New Overheating Clue

[shotgunslade]

OK. So, I'm going to get an underdrive pulley. Who here has experience with them? Is there any reason I should cough up 206 bills for the Unorthodox Racing Aluminum Billet vs. the $45 job in an eBay shop?

[shotgunslade]

Used the Search button. no need to reply to previous post. Seems like the Unorthodx Racing is the way to go.

[SVXRide]

Dan,
You might want to see what LAN would charge you to machine one up. He's got the set up already from the Stage 3 SC work.
-Bill

[SUBBYRU89]

Originally Posted by Hocrest

I think you know this I just want to make sure that you know that an underdrive pulley won't have any effect on the waterpump. Since the waterpump is driven off the timing belt.

1-2-Tree..

[YourConfused]

Originally Posted by shotgunslade

OK. So, I'm going to get an underdrive pulley. Who here has experience with them? Is there any reason I should cough up 206 bills for the Unorthodox Racing Aluminum Billet vs. the $45 job in an eBay shop?

I got a $25 pulley off ebay (OBX) and have had it over a year. I see no reason that it isn't adequate. It is metal, round, has the grooves and works. Why pay so much for something else that cost more due to advertising? 6061 is 6061.

[dynomatt]

How does the underdrive pulley work for the water pump? The pump is driven off the cam belt, so teh only thing I could see would be to machine up a large sleeve that would press fit on the water pump pulley.

Has anybody got a picture of what you are talking about with respect to underdrive pulleys?

Thanks,
Matt

[Trevor]

Originally Posted by RallyBob

Just about every race engine I've ever built I've had to slow the water pump pulley down on. Most factory water pumps are driven a little bit faster than the crank speed. Running an engine at 1000, 2000, or even 3000 rpms higher than the factory anticipated usually results in cavitation and overheating.

For a street engine I typically slow the water pump down by the same amount it is overdriven. In other words, if the pump spins faster than the crank by 10% (example: 6600 rpm pump speed @ 6000 rpm crank speed) I will reduce that speed to 90% of the crank speed (5400 rpm pump speed @ 6000 rpm crank speed). Nothing scientific about it, just trial and error on my part. For racing I will knock the overall pump speed down by 35-40%, but this can create overheating at slower speeds so it's not recommended for road use.

There is everything scientific about what you have described. If the pump speed is such that the outlet is restricting flow, cavitation will occur.

A crude method of prevention, is to drill a hole through each impeller blade so as to reduce efficiency and provide for increased RPM. This was a cure used with the old flat head Ford V8 engines. Unfortunately the SVX set up does not allow for a sufficiently oversize pump pulley, so that this could be an answer to the problem.

[shotgunslade]

Dave(indirectly through SubbyRu89):

Thanks for the correction. Didn't realize the water pump was not driven off the main crank pulley. Don't expect there's any easy way to slow down the water pump. Anyboyd got any ideas?

[TomsSVX]

Trevor mentioned the "old school" way of slowing it down. Cheap effective, but not exactly good practice. Reducing the efficiency of the pump may be our only route in this area

Tom

[svx_commuter]

Well this is a thread of interest to me. I have been designing centrifugal pumps for about 30 years now. I had no idea this was a problem in car engines. Cavitation can drop the pressure on a centrifugal pump for sure. This usually happens at very high flow rates. Well it is just more critical at high flow rates.

The pump is variable speed as the speed goes up the pressure increases and the flow rate increases. The flow rate is determined by the point of intersection of the system curve and the pump curve.

For a fixed speed, starting at zero flow the pump normally has the highest discharge pressure. This is usually the maximum pressure at the fixed speed. As the flow thru pump increases the pressure drops for this fixed speed case. The flow rate thru the pump depends on the system resistance. The system resistance is how much pressure is required to push the volume of water thru the engine, hoses and radiator. This system curve requires more pressure as the flow rate increases. The system curve changes with the flow rate squared. The flow rate thru the pump is where the system curve and pump curve intersect.

The other part of this is that the pump requires NPSH, Net Positive Suction Head. This curve is plotted as NPSH versus the flow rate. As the flow increases the NPSH also increases. So yeah the flow can get high enough to run out of NPSH and then the pump produces no pressure.

This can be corrected by limiting the flow of the system. This involves increasing the system resistance to limit the flow. Installing a valve and throttling a valve will decrease the system flow rate and solve high flow cavitation problems.

Then there is the other problem. AIR Centrifugal pumps do not pump air and water. Air can stop a pump from pumping. The spinning impeller acts as a centrifuge. Any air in the water will get pulled to the center of the impeller at the inlet. The water goes around the outside of the air bubble. The air will stay there blocking the flow and a more air get pulled out it will add to the blockage. If there is enough air in the coolant system it will block the flow and the pump will stop pumping.

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

[Trevor]

John,

Questions must be raised to ensure members receive exact information. Please do not take offence as a result of my queries. The issue probably comes down to the use of words.

For a fixed speed, starting at zero flow the pump normally has the highest discharge pressure. This is usually the maximum pressure at the fixed speed.

A rotary vane pump has the highest reliable discharge pressure at a maximum established (fixed) speed, and this is the point of reliable maximum flow. At any speed exceeding this speed, the rotor will run in advance of the flow and cause cavitation. One can think along the lines of a marine engine running at full power and turning a propeller which is too small.

The other part of this is that the pump requires NPSH, Net Positive Suction Head. This curve is plotted as NPSH versus the flow rate. As the flow increases the NPSH also increases.

You indicate that the pump requires a net positive suction head. I assume this means it must pump against a pressure at the outlet, above that which is statically present at the inlet and as a result the pump chamber must be full at start up. Primed, is the common term.

So yeah the flow can get high enough to run out of NPSH and then the pump produces no pressure.

An increase in flow requires an increase in pressure, i.e. the pump will be pumping against an ever increasing head in order to increase flow. How can there be a point of zero pressure? However there will be a level of pressure and flow which if exceeded, will result in the pump impeller inducing cavitation.

This can be corrected by limiting the flow of the system. This involves increasing the system resistance to limit the flow. Installing a valve and throttling a valve will decrease the system flow rate and solve high flow cavitation problems.

Throttling a closed loop system at any point, will increase input pressure as the affective means of reducing the upper flow rate. If the flow rate at a point of increased speed, falls below the capacity of the pump, the impeller will run in advance of the flow, resulting in serious cavitation. Any form of throttling will exacerbate the problem of cavitation within the pump at increased RPM.

The cure I have suggested, i.e. to reduce the efficiency of the pump, could allow for operation at higher speeds without cavitation.

[Dessertrunner]

Trevor
I won't attempt to answer all your questions but when thinking about the water pump on a car you need to remember that the pressure on the out let is also the pressure on the inlet less the friction loss. That is why on start up it needs to most power.
Tony

[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.

[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.

[Trevor]

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.

Thanks for a well written decisive explanation. Thankfully you are no doubt correct in your interpretation of the text.

I had the affect of kinetic energy as a result of the circulating medium in mind, but had discounted this in view of the wording of what was described. (I am immediately aware that there are those who will interpret my words as a means of bragging/excusing, that I knew it all anyway, but who cares?)

Your practical experience brings another possible means of correction into the picture, which certainly warrants investigation. Your further thoughts will prove valuable and will be awaited with sincere interest.