KX500 1985-1986 WATER PUMP COVER ON A LATE MODEL KX500

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Time for prognostication.. Theory vs reality

The first question is can you swap them?
The answer is YES!!!

KX GURU RACING KX500 WATER PUMP COVER 85-86 ON A 89-04 101.jpg (296.16 KiB) Viewed 5843 times

KX GURU RACING KX500 WATER PUMP COVER 85-86 ON A 89-04 102.jpg (342.32 KiB) Viewed 5843 times

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The next question is, "Why would I want to swap them?"
What we are looking for is greater cooling efficiency.
After all about 1/3 of the BTU's generated by the fuel burning is going out through the cooling system.
In part one of the goals we want to address is reduced cavitation.
We have long known that the 89-04 cooling system experiences cavitation at higher RPM's.

Adam Millar (CR500 land speed record holder and fellow KX Guru insider member) ran some cavitation tests and found that the 89-04 water pump cover creates cavitation at 8200 RPM's
The 85-86 cover starts cavitating at about 8900 RPM
Since the drive ratio for the water pump impeller vs crank rotation is 1.18 : 1, (Thanks Alex Ward) that is for every 1 crank revolution the impeller is rotating 1.18.

8200 rpm crank speed translates into 9676 impeller RPM
8900 rpm crank speed translates into 10502 impeller RPM

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Here is a clear disadvantage of the early model pump cover..

KX GURU RACING KX500 WATER PUMP COVER 85-86 ON A 89-04 103.jpg (429.05 KiB) Viewed 5839 times

KX GURU RACING KX500 WATER PUMP COVER 85-86 ON A 89-04 104.jpg (408.94 KiB) Viewed 5839 times

110 grams for the older, 90 grams for the newer...
If every gram counts....

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Here's a quick look at the inside of the covers.
First, look at the sharp edge vs the rounded edge..

KX GURU RACING KX500 WATER PUMP COVER 85-86 ON A 89-04 105.jpg (486.48 KiB) Viewed 5838 times

Then look at the intake shape... Round always beats oval for water flow

KX GURU RACING KX500 WATER PUMP COVER 85-86 ON A 89-04 106.jpg (357.4 KiB) Viewed 5838 times

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The other thing is that these parts are cast...
So, the water flow area is tapered..
Measuring on the inside smallest cross section area you'll find that the older cover is 13% bigger.
And it would be a simple matter to enlarge it slightly....
But again, should you?

More flow is great right?
Sure, the correct amount of flow is very necessary..
But too much flow will reduce the time that the radiators have to reduce the heat from the coolant.
How much is too much?
This is what we will try to find out...