8 replies to this topic

[stevegrabba]

OK guys bare with me on this!!

 

When running an advance and retard there are two common places to pick up the vacuum from at the top side, rather than the distributor.  Generally these are referred to as the port or the inlet manifold.  When running twin SU carbs, such as HS2 or HS4 there is often a small nipple just in front of the butterfly on one of the carbs that is often used to provide the vacuum connection through to the advance and retard on the distributor.

 

Now for the tricky bit!!  At idle, if you use a vacuum gauge and take a reading at the manifold connector you should get a reasonably steady reading of 17 to 22 inch mercury pressure on a relatively standard spec 1275 engine.  If you blank that up and take a reading from the port connector you'll probably get 0 inch merc pressure.  This is well documented.

 

So.....keeping in mind that vacuum a typical advance and retard unit has numbers stamped into it of for example 8 14 10.  And these numbers mean that the advance and retard unit will start to operate at 8 inch merc pressure....will be fully advanced at 14 inch mercury pressure and that full advance will add 10 degree of advance to the timing. (Just a random example!)

 

So...if the vacuum feed to the advance and retard is connected at the port position, and the vacuum there at its highest during idle or deceleration is minimal or zero.  Then, isn't it logical that it will never actually activate any movement of the advance and retard unit?  Hence it becomes solely mechanical timing advance which occurs.

 

Hence, with twin SU carbs the moral of the story has to be A) why bother putting the vacuum nipple in front of one carb.  b) Blank the carb nipple off and take a vacuum connection at the manifold to the advance and retard unit instead.

 

You may read on the net that many car manufacturers started to use the port connection due to emissions requirements and changes etc.  But wasn't that one of the key reasons for using a vacuum advance and retard unit in the first place?  Which the port connection then prevents from functioning.

 

Please tell me if I'm going bonkers, or if BMC lost the plot on various minis, midgets, MGB's and other cars which may also have had port vacuum nipples connected to advance and retard units??

 

Food for thought anyway!

[nicklouse]

ah yes that "issue"

so reading

http://www.theminifo...um#entry3401569

[ACDodd]

Ported vacuum is what you need, no vacuum at idle, but vacuum at part throttle when cruising, this gives the ignition advance you need to efficient at part throttle running and no advance when idling to reduce idle emmisions. The best of both worlds.

Ac

[stevegrabba]

Thanks, I guess I'm a little surprised that the port vacuum is created to a sufficient pressure considering that it is zero at idle.

 

mmmm...it sounds like a little experiment is required!! 

 

 

At the end of the day if it doesn't get to 8 inch mercury pressure, I might as well use the advance and retard unit into a bicycle bell!! ha ha

Edited by stevegrabba, 11 October 2017 - 02:04 PM.

[ACDodd]

Its zero at idle because the port is covered by the butterfly.

Ac

[Spider]

You thinking is correct though, if I can perhaps clarify, the Vacuum Advance unit fitted on these cars on;y provides advance, though there are some about that can do both Advance and Retard.

 

There's a bit more info in this thread (give give the photos a moment to load, they are on Photobuck but will load thanks the Deathrow's work)

 

http://www.theminifo...dvance-fitting/

[panky]

Just looked through the linked thread, very interesting. 

[timmy850]

Vacuum is at it's highest during coasting with the throttle slightly open. Coasting and part throttle cruising is the time when you want additional advance. When under load/full throttle you'll get 0-5 Ins Hg, which means you get no vacuum advance and all mechanical advance. 

 

My car gets 25+ Ins Hg during coasting

Attached Files

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Edited by timmy850, 12 October 2017 - 12:24 AM.

[stevegrabba]

I've done a little experiment and can confirm that there is a good healthy vacuum present from the 'port' connection, from approx 1700 rpm upwards and rising with steady increase of rpm.

 

At approx 3500rpm it was up towards 15 merc inches and hence full vacuum advance would have occurred via an advance and retard unit. FYI- the test was done in my garage with the engine not under load etc.