Vac Advance (Atmosphere side blocked or not?)

[ChimpanLucky]

Hi all, a quick question.

I noticed that unusually for a vacuum advance capsule, the one fitted to our cars has a take off on the atmosphere side of the diaphragm.

Does anyone know why?


I've seen it blocked with a bung like this:





But also left un-blocked like this:


And here's a TVR engine bay that also shows the atmosphere side of the diaphragm with no bung (ie open to atmosphere).




Is the atmosphere side of yours blocked with a rubber bung or unblocked & open to atmosphere?

Logic dictates if this atmosphere side take off is bunged you would need more vacuum to deliver the same amount of advance compared with the bung removed.

Actually I'm intrigued as to why it has the feature at all?

Every other vac advance capsule I have come across has been a completely a sealed unit.

Can someone enlighten me?

[TVR Beaver]

You can't get more vacuum than zero... so the question is.. what does it pull now.. the range is atmospheric to zero?

I have mine plugged BTW but its a fair question... If air can't get in.. how can the diaphragm move? it's rubber so does it just expand instead of deflecting?

I was told that the second input was for a vac tube that went to the plenum side of the throttle disc... so it would pull back when the throttle was closed?... ?? don't know if that’s correct?

Sorry.. not a lot of help.. but thought I'd comment as I've wondered the same

[EGB]

On my new vac advance from Trev, the bung is as on the first photo. Sucking on the plenum connecting tube makes no noticeable difference to my suck whether the bung is on or off.
I also noticed that the connection had come adrift and I did not notice any difference in driving behaviour or tick over. I bought a new one since on the old one, I could suck air because the plasticky diaphragm after 11 years of use had cracked. My Griff still accelerated and went like a boost rocket without any vac advance.
Steve Heath goes into a lot of detail on this and wonders "why it is needed (or not)". Very confusing. I suspect there are a lot of Rv8 TVRs out there with cracked diaphragms but still going like rockets! The 1960s Cooper S for standard road and rally use never had a vac advance. The Jury is out!

[ChimpanLucky]

Well I totally understand the theory behind a vac advance, it's 1940's technology and purely an economy device, no mystery there.

What I would love to know is why the vac advance capsule on our cars has an outlet on the atmosphere side of the diaphragm?

Almost every vac advance capsule I have come across is a sealed unit apart from the vac side.

I say almost because the only other time I have seen outlets on both sides of a vac capsule, it was sold as part of a Capri turbo kit made by Turbo Technics.

I believe the idea was to apply negative pressure to other side of the diaphragm to retard the timing & prevent knock under boost conditions.

Kind of using the vac advance in reverse, a "push me pull me" vac capsule if you like

But why is there an outlet on both sides of the capsule on our cars?

Anyone know?

[EGB]

Could be a very fine tuning mod. With bung on there could be slightly less advance for certain engine set ups. I am very probably wide of the mark. Still a mystery! Perhaps Spend can come in on this.

[A900ss]

Apologies for thread revival but I notice that my advance doesn't have the rubber bung on the back.

Does it need one?