Even with a totally original engine i.e. worn parts the original
recommendations for timing may be of no use given that many characteristics
will have changed but more importantly the fuel you are using is probably
very different now to then. The original spec for a 77 with CEI (41813 on
an 18V 883/884 or 890/891) was a dynamic of 10 degrees at 1500 rpm which
gave a max advance of 15 degrees at 2600 rpm, vacuum disconnected in both
cases. This is very little centrifugal advance compared to North American
low compression engines, which could be as high as 40 degrees at 4800 rpm,
and is probably down to the difficulties in meeting the emissions
requirements in later years. The vacuum capsule should start at 3 inHg and
give a max advance of 24 degrees at 11 inHg, all in crankshaft degrees not
distributor, the capsule should be marked '3-11-12'. Vacuum advance was
manifold derived and available in 4th gear only.
The simple answer is that you run it with as much advance as possible
*without* pinking at any combination of load, revs and throttle opening.
The long answer is to put it on a rolling road and determine the best curve
for your particular engine, then tweak the distributor springs and advance
stop till you get it. Time consuming enough with a 'performance'
distributor without vacuum advance, much more so with vacuum advance at
anything other than full throttle. But with the original distributor you
are still going to be down on centrifugal advance and so be losing power
(and gaining heat) at higher revs. Some places offer the so-called
'Eurospec' distributor as a cure-all, but many of these are the 41610
distributor used in the UK until the end of production which gives a max of
24 degrees at 6400 rpm, with the same vacuum capsule as yours, but this was
for a *high* compression engine. As such it can only be a poke in the dark,
and whilst probably an improvement it will still have to be set by trial and
error for no pinking, and even then probably only touch the ideal curve for
your engine at one places on the curve.
PaulH.
----- Original Message -----
From: <mark.d.melvin@jpmchase.com>
To: <mgs@autox.team.net>
Sent: Wednesday, November 12, 2003 9:47 PM
Subject: Found some history on my '77 / Piper 270 cam / Timing startpoint?
> So to the topic of timing. What does the list say I should do for timing
on
> a '77 overbored 18V with the Piper 270 Cam, a CEI electronic distributor
> (proper for '80, but was retrofitted to this car), and the DGV carb? Web
> resource I found says the CEI distributor should be set to 10 BTDC at
1500,
> as this distributor has a wide vac advance configuration.
>
> What's a safe starting point for the timing? 10BTDC at 1500 seems awfully
> retarded to me.
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