View single post by Mark Rosenbaum

Posted: 10-13-2005 03:37 pm

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Mark Rosenbaum

Joined:	03-12-2005
Location:	Kingman, Arizona USA
Posts:	532
Status:	Offline

It's my understanding that the various timing changes were the factory's response to the changes in emissions specs (primarily in the US) for the 1973, 1974, and 1975 model or calendar years (depending on how various local laws were written). Do recall that the level of understanding regarding exhaust emissions was far less back then.

For the record, a careful reading of the shop manual along with other factory documents gives me the following values for the static timing on the JH and GT:

Cars with Del'lortos:
     1972: 12°BTDC, all markets
     1973: 12°BTDC, all markets
     1974: 12°BTDC, all markets (chassis to 18900)
     1975-up: 02°BTDC, all markets (chassis 18900 up)

Cars with Strombergs:
     1972: 10°BTDC, all markets
     1973: 10°BTDC, all markets (engines to 4803)
     1974: 08°BTDC, all markets (engines 4803 up, chassis to 18900)
     1975-up: 08°BTDC, North America and Japan (chassis 18900 up)
     1975-up: 05°BTDC, Europe and Australia (chassis 18900 up)

To further confuse the issue, the cam timing changed from 115° MOP to 110° MOP at engine 4030, and again to 110°/100° MOP at engine 10480 (for North American cars only). Additionally, it appears that many (not all) 1975-up cars also had the air injection pump, even when not required for a particular market, and it's my understanding that the final cam timing worked less than abysmally only with these air injection engines.

IMHO the only reason to pay any attention to any of these numbers is when one must comply with regulations that involve ensuring that settings remain at the factory values (whether or not those values actually do result in low emissions).  When that is not the case, one should use whatever settings give the best overall engine performance. For a stock low-compression 2 liter 907 engine with the 'C' cam -- the vast majority of JHs -- this happens to be with a 110° MOP and ignition timing 10° to 12° BTDC regardless of carburetors used. If, for whatever reason, it develops that an individual car happens to run better with different settings, use them instead -- nobody¹ will mind.

About the only thing one really needs to be careful of is to ensure that the ignition spark never occurs earlier than 38° to 40° BTDC at high engine speeds (again, for stock engines), as the engine will likely ping ('pink') or even knock (detonate) if the timing becomes further advanced. Fuel octane, type of ignition system, spark plug brand and heat range, cam selection, intake and exhaust porting, supercharging, and mechanical compression ratio, will all affect this limit, so again, different settings may apply to individual cars.

1. Smog nazis excluded, of course.