Timing - Team Camaro Tech
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post #1 of 9 (permalink) Old Jun 13th, 19, 09:21 AM Thread Starter
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Randy Brutout
 
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Timing

Gentlemen,


I have a 1968 Camaro SS with a 396, 375hp, manual transmission. Can someone tell me what the initial timing is set at?


Thank you!

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post #2 of 9 (permalink) Old Jun 13th, 19, 10:08 AM
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Re: Timing

according to the book I have
8-396 375hp
43N -.035 plug + gap
.019 points setting
28-32 dwell
18436572 firing order
10 BTDC
750 5 idle with AC on
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post #3 of 9 (permalink) Old Jun 13th, 19, 10:36 AM
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Re: Timing

Chevrolet specs per the Chassis Service Manual is 4 degrees before top dead center.

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post #4 of 9 (permalink) Old Jun 13th, 19, 11:51 AM
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Re: Timing

How to Time a Big Block Chevy
by Don Bowman

Big-block Chevy timing is pretty well standardized unless major modification have been done. The timing on stock 396, 402, 427 and 454 engines is 32-34 degrees total advance at 3500 rpm. This is for an engine with stock camshaft, heads, stroke, and a compression ratio of 10:1. The high-performance versions are slightly higher with a maximum advance of 38 degrees on premium fuel, if the camshaft warrants it. It depends on the overlap of the valves as in a split centerline and total advance on the intake valve. Total advance is the most important setting; however, the engine runs better at an idle and off-idle retarded to an average of 18 degrees BTDC with the vacuum advance operating.
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Originally Posted by JohnZ
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph

Last edited by flat tire; Jun 13th, 19 at 12:23 PM.
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post #5 of 9 (permalink) Old Jun 13th, 19, 12:28 PM
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Re: Timing

Chevrolet specs per the Chassis Service Manual is 4 degrees before top dead center. John states 10. It's up to you. (But between you, me and the lamp post, I would go with what Kevin posted from John; use full manifold vacuum, not ported, and a vacuum advance can that's "all in" at 8mmHg)

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post #6 of 9 (permalink) Old Jun 14th, 19, 02:10 AM
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Re: Timing

How come full vacuum is used over ported? Excuse my lack of understanding on this. I always figured that ported vacuum would allow advance to gradually advance the spark as the engine rev'd, and not bog an engine with full advance at idle.

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post #7 of 9 (permalink) Old Jun 14th, 19, 03:18 AM
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Re: Timing

Full manifold vacuum is the best way. Ported was introduced later strictly to lower emissions which innturn had a negative affect on performance.

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post #8 of 9 (permalink) Old Jun 14th, 19, 06:36 AM
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Re: Timing

Quote:
Originally Posted by bucko View Post
How come full vacuum is used over ported? Excuse my lack of understanding on this. I always figured that ported vacuum would allow advance to gradually advance the spark as the engine rev'd, and not bog an engine with full advance at idle.

Here is the full article from John Z that should fully answer your question with all the gory details:
Attached Files
File Type: pdf timing101.pdf (2.09 MB, 5 views)
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post #9 of 9 (permalink) Old Jun 14th, 19, 06:43 AM
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Re: Timing

Hooking your vacuum advance canister to a full time vacuum source is the preferred method for optimum engine performance BUT... be careful if you connect your vacuum advance canister to a full time vacuum source without knowing what your initial timing is set to (no VA), total mechanical advance, how much crankshaft degrees of advance the vacuum canister will provide, and at what vacuum level will the vacuum advance canister be at full pull (all vacuum advance achieved).

I'll use a popular performance advance curve as an example: 15 initial timing at idle (vacuum advance disconnected) and 21 mechanical advance - for a total of 36. At idle, manifold vacuum is at 12 Hg.

A lot of vacuum advance canisters can provide up to 20+ degrees of additional advance. If you just connected your vacuum advance canister to a full time vacuum source and did not control the amount of vacuum advance degrees with a limiter plate, (using the performance curve shown above) you could have too much initial advance at idle and too much total advance while part throttle cruising.

When connecting your vacuum advance to a full time manifold source, you also need to look at what vacuum level (Hg) all of the vacuum advance is in by. If your vacuum advance canister needed 15 Hg of vacuum to fully advance/completely pull-in... but your engine was only achieving 12 Hg at idle, you will most likely have an inconsistent/rough idle due to the rpm fluctuations caused by the vacuum advance constantly changing the amount of advance degrees... especially noticeable if you are running a larger duration cam.

By installing an adjustable vacuum advance canister with a VA limiter plate, you can control both the amount of advance the VA canister is providing (degrees) and at what vacuum level (Hg) the vacuum advance canister will be at full pull/completely advanced.
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