cavitation

PostPost by: Ennva » Thu Jul 24, 2014 1:01 pm

Now heres a subject for those technically minded, how do you determine if cavitation is occuring and then what do you do?

many thanks
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PostPost by: rgh0 » Thu Jul 24, 2014 1:09 pm

Cavitation is formation of vapour bubbles in the low pressure suction areas of the pump. When a pump cavitates its pumping efficiency drops significantly.

It can be caused by a flow restriction in the suction lowering the pressure at the suction or an engine very hot bringing the coolant closer to its boiling point or the impellor operating at higher speeds than design.

The pump works OK at 9000 rpm in racing engines so engine speed should not be your problem
Your engine is not boiling so high temperature should not be the problem
You may have a flow restriction in radiator or hoses or thermostat or head / block but if this the case then cavitation is only a small addition to the base flow restriction problem that will limit your cooling.



cheers
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PostPost by: CBUEB1771 » Thu Jul 24, 2014 6:59 pm

Cavitation can be quite difficult to detect because it is a transient phenomenon. Once the vapor bubbles have moved away from the offending low pressure region they will collapse as the vapor condenses back to liquid. The condensation process can be quite destructive because of the large amount of energy released in small volumes. This can result in serious pitting of metal surfaces in the regions where condensation occurs. Pitting at the outer diameter of the water pump impeller might indicate that cavitation has occurred. Rohan's comments are quite reasonable.
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PostPost by: JJDraper » Thu Jul 24, 2014 10:47 pm

Caviitation in boat props looks like pitting, so I guess the same sort of damage would show on any areas of the pump where cavitation occurs. As Rohan suggests, high rpm doesn't seem to be a problem. I am more concerned about low rpm inefficiency! Anyone else notice higher engine temps at prolonged lower road speeds (say 50mph) in top? Speed up and the engine cools down..

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PostPost by: 69S4 » Fri Jul 25, 2014 7:26 am

[quote="JJDraper"] Anyone else notice higher engine temps at prolonged lower road speeds (say 50mph) in top? Speed up and the engine cools down..
Jeremy[/quote

I get that with my narrow rad S4 all the time. Round town the fan keeps things under some sort of control but between 30 and about 50mph seems to be too hot for the fan but too slow for airflow and the temp rises. The engine is at its happiest (temp wise anyway) at about 65-70mph. I'm sure it's something to do with the way air exits the engine compartment as on a few occasions I've driven with the bonnet catches released and the temp drops rapidly.
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PostPost by: MyLotus » Fri Jul 25, 2014 10:24 am

My reply does not deal with cavitation but rather the lack of cooling provided by the narrow S4 radiator. I came to the conclusion that the narrow rad is not sufficient when I engaged the heater while to engine temp was rising above 95 degrees. The temp dropped immediately by 10 deg. Of course, I was cooking but the experience was proof enough for me to get a wide rad which I am in the process of installing. Will advise on the results.
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