Ignition timing set-up

Wed Aug 27 18:36:16 GMT 1997

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Its been awhile since I looked at the final advance curve I put in my Holley 4Di
projection system on my 302, 224 at .05 .520 lift 9:1. In the Holley system the 
ignition is set by a map with RPM and throttle position as the x and y 
coordinates respectively.  At idle, 700 RPM and no throttle, the advance is set 
to 10 degrees and ramps steeply to 42 degrees, max allowed, by 3000 PRM.  I have
more than 10 degrees advance at part throttle and low RPM.  I had to run this 
advance at idle to pass admissions testing.  I backed off the advance pretty 
quickly once the throttle opens more than half way as a preventive measure to 
avoid knocking, but I have not tried to set a real performance limit as yet 
since the FMX tranny broke after a couple of months.  I had a lot of problems 
with backfire with the engine cold which was cured by adding in the advance a 
lot quicker.  I guess this lines up with what everyone is saying about getting 
as much initial mechanical advance as possible at idle, this system lets you do 
more than springs and weights, but has no vacuum input, just throttle position. 
me

______________________________ Reply Separator _________________________________
Subject: Re: Ignition timing set-up
Author:  owner-diy_efi-outgoing at coulomb.eng.ohio-state.edu at SMTPGATE
Date:    8/26/97 10:48 AM

>
> 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.

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In-Reply-To: <9708261429.AA23908 at peaches.ph.utexas.edu> from "Tom Cloud" at Aug
26, 97 09:29:52 am
Date: Tue, 26 Aug 1997 10:48:31 -0500 (EST)
To: diy_efi at coulomb.eng.ohio-state.edu
Subject: Re: Ignition timing set-up
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