SBC Timing? - Chevelle Tech
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post #1 of 9 (permalink) Old Mar 24th, 09, 11:36 PM Thread Starter
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Kyle
 
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SBC Timing?

What should the all in timing be on a SB350 and at what RPM? Is it Cam and Combo Dependent?

I have the following combo:
350ci SBC
MSD HEI Stock Style Distributor
Stock 1.94 heads
Eddybrick Performer Intake
1 5/8" full length Dynomax headers
Holley 670cfm Street avenger carb (properly tuned)
Unknown mild street cam
stock 2-bolt bottom end

Well built TH350 w/2600-2800 stall converter
12 bolt rear w/3:73 and locker

Yada, Yada, Yada

I know some of the specs are moot for setting the timing, but I layed it all out on the table just in case it helps.

I have set the initial timing @8 degrees BTDC (vacuum disconnected) as a start. After setting this I checked the timing and it is all in @24 degrees BTDC by 2400 RPM.

Am I way off or is this close to where I should be. I have a weight and spring kit to change the curve of the Distributor and the vacuum canister on the distributor can be adjusted to change the vac advance.

I thought I should be closer to 34-36 degrees all in? Where should I be with this Mini Mouser?

Thanks for any help.



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post #2 of 9 (permalink) Old Mar 24th, 09, 11:38 PM Thread Starter
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Kyle
 
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Re: SBC Timing?

Oh and my vacuum canister is being hooked up to the ported vacuum of the carb and my total intake vacuum @ idle is ~17" hg.



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post #3 of 9 (permalink) Old Mar 24th, 09, 11:57 PM
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Re: SBC Timing?

How high did your rev it while checking total? All of out-of the-box MSD's I've timed didn't reach total until nearer to 4k. If you're getting 17hg, unless you're running a very high idle speed, it also means that your cam is stock or near stock specs. 36 total is a good place to start. Set it there and then check initial. If your want total in sooner or want more or less initial, refer to the MSD instruction manual's chart.
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post #4 of 9 (permalink) Old Mar 25th, 09, 12:27 AM Thread Starter
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Kyle
 
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Re: SBC Timing?

I went to around 4k RPM and there was no change between 2.4k and 4k RPM. According the little yellow tag I found that was originally attached to the Distributor it comes factory with heavy springs to limit the mechanical advance until later in the RPM range, something about limiting detonation on heavily modified engines getting too much advance too fast. Which agrees with what you describe above. But if that were the case why would I get no change above 2.4k rpm's?

As for the cam, it could be stock. I was told by the previous owner that the cam was mild street, but he had his head so far up his a$$ he didn't know if the sun was yellow or blue. Needless to say... I dunno what it is!



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post #5 of 9 (permalink) Old Mar 25th, 09, 10:33 AM
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Re: SBC Timing?

It might be that the springs are too heavy and won't let it advance further until it reaches a higher rpm than you went to. If that is the problem, swapping in the lighter springs should solve it. I haven't set up their HEI but all the others seem to come with a 21 bushing installed. If that's what yours has, 8 initial would result in only 29 total. Typically, I start with 16 initial for 37 total and change springs to get it all in by about 2800. On real light cars with low gears I can bring it in a lot sooner than that without problem. If you need more initial you need to reduce mechanical.
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post #6 of 9 (permalink) Old Mar 25th, 09, 11:00 AM Thread Starter
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Kyle
 
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Re: SBC Timing?

Thanks Jerry. I'll see what I have in the distributor now for springs and bushings. I'll try taking it to 4500 to see if it advances further and post back.



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post #7 of 9 (permalink) Old Mar 25th, 09, 11:26 AM
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George
 
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Re: SBC Timing?

With your combo: 10-12 degees initial, 24-26 degrees centrifugal and 36 degrees total.
'alll in' by 2400rpm is too low for your combo, 3500rpm+ 'all in' is better suited to your combo.
If the HEI is a CCC unit don't forget to disconnect the 'control' wire and discnnect vacuum advance when checking timing
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post #8 of 9 (permalink) Old Mar 25th, 09, 1:05 PM
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Matt
 
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Re: SBC Timing?

Aside from bone stock, my SBC has a very "mild" cam, intake, and carb.

I am able to run 16 initial, 36 total all in by 3k rpm. My vac can is hooked up to ported and adds another 16 during cruise.

It's a low compression motor and runs on 87.
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post #9 of 9 (permalink) Old Mar 25th, 09, 2:06 PM
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Smile Re: SBC Timing?

With mild street cam start with 16-18 base + 18 deg mech in dist all in by approx 2800 rpms ,then you can tweak the timing a deg or 2 up/dn to find the sweet spot.

Then you will have approx 36-38 total by 2800rpms which is a good place to start but the 10 deg base timing you started with is way too retarded by at least 6 deg if not 8 deg .

Base timing being that retarded results in significant power loss esp below rpm that mech adv is fully in/poor soggy throttle responce/wasted fuel-poor fuel mileage/hotter running motor/etc.

Run 93 fuel with this ign timing curve and only try lower oct fuel if the 93 doesnt detonate.

Scott

SCOTT
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2002 MAXIMA (DAILY DRIVER/1 owner,GOT 3/2013 w-44k miles)
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