Ignition timing set-up

[Clare Snyder]

>> 
>> Help ... I've done a lot of work rebuilding a Ford 351W for
>> a Bronco and am experiencing similar problems to what I had
>> when I first pitched the old variable venturi carb and EGR
>> for efi and a regular distributor and DuraSpark ignition
>> off a different vehicle.
>> 
>> Here's the problem -- timing!  (at least that's what I *think*
>> is the problem)
>> 
>> If I set the timing to give good low end response (requires
>> 20 to 25 degrees advanced at 600 rpm, no vacuum), I get
>> clatter under load at high rpm's.
>> 
>I've been battling much the same problem with my 351W 
>DuraSpark/MSD-6A combo.  I run a fairly warm cam (Comp-cams
>dual-pattern hydraulic), & recently switched from a Holley
>650 to an Edelbrock 750.  I've tried both ported & direct
>vacuum advance.  I've turned the vacuum advance adjustment
>screw to its limit for least advance, and run direct
>vacuum presently, with about 10 degrees idle advance (mech).  I 
>ended up setting the carb fairly rich to reduce pre-ignition(?)
>at part throttle accel (like trying to maintain speed on
>an uphill grade in 5th).  Then my O2 sensor says I'm a bit
>rich on level ground.  Playing with the timing light in 
>the garage, I see way beyond 30 degrees advance at 3000 RPM
>with the vacuum hooked up, causing pre-ignition.
>
>Like you, best idle is obtained with about 20 degrees 
>advance (vacuum + mechanical).  I wouldn't recommend
>setting idle mechanical advance beyond 12 degrees, or
>your total advance at 3000 will be over 30 (no vacuum),
>and thats not good.  Although idle and startup suffer,
>I've had my best high-rpm operation with the idle
>advance (mechanical only) set around 6 degrees.  Again,
>this keeps the ignition from being over advanced at
>high RPM.  I have a terrible time with idle at 6
>degrees though.
>
>Realistically, most of my problems are due to attempting
>to operate the engine outside its optimum point.  I have
>a cam, carb, intake (Torker-II), and heads (Dart-II) that 
>are intended for 3000-6000 RPM, yet I want to cruise at
>2000 RPM in 5th.  If the Vettes and Camarobirds can run
>so well and cruise at 1500, why can't I, right?  The
>reason I can't is that I don't have EFI and EST, and
>to obtain my HP, I've compromised the low end.  With
>megabucks of engineers, computers, labs, etc GM can
>analyze everything to the last nano-whatever, and balance
>the whole system.  You and I are just hacking around
>the edges, experimenting by trial-and-error, with a
>very limited number of trials.  
>
>Supposedly you can take the distributor apart and bend
>some tabs to limit total mechanical advance.  I've
>tried, and found the tabs very hard to bend.
>
>I haven't tried hotter plugs, but maybe I should.
>
>So whats this have to do with EFI?  Only an understanding
>of what works and what doesn't, and why I need an ECM!
>
>Oh, and another reason not to set your idle advance so
>high is wear and tear on your starter.  I've burned up
>a few starters trying to start a warm engine with too
>much advance.
>
>
>-- 
>Matthew D. Sale,  IC Development Engineer, Delco Electronics Corp.
>msale at iquest.net   http://www.iquest.net/~msale
>'69 Mustang 351W 5-spd (13.464 at 103MPH using cave-man technology).
>
>All responses are my own and should not be mistaken
>for those of Delco Electronics or General Motors.
>
Back in the "good old days" before EGR et al, I used to fool around with
mopars - mostly slant sixes, with a relatively stock 170 cid unit pulling
206hp at the rear wheels at 6000 RPM on the dyno. It was a bit of a dog off
the line, but so what? I found setting the timing to about 4 degrees with
vac disconnected, then connecting the advance to manifold, not ported,
vacuum, gave me the best results. The dist was recurved to 273 hyper pack
specs, more or less, and the valve timing was modified by reducing valve
lash to 1/2 specified, with 15 thou off the head surface. Other than that,
carburetion jetting (with a 225 carb) was about all I had to change. With
vac advance at idle, the idle was smooth. When under load, the vac dropped,
retarding timing to avoid ping. At high speeds, a bit of vac advance came
back in, more at light load, and less under load, effectively eliminating
ping with 89 octane fuel.

You might want to try something like this approach. Try a 351 Boss advance
curve. Set the power jet a bit richer than stock, with main jets leaned back
to stock setting. A restrictor plate on the 750cfm carb may also help, by
reducing the effective compression. This will also tend to provide a better
vac signal for the vac advance (manifold, not ported).

Worth a try anyway.