6 Vane or 12 Vane pump which is best

[Ian Durrant]

I could go on discussing vane pumps and piston pumps all night. I used to work with the manufacturers and designers of these things. I used to love the complications and intricacies of finding the best solutions to quite difficult and not very well thought out machine designs. I also used to design and help build rock crushers for a living for a while. From design to commission they were very entertaining and quite often the theory in the office didn't match the field event!
The maths part...if the overall swept volume of the centrifuge doesnt change then the centrifuge type pump cant move any more water JUST by increasing the number of vanes.(taken to the limit... there could be 24 vanes on the impeller. The pockets would be even smaller. Double again and the pockets are again reduced keep on adding vanes and eventually there wouldn't be enough room for water at all...) . More to the point here if the effiency of the impeller is increased by closing up the tolerances between the impeller and the cover then the pump will have less losses and so an increase in flow would be seen.
I seem to remember too that the shape of vanes was changed from spiral in the 6 vane to straight in the 12? Is this the case? This may also have an impact on the efficiency but I cant figure that out at this time of night.
With the dirt that can get generated by rusty chambers in engine ways and radiator cores these closer tolerances would be more susceptible to wear by debris passing the impeller and wearing the edges of the vanes and the surface of the cover. The increased flow would still be seen but would drop off faster as the impeller and cover wore away
There's no doubt that more flow definitely means more cooling and an improved efficiency of the pump would make up for the extra power required (i would reckon it is unnoticeable anyway.) My point earlier was that pumps arent "powerful" at all but are power users.

Ah OK then I misread your post, sorry! Though I am not at all sure that extra vanes don't mean more flow because of complex fluid dynamics involved. Triumph engineers clearly believed there was a difference.

My main point is that a more efficient pump must mean more flow. More flow does mean more heat dissipation. The amount of extra energy the pump consumes is utterly insignificant in the total system since the most heat is generated by the power thats moving the vehicle not (by a long way) in running the pump.

[73stagman]

My main point is that a more efficient pump must mean more flow. More flow does mean more heat dissipation. The amount of extra energy the pump consumes is utterly insignificant in the total system since the most heat is generated by the power thats moving the vehicle not (by a long way) in running the pump.

The mostly we be in agreement me thinks

[Ian Durrant]

The mostly we be in agreement me thinks

Yes! its late. Sorry again

[73stagman]

yeh Im off now to think of other things the Walrus said .. ceiling wax and bits of string or something like that anyways

[Mark S]

The 6 vane is a better design for a pump impeller, but it's not as simple as that. Pumps are generallydesigned to run at a particular speed and the 6 vane works well at higher revs where it will produce good pressure and hence excellent flow. Unfortunately at low revs itstruggles to shift enough water in the Stag engine.

The 12 vane performs better at low revs because there are double the number of vanes but the tangential design results in it being considerably less efficient at higher revs whereit will tend to churn(NOT cavitate!!!) rather than cut into and pick up thewater from the eye and force it into the volutes.

There is insufficient room to cast 12curved vanes on the impeller as there would be more vane than space to allow the water to flow. Since Stags and Dolomites tended to overheat after a good high speed run when they came to a suddenstop (or even on tickover) the 12 vane suited the easy cheap fix methods adopted by BL at the time.

Anyone that tells us that a 6 vane pump works best in a racing Stag "because it produces less flow allowing the water more time in the radiatorto cool" is talking rubbish.It's better because it produces MORE flow at HIGHER revs. Increased flow will ALWAYS result inbetter cooling.

There seems to be many misunderstandings about the Stag's water pumps, radiators, pressure caps, expansion tanks, header tanks, plumbing, cooling fans etc.and reasons for it not performing.

Which pumpis best? It depends how you drive, around town or open road,if your engine overheats and under what circumstances. Unfortunately there is no right answer IMHO.

Have we gone full circle yet?
Spin the 12 vane pump too fast and it will tend to shear the coolant rather than pick it from the eye and fling it outwards.

[jleyton]

My brain hurts with all this too - and I've only been reading it

There's just one thing I'd add which I don't think has been mentioned so far and it's from Peter at LDparts. In one of his little news features he commented:

" The 6 vane impeller, with a top nut added, when offered into a 6 vane cover, demonstrated a huge clearance between the vane edge and cover beveled face.

When undertaking the same comparison with a 12 vane cover we found the gap between the vane edge and the bevel is far less."

I guess this could also be a factor - just don't ask me to analyse it's effects , and if somebody has mentioned it already then I apologise !

Cheers

Julian

[V Mad]

The reason I restarted this topic is not about whether the 6 or 12 vane pump works best (sorry to go off at a tangent), but to question the often held theory that more flow = more cooling (I think that is Mark S opinion?). As I said I think (in my laymans opinion) it is not as simple as that, and I suspect there is an optimum flow for max heat transfer.

The links given by Stagmaniac make interesting reading for me, but it is difficult to interpret the mechanism at work in a car cooling system such as the Stag system. I think the time needed to explore this fully exceeds the spare time I have available!

If we could set up a stag test where coolant flow could be independantly varied and the coolant temp measured under constant running conditions then we could test the theories. (I know that some cooling system tests were done, as published in Technical Reprints; but they did not have control of coolant flow).

I dont suppose we will ever solve this one!

[Westfaling]

Chris
The fluid dynamics of flow within a heat exchanger (radiator) are very complex. Assuming the flow through the heat exchanger doesn't exceed certain upper limits when turbulent flow will reduce heat transfer in the rad., then more flow will transfer more heat. If we accept that the 12 vane impeller produced more flow the question seems to be whether the 12 vane pump exceeds this upper limit. if it does not then it must help. Of course this whole thing is complicated by it being a variable speed pump.
Two points that I think are relevant:
1. Why did BLMC change the pump impeller - They knew that they had overheating issues.
2. The external water pump installations seem to improve (cure?) overheating. Please correct me if anyone's found different! Can this be for any other reason than increased flow??
Nick

[73stagman]

My brain hurts with all this too - and I've only been reading it



" The 6 vane impeller, with a top nut added, when offered into a 6 vane cover, demonstrated a huge clearance between the vane edge and cover beveled face.

When undertaking the same comparison with a 12 vane cover we found the gap between the vane edge and the bevel is far less."

I guess this could also be a factor - just don't ask me to analyse it's effects , and if somebody has mentioned it already then I apologise !

Cheers

Julian

this was my point exactly. The poor design of the six vane impeller and cover assembly as a unit they both make the "pump" as neither are a pump on their own. The Improved design of the 12 vane impeller AND COVER almost certainly makes the difference.
External pump? Don't the majority of car engines use a belt driven external pump? Triumph designers, not for the first time, were out on a limb! But they were responsible for some of the things we now take for granted. Dash board mounted air vents for one! More power to them for trying something different that migth even have worked.
Things might have been very different if the quality of the overall engine builds were better. Sales may have been better so that more money may have been available to develop the internal water pump and improve the cooling system. There may even have been a MK3 (with an all aluminium V8 engine with external water pump! Maybe developed even further with fuel injection and things

The basic principles are all in this thread. More flow, more coolant, bigger radiators more efficient pumps, better filling arrangements. Many of these are available now as bolt on kits. So lets keep cool!

[martin]

Just to stick my non expert oar in, the benefit of the exterior pump, is that it sits lower, and dramatically reduces the risk of cavitation, and takes gear drive out of the equation. Another thought, what percentage of Stag owners actually suffer with cooling problems ? I don't, and in 8 years - God help me for tempting fate - I never have. These cars are old ! they went through a long phase when they were not worth a lot, as with all other old cars, that led to poor infrequent maintenance, and now, a few are paying the price.
I don't think there's an issue here guys, the discussion has been VERY interesting as an esoteric exercise, but to new owners, must be a bit scary. Keep the thread going - but don't worry about it ! Martin.

[V Mad]

As Martin says, this discussion is not about any cooling system problems. For me it is an interesting exercise for my old engineer's brain trying to keep it working. Same with the stag if you dont use it you lose it!

For any prospective stag owners, the stag is a great car, and if looked after properly does not overheat.

[Mark S]

CAVITATION - AAARGH that word again!

The likelihood of conditions which would cause cavitiaton to occur in this pump is remote.

The biggest problem with the Stag pump is with pump prime. If AIR enters the pump it's performance is drastically reduced until the air has cleared the pump. If the pump draws too much air in it will become completely gas-bound and practically stop pumping!

Two sources of air are from higher up in the engine where it is piped straight to the pump inlet
1) The rad by-pass, which should be shut under overheating conditions
2) The heater return. If your heater is gurgling you ARE already suffering reduced coolant flow due to reduced pump performance.

The cooling system seems to be prone to drawing in air when it cools (and dips below atmospheric pressure) - most likely through the pump mechanical seal. IMHO. But also through other leaks, cracked rad neck etc.

The original expansion tank system allows this air to be trapped in the system a little more every warm-up/cooling cycle, then feeds it to the pump inlet!

A lower mounted pump will help but you must still be feeding the by-pass and heater returns to the pump inlet for the system to work properly. The answer is to stop air getting to the pump in the first place.

The best, obvious and cheapest solution is a high header tank which is set up to allow a small amount of coolant from the highest point in the system to continuously circulate through it, trap this air and vent it through the pressure cap as the coolant expands every time the engine warms up.

[milothedog]

"The cooling system seems to be prone to drawing in air when it cools (and dips below atmospheric pressure) - most likely through the pump mechanical seal. IMHO. But also through other leaks, cracked rad neck etc."

At a potential 20 psi surely if it was drawing air it would leak from the same place as well,wouldn't it, when the system cools it is a function of the pressure cap to allow the system to draw in air to prevent collapsed hoses etc, if what you theorise is occurring then the first step would be to change the pressure cap. just my 2p worth.

Ian.

[Dieter H. Marschall]

Hello,

Matthias Lindner drives a Stag and is professor at Mannheim Technical University. He has a simple philosophy: I believe what I can measure. So he tested both the 6 vane and the 12 vane pump in his lab. He found the 12 vane one more efficient.

Kind regards,
Dieter.

[Mark S]

[QUOTE=milothedog;153791At a potential 20 psi surely if it was drawing air it would leak from the same place as well,wouldn't it, when the system cools it is a function of the pressure cap to allow the system to draw in air to prevent collapsed hoses etc, if what you theorise is occurring then the first step would be to change the pressure cap. just my 2p worth.

Ian. [/QUOTE]

Air is much less viscous than coolant and will find it's way through leaks that coolant will not, and it doesn't need a high differential pressure to do it. The radiator cap needs a slight vacuum to open so air will and does find it's way in.

Leaks and weeps seem to be common place in the Stag, that's why I use K seal. Some leaks may not even be noticable but will draw air in. The pump mechanical seal is not a "seal" a such but runs on a cushion of coolant a few molecules thick continuosly leaking out under the impellor, another possible source of air ingress if the vacuum dips below the head of water above the pump.

If a Stag engine doesn't need topping up every few thousand miles or draw air in I think the owner should consider themselves very lucky.

Where would you suggest that the air comes from.

My engine dosn't lose much coolant, it could even lose a litre before the low level switch operates and the top hose would still be full at that point. I'm not paranoid about air ingress and I don't have to remove the rad plug to check the system is full.