Which is the max water pump to crank pulley ratio?

[dfom]

Hy everybody,

I am doing some calculations for choosing pulleys for my Fiat 2L TwinCam. I´ve found two pulleys from a supplier that would fit, but i´m unsure about the maximum allowed ratio between the water pump pulley and crank pulley. The original rate is 1,25, meaning that water pump is 25% faster than engine RPM.

With these pulleys, found at a local supplier, the ratio would be 1,45 (water pump 45% faster than engine).
My engine will not rev higher than 6500 RPM, because it has cast pistons and rods.

Would it be possible to run at the above ratio (1,45) without water pump cavitation at 6500 RPM?

Bye.
Daniel Melo - Brazil

[Guy Croft]

I can offer a rule of thumb about this.

Without doing detailed experiments one might reasonably assume that for standard red-line rpm use, the standard ratio is right to avoid cavitation (I am assuming that the phenomenon can actually exist at high speed - I don't know any computational method of working it out), so if you run the engine much faster than std redline (and from exp I mean speeds over 7500, because I have not encoutered any cooling system problems up to and even over that speed with the std ratio) then you would certainly think about reducing the pump speed to bring it back into line with the original designer's intent - to stop cavitation at that higher speed.

On my NHRA 2 liter which raced at 9500 and fractionally higher I ran 1-1 ratio and never had a problem with circulation; mark you, it was a toothed belt drive.

Summary:

std engine std ratio 1.25 at 6500 = 8125 rpm @ pump - known OK
GC eng std ratio 1.25 at 7500 = 9375 @ pump - proven OK (EDITED DUE TO GC ARITH. ERROR)

Your suggested ratio 1.45 at 6500 = 9425 @ pump - bit high but might be OK based on my exp.

Give it a try if you have no other choice, report what happens!


Worth mentioning 'en passant' that high rpm use with conventional Vee belt drive can cause the belt to twist and fly off. Odd but true.
The highest I have run a Vee belt is 8200 and I know of others who have done the same. Mind you, it has to be securely tensioned.. but that speed is not in excess of the design belt speed. A 'cogged' design is much less likely misbehave than a plain Vee section. Vee belts are pretty inefficient really because they slip. That's one reason they get hot and degrade. 'Micro Vee' is much safer on a high rpm motor, better still a 'toothed ' (3/8" automotive trapezoidal) because they don't exhibit any slip at all.

GC

[dfom]

Guy,

Thanks again for the excelent information.
I will try the pulleys and report the results.

Bye
Daniel Melo - Brazil.

[RedLexus]

Sorry for hi-jacking the thread , but I had a similar query... My Rally Car (1.3 EnduraE Ford Ka) has exhibited hot running temps when on a stage in summer , I have moved the radiator , replaced it , supplied extra cooling air and cleaned the coolant system , Would a slight reduction of the gearing to the water pump help do you think Guy? I take it to about 6750-7000 rpm max . The standard Ford Water pump is still fitted .