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Stamped vs. Machined water pump impellers

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

RE: Stamped vs. Machined water pump impellers

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

2.  Costs more money.

Mike Halloran
Pembroke Pines, FL, USA

RE: Stamped vs. Machined water pump impellers

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...

RE: Stamped vs. Machined water pump impellers

(OP)
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

RE: Stamped vs. Machined water pump impellers

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

RE: Stamped vs. Machined water pump impellers

(OP)
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

RE: Stamped vs. Machined water pump impellers

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.  

RE: Stamped vs. Machined water pump impellers

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.

RE: Stamped vs. Machined water pump impellers

Quote:

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

RE: Stamped vs. Machined water pump impellers

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.

RE: Stamped vs. Machined water pump impellers

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Stamped vs. Machined water pump impellers

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

RE: Stamped vs. Machined water pump impellers

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.

RE: Stamped vs. Machined water pump impellers

(OP)
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

RE: Stamped vs. Machined water pump impellers

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 :)

RE: Stamped vs. Machined water pump impellers

(OP)
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.

RE: Stamped vs. Machined water pump impellers

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Stamped vs. Machined water pump impellers

Quote:

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

RE: Stamped vs. Machined water pump impellers

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Stamped vs. Machined water pump impellers

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.

RE: Stamped vs. Machined water pump impellers

oldrotary1  geesamand I have some  ideas you can try. since 40% of the cooling is done with oil and an oil cooler try increasing or adding to the oil cooler capacity or add another cooler which will increase your oil volume,this can also be done by adding a modified deeper or wider pan as well (to increase volume)which will help to keep the engine cooler as a , or if you can afford one moroso makes a beautiful alum pan with baffles for racing. you should also remember that if you base your rad capacity against the factory rated horsepower and you modify your horsepower say by 50% you should also icrease your rad by the same amount,I think you could ad some ducting to force extra air to the rad, I have done a lot of research on this as I am curently building a waterpump that will not cavitate even as high as 20000rpm  

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

If its a track car have you thought about running an electric water pump. Did this on a Mini (classic) solved all my cooling problems.  

RE: Stamped vs. Machined water pump impellers

Electric pumps may be the hot setup, I won't argue that.  They   require quite a bit of plumbing and modifications and, in the clubs I race with, are technically NOT legal...This is not to say I have not seen some of my competitors using them...I just don't push the issue as I don't see any great performance advantage in their use.  My 63 Austin Cooper uses a stock type pump with a slightly modded impeller and a smaller dia pulley with the aforementioned restrictor for decent cooling at Socal tracks...read that as some pretty hot days in the summer.

Rod

RE: Stamped vs. Machined water pump impellers

(OP)
Thanks for the ideas.  Water pump upgrade is only part of the cooling package, of course.  For price and classfication reasons I'm not adding extra oil coolers, oil pan size, or radiator.  Ditto on the electric pump - installing this improved water pump was just too easy by comparison.

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

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

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

kevindurette repeated
"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 great impact of the Japanese cars was largely due to them being trouble free for at least twice the warranty period. In my opinion, that raised the bar enough so others had to follow to keep some market. Warranties then escalated from one to three years or more.

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

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

RE: Stamped vs. Machined water pump impellers

Twice? My horrible Corona is now 24 years old. Last year I had to replace the the clutch master cylinder ($70). Two years before that is was the thermostat housing ($20). When will this unreliability end?

 

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

It goes back to the saying "Good enough is best." The cavitation point of the water pump pulley won't affect its longevity when loafed around on the street.

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 pressurised closed circuit such as an automotive cooling system with an impeller running is a casing with such wide clearances etc, it is unlikely what you see is cavitation. Unfortunately cavitation is a poorly understood phenomenon used by many to describe actions/effects which are not cavitation.

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.   

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