Stamped vs. Machined water pump impellers
Stamped vs. Machined water pump impellers
(OP)
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
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





RE: Stamped vs. Machined water pump impellers
2. Costs more money.
Mike Halloran
Pembroke Pines, FL, USA
RE: Stamped vs. Machined water pump impellers
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...
RE: Stamped vs. Machined water pump impellers
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
RE: Stamped vs. Machined water pump impellers
ditto the "insufficient stock system"?
What is the radiator pressure cap rating?
Increased inlet pressure is one key to preventing cavitation
RE: Stamped vs. Machined water pump impellers
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
RE: Stamped vs. Machined water pump impellers
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.
RE: Stamped vs. Machined water pump impellers
RE: Stamped vs. Machined water pump impellers
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
RE: Stamped vs. Machined water pump impellers
RE: Stamped vs. Machined water pump impellers
Regards
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RE: Stamped vs. Machined water pump impellers
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
RE: Stamped vs. Machined water pump impellers
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.
RE: Stamped vs. Machined water pump impellers
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
RE: Stamped vs. Machined water pump impellers
RE: Stamped vs. Machined water pump impellers
RE: Stamped vs. Machined water pump impellers
Regards
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RE: Stamped vs. Machined water pump impellers
How does that work, Pat? I agree that it can be done, but cheaply and easily???
Rod
RE: Stamped vs. Machined water pump impellers
A fully machined blade of the original size will be more efficient at higher rpm, but will cost a packet.
Regards
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RE: Stamped vs. Machined water pump impellers
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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.
RE: Stamped vs. Machined water pump impellers
RE: Stamped vs. Machined water pump impellers
The stock impeller was always stamped steel? I'm not aware of street driven RX-7 having a bad rep for cooling issues, unless 100,000 mile seal reloads of legend are related.
Mike H, do you believe any product is engineered to expire at the end of the warranty period?
Exceeding the warrantee period is certainly important for a manufacturer's cash flow, short term and longer. A friend was a zone tech rep for GM, and used to say the Fire-maro T-Top warrantee issues were burning thru his pension plan.
But I get the feeling today's most successful mfrs are the ones whose products' have earned reputations for appliance-like reliability well beyond the 100,000 mile mark, factory warranty be hanged.
RE: Stamped vs. Machined water pump impellers
RE: Stamped vs. Machined water pump impellers
Rod
RE: Stamped vs. Machined water pump impellers
Thanks for the suggestions. I have it installed currently, and while it will be hard for me to demonstrate an improvement, others have found that this unit helps.
Regarding general rotary reliability, this is only a problem at high RPM. So warranty is a non-issue, this only comes up when on the track, and those guys have traditionally upgraded the rest of the cooling system or gone electric since there was not a drop-in water pump upgrade before.
RE: Stamped vs. Machined water pump impellers
2. Costs more money."
Yep. That's the end of it. Cars are made to sell. Dreamers and enthusiasts end up spending more money than they'll ever make. The companies aren't run by engineers; they're run by accountants, investors, and other business people.
I wish this wasn't the case. There are many things that would make a car so much better that might all add up to only a few extra grand in the end, and avoiding the free use of stamped components is definitely one of them.
RE: Stamped vs. Machined water pump impellers
"1. It gets through the warranty. End of discussion."
I'm pretty sure I'd heard of some vehicles or products actually lasting beyond the warrantee. Actually I question the ability to engineer and manufacture a product to last precisely X hours or miles. Certainly there are plenty of examples of products expiring before the warrant ended.
Lots of mission statements include something about exceeding customer's "expectations". That is not complete nonsense. Brilliant marketing and buyer's incentives can only sell so many ---------- (insert name of extinct product).
RE: Stamped vs. Machined water pump impellers
The stock parts work fine for the use they were designed for. If you change the use it up to you to solve the consequential problems YOU caused.
Regards
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RE: Stamped vs. Machined water pump impellers
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Stamped vs. Machined water pump impellers
I think it'd be cool to design a car with "Easter eggs" throughout... such as a valvetrain that uses exotic materials to survive 20 000 rpm. Nobody will know until they start playing with it, but once they do, the popularity of the car will explode among enthusiasts. This same thing happened to the Toyota Supra's 7M-GTE, which can handle 800 horsepower on the stock bottom end.
I think GM should make a new Nova. They should put the ECOTEC in an expansive engine bay, sitting on an adapter engine cradle. Once you take the cradle out, it'll open up to the stock mounting points for a Chevy Big Block. That'd be cool as hell. The car would survive EPA testing but be a complete "Easter egg" to enthusiasts.
RE: Stamped vs. Machined water pump impellers
In a automotive water pump it could well be recirculation of the flow round the impeller or corrosion-erosion or a combination of both.
A few photo's either posted here or preferably in the pump engineering forum would achieve some knowlegable comments.