Re: difference between compression stroke TDC and expansion stroke TDC

[Scott Fisher <sfisher71@domain.elided>]

--- Douglas Kunzman <doug.kunzman@domain.elided>
wrote:
> Hi
> 
> What is the easiest method to tell the difference
> between TDC compression
> and expansion on a 84 Spider ?

I presume you mean compression and exhaust, as
expansion immediately follows TDC on compression.

One simple, sure fire way that uses no tools: pull the
#1 plug and have an assistant crank the car while you
hold a thumb or finger near the spark plug hole.  When
the air poofs out of the plug hole, that's
compression.  At least that's how we identified #1 TDC
on the compression stroke when we replaced the
distributor in my '74 Spider a couple months ago.  No
doubt we'll now hear all sorts of learned explanations
of why it can't work. :-)

I'll add that this is a good approximation for
learning which is the orientation of the #1 plug wire
when you change distributors.  Having determined
through feeling the compressed air that you're on the
compression stroke, you should still use the timing
marks on the crankshaft pulley to dial in the
distributor for the right timing, both static and
dynamic.  For that, the old test-light-on-the-points
trick is still effective to get to static timing; for
more info on that (or on the ohmmeter equivalent), see
http://www.alfaclub.org/techstff/tuneign.htm.  

Dynamic timing will vary from year to year, but is
critical to good performance; in my celebratory test
drive with the car running but set just to static
timing, it was enjoyable, but setting the timing to
full advance at 5000 RPM as in the final steps of the
referenced Web page, it was a lot more fun.

Best,

--Scott Fisher
  Tualatin, Oregon
Great stuff seeking new owners in Yahoo! Auctions! 
http://auctions.yahoo.com

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