Stamped vs. Machined water pump impellers

[geesamand]

I'm evaluating a new water pump for a sports car. The stock water pump is known for cavitation at high rpm, so traditionally the track dogs changed to an underdrive pulley.

Now a shop is producing a water pump replacement that has an impeller built from CNC'd billet aluminum. Supposedly it does not see significant cavitation all the way up to red line.

Questions:
1) Given that this car was considered to be a very well sorted sports car, why would the OEM still use a stamped design that cavitates? (The car is known for an insufficient stock cooling system)

2) Could there be any downsides to a water pump that is more efficient? Perhaps more hp draw?

Thanks in advance for your thoughts.

Dave

 

[MikeHalloran]

1. It gets through the warranty. End of discussion.

2. Costs more money.



Mike Halloran
Pembroke Pines, FL, USA

 

[RossABQ]

I don't think your description of the new impeller as "more efficient" is the real story. I suspect that the stamped impeller has sloppy clearances and a poor flow pattern around the blades. A machined impeller can have separate profiles on the front and back sides of the blades, tighter clearances to the pump body, and other features that prevent cavitation. It doesn't necessarily pump more at a given RPM (neglecting whether there is cavitation on the stock pump). It simply is better behaved.

I also suspect that the OEM mfr made a trade off for pump performance in traffic with the A/C on vs. the far less frequent event of continuous high-RPM operation. And yeah, it was cheaper...

 

[geesamand]

I had considered the question of trading off low rpm performance with high rpm peformance - it makes sense.

I may have been incorrect in using the term pumping efficiency - but clearly the stock pump cavitates and loses performance at higher rpm. So if the aftermarket pump doesn't cavitate it will perform better.

Dave

 

[Tmoose]

What are the symptoms of the alleged cavitation?
ditto the "insufficient stock system"?

What is the radiator pressure cap rating?

Increased inlet pressure is one key to preventing cavitation

 

[geesamand]

Pressure cap is 1.1bar IIRC.

I have not seen hard evidence of cavitation firsthand, but I haven't been in a position to compare. I know that racers have seen reduced water/oil temps when using an underdrive water pump pulley.

Dave

 

[Snork]

Billet vs. stamped impeller is not, on it's own, indicative of performance. It's possible to have a very poor design done in a very pretty billet part.

A couple of things to compare from one impeller to the next:

A impeller with blades having an angle closer to tanget with a circle centered at the shaft center will have less tendency to cavitate than a blade with an angle that is less close to tangency.

Check how close the blades run to the cover. You can do this by putting some clay on the blades, and then installing the cover so it pushes the clay down. Remove the cover and carefully cut the clay parallel to the shaft axis, then measure the distance from the blade to the top of the clay. The smaller this distance the better the fit and the less re-circulation you will have.

Does the hose leading to the inlet of the pump collapse at high engine speed? I know that's a tough one to check, but maybe a small video camera, or reving the engine when it's in neutral. If the hose does collapse it will limit your inlet flow and the pump will cavitate.

There are many more things to look for, but these two are a good start.

 

[mcroadracer]

Our roadracers cooling problems went away when we switched to Evans Coolant. Without water, cavitation is seriously limited at any RPM. Running the pump faster will then lower coolant temps.

 

[evelrod]

Our roadracers cooling problems went away when we switched to Evans Coolant. Without water, cavitation is seriously limited at any RPM. Running the pump faster will then lower coolant temps.

No road racing club that I compete with here in the U.S. (SCCA, VARA, HSR-West) allows anything but plain water or water plus WaterWetter.
No EGW or PGW coolants allowed.

As to the OP---Mike has the definitive answer, IMO!

Rod

 

[mcroadracer]

Depending on the racing club, we have restrictions on coolants as well. My club allows Propylene Glycol but not Ethylene Glycol. If your club(s) exclude anything but water, you are back to pump efficiency and cavitation issues. Hp draw due to the pump is minor especially if the pump keeps water from boiling. Certainly the cost of boiling water is higher than pump hp draw. The fan is a bigger drag, but again, you have to do whatever it takes to cool the liquid. A high flow radiator might help fight pump cavitation. We remove the T-stat whether using PG or H2O to increase flow. In some circumstances, the T-stat restriction raises the pressure in the engine helping avoid local boiling. I think a higher pressure cap and no flow restriction would be best. I know of a Corvette team that has an air valve on their cap. They actually pressurize the system with compressed air before even starting the car. F1 cars used to run at 90-100 psi before the rule change.

 

[patprimmer]

I would be very surprised if a fully open thermostat made much restriction to flow.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

In order to restrict coolant flow to the point that it raises the pressure in the engine, presumably to aid in reducing pump impeller cavitation, one would need to use a specific flow restrictor...more than a std. Tstat. One of my vintage racers (the Mini) is notorious for overheating. One of the cures that I use is a flow restrictor sleeve with a series of small holes. On the Lotus heat is not an issue, but I still use a 5/8" restrictor in place of the Tstat. Both cars have std. pulleys on the waterpumps and engine speed is restricted to a max of 7800 and 8600 respectively.

Using very high pressure caps is not the answer in my case as neither engine will tolerate much over 15 psi. due in part to very old designs for sealing them. I find that 15 psi works quite well, from a practical point...There is the 800 lb gorilla to worry about (the law of diminishing return), especially when it comes to $$$ !

Rod

 

[izzmus]

The link does not work but the forum title is familar. I race Mazdas as well.

The cavitation problem is well known and Racing Beat has a chart of when given water pump/pulley combinations begin to cavitate.

Personal experience is that the particular engine I use will run hot with sustained revs over 6000rpm. Underdrive pulleys raise this level. It looks and quacks like a water pump problem.

 

[geesamand]

Here is an updated picture.

I can certainly believe the OEM cheaped out on a part that doesn't perform well for extended high RPM use - since that's not likely to cause any issues within warranty.

What does surprise me is that this pump issue has been known for decades and nobody has directly addressed the issue before. Considering the amount of bogus "upgrades" for sale out there I wanted to understand more.

Thanks all,

Dave

 

[JonKBmw]

Are we by chance talking about BMW pumps? And if so, are we talking about the OEM tack welded/stamped impeller vs a stewart warner unit? Just curious as I recently had a similar discussion for similar reasons

 

[geesamand]

Nope. This application is a Mazda RX-7. But I'm sure the issue is relevant for any engine that redlines at 8k or more.

 

[patprimmer]

If Pump cavitation is a problem, it can be easily and cheaply prevented by OEM by having freer flow into a pump with a wider blade to increase capacity. It can then be turned slower via pulley size changes, to maintain low speed flow, but avoid high speed cavitation.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

it can be easily and cheaply prevented by OEM

How does that work, Pat? I agree that it can be done, but cheaply and easily???

Rod

 

[patprimmer]

A wider pressed metal blade wont cost much more than a narrow one. Same number of processes, just a bit more metal.

A fully machined blade of the original size will be more efficient at higher rpm, but will cost a packet.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MechanicalChef]

The easiest solution would be simply remove the entire engine and swap in an American V8. :evil laugh:

------------

The inside joke here is the the die-hard rotary guys HATE thought of losing the Wankel. Merely bring up the characters "V" and "8" are enough to give some of them seizures. I know a bit about RX-7's as I own a 2nd generation turbo-car. But, I'm from the other camp of people who aren't reluctant to rip out the heart of a 7. Surprisingly, people claim handling is only slightly affected by the engine and trans swap. In return, the owners reap the benefits of torque, cheap widely available parts, and that brilliant chassis.