setting initial timing

small block chevy. unknown compression ratio. unknown cam. automatic trans with 3.42 gears. weiand dual plane intake, 600 holley vacuum secondary factory HEI out of god knows what. chevelle manifolds and 2.25 dual exhaust.

since we don't know much about this thing, I have no idea what to set initial timing to. no mechanical or vacuum advance at idle has been confirmed, so were talking pure initial timing.

what methods are there to determine best initial timing? I was honestly thinking of getting the thing to idle as low as possible, and hook up a vacuum gauge. keep cranking in timing until the manifold vacuum starts to drop off, then backing it off 3-4 degrees. what is wrong with this approach? I know the rest of my timing curve will be all sorts of out of whack after that, and I will address that by the same means I always have: dial back timing light to get an idea of total timing, figure out how much total mechanical it can stand before detonation on junk gas/heavy load/high throttle, and then figure out where the curve is happy. then add in as much vacuum advance as I can.

but this post is about initial timing, and how to determine where to set it. please advise. wanna get this done and get the ignition started to be sorted prior to getting the new carb dialed in. currently its at 14 degrees btdc, and starts like theres barely any advance in it. (unlike my mopar that has 12, and will kick back on the starter hot)

Michael

start at 12 BTDC with the vacuum advance unhooked and go from there. if it's a smallish cam, this will be in the ballpark.. if it's a bigger cam, it will want 18 or even 20...

Initial really doesn't mean much. I always set the max timing and let the initial fall where it may. Start at 34 total timing (not counting vac. advance), with 36 being the norm on stock heads. Most HEI's add 20* mechanical advance, FYI. You might as well add a distributor spring kit while you're messing with it, it'll allow the timing to come in much sooner. As it sits you probably won't see max mechanical advance till 4500 RPM or more, when you want it all in by about 2500-3000. Don't change the weights, just the springs. Oh, and hook your vacuum advance up to a manifold vacuum source, not the timed port on the carb. The timed port was an emissions crutch invention.

ross2004 wrote: Initial really doesn't mean much. I always set the max timing and let the initial fall where it may.

Exactly this. Once you have your timing curve where you want it, and max timing where you want it, if it starts and idles, who cares?

ross2004 wrote: Oh, and hook your vacuum advance up to a manifold vacuum source, not the timed port on the carb. The timed port was an emissions crutch invention.

I hear this a lot but still dont get it. The only difference between the two is on ported vacuum, when you jack the throttle open when you need power the most, VACUUM RISES, rather than falling off like on manifold vac. Other than that they read the same. Why would you want less advance when you want the most power?

At least on a fiat 1116 single cam motor with a weberesque carb, the car falls on its face on manifold vac at launch, because its got no timing advance, being ta low revs and seeing no vaccum. Car rips right out of the hole on ported vacuum.

If its a vacuum retard distributor then manifold vac makes sense.

Give the car what it likes I suppose- our SBC Lemons car (350, Holley 600 vacuum secondary carb) idles 100% better on manifold vacuum rather than the timed port. Manifold vacuum will give you advance at idle, when you want/need it, timed won't. This is why the timed port was invented- reduce timing advance at idle to shift the combustion later in the cycle and help with emissions. After that, say light cruise, I agree there's probably very little difference in the vacuum signal between the two. Our automatic 350 would drop about 400 RPM going into gear with the timed port, and only around 100 on the manifold port. It idles so strongly on the manifold vacuum I had a hard time getting it to idle low enough. All this is on a Holley, perhaps Webers are different?

http://www.corvetteactioncenter.com/forums/c3-technical-performance/60830-ported-vs-manifold-source-vacuum-advance.html

This is a stick car, and it idles great once warmed up(no fast idle system, hotwired choke). I run 12* initial on it, 2 more than factory specced. Probably doesn't matter as much on a big v8 auto car that only takes off a few times a day anyhow. When you have 55hp in a street car, you want it all, immediately, often, from a stop.