[Mgs] Timing discussion

[PaulHunt73]

"until you notice preignition"  Is a bit drastic.  Haynes says to drive at full throttle in 4th from 30 to 50 and advance until faint pinking can be heard.  However I've noticed that I can get pinking at part throttle and opening the throttle further stops it, either because the bigger charge cools the combustion chamber more, or the vacuum advance backs off.  So I use the criteria 'at any combination of throttle, revs and load' and keep it just short of that.  

The reason Haynes says that you can benefit from advancing it over book values is that with the level of engineering used in our engines some will be more prone to pinking than others,and the book caters for the worst case, and maybe with a bit of a safety factor.  That was definitely the case with an A-series in the 70s, which as a new car and dealer serviced always came back running like a dog (comparatively) as they always put it back to book.  But on my 73 MGB I couldn't run more than book, even on leaded, and have to retard slightly from that on unleaded even 99 octane.

Whether it pinks or not also depends on terrain.  Living in a relatively flat part of the world I don't get pinking, but when touring in Wales and other mountainous areas it can.  The 25D4 distributor is a great benefit here as it allows one to retard the timing a degree or two very easily, also if one can't get higher octane fuel in rural areas.

Completely agree with your reasoning behind centrifugal and vacuum advance, but:

"The other is pressure operated, offering advance when the twin turbos raise the plenum pressure above ambient."

Are you sure that isn't vacuum retard?  If the turbos were increasing the pressure I'd expect that to make spontaneous combustion more likely, needing retarding to control.

PaulH.
  ----- Original Message ----- 
     I write to open a discussion about ignition timing. Over on the Triumph forum ( Apologies for using profanity. ) a member wrote, "Just advance the timing until you notice preignition." Now I recognize that Triumphs use farm implement engines, but that advice seems a bit sketchy. So let me write a preface then ask a question.
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