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Re: [T3] A question about oil pumps

On 26 Aug 2006 at 21:11, Dave Sanderson wrote:

> We have a pump for a flat  cam, a flat cam and yet it was too long -
> but only by a very small  margin. I filed the end of the housing, that
> is the section that  sticks towards the cam and holds the gear that
> interfaces with the  cam, by a thou or so and now, voila no leaks.
> This is a new 30 mm  pump from Shadeck. Has anyone else had this
> experience. Is this  something that happens with these new Brazilian
> pumps? 

I haven't seen this problem before, but I haven't used any of those 

When you look at the history of VW pumps, you realize that they 
increased the pump capacity by a LOT over the years. The gears 
started out as 19mm, then 21, and finally 26, IIRC. I've generally 
gone with aftermarket 25s (with flat cams) or OEM 26s (with the later 
dished cams) just because I found that those parts fit together well 
and I knew that the late engines didn't seem to have any of the 
lubrication problems that the early ones had. In other words, there 
was clearly a weakness here, and VW solved it in stages ending with 
the 26mm pump.  

When I've tried other oversized pumps, I've often found that they 
would either hit the fan housing or the rear mount crossbar (on late 
engines.) It seemed clear that most of the oversized pumps were never 
designed with a type 3 engine in mind.  

Another consideration is the power required to drive a larger pump. 
The power is proportional to the flow rate x the pressure. Since the 
pressure goes up when the flow rate goes up, the power requirement 
goes up ~(flow rate)^2. If there's really no need for the extra flow, 
then this is just wasted power that can never get to the wheels. That 
power must be transmitted thru the same cam gears and the tang on the 
pump shaft, and the increased forces must be supported by a pump 
housing that now has a shorter support bore. The fact that you had to 
shorten that bore (only slightly) indicates that the maker tried to 
fit as much support in there as they possibly could.

So this is another case where bigger is not necessarily better.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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