water pump cavitation

[John Fitzpatrick]

What's the max water pump RPM before cavitation begins?

[Pete.]

I am not trying to be a smart arse but the answer is how long is a piece of string?
Many factors cause cavitation, not just the RPM of the pump. The type of coolant, temperature and pressure are all factors along with the type of pump.
Do a google search using cavitation and you can read for about three days.

[Russ Noble]

Originally Posted by Pete

the answer is how long is a piece of string?

Haha Pete, that was exactly what I was going to say.... but thought better of it!

John is this an actual problem or a theoretical problem? I have never come across anyone with cavitation problems. That might just be because I haven't recognised it as such! No doubt it does actually happen and presumably leads to overheating, but how can you tell that it is cavitation causing a problem and not something else? Or vice versa.

Needless to say I haven't done Pete's suggested search as I don't have the three days to spare!! And as yet don't have a cavitation problem.

Cheers

[John Fitzpatrick]

Okay, let me rephrase the question. Boss 302 water pump with a 28 tooth gilmer drive pulley. What size crank pulley would give a satisfactory drive ratio? Maximum engine RPM 6500, 20 psi radiator cap.

[Howard Jones]

I think the question is, at what percentage of engine speed should the water pump be driven.
I bet the first answer is it depends on what the car is being used for. So..... 1. if the car is driven on the street most of the time and spends 30% of the time at low speeds (<30mph)and low revs (< 3000rpms) and 2. run on track with a engine speed range of 4000-6500 rpms and vehicle speeds of 80-140mph.

Assume the radiator and fans are adequate and everything else is correctly designed.

My guess is 1. 80% 2. 60%

[John Fitzpatrick]

Thanks, Howard. The car will spend most of it's time on the track but I want for it to be usable as a touring car (minimal city driving) also, so I don't want to underdrive the pump to the point that overheating could be an issue. In the absence of any specific info, I'll just assume 5,000 RPM pump speed isn't going to cause cavitation and get as close as I can to 75%--about 22 teeth.

[Sandy]

Likely a good starting point might be looking at Stock Pulley for the SBF or some of the MARCH Brand pulley's. March's web site might also give you some magic numbers like xxx% under driven which would get you to a good starting point. I can say that on my Race Mate's (Sorry Russ, BTW did you ever get the used one??) they are heavily under driven vs. the stock, but it does not work well when tooling around town unless moving, but will have no problem rolling on the track at 7500. Start with the march site and see if you can find some info.

Sandy

[Rick]

John: I delt with this very issue several years ago with a fresh 64 289 Hipo for my Cobra replica. I had been collecting parts for some time and purchased a reman waterpump from a parts house as a last minute fix. It was the type of aluminum pump without a back plate. The symptoms of cavitation were that with a thermostat in the cooling system the guage would slowly rise to the point that the thermostat opened then the guage would drop 40-50 degrees then it would start climbing and push the guage past 200-210-230 very fast then all of a sudden it would instantly drop again, and start over. It was not an air pocket as I purged the radiator and manifold both. This went on for several months. I discussed this with several people, without making progress, then one day I decided to purchase a Ford OEM reman pump. Upon opening the box it was immediately apparent that the impellor was different (cast iron with curved blades insted of stamped sheet steel wit straight blades). I installed it and the problem went away entirely.
As for the pulley size and drive ratio concerns. I just stepped outside and carefully measured them and the OEM single V belt crank pulley is 6" diameter and the water pump pulley is 5.75". Keep in mind that there was no A/C or power steering being driven by these single belt setups. I tried running a small Moroso aluminum lower pulley that is about 3.5"- 4" some time ago but felt that the alternator would not be charging enough so I discontinued using the small pully (on the crank). My car was and continues to be primarily street driven.

PS:
All I can imagine was the thermostat closed when the temps dropped then it began to heat and the gauge shot up and cycled. At times I would be cruising down the street after it had time to warm up and the guage would slowly creep up, and then instantly in (2-4 seconds) droop to 140. I have no other expalnation other than the impellor design. The radiator had plenty of capacity as it never showed sighns of over heating after the water pump was swapped out. Nor did it show stabilized high temps with the first water pump. Just the frequent temprature spikes.

Rick

[Russ Noble]

Sooooo.......

Who on here has actually experienced problems with cavitation?

And to look at the other side of the equation, how many guys have experienced overheating at low speeds due to underdriven waterpumps?

(BTW Sandy, I took Jay up on his offer and he said he would get back to me but I haven't heard any more. Must be a feature of Racemate deals!!)

Cheers

[Russ Noble]

Ah, Rick posted while I was typing.

Interesting that the temperature cycles rather than just getting progressively hotter which I would have thought would have occurred.