This is something I've wondered about for a while, and with the resurrection of the JCW thread (smaller pulley or not) I thought I would ask it now.

I'm really interested in swapping in a 19% reduction pulley sometime after break-in. One of the biggest detractors I've seen so far is the concern over cavitation of the water pump.

Now, in the JCW thread, there is some argument over whether the JCW kit includes different gearing for the water pump. Ryephile says that its only a rumor, but according to Andy@ross-tech.com FURIOSO has observed the JCW kit turning the water pump more slowly than stock.

My question is, would it be possible to change the gearing for the water pump to prevent cavitation on the 19% pulley? And if so, how hard would it be to accomplish? Seems like a way to increase reliability without having to go with a larger pulley.

Sorry if my lack of technical knowledge makes this a stupid question (i.e. impossible to accomplish).

Doug

 

I think we need to ask some basic questions. Opinions vary on the answers so far.
You can try to talk to Randy webb and Eric Savage to get an opinion and post them on this thread if it helps.

Question #1 Cavitation- theoretical or real?
Is cavitation truely a problem for the 19% reduced pulley?
Is it a problem for all reduced pulleys or just more of a problem for the furthest reduced pulleys? And under what conditions is it a problem?

Why I ask is that for all those running 19% pulleys now we have not seen any cavitation problems in normal use driving street, autocross or light track. So how much of a problem is this when it is not detectable, causes no heat problems, etc under this type of usage.

Question #2 Water pump gearing- Fact or fiction?
If cavitation is not much of a problem with normal use even with a 19% pulley installed then it doesn't matter what the gearing is on the water pump. Naturally having better gearing that helps the reliability and longevity of the water pump is a good idea.

Determining if the JCW kit includes any change to the gearing for the stock water pump would be a plus for those who have invested in JCW. Maybe you should ask JCW for an answer as was suggested.

 

i have noticed no cavitation with the 19%...if it were to be a problem it would be in the upper RPM range, and actually the reduced pulley would cool better at low RPM due to the extra flow, now comes the question how amny of you stay at high rpms to be a problem.. it would have to be a sustained time at high rpms, with the pump cavitating(if it does) to be a problem otherwise if you were to allow the rpms to drop any (aka gear shift) it would recatch ,move water again until the point of cavitation, now i tested this a while back to see if it were cavitating (wish i took pics) i used a piece of lexan pipe inbetween where the vent screw is (on top of the radiator) and watch the flow to look for either arir bubbles or flow movement to stop(i admit this is not an absolut best way to check but gives a general idea) i had a friend take the car to redline (on a dyno) and watched the lexan pipe, no flow stopping or air bubbles went by(the reason for looking for air bubbles is, that what cavitation does, the impeller spins so fast that it ends up spinnning in a "bubble of air" unable to grab the water any more), Now like a stated before that is not an absolute way to test but if there were to be a problem i would be able to see it. I dont feel the cavitation problem exist, not to mention most well pumps turn much higher rpms than we are without cavitation problems......

 

the gears driving the water pump are inside the rear housing of the blower...not easy to get at. Tomorrow, if I get a chance, I will measure the ratio on my spare blower and report back.

 

Jlm hows that "short" block coming? ever get you pistons and rods worked out and all.......

 

still waiting for those special rollerwave pistons.

 

damn that sucks i cant wait to here!!!

 

Thanks for all the info - just trying to do my due diligence on any aftermarket parts. :smile:

 

>>i have noticed no cavitation with the 19%...if it were to be a problem it would be in the upper RPM range, and actually the reduced pulley would cool better at low RPM due to the extra flow, now comes the question how amny of you stay at high rpms to be a problem.. it would have to be a sustained time at high rpms, with the pump cavitating(if it does) to be a problem otherwise if you were to allow the rpms to drop any (aka gear shift) it would recatch ,move water again until the point of cavitation, now i tested this a while back to see if it were cavitating (wish i took pics) i used a piece of lexan pipe inbetween where the vent screw is (on top of the radiator) and watch the flow to look for either arir bubbles or flow movement to stop(i admit this is not an absolut best way to check but gives a general idea) i had a friend take the car to redline (on a dyno) and watched the lexan pipe, no flow stopping or air bubbles went by(the reason for looking for air bubbles is, that what cavitation does, the impeller spins so fast that it ends up spinnning in a "bubble of air" unable to grab the water any more), Now like a stated before that is not an absolute way to test but if there were to be a problem i would be able to see it. I dont feel the cavitation problem exist, not to mention most well pumps turn much higher rpms than we are without cavitation problems......


I don't actually know what the insides of the water pump look like (never having seen one outside an engine compartment), but I would imagine it's basically a fan-type mechanism of some sort which pushes water along. If cavitation is caused by the "fan" being so fast that water isn't moving, wouldn't the observed JCW behavior of slower rotation tend to prevent cavitation? And if so, wouldn't a smaller pulley have more of a preventative effect? Pardon my engineering ignorance, I'm trying to make sense of this.
-Cor.

 

the as measured gear reduction is almost 2:1 (I didn't open it up to count teeth.) this means that if the blower is turning 14,000, the pump is turning 7,000.

for you number crunchers:

crank pulley dia: 5.4"

stock pulley dia: 2.59" (ratio= 2.084; 6800crank rpm=14,177 blower rpm=7,088 water pump rpm)

15% (P&D) dia: 2.20" (ratio=2.45; 6800=16,690=8,345)

19% (P&D) dia: 2.10" (ratio=2.57; 6,800=17,485=8,742)

 

So a 19% reduction in pulley size equates to a 23% increase in water pump speed. Sounds like a significant increase, but I don't know much about water pumps so no idea if this is a problem. Like someone said, I would tend to think that unless you spend a lot of time at high rpms you aren't in much danger.

Just for note, you have to pass 5500 RPMs to exceed the normal water pump speed @ 6800 rpm on a stock pulley.

(7088*2=14776 blower rpm / 2.57 ratio = 5515 crank rpm)