Jaguar went out of its way to indicate "top up - here; top down - here" on the reservoir. This is because a not-insignificant change in fluid level happens when we raise / lower the top.

The difference has to go somewhere, but let's table that for the moment.

Anyone with a ragtop can observe the change and report back for us. If memory serves, on my car the level change is something on the order of the spacing between the two markers.

Can someone take a look please?

 

Based on my observations, the fluid level with top opened and closed matches precisely the JTIS descriptions.

Doug

 

Here is the newbies two cents worth.
When the rams are full extended, the entire volume of the housing is filled with fluid. When the rams are retracted, that same volume is partially filled by the shaft which leaves a smaller volume filled with fluid. Less fluid in the retracted ram means more fluid in the reservoir.

 

Goodness me.
The top moving rams are dual action.
To extend them fluid is pushed under the piston and the volume of fluid in the ram fully extended is
stroke x piston area
To retract fluid is pumped in above the piston and the volume of fluid in the ram fully retracted is
stroke x piston area MINUS the volume of the ram rod.
Fully extending the ram returns less fluid to the reservoir than fully retracting it.

 

You may have hit on the answer Gary.

Where were you yesterday Steve? Maybe an OF but you haven't lost it yet!

 

Norri,
I hate to tell you but I was working!

 

The timing of these posts is perfect. A couple of days ago I was retracting the top when all of sudden it wouldn't come off the windshield header. I've had the car for two years and the top has worked great until now. I figured I would start the engine to raise the voltage on the pump and viola the top went down. I made a note to myself to check the petcock on the pump after I returned home. Sure enough the petcock was not tight going clockwise. I was surprised it was only slightly loose (it turned about 1/8th of an inch. During this exercise I noted the fluid in the pump reservoir changed less than a 1/8th of an inch ( little over the top hash mark to a little under the hash mark) between top being fully closed and fully open.

 

Yes, that's what you'd expect. The volume occupied by the ram rods isn't much but does affect the level in the way you note.
Fully retracted (top closed) = more fluid in reservoir.

 

Thank You! I am presently not with my xk8 so I was unable to make the test myself.

 

Yes indeed. I wish I'd thought of your lucid way of saying it.

As we raise/lower the top, the fluid held in the reservoir changes by exactly the same amount as the fluid held in the rams changes (one going up as the other goes down). The "flow rate in = flow rate out" argument does not hold.

The two reservoir hash marks do not mean "full" and "add". They both mark a "full" condition, the lower one when the top is up, the upper one for top down.

 

The flow rate in does equal the flow rate out, the displacement is different due to the rod creating the level difference.

 

When the ram is in motion, the fluid flow rate out does not equal the fluid flow rate in. Say we're extending the ram. For every inch of piston travel, fluid enters at a rate proportional to the radius of the tube. On the other side of the piston, fluid exits at a rate proportional to the radius of the tube minus the radius of the ram rod. (Actually, all this is really radii squared, since cross sectional area is what matters.)

If the flow rates were equal fluid could not accumulate in the ram.

I'm sure Steve will have a better way to say that.

 

Numbers, just for the hell of it:

The two hash marks on the reservoir are about 3/8" apart, and the surface area of the fluid there is (very roughly) about 6 square inches.

So if the fluid level were to change by the separation of the hash marks, that corresponds to about 2.25 cubic inches, or about 37 ccs of fluid.

That's how much fluid would then be going to the rams to extend them.

Hows that for a useless factoid!

 

OMG! Let it go!

 

Very informative. And creative.

 

We're only kicking the ball around, none of this is really important is it, as long as the fluid remains between the lines at all times?

There is a flow and return to the ram, it is the same pump same dia hoses and I assume the same dia port into the cylinder at each end the flow must be the same.
The 37cc that you mention is the difference in volume between the top and bottom sections of the cylinder created by the rod.
I'm sure the capacity of the cylinder is much more than 37cc.

 

Yeah, it's just an exercise at this point. But we learn stuff that way, no?

We can continue to kick around or not, as you like. But for the reason given above the flow rate out of the ram is not the same as the flow rate into the ram. That requirement would exist only if the ram held a constant volume of fluid, which it does not. The difference in outflow v. inflow is just the rate at which the ram is absorbing fluid as it expands or dispensing fluid as it contracts.

37 cc was just a calculation for fun. The number has no special significance.

 

I enjoy a good kickaround.

I'm pretty sure the cylinders are normal double acting units as below.
We know the same pump is pumping to either port depending on valve position the flow into the cylinder must be the same at both ends.
The volume pumped will not be the same due to the rod, this difference will cause the reservoir level to rise and fall by an amount equal to the volume displaced by the rod.

 

Well I just finished dinner and thought instead of beating the dog I'd beat the dead horse
Attached are pics showing the fluid level in the pump reservoir when the top is fully closed (that's the pic where the fluid is even with the top hash mark) and fully open (the fluid level is above the top hash mark). Ma! is the horse dead yet or should we keep beating this thing? Ah, it's great we have the freedom of expression---now let's go watch the Olympics.

 

Norri,

OK, got it. In putting together a response, tomorrow maybe I'm afraid, it will help me to know if you agree or not with the following:

Consider a black box, with unspecified contents, having incompressible fluid input and output ports. Statement: the rate of fluid outflow is equal to the rate of inflow if and only if the volume of fluid, if any, contained in the black box is constant. Agree?

I think this statement is nailed-down enough to permit a yes or no response.