heinrich

Actually you can add in yet another factor, namely that each rotating assembly of the system as the engine revs faster, becomes its own little gyro (no, not the kind with yoghurt and tahini). Each item starts keeping the belt from pulling it sideways, including the pump. In a very tiny way, these several gyros keep the belt from damaging the pump bering. Also ... you could calculate the incremental cooling efficiency of the system as revs increase and coolant swirls past the bearing bringing it to optimal temperature and expansion and keeping it there, as opposed to a closed, unrefreshed system .. as wel as the aditional oil your cam and crank berings get from higher oil pressures, which at low rev high gears do not exist. Plus, there is less chance for detonation at higher revs. thusly:

low gears == less water pump failure.

toofast928

Wow I started a debate....
If you constantly stomp on the gas stop light to stop light, the weakest link will eventually fail. For the 928 it's the WP.
Not robust by design, under "normal" driving the bearings wear, the shaft wobbles, the seals are then compromised, and coolant leaks thru the weep hole. If allowed to continue, coolant will seize the bearing compleatly. WP failure is common around 60 -70,000 miles.

If in first gear and stab the accelerator, the early 5 speeds have very low final drive and first gear ratio. The rapid RPM or the rapid rate of RPM causes the ***** in the bearing to slide rather than roll in the race. The slide causes flat spots on the ball or the race. Then the WP dies before it's time.

The statement " water pump will fail sooner under hard acceleration in the low gears" is generic. It includes any high HP car. The 928 is just a-little more susceptible to failure from this driving habit because of the bearing design.

Nothing to loose sleep over, no need to redesign the WP. Just be aware and inspect the WP weep hole every time your timing cover is off.

Now back to designing the 16V x pipe.....................

heinrich

So, Mark Anderson and Mark Kibort are seeing water pump faiures left-right-and-centre then. I'm not sure where your info comes from Toofast but I have ***never*** heard of a 928 water pump breaking as a result of first-gear accelerations. In fact I do it several hundred times a day, and my pump was new 6 years ago on my S4 5spd, and I've owned more than 10 928's (I lose count).

Show me one example.

So, the statement that the water pump is the weakest link on the 928 .... I question that too. How many failed water pumps have we seen and why did they fail? It is maintenance.

Kibort and Anderson and the mouse racers had better stop racing if it's going to kill their water pumps. Really -- this is the very first time I have heard that theory. Ever. It isn't correct.

heinrich

About the pump being the weakest link ... what about thrust bearing? Valve seals? Rings? Cam timing gears? Radiators? Synchros? Torque tubes?

heinrich

Oil pan gaskets? Oil cooler lines? Alternators? Starters? Vacuum systems? Fuel injectors? Coils and ignition wires? Distributor caps/rotors? These are all items that fai more often than water pumps.

heinrich

2/6 rod bearings?

Jim bailey - 928 International

Simply stated the more RPM you use the sooner EVERYTHING wears out belts and pumps included. Drive around in first gear all the time and things wear more quickly belts are probably NOT going to last the 60,000 miles that Porsche thinks they will. The switch to plastic impellors was done for reasons known only to Porsche, that it was to save engine blocks is conjecture. The plastic has a small knurled metal insert to which the plastic is molded and that insert is pressed onto the shaft the failure is the insert gets loose and the impellor does not turn and at speed the engine overheats . Porsche in a tech bulletin indicates that minor seepage out the weep hole is NORMAL and does not indicate that the pump needs changing (and they would not pay under warranty). I think I have only seen one block where the metal impellor machined away much metal so that failure is very very rare. When it comes to "rebuilt parts" there are no standards, no minimum quality, so the question of who is doing the rebuilding becomes a very critical issue as well as who you buy the parts from. When we sell our rebuilt pumps for $119 exchange from a rebuilder that we have used for 15 years (who also rebuilds for Porsche Cars N A) I have a hard time understanding why people would want to "try" pumps from some unproven source. Particularly when you have to believe that most have never seen a 928 water pump actually installed on a car and may have no idea how important it is to timing belt life.

mark kibort

I think your right here too.

if you look at the layout, you can see as the crank accelerates, it probably increases forces on driver side cam more than any other rotating element. the rest of the rotating components are not going to see any substantial increases of force. accelerating the rotating masses, increasing wear is a streatch too.

sometimes higher revs equal higher wear, but sometimes the reduction in vibrations outweigh those effects, which is charateristic of higher rotational speeds. the water pump is spinning at very fast speeds. I forgot the gearing, but its more than 2-3 x of the crank speed.

do we spend more time at higher revs in the lower gears?? dont know, probably depends on the driver!

however, i think most of the good waterpumps are in the cars that are racing. (bar my defective ones!)


Mk

mark kibort

good point. I think the cooling is the most important part, even more infuential than rpms. (and lubrication) I think most of wear is at start up anyway, so you want your water pump to last, leave your car running
theoretically, once oil is circulating in the system, and components are at temp, there shouldnt be any bearing wear in the engine!

Mk

Jim bailey - 928 International

Higher RPM basically is always going to be higher wear. The more times a piston goes up and down the more wear. The more times a bearing turns the more wear. Obviously the left driverside cam sees the most load since the crank pulls the belt down toward it and all the work of turning both the cams and water pump loads up on the driverside cam the water pump sees only the load from the drag of the passenger cam and the tensioner roller. The tension roller is on the loose side simply taking up the slack. The inertia flywheel effect given the diameters of the cams etc. would be rather low. Now are we looking at torque or horsepower the load on the belt or the amount of work being done ?

SharkSkin

I thought I made it pretty clear that I wasn't convinced that redlining in lower gears leads to significant or even measureable reduction in WP life.

I was mulling over the fact that on the front of the engine everything except the crank is driven by the crank via the belt, all those parts have mass, therefore they have inertia. The inertia may be low as Jim B says, but it is there. There is also inertia in the valves, the portions of the springs that are moving, the lifters, etc etc. Given that all of these parts (a) are driven by the belt and (b) have inertia, the belt will see a higher load at higher rates of acceleration -- QED.

Again, there are lots of good reasons why WPs fail, this difference in load may not even be measurable. Hopefully I've made myself clear this time.

BTW, I agree with Jim, high revs make everything wear faster. For that reason, and that reason alone, I don't go to redline every time I shift. In fact I almost never shift above ~5700 because it's accelerated wear that serves no purpose whatsoever. I consider 5700RPM to be my redline. Usually I shift below 4K unless I have good reason to wind it up.

FBIII

The acceleration rate of an engine in first or second gear pales in comparison to how fast it will rev in neutral. I can't think of one automaker that has a disclaimer or warranty void for the occasional blip of the throttle while in neutral. Hell if that was so dangerous they would put governors on engines to limit their acceleration rates.

CWO4Mann

How about an electric water pump? Didn't we hash that over here last year?

Daniel Dudley

I can't remember the name of the speaker at last year's Frenzy who gave the talk about PM service, but I do recall that he said his shop would no longer install rebuilt WPs on 928s. He said it was too much of a crap shoot.

Having said that, I have a 928 International rebuilt on my 80. It was a replacement for a rebuilt that only lasted 500 miles .

Would I install an Auto Zone rebuilt ? Probably, if it was my labor and it looked and felt good. Heinrich is right; proper tension is critical .

As for the design; very efficient, logical and elegant. Also very stupid in hindsight. Several other manufacturers tried to drive items off the timing belt in that era. Things like power steering pumps and smog pumps. Bad idea. I don't think anyone does that now. Not to mention the additional cost of service when you ''just'' need a water pump or three. LOL

heinrich

No somehow I had missed that Dave, I didn't intend anything personal man :