Topping up SU piston dampers

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Jeremy
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#1 Topping up SU piston dampers

Post by Jeremy » Wed Oct 13, 2021 1:38 pm

3 (probably trivial) questions if I may:
1. How often should one put oil in the carb dampers? The big lube/servicing chart says only once every 3000 miles. I check them much more often, and they always take a squirt of oil. (I fill only as far as the ridge, with engine oil).
2. Should I be able to detect any difference in the throttle response when I've done it?
3. Where does the oil go?
(The carbs were rebuilt by Rob Beere and AFAICR they've been like this from day one.)
Thanks all,
Jeremy
1967 S1 4.2 FHC

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bitsobrits
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#2 Re: Topping up SU piston dampers

Post by bitsobrits » Wed Oct 13, 2021 2:10 pm

I check mine once a year, sometimes every other year. I do about 1000miles/year, btw. No need to do it any more frequently than the recommended interval.

If you are filling to the top of the 'ridge' seen down the hole in the suction chamber housing (which would be be the top of the air piston) then I would say you are over filling. Filling to the top of the air piston means that when you re-insert the damper unit, an amount of oil equal to the volume of the damper unit is displaced and fairly quickly disappears down the interface between the piston and the suction chamber 'bell'. Good for lubing the piston to chamber interface, but it doesn't need nearly that much.

Filling to about 1/4" below the top of the air piston will displace only a very small amount of oil, just enough to keep the piston lubed until you check again in 3000 miles or so.

And unless the piston cylinder was very low on oil, you will not detect any change in throttle response by adding oil. As long as there is enough oil present for the damper unit piston to be submerged, then you're good. Any more than that has no impact. The oil reservoir in the air piston is oversized enough that you needn't do frequent checks.

I do believe the only reason it is recommended to check at the 3000 mile interval is to cause you to overfill a bit and thereby provide a bit of oil to the piston interface. If you slowly and carefully remove the damper unit and don't lose any oil in the process, you will find the oil level to be remarkably consistent over many thousands of miles.
Steve
'65 S1 4.2 FHC

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Gfhug
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#3 Re: Topping up SU piston dampers

Post by Gfhug » Wed Oct 13, 2021 7:20 pm

Jeremy, you’ve asked questions I’ve wondered about and not been brave enough to ask!
Steve, thanks for your thoughts.

Question 2, would throttle response differences only be seen at wide open throttle?

Geoff
S2 FHC Light Blue
S2 OTS LHD - RHD full restoration

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#4 Re: Topping up SU piston dampers

Post by Jeremy » Wed Oct 13, 2021 8:35 pm

Many thanks Steve. An interesting and v helpful response!
Jeremy
1967 S1 4.2 FHC

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angelw
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#5 Re: Topping up SU piston dampers

Post by angelw » Wed Oct 13, 2021 9:13 pm

Geoff Wrote:
Question 2, would throttle response differences only be seen at wide open throttle?
Hello Geoff,
You won't see any difference in throttle response per se. The reason for the damper feature is to cause a hesitation of the ascent of the Air Piston and therefore, momentarily limit the gap between the Bridge of the Carburetor and the bottom face of the Air Piston during rapid acceleration when the throttle butterfly openings are suddenly increased (or wide open). The increased opening of the butterflies allows for a greater volume of air to be pulled into the cylinders, but because the gap between the Carburetor Bridge and the Air Piston remains relatively small, the air passing through this gap and past the top of the Metering Jet, must speed up. This in turn, sucks more fuel for the given volume of air into the air stream, enriching the fuel/air mixture for a slight boost to acceleration.

Regards,

Bill
Last edited by angelw on Wed Oct 13, 2021 9:33 pm, edited 1 time in total.

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#6 Re: Topping up SU piston dampers

Post by Gfhug » Wed Oct 13, 2021 9:25 pm

Thank you, Bill. Always learning from the generous input from people like you and Steve above.

Geoff
S2 FHC Light Blue
S2 OTS LHD - RHD full restoration

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malcolm
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#7 Re: Topping up SU piston dampers

Post by malcolm » Thu Oct 14, 2021 8:26 am

I just check on a regular basis whenever the bonnet is open for whatever reason! Often need a top up. I probably do overfill slightly, but don't think that matters?
Malcolm
I only fit in a 2+2, so got one!
1969 Series 2 2+2
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#8 Re: Topping up SU piston dampers

Post by bitsobrits » Thu Oct 14, 2021 6:39 pm

The only reason you need a top up is because you are checking them often! Best to leave them alone.

While it's probably not actually harmful to dump extra oil into the air piston, the overflow ends up all over the top of the piston platform section, and most eventually makes its way into the engine. SU engineers took great care to design these precision instruments and calibrate the air piston weight and suction chamber spring strength for a particular application, then someone comes along and dumps excess oil in, effectively adding weight to the air piston. :shock:

It's your car, and if that doesn't bother you, then it's your call. Bothers me just thinking about it, but then, I'm that way. 8-)
Steve
'65 S1 4.2 FHC

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#9 Re: Topping up SU piston dampers

Post by Jeremy » Fri Oct 15, 2021 8:10 am

Thanks again, Steve and Bill
Jeremy
1967 S1 4.2 FHC

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tonyabacus
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#10 Re: Topping up SU piston dampers

Post by tonyabacus » Fri Oct 15, 2021 7:47 pm

Jeremy
The main purpose of the oil is to provide some control over the piston in relation to the damper, which by its very name gives a clue as to its operation. As you increase pressure on the accelerator pedal the piston tries to rise, depending on how fast you press the pedal it can try to rise rapidly. As it does so because of the tolerances between chamber and piston it can effectively try to rattle its way to the top if you jab the throttle hard and quick. The oil in the chamber acts in much the same way as that in a shock absorber, trying to provide a smooth ride as the piston ascends. The type of oil used is normally something in the way of a 20 mono grade.

However I have known and used a variety of fluids over the years in racing to try and gain any small advantage, so things like bicycle oil, brake fluid, hydraulic fluid, all have been tried as their viscosity allows for different speeds to which the piston rises. But for general use normal single grade oil is fine.

Perhaps this site below will provide some better understand of the different types of SU carbs, their components and what they do, including topping up of the chamber.

https://sucarb.co.uk/technical-su-carburetters

Tony

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angelw
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#11 Re: Topping up SU piston dampers

Post by angelw » Fri Oct 15, 2021 8:56 pm

Tony Wrote:
As it does so because of the tolerances between chamber and piston it can effectively try to rattle its way to the top if you jab the throttle hard and quick. The oil in the chamber acts in much the same way as that in a shock absorber, trying to provide a smooth ride as the piston ascends.
Not really Tony. Read my previous Post for an insight into the real purpose of the damping system, or go to the Link you Posted, as it basically says the same.

Regards,

Bill

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