cfm case has about 2x the total heat flow as the low cfm case. That is still true. But for local heat rejection from the exchanger:

To properly look at the vent flow case, you need to assume an inlet flow temperature ... 30F outside air would be reasonable for my example. heat rejection rate will be cfm x dT x cv. So the heat flow rate ratio at hi vs low speed is:

(2 (120-30) cv )/(1 (150-30) cv) = 1.5, so 50% more heat rejected even though the vent air is 120F vs 150F.

Thanks for the bust ryan. :)

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For max coolant heat transfer, you want low viscosity, high heat capacity and conductivity, and high velocity. Evans NPG has made great efforts to reduce viscosity, and used to sell higher flow rate pumps to make up for the inferior heat transfer properties of their old product.

Easy to check lower limit on velocity issue ... trickle flow. Engine warms up stat opens and you get 100% efficient radiator as 190F coolant entering rad drops to ambient just after entering the finned tubes. But gross heat rejection rate is zip since there is negligible coolant mass flow, and soon your overheating like crazy in the engine as the initially ambient coolant creeps through the engine and is at a raging boil early in it's journey.

95MX6: I agree with all of that, except that I underestimated energy transfer. :)

My point was that coolant has to remain in the radiator long enough to be cooled. At some flow rate (which is perhaps unattainable), coolant will not be in the radiator long enough.

why is this not a sticky?

It is linked in the FAQ sticky.....

And this basically goes right back to what I was referring to in this post. There are a lot variables here that need to be taken into consideration, and simply boosting the coolant flow rate via faster pump impeller speed or however based on just heat transfer principles is taking chances with your engine.

I don't mean to detract from the discussion here, but it seems like some real-world testing would settle most of the disagreements in this thread. Unfortuneately, I'm hoping to buy a house in 2007, so it might be a while before I do much testing on my own car.



Here's my plan, I'd be very pleased if anyone else wants to do something similar:

1.) MEASUREMENTS, recorded by a data-acquisition unit, such as the ones available from www.aimsports.com
Install new temperature senders (or monitor OEM sensors) in the following locations:
-Before engine (lower radiator hose)
-Inside engine (OEM sensor in rear housing, used for OEM temp gauge)
-After engine (Thermostat housing. Might use the OEM sensor, might tap and install additional sender)
-Oil temps before & after cooler(s). Oil temps would tell you if your coolant is absorbing more or less heat from the motor.


2.) TESTS, don't forget to document ambient temps. Perform at least 3 trials of each test
-Time to warm up, from 180F - 210F, at idle with fans off.
-Time to cool down, 210F - 180F, at idle with fans on.
-Hill climb, 3rd gear, constant speed with nearly constant boost.


3.) VARIABLES
-Coolant mixture (% water/glycol)
-Thermostat (OEM, drilled, gutted)
-Fan control
-Ducting
-Vented hood vs stock
-Aftermarket radiator
-Oil cooler, etc...

Eventually I'd like to try some of the less common mods such as electric water pump, grooved water jackets, even Racing Beat's all-aluminum housings, but those might be years away if I ever get around to them.


-s-

^just don't sell the 7 ;)

May want to reduce scope, and include a few trips to a load based (mustang) dyno. Could check steady state temps with oem fans on high, consistent load. Winebagos have a way of busting hill climb runs.

Also, for cavitation, with oem t-stat, a pressure measurement where the heater hose comes off (orifice/restrict heater hose) would show pump head at high rpm, and related loss if cavitation occurs.

But tests are much easier said than done.

Hey that heat light, on the dash is that engine or exhaust warning heat, mine comes on and goes off at wierd times, like after its been switched off for 5 minutes, does it mean something is hot and if so what its a crazy world.

The red one by the security light for the stock alarm is for a temperature sensor that detects if your stock cat is overheating, if it's staying on for awhile after driving a bit you may have a clogged cat that needs to be replaced.

-Andy

Cheers, i dont think i have any cats though, My car ROARS like a V8, cheers

just out of curiosity, i was wondering if a coolant temp of 103C is normal on a stock radiator and water pump... i noticed that my temp was starting to go up the other day and then discovered yesterday that my thermostat was no longer in working order... so i guess i have two questions then... do i need to install a new thermostat(pros and cons), and on a modified 93(mainly a new engine out of a 97 and upgraded intercooler) running the oem radiator and water pump at what temp should i start getting worried about the coolant temp
oh yeah, it does this at idle... when driving it will drop down to about 90C

if this has already been covered please post a link to where the thread is. ive been trolling for the better part of the week with little success as to my questions
thanks

Your coolant temps should generally go up and down within a particular range. On my stock FD the high end is 210F (about 99) and the low end is 180F. The fans turn on at 210 and the fans stop when it gets to 180. I have a 2nd gen thermoswitch so the 210 might be more like 215 or 220 if you have the original 3rd gen thermoswitch.

If you have any doubt about your thermostat, replace it.

Dave

does your car tend to get that hot while idling or when running hard?

Mine will get that hot both while driving or while idling.

It has only ever been hotter when I was on a track and flogging it.

Dave

sweet, thanks that gives me a little better idea what im looking at here

and what would be the pro's or con's to just removing the thermostat entirely?

Do not remove the thermostat without plugging the bypass hole in the thermostat housing. Hot coolant will bypass the radiator and recirculate straight back into the engine, and your car will overheat very quickly.

https://www.rx7club.com/attachment.p...5&d=1152905872



If you want a little more coolant flow, I recommend drilling a few small (1/8") holes around the the outer ring of the thermostat.

-s-

awesome, thank you... you guys have been immensely helpful

why is a suggested modification replacing the stock AST 0.9 cap for a 1.1cap on the first page????

Using the 1.1 bar cap will raise the boiling point of the coolant without sacrificing heat transfer. The other way to raise the boiling point is to add more glycol, but that won't transfer heat as well.


Another thing to consider: when the pressure cap vents coolant out of the system, you don't get that coolant back until the engine shuts off and cools down. This decreases thermal mass, which is bad. Sorry I don't have a better explanation, but trust me you want a higher pressure cap if your hoses and water pump can withstand it. It's not uncommon to find pressure caps as high as 30 psi (2.0 bar) on purpose-built race cars.

-s-

Do not remove the t-stat, period, it is pointless, even with plugging the bypass. The stock ecu doesn't even turn the fans on to LOW speed until 221F (105C). Get a PFC, do the fan mod, or install an FC thermoswitch.

Very helpful thread. Thanks for the hard work.