New Overheating Clue

[Trevor]

Originally Posted by IdeasMan

I agree with shotgunslade. A flow control valve is the most simple solution. Using a swirl pot and/or electric water pump would also help. I haven't seen any real good swirl pots out there. You're probably better off making one custom anyway to get the best fit and performance. A poorly designed one can make things worse. I'll try to post pics/diagrams later.
I prefer the Davies Craig electric water pump to others. It has a good impeller design and its temperature controller only runs the pump as fast as it needs to to keep the temperature you set. It can be used as a total replacement or as a booster.
You could get a nice aftermarket radiator to keep temps down. You could also go all out with stainless steel braided piping and AN fittings.

More good good stuff.

I was told many years ago, that a major problem at high RPM can be the pumping of the water at a pressure, whereby the speed of flow through the radiator becomes such that there is too short a time for the fluid to be cooled via the radiator. This makes sense, but I have never read anything along these lines.

Comments and or experienced advice regarding this, could be of value.

[SVXRide]

Originally Posted by shotgunslade

Trevor:

A few clarifications of terminology. As you mention, a centrifugal pump creates positive pressure on the outlet side and negative pressure on the inlet side. It is also adding kinetic energy to the fluid by accelerating it. As the fluid circulates through the system, friction dissipates the positive pressure induced by the pump.

The problem occurs when the fluid reaches the inlet of the pump. Theoretically, all of the positive pressure induced by the pump will have ben dissipated by the flow at that point. If the negative pressure induced by the pump inlet causes the absolute pressure of the fluid to fall below the vapor pressure of the fluid at that point, it boils. This pressure drop at the inlet is high, because the fluid is being accelerated from a low velocity before the pump inlet to a high velocity at the pump discharge. The higher the pump speed, the greater this velocity difference, and the greater the pressure drop through the pump inlet. That is the net positive suction head problem. The warmer the fluid, the higher the vapor pressure, the bigger the problem. It is similar to propellor cavitation, except that a propellor is not in a housing (a volute). Because of this housing the cavitation is mostly limited to the throat of the pump.

Increasing the pressure drop near the pump outlet is effective, because it increases the pressure drop through the system, thereby decreasing the fluid flow rate at any specific impellor speed, thus, decreasing the velocity at the pump inlet. The pressure drop at the pump inlet is thus decreased, increasing the absolute pressure of the fluid at that point and avoiding boiling.

Actually, the solution to our problem might be a Griswold flow control valve. This is a needle valve that is held open against the flow by a spring. Greater pressure on the upstream side pushes the needle into the orifice against the spring pressure, reducing the open area. They are designed to maintain nearly constant flow over a wide range of upstream pressure. We use them all the time in the HVAC industry to facilitate hydronic system balancing. I will look into this.

OMG, I just had a flashback to my junior year fluid mechanics course!
-Bill

[Dessertrunner]

I would support your point Trevor about the "Water going to fast". From my experiance with refrigation and blast tunnels these things don't happen. In its simplest form a radiator is a means to relocate heat from the water to the air passing past or throught the cores. The factors that effect the performances of the unit are:-
Difference in Tempreture of the water and air, put a higher pressure radiator cap on you raise the tempreture of the water going throught the core that in turn means 1 cubic metre of air can take away more heat. Higher the difference the greater the amount of engery relocated to the air.
Volume of air passing the radiator at any given time and the amount of fin surface area it contacts.
Volume of water in the cooling loop. The point being that if the water passing through the engine is to slow or to hot it will not be able to relocate the heat as fast as the engine generates it.

In my opinion the SVX engine under extreme loand needs the return line to the pump redirected and the thermostate to drill a small hole. You need to keep the biggest possiable difference between the water into the engine and out of the engine possiable. The same as you need the biggest possiable into and out of the radiator.
Tony

[shotgunslade]

I love fluid mechanics. It makes my day.

Trevor:
The issue of the water moving so fast thru the radiator that it has a lower temperature drop is correct. However, since the heat transferred from the water to the air is a function, on the water side, of both the temperature drop in the water and the mass flow of water through the radiator, the greater mass flow of water more than makes up for the lowered temperature drop. In general, with a fluid to fluid heat transfer device, the more fluid you have moving through the two sides, the more heat transfer occurs, even though greater mass flow results in a lower temperature change on each side of the heat exchanger. Higher flow rates also mean higher velocity through the radiator passages, which increase the heat transfer coefficient between the radiator tube walls and the coolant.

The only place higher flows will really hurt us is on the inlet of the pump, because it is both a constriction and because it is the farthest away from the pump outlet, thus has the the lowest static pressure in the system. It only hurts us there if the static pressure falls below the vapor pressure of the fluid.

I've been running Water Wetter and a 33% anti-freeze concentration. While the lower anti-freeze concentration may reduce the boiling point of the coolant, it also increases the specific heat of the coolant, so a given temperature change at a given mass flow rate results in a greater heat transfer. Water wettter reduces surface tension and increases the surface film heat transfer coefficent of the coolant.

Ahhhh, physics.

[CorSVXette]

Originally Posted by svx_commuter

This is why it is absolutely important to get all the air out of the system after the coolant system has been refilled. My experience with the SVX has shown me that it takes at least two warm-up and cool down cycles of the SVX coolant system to get all the air out. The SVX engine has lots of places for air to get trapped. I know when the system is filled up “hard” with no air as this is the point were the coolant level stops changing when the engine cools off.

Take care of that SVX,

John

OK I'll ask again. How do you get all the air out?

[cdigerlando]

Originally Posted by CorSVXette

OK I'll ask again. How do you get all the air out?

Like he said. Fill the system. Run it through a heat cycle. I also recommend running the heater to help purge the air. After it heats up, allow it to cool. Make sure your reservoir is full. Do the same thing a second time. Then drive it keeping an eye on your reservoir.

Why reinvent the water pump if running higher pressure prevents the cavitation? If you are building a race car, do what rallybob does to lower the pump speed. Should be very easy to machine off some of the diameter on the water pump.

By the way, STi caps are about half the cost of the crucial caps and are very close in their pressure settings.

STi's have a different water pump impeller. They also have heat issues though, so I'm not sure that this cures the problem we have been discussing.

Question is, how is rallybob's car running now that he added a new cap? Still no problems? That would be the bar. If he isn't having problems, none of us will.

[TomsSVX]

Bob had a great idea for filling the system. A large(tall) funnel in the cap of the radiator and a lot of patience to make sure all bubbles are out. Once the bubbles stop, leave the funnel in and start the car and let it get to running temp so the thermostat has a chance to open. Once no more coolant is allowed to get in, remove the funnel (a drip pan and quick hands are a must here) and replace the cap. top off the resevoir and allow the car to run with the cap on and take it for a 20 min drive. Bring it back in, let it cool completely. Remove the cap and add fluid as needed.

Tom

[SVXRide]

Originally Posted by TomsSVX

Bob had a great idea for filling the system. A large(tall) funnel in the cap of the radiator and a lot of patience to make sure all bubbles are out. Once the bubbles stop, leave the funnel in and start the car and let it get to running temp so the thermostat has a chance to open. Once no more coolant is allowed to get in, remove the funnel (a drip pan and quick hands are a must here) and replace the cap. top off the resevoir and allow the car to run with the cap on and take it for a 20 min drive. Bring it back in, let it cool completely. Remove the cap and add fluid as needed.

Tom

A caution here is that it may take a while (20 min?) for the engine to come up to temperature (thermostat opening) without the rad cap on.
-Bill

[Hocrest]

When I need to fill the empty/drained cooling system, I fill the radiator with coolant (just to the top of the fins), then I disconnect the top hose from the radiator and use the hose to fill the engine with water. Reconnect the hose, top off the radiator.

This fills most of system and it works out to be close to 50/50.

Then I run it with a funnel in the cap hole and top it off as a few bubbles come out.

[Trevor]

Originally Posted by shotgunslade

I love fluid mechanics. It makes my day.

Trevor:
The issue of the water moving so fast thru the radiator that it has a lower temperature drop is correct. However, since the heat transferred from the water to the air is a function, on the water side, of both the temperature drop in the water and the mass flow of water through the radiator, the greater mass flow of water more than makes up for the lowered temperature drop. In general, with a fluid to fluid heat transfer device, the more fluid you have moving through the two sides, the more heat transfer occurs, even though greater mass flow results in a lower temperature change on each side of the heat exchanger. Higher flow rates also mean higher velocity through the radiator passages, which increase the heat transfer coefficient between the radiator tube walls and the coolant.

The only place higher flows will really hurt us is on the inlet of the pump, because it is both a constriction and because it is the farthest away from the pump outlet, thus has the the lowest static pressure in the system. It only hurts us there if the static pressure falls below the vapor pressure of the fluid.

I've been running Water Wetter and a 33% anti-freeze concentration. While the lower anti-freeze concentration may reduce the boiling point of the coolant, it also increases the specific heat of the coolant, so a given temperature change at a given mass flow rate results in a greater heat transfer. Water wettter reduces surface tension and increases the surface film heat transfer coefficent of the coolant.

Ahhhh, physics.

We should be talking on this over a beer.

I understand exactly what you are saying and agree with you. Therefore the early theory put forward must have been wrongly attributed and the results must have come about through eliminating cavitation.

“Not Ahhhh, physics.” Quite the reverse, an interesting and worthwhile sorting of the issues involved. Thank you.

Cheers, I am in fact having that beer. Trevor.

[Dessertrunner]

If you want to slow pump you have to increase the diameter of the pulley. Machining it will make it go faster.
Tony

[cdigerlando]

Originally Posted by Dessertrunner

If you want to slow pump you have to increase the diameter of the pulley. Machining it will make it go faster.
Tony

Da ya. My bad. Do you think there is enough play in the timing belt to do much?

[cdigerlando]

Originally Posted by TomsSVX

Bob had a great idea for filling the system. A large(tall) funnel in the cap of the radiator and a lot of patience to make sure all bubbles are out. Once the bubbles stop, leave the funnel in and start the car and let it get to running temp so the thermostat has a chance to open. Once no more coolant is allowed to get in, remove the funnel (a drip pan and quick hands are a must here) and replace the cap. top off the resevoir and allow the car to run with the cap on and take it for a 20 min drive. Bring it back in, let it cool completely. Remove the cap and add fluid as needed.

Tom

That is a good method too, but don't forget to run the heater to get the air out. I would do it after you come up to temp of coarse, and fill if you see any air.

[Dessertrunner]

Given the ongoing issue of the engines overheating when pushed hard I have decided to run a trial. I have fitted two new tempreture guages and senders. Over the last 3 days I had to pull down my 450,000 engine because the old dear finally blew a head gasket so while I had it apart I fitted the sender etc. The first is on the pipe going to the radiator but under the manifold, the current SVX senders are before the second bank. The second sender is fitted to the bottom water pump cover before the thermostate so I can test the water coming back from the radiator to see if it has any more capiciaty. Now I should be able to get 3 tempretures to record:-
Air temp from the Air Con display.
Water temp out of engine.
Water temp into engine.

Here are some photos of the set up, all that is left to do is the final wiring.
Tony

[IdeasMan]

I couldn't find any good pics so I drew some (see below).

The swirl pot should be as big as possible to accommodate expansion. The height needs to be enough so that in a max-g turn the lower exit is still submerged and the top entry isn't. The swirl pot should be at the top of the system. Otherwise it will fill up when you stop the engine. The coolant level can be adjusted by raising or lowering the swirl pot. Some people like to use a clear Lexan top so they can check the level. It also looks cool when the car is running. You should be able to get the components at a radiator place if you want to make it yourself. Don't forget to pressure check it. Otherwise any respectable radiator place should be able to make it for you.

Recap: Water Wetter - good, cheap, use it.
Sti pressure cap - good, probably worth the money
Flow control valve - good, less expensive alternative to an electric pump
Swirl pot - great, especially if you are racing
Electronically controlled electric pump - great, for racing or daily driving
Aftermarket radiator, hoses - great, looks great, works great
Properly filling your cooling system with the right mixture for your application - Paramount, absolutely necessary for the rest of this stuff to work