Miata Mailing List: September 1993, Message #109
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   From: capek@peach.rtsg.mot.com (Tom Capek @708-342-5639)
Subject: oil filters
   Date: Wed, 15 Sep 1993 09:59:12 -0400

Just got delivery of several oil filters from Honey Mazda.  I hate to burst
your people's bubble, but stock Miata filters do NOT have an anti-drain back
feature.  Just look inside the central core, it goes the entire length of the 
filter.  If it didn't, then I'd believe there might be this feature.  Supporting
arguments:

1) When removing filters, no oil drains out.  Right!  It's all drained back 
into the pan.

2) Some people hear lifter clatter for about three seconds after startup.  
Right!  The oil pump is in the process of re-filling the filter and the oil 
passages and hasn't lubricated the lifters yet.

3) There is a slight delay until oil pressure builds up upon startup.  Right!
Empty oil passages and filter essentially offer no restriction to the flow of 
oil until they are full, then allowing oil pressure to build up.

I didn't check, but most domestic filters that I've seen have a diverter valve
at the opposite end of the threaded end that will allow oil to bypass the
filter media if it is clogged, thus maintaining oil flow, but with unfiltered
oil.  I'll look to see if Miata filters have this feature.

Finally, due to the superior cling properties of synthetic oil, lifter clack
is reduced because more lubricant remain on these parts after shutdown.  Just
some of my observations learned in Life 101.

tom capek

p.s. 14BTDC timing doesn't seem like an upper limit on my car.  Will advance
in steps until either difficult hot starting or knock occurs - reports to 
follow...