396 Timing - Chevelle Tech
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post #1 of 9 (permalink) Old Jun 21st, 19, 5:37 PM Thread Starter
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Phil
 
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396 Timing

I have a .060" over 396 big block with large oval port heads, Edlebrock Performer RPM intake, Holley 3310 vac secondary carb, approx 9.5-1 compression, original points distributor, Hooker Comp headers, Flowmasters, 3.73 gear and Muncie M21 trans. My question is, what vacuum advance should I have? Also, what should my base timing be at an idle with vacuum advance disconnected? What should my timing be at an idle with the vacuum advance connected? What should total timing be and what RPM should it all be in? I know that's a lot of questions but, I know you guys have a lot of experience. Will you please help me out?
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post #2 of 9 (permalink) Old Jun 21st, 19, 5:38 PM Thread Starter
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I forgot one more important spec, Comp XE274 cam.
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post #3 of 9 (permalink) Old Jun 21st, 19, 6:41 PM
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Re: 396 Timing

I limit the vac advance to 10*, set total without vac advance hooked up to 34/36* and let my initial be whatever it winds up being. Then with the vacuum advance hooked to manifold it will be whatever it was + 10*.

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post #4 of 9 (permalink) Old Jun 21st, 19, 7:23 PM
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Re: 396 Timing

Quote:
Originally Posted by oldcutlass View Post
I limit the vac advance to 10*, set total without vac advance hooked up to 34/36* and let my initial be whatever it winds up being. Then with the vacuum advance hooked to manifold it will be whatever it was + 10*.

x2
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post #5 of 9 (permalink) Old Jun 22nd, 19, 4:00 PM
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Re: 396 Timing

BAD advice to "let the initial timing fall where it will". There needs to be defined settings, even for initial.

Case in point, last year of the Pontiac 455 engines, distributor had 40 crankshaft degrees of mechanical timing, very stiff mechanical advance springs, and used ported vacuum advance, and, the total mechanical timing the engine was run at was 36 degrees. What was the initial? 4 DEGREES AFTER TOP DEAD CENTER, 4 degrees retarded at idle. Engine ran like it was chained to a very large boulder.

Now, I would think that engine above would like

14 INITIAL
10 vacuum advance, run on full manifold vacuum sourcing, NOT PORTED VACUUM
24 IDLE degrees after engine starts
22 MECHANICAL, for an INITIAL and MECHANICAL total of 36

Build a vacuum advance stop for the degrees on the vacuum advance can to set its 10 crankshaft degrees, no matter if it is stock, or adjustable.

FREE info packet with pictures on how to do the vacuum advance stop 3 different ways, rubber stop, home made metal, Crane 99619-1 adjustable stop plate, send email and ask for it:

[email protected]
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post #6 of 9 (permalink) Old Jun 22nd, 19, 5:37 PM
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Re: 396 Timing

No Dave I'm not wrong and your comparing apples to kumquats. We aren't talking about a Pontiac. Most Chevy HEI come out of the box with around 21* of mechanical advance built in whether its GM stock or chinesium. So 34/36m total-21 mechanical =13-15* initial + the limited 10* of vacuum adv. Wait, we are talking about the same settings... These are a starting point as some engines actually like more initial and/or total timing. Dave, don't get me wrong, I do respect your knowledge. What I don't like about the new distributors is the rate in which the advance comes in 3500-4000 RPM to be all in. I prefer it all in by 3k.
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post #7 of 9 (permalink) Old Jun 23rd, 19, 3:49 PM
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Re: 396 Timing

Oh, PLEASE, timing specifications are UNIVERSAL, there is NO "set the total and don't worry about the initial", not back then, not now, never in the future. EACH area MUST have a definate setting that works, NONE of them can fall to fate, the fate of not being set right, and then falling where they will.

The Pontiac reference was simply to show that when a curve has 40 degrees in it, and the total is set to 36 deg/BTDC, the INITIAL will NOT be where one really wants it to be on an engine.

I AM NOT WRONG.

Lets use your settings above

INITIAL: 15
VACUUM: 10
MECHANICAL: 21

And a target of:

IDLE: 24/25
TOTAL: 36

Initial of 15, plus vacuum of 10, on full manifold vacuum: 25
Initial of 15, mechanical of 21: 36

Basis is good, and ISN'T the disaster the Pontiac was saddled with. MOST GM HEI's were set at an arbitrary 18 degree mechanical curve, for emissions use. Later small blocks used very conservative initial of 8 deg/BTDC. Total for one of them was 6 + 18, for a total of 26

Now, why did that work for those engines? Well, because they were also set up to have as much as 24 degrees of PORTED vacuum advance, to add a second acceleration ignition setup. Depending on the specific cruise vacuum level on ported, any level of vacuum advance degrees, from 0 to 24, could be in use. Assuming the initial was the conservative 8, and the mechanical was to the rpm point it gave 12 of those 18 degrees, (20), and, the vacuum advance was being run at a vacuum that gave it 16 degrees, the combined total would be 36. 8 + 12 + 16 = 36.

Remember, GM HEI was NOT designed as a performance distributor, but as a universal emissions unit, with vastly different curve setups, just at 437 of them, with the ONLY performance curve coming from the late 70's/early 80's trucks and some few suburbans, that went on to become the ZZ series carbureted crate engines. 41 weights were found in Buick, Olds, Pontiac, 375 center, different Buick, Cadillac, Pontiac, Olds, and a few AMC that ran large HEI. Where were those same weights and centers run together, other than those Chevy engines listed above? ZZ crate engines only, NOWHERE ELSE.

How many degrees are delivered with the ZZ set? 22 crankshaft degrees. ZZ4 initial, 11 degrees. ZZ total: 11 + 21 = 32, 11 + 22 + 33. ZZ4 vacuum advance can deliver up to 12 crankshaft degrees, and GMPP orders us to use PORTED vacuum, so, idle stays 11, totl stays 32/32, but, as much as 6 more vacuum degrees can be added at set cruise rpm's, but upon acceleration, up to 11 + 22 + 12 = 45. They run hot, aren't especially drive able, don't accelerate well set up like that. Which subtle changes, the problems go away, 11 initial, 22 degrees of mechaiical, and making a 9 degree stop for the vacuum advance, changing one spring to make the mechanical advance faster, and connecting the now stopped down vacuum advance, those engines wake up, don't over heat, and get reasonably good fuel economy. Some people idle their ZZ4's at 12 degrees, no problems. The springs on the mechanical, as GMPP set the curve up, start point is 1,400 rpm's start, well past 5,500 rpm's limit. Chancing just one of the stock springs, or, using a spring set from a kit, to get start at 100 rpm's over out of gear idle rpm's, and limiting to 3,000 to 3,100 rpm's, does the trick on those setups.

Same setup is used in the ZZ430, which is a ZZ4 with a hotter cam, the above is only off in the mechanical advance speed.
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post #8 of 9 (permalink) Old Jun 23rd, 19, 9:42 PM
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Re: 396 Timing

Quote:
Originally Posted by oldcutlass View Post
I limit the vac advance to 10*, set total without vac advance hooked up to 34/36* and let my initial be whatever it winds up being. Then with the vacuum advance hooked to manifold it will be whatever it was + 10*.
This is what needs to be done. Not everybody has a drawer full of centerplates/weight/springs to give the magical "correct" curve. Set the total and let the initial fall where it does, and if your starter doesn't balk and you have a nice idle and crispy throttle response, your done.

And that cam will want as much idle timing as you can give it, but again, your constrained by what your distrib will give you unless you want to make this a research project.

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post #9 of 9 (permalink) Old Jun 24th, 19, 2:24 PM
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Re: 396 Timing

I used to have an adjustable vacuum can on my HEI and after a lot of experimenting found that my 454 ran best with about 50-52 degrees of cruise timing (12* initial, 20* mechanical, 20* vacuum advance, as well as 14/20/18). I've read accounts of using as much as 54* cruise timing. The diaphragm on that adjustable can eventually went bad and now I have a factory AR10 can, with the same curve. Unlike other adjustable cans, the allen wrench setting on the one I used (part of an older Summit upgrade kit) definitely affected the amount of timing by limiting the travel of the arm, instead of when the timing came in.

With about 51* cruise timing, I haven't had any surging or pinging at part throttle. I've put part throttle load on the engine going up hill with a road divider providing sound reflection and still no pinging or rattling. No bucking or lurching at highway speeds. I used to run the 46-48 cruise timing and noticed that the extra couple of degrees made the pedal pressure extremely light when cruising and that highway mpg increased about 15-20% (about 1-2 mpg, but still something).

Dave, I notice your cruise timing recommendation is about 46*. What's your opinion on running 50*+ if there are no adverse symptoms?
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