So you are saying these water vapor bubbles will distory the water pump blades?? Hmm. I never knew that.. But I think what some people think is that these bubbles are travelling thru the cooling system and actually making the engine run hotter. That I don't believe... If I'm wrong correct me please!

 

http://en.wikipedia.org/wiki/Image:T...ancis_Worn.JPG

In the wiki, the subject of discharge cavitation is the one that applies to water pumps. In a nutshell, if you drive a pump with enough velocity, the flow stops and it essentially spins in a bubble of water vapor. That's a crude description, but it visually resembles how your blender can spin without circulating any material if the mix is really thick and you run it at very high speed.

 

a great example of cavitation is my Yamaha GP1200R before I sealed the pump....the impeller spins so fast off the bottom that tiny pockets of air are entrained, causing damage

an aftermarket "swirl" prop and improved pump sealing fixed it...and transformed the hole-shot

Sea Doos are slow

 

Increasing pressure can make a huge difference. Submarines cannot answer a flank bell near the surface because of cavitation. They can and do answer flank bells at depth. The only thing that changed here is pressure.
These are very big props/screws. Have you considered the velocity at the tips?
Centrifugal pumps require Net Positive Suction Head (NPSH) to avoid cavitation. Once this is lost (doesn't need to be a sustained loss), the pump cavitates until NPSH is returned.
Sustained cavitation will destroy an impeller. A cavitating pump sounds like it is pumping gravel.
Or decrease velocity, decrease temperature, change impeller design, use something other than water, etc.

 

good read...nice write up, Scott.

 

Very impressive write up ! , I am in the process of doing further up grades to my cooling system and was looking about for the coolant flow diagram , thanks for that .
So are there any Modified 7's with big frony mounts who want to contribute some information , presently I use the Koyo rad , the large NPR FMIC (2.5" thick) and no thermostat , my water temps stay around 190F generally , in traffic however it takes a while but it can get up to about 210F , my oil temps drop to 150F on the highway and equals the water temps whilst in traffic .
I want to upgrade my intercooler to a big 4" one , so I am in the process of upgrading my cooling system in anticipation of higher temps caused by the added restriction (blockage) caused by the thicker FMIC , I am presently installing an electris Miziere 55gpm pump , replacing my thermostat and upgrading to the tripple pass Koyo NFLOW radiator which will be fully ducted , I hope these changes will allow me to run my "monster " front mount without ANY issues.

 

Yep. Another thing to consider when trying to increase flow rates: Faster coolant flow means less time in the radiator. This translates to less coolant heat loss, and hotter coolant entering your engine.

 

That's not what heat-transfer equations say! The problem is that you're thinking about cooling a tiny bit of coolant (e.g. a molecule of water), but there's an incredibly small amount of heat in there. Let's think about this from the other side: should we slow the fans so the air has more time to heat up?

 

It's a fact, that something hot will cool more if it is exposed to a cooling environment for a longer period. Not sure what you are getting at with your ridiculous air speed comparison. It isn't in any way relevant.

 

the air is the cooling "agent" you want as much as you can moving thru...there is a balance to find...the right speed thru the motor to pick up the heat and dump in the radiator. I think Mazda got it right.

 

I agree dubulup!

 

...and while your coolant molecule spends all that time parked in the radiator it's not making its way back to the engine to pick up more heat.

The thermo equations say that the faster you pump the coolant through the system the more heat transfer you get (and they're right).

If you focus on one single loop of one single water molecule pumped at higher than stock speed yes it spends less time in the radiator per pass, but it's a closed loop. It also spends less time in the engine per pass. On the other hand that single molecule is moving faster so it returns to the engine and radiator more often in a given span of time. That's why higher coolant flow rates are more efficient.

As Eggie was alluding to look at the radiator. The radiator transfers the heat of the coolant to the air. So the radiator runs cooler if the airspeed is really slow so each air molecule spends more time in the radiator fins? Heck no! High air speeds drop the radiator temp faster because they bring more air molecules into contact with the radiator in a given period of time. Whatever your cooling media is the faster you move that media through the system the more efficient your heat exchanger is.

 

Almost all the info I've read about using the high flow electric pumps , mentions over cooling and the need to replace the factory thermostat to regain "stock like " coolant temps , I also read that some racing team (or the other ) had to block off part of their radiator to get the temps up, and as we know all these pumps do is maintain a constsnt and high flow through the system.

 

Let's not forget that heat transfer is related to the temperature difference between hot and cold sides. IOW, as soon as a bit of coolant hits the radiator and loses any heat at all, you want to get it out of the way so that hotter coolant can take its place. We aim to maxime the system's overall cooling ability, not the temperature at any given point.

 

I'm not disputing that. In fact, if the coolant moves too slowly, the coolant inside the engine will overheat more quickly.
I never said anything about slower air speeds thru the radiator being a benefit.

What I meant is that coolant running to fast won't cool properly. It may be physically impossible for this to happen. For example, coolant moving thru the system at 1000 mph won't cool properly. Obviously, there is some point at which heat exchange will not work very well.

 

I'm going to sound like a mega-engineering-nerd here, but anyway: It's not that simple.

While you want to get the hot coolant against the coldest surface possible for cooling, the heat will only transfer so quickly. So if you drive the coolant past a very long surface, you will remove more total heat. But as the coolant gets closer in temperature to the cooling surface, the heat transfer slows down and the cooling medium (air) gets warmer too. Or if you slow down the coolant flow, the radiator takes out more total heat but the coolant coming from the engine is now slower moving and arrives hotter. Plus, any changes in cooling surface shape rarely happen without affecting the airflow over them. And on and on - any change in the design is a tradeoff.

So really any change in an intercooler or radiator is very complex, and the tradeoff takes place in several dimensions at once. The experts in the heat exchanger industry use programs based on differential equations to try out different configurations. It can subtly difficult to improve a heat exchange system. Lacking fancy software and detailed measurements of the situation, testing is what works. In many cases, racers know what works because they innovate by brute force testing. While it's not always pretty, their results work and have solid data backing up the improvement. IMHO, our best bet as enthusiasts is to keep the system well maintained and follow the race crowd with a studious eye to pick up on their bits of wisdom. So while we can't all move our intercooler under the rear wing, we can prop open the front bumper cover for more frontal area.

Dave

 

I agree 100%. OTOH, I still believe that my last post gives an easily understood example of why higher mass flow increases heat transfer. All else being equal...

 

You're wrong.

When it's a hot summer day and you sit in your car with the a/c on would you like the blower on low or high? High increases the mass flow of the air over you and cools you off much more quickly than low speed can.

In the engine the faster the coolant flows the quicker the coolant moves between the engine and radiator and therefore more coolant molecules travel through the engine and radiator in any given span of time. This increases cooling efficiency. Again look at the radiator and air example. The radiator sheds its heat into the mass of air going through it. The engine sheds its heat into the mass of coolant flowing through it. More airflow cools the radiator more efficiently and more coolant flow does the same for the engine. The exact same principle is at work in both cases; they are not different from eachother.

 

there are cases where coolant can move too fast PERIOD

faster doesn't equal better in all cases...just like bigger isn't always better.

"Under cooling" is what happens when the coolant flows through the engine too quickly. Since one of the jobs of the thermostat is to not only control engine temperature, it also controls the speed of the coolant through the system. If the coolant flows too fast, it does not have time to pick up engine heat and results in what's defined as "undercooling". Since "undercooling" usually does result in "overheating", it's common to think of the two as the same.

this is probably a bad example...

Put a thermometer in the center vent and turn the heat on HIGH and fan speed on LOW. After five minutes or so, note the temperature. Then put the fan speed on HIGH. You will see that the temperature has dropped because the air is moving too fast to pick up all the heat (in the core) it is capable of carrying. Not enough to call the heater bad, but enough to make a difference.

I also find the the AC blows cooler air on the III setting than IV in most cars...

 

True, but misleading once again. The point is not the temperature of the air coming out of the vents, it's how quickly the air coming out of the vents makes YOU feel colder.

Higher mass flow of air across your body makes you feel colder even though the air temperature is no lower. You experience this all the time. It's called the windchill factor. The air isn't any colder but since more of it is blowing across you you do in fact FEEL colder.

 

...which, outside of a perfect world, it never is unfortunately.
Exactamundo. There are a too many variables here to simply go about making coolant flow rate modifications based solely on basic heat transfer equations.
My statement "pressure won't necessarily change anything" was more in reference to increasing pressure in the FD's basically stock cooling system; a change of 5 psi probably won't make that much of a difference. Yes, subs can go to flank speed at depth, but doing so still creates some cavitation; it's one of the reasons why they don't travel at that rate much, because doing so quickly gives away their position due to the noise generated by the props.
That's why I used the term "comparatively"...
Thank you for the clarification, but I was referring to sustained high-rpm operation of the FD engine, meaning that cavitation basically occurs only if you're tracking the car at higher rpms for extended periods. If you're driving on the street, your rpms will quickly drop to the point that NPSH will return and regularly scheduled programming can resume...

 

I think we are all going off in wrong directions??!?!?!

in the case of "windchill factor"...do we want our motors feeling colder? or actually running cooler...

we want the heat to actually transfer which takes more time...then give a windchill, right?

 

It's the same thing! You ARE colder due to windchill.

 

this will be my last post on this subject matter...as I feel you are getting aggravated with me

wind-chill is a man made measurement as the nervous system in our bodies will interpret the temperature outside a little different than actual temperature...our little 13B's don't have feelings, even though some can be quite moody.

 

A good example would help

Good example to show fast is good.

What you are missing is the temperature of the coolant exiting the heater coil in both examples. With the high fan flow case, even though the exit air is not as hot, it's 200% increase in flow rate more than offsets the slight drop in ABSOLUTE temperature (150F to 120F means (120+460)/(150+460) = .95 or 5% drop in degR. This is a much higher heat flow rate off the heater coil, so the coil water exit temperature will be lower.

Faster air flow meant more heat pulled off the heater coil, although the air exit temp was lower.