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Why were 1935-on Chrysler products timed ATDC?


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Here's something none of us can understand.   We were talking about timing, noting that most immediate prewar US cars had tune up specs showing 2 to 7 degrees BTDC,  other than the 260-ci ohv nine-mained Nash straight eight up to 15 degrees BTDC 1935-37, nine degrees 1938 or '39.   But the point is,   all in MoToR's Manual were and are Before Top Dead Center   e x c e p t   Chrysler products.  
  Why would Chrysler products alone call for tuning specs of  ATDC (after top dead center)?     The laws of physics,  those for four-stroke i.c. engines don't  change.    No other automaker lists anything other than BTDC.    For example,  according to the 1935-42 and 1935-53 MoToR's Manual,  1935-38 Plymouth 4 degrees ATDC, 1939-42 TDC;   1935-36 DeSoto Airflow Six 5 degrees ATDC,  a couple less ATDC 1937-42.   This ends the  War II lower octane gas idea. 

  So our question remains.  In fact, all the more mysterious.   Why just Chrysler of all automakers?

Edited by Su8overdrive (see edit history)
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I assume you are referring to the 323 Chrysler eights...

I too have wondered about the two degree's ATDC setting. 

My 1953 Didge truck with the big 413 six is also 2 degree's ATDC.

Two barrel Stromberg AAV carbs were used on the 1946 thru 47 Chrysler eights...a single barrel Carter Ball and Ball on the 1948 engine. Both variations still set at 2 ATDC.

But 1949-50 Chrysler eights with the single barrel Ball and Ball carbs were now set at TDC.

I have always set them a couple degree's before TDC especially needed  now days because of the ethanol fuel.

Wish I had a good answer for you.


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Wild guess: Maybe the advance mechanism(s) in the distributor was designed for more advance when running. A static setting of little after TDC might help in starting but as long as the total advance (static setting + mechanical advance + vacuum advance) gets you where you need to be once the engine is started it should run the same as all the other brands.


I've not looked at the advance curves for very many cars so I can't say the above guess has any merit. Just tossing it out as a possibility.

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If I remember correctly the vacuum advance on these cars was disconnected when you set the static timing at idle speed. The vacuum supply is manifold vacuum so when you reconnect the vacuum line the distributor advances the timing  for the full vacuum advance degrees. When driving the vehicle the vacuum drops as load increases and retards the timing from the full vacuum advance. Mopar must have found that the engines started better hot with timing retarded or another similar benefit came from the retarded base timing.


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