Ignition timing set-up

[Gary Derian]

 I think you're on the right track with your last statement.  You need a
"short "distributor curve that begins at 20 to 25=B0 and ends at  30 or s=
o.
I don't know all about  Fords but Chevys like 36 to 38=B0 max.  High
performance cams lose cylinder pressure at low rpm and can therefore
tolerate more low speed spark advance than a stock setup.

Usually, the onset of detonation is the limit for spark advance but it is
possible to have optimum timing less advanced than the detonaton limit.  =
I
you had 8:1 compression and used 108 octane fuel, the optimum advance cou=
ld
not be determined by listening for detonation.

When I set up my engines, I use low octane fuel and listen for detonation.
Make sure your mufflers are quiet enough to hear the engine noises.  Mayb=
e a
knock sensor could be used with a scope to see detonation.  Anyway, wigh =
the
engine warmed up, make an acceleration run from idle to redline in 2nd or
3rd gear.  The goal is to have a consistent amount of detonation the whol=
e
way up.  This will determine the optimum advance curve for full throttle.
Experiment with advance weights and springs to achieve this.  Mark your
damper and measure your advance curve with a timing light for each test.
This will keep you on track.  Once the optimum curve is achieved, retard
slightly so there is no or almost no detonation.  Then add vacuum advance=
,
as much as possible to improve part throttle economy.

This is much easier to do with an engine dyno and lots of computerized st=
uff
but it can be done with an ear and a timing light.

I have heard many times that lack of backpressure causes a loss of torque.
I don't believe it for a second.  Backpressure can be tolerated but has n=
o
benefits other than a cheaper and lighter exhaust system.  Anyway, at low
rpm, even a restricted exhaust offers little backpressure because the
exhaust flow is low.  Exhaust pulse timing can be changed by changing
exhaust systems which can affect carburetor calibration.  Make sure you
install a balance pipe in your new exhaust about 36 inches from the engin=
e.
This will smooth out the exhaust sound so you need less muffler for the c=
ame
noise level and provides a spot for pressure waves to reflect back to the
engine to help tuning.

Gary Derian <gderian at cybergate.net>

-----Original Message-----
From: Tom Cloud <cloud at peaches.ph.utexas.edu>
To: EEC-EFI List <eec-efi2 at eelink.umich.edu>;
diy_efi at coulomb.eng.ohio-state.edu <diy_efi at coulomb.eng.ohio-state.edu>;
Bronco Group <bigbroncos at off-road.com>
Cc: Todd Knighton <knighton at net-quest.com>
Date: Tuesday, August 26, 1997 11:59 AM
Subject: Ignition timing set-up



>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.
>
>If I back the timing off to 10 degrees at idle (closer to a stock
>engine setting, it runs okay at high rpm's but is disappointing
>at the low end.
>
>I've set the timing to a compromise position of 18 degrees
>and now I hear something that "could" be something loose
>rattling (that's really what it sounds like) at exactly
>"neutral" throttle (i.e. no accel/decel) above about 1800
>rpm.  This bothers me, as I'm afraid it's pre-ignition (or
>whatever it's called   8^)   so I'm going to set the timing
>back some more before I drive it home this afternoon.
>
>Here's the questions
>
> - does anyone know which way to turn the adjustment screw
>inside the Ford vac advance cannister (well, obviously you
>can turn it *either* way  ;-)   .... but which does what ??
>
> - How should timing **really** be set up ??  I know people
>who rebuild engines and race and they set their base timing
>at least as far advanced as I if not farther (they set it
>up until it kicks back starting and then back it off a tad).
>
>I had this problem with the stock engine (well, it had a
>Performer intake and had the EGR removed).  Now I've got
>a "Lightning" short block (supposed to be 8.8:1 CR) and
>an E-303 roller cam with headers (still have the 2" dual pipes --
>but not for long, going to 2.5" duals) and GT-40 cast iron
>heads, C-6, stock stall convertor and 3.50:1 diff.
>
>I keep hearing that the lack of exhaust back pressure
>might be the cause of my lack of low-end torque (which
>I can get back by advancing the timing, BTW) but then
>I've read that relieving exhaust back pressure **never**
>hurts low-end torque -- only that the engine needs to be
>set up for it .... that removing exhaust back pressure
>**always** increases torque and hp at **all** rpm's --
>and I've seen dyno curves to support that claim.
>
>It seems to me that I need a distributor curved to start
>at 15 to 18 degrees advance and go to a maximum of 30 degrees
>or so at 2500 rpm ????    And then, how should the vacuum
>canister be connected (manifold or ported/"spark") -- and how
>much advance/retard should it introduce ??
>
>If you can help me with this, I'll be eternally grateful and
>will tell all my friends (oops, friend  ;-)  how wonderful
>you are  ;-)
>
>Tom Cloud
>
>        He laughs last thinks slowest
>