FE-FC Holden Discussion Forum

I'm not convinced that I am getting good vacuum advance. The diaphram on the unit is good. My question is does it really matter where its connected on the manifold side? I run twin strommies and attached the pipe to the base of one carby only with the other blocked off. Is this the proper way of doing it or should I have a T and connect it to the base of both carby's or just use the manifold attachment (which I was hoping to use for a brake booster)

Thanks

Rod

Rod I don't think you will get enough vacuum from the base of the stromberg (or from both strombergs hooked together). You can buy a standard adaptor to fit both a vac line and a booster line to the manifold.
Later model carbies have vac lines at the base but these produce better vacuum by different design.

Ken

Cheers Ken. I thought this maybe the case.

What are others doing with twin strommies? Are we doing the same as Ken.

Rod

Hi Ken and All,

The reason I originally posted this topic was that if I had a single / standard stromberg setup the vacuum pipe goes to the base of the carby which is a small opening. By going to twin carby's I am making the assumption that the vacuum would not be as great as its only drawing on vacuum from one carby hence advance would be retarded some what.

However by going to the vacuum outlet on the manifold the vacuum would be drawing from both carby's. The problem that I can see with this set up is that the hole is alot larger and I assume more vacuum than standard would be developed hence the potential for over advancing. I am assuming this is why on a standard setup the vacuum wipers go to this point because they need alot more vacuum to drive the motor.

In the manual the vacuum advance is operational between 6 and 13 inches of mercury.

I would be interested in what set ups other have got.

Cheers

Rod

There are two types of vacuum. Manifold vacuum is used for brake boosters and vacuum wipers ect. You get more vacuum when you take your foot off the gas. That is why vacuum wiper work so well when you lift your foot and the same with brakes. When you take your foot off the gas you use the brakes.
Distributor vacuum is different because it increases when you put your foot down.
I'm no expert but i think it is positioned in the carby above the butterfly. As the butterfly opens the vacuum to the distributor increases.
Graham

I think you have a point there Spanner, vaccuum for the advance must increase as you put your foot down. I don't know what the vaccuum difference is between single and twin strombergs though.

Probably need to measure with a guage.

Ken

Spanner, you are correct in saying that the vacuum for the dizzy is normally taken above the butterfly.  However, when you accelerate there is almost NO VACUUM.

Vacuum advance is purely for economy.  When you are cruising with light throttle application, you have a high vacuum reading of 20 In/Hg or so.  This is when your vacuum advance module increases your dizzy advance by 6-12 degrees depending on model.

When you have your foot down there is no vac advance, otherwise it would ping like crazy while running total advance with that sort of load.  

Its another one of the reasons you get such poor economy punching into a headwind or towing as you need more throttle application to hold the same rpm.

Performance ignitions dont have vac advance as it is not required, and if it slips internally if just disconnected will cause the timing to be too far advanced.

Hi Rod,
I've fitted a number of twin strombergs to early Holdens and have only fitted the vac advance to one carby (the brass fitting above the throttle plate).I don't think it would make any difference in vacuum if it was connected to 2 carbies.
Dave EH,
Mechanical advance is the second method of ignition timing advance. As the distributor spins fast enough to activate the mechanical-advance weights, the engine receives initial timing, mechanical timing, and vacuum timing under part-throttle conditions. As the engine accelerates to wide-open throttle, the vacuum drops, eliminating the vacuum advance timing. For example, part-throttle total timing would look something like this: 10 degrees initial + 10 degrees vacuum + 20 degrees mechanical = 40 degrees of total timing. At wide open throttle there is no vacuum present and the vacuum advance timing is eliminated, giving your engine a total of 30 degrees timing. The reason your engine is able to cope with more timing at part-throttle is because only a limited amount of air and fuel make it into the cylinder at part-throttle. Lower cylinder pressures enable the combustion process to start sooner and help improve part-throttle response by increasing torque.
ACE  8)

Thanks Ace ;)

Isn't that what I said ???

Good to have your input.

Dave