Need Timing Advice - Team Camaro Tech
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post #1 of 2 (permalink) Old Aug 20th, 03, 09:24 AM Thread Starter
Senior Tech
 
Join Date: Sep 2002
Location: Greensboro, NC, USA
Posts: 109
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All,

I hope it is okay to post here and in the performance section. If it is not, please tell me and I won't do it again.

Anyway, I have a 406 small block with moderately high compression (10:1) and a crower cam, part # 00243, specs are all listed at the end of the email.

My problem is in setting timing. It seems my car likes alot of initial timing - around 19*. With this, my total advance is around 35* (vacuum not attached). I am still messing with how much and what kind of vacuum advance to run. I have an adjustable cannister, so I am leaning toward using a little, say <5*, manifold vacuum. My concern is seeing all the posts that recommend initial timing of roughly 12* advance, total of 34-38*. If I set my initial at 12*, my total, using my adjustable timing light, is around 25* WOT. I certainly don't want any pinging, but 25*? This seems a bit small. I realize that the 12* was for stock applications, so I'd like some advice from those wiser than myself -not terribly difficult- as to a rough idea of what my timing should be. Feel free to recommend anything.

Specs for cam are:

Grind Lobe Center: 284HDP 112*
Advertised Duration: Intake - 286*, Exhaust - 289*
Duration @ .050": Intake 225*, Exhaust - 230*
Gross Lift 1.5/1.5: Intake - .454", Exhaust - .463"

I have an Accel HEI ignition. Currently, I am running an Edelbrock Performer 600CFM carb -I know its too small, but I drive my car everyday, it helps on gas. I have an 350 Auto transmission, with a B&M Holeshot 2000 converter, thinking of upping to 2400.

I appreciate any advice you can give.

As always, thanks and God bless.

Mike J.
67 406ci named "Gideon"
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post #2 of 2 (permalink) Old Aug 21st, 03, 02:14 AM
Senior Tech
Mark
 
Join Date: Feb 2002
Location: LR AR
Posts: 2,372
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How do you know it is set for 19° initial? Something is not Kosher here. You may be idling on the curve due to having the advance springs too soft.

Lets start with a basics. Measure the circumference of the balancer and divide that dimension by 10. Mark a line on the balancer that is just to the right of the TDC groove by that dimension. This new mark is the 36° advanced mark. I like to use a sharpie to make the mark as it is easy to see and won't wash off in the rain. The dial back timing lights can be inacurate especialy if they are cheap. This mark serves the same function but is more accurate. Use this mark to set the timing at the top of the curve from now on.

Take one advance spring off the advance mechanisim and hook up the timing light. Rev the engine up till the timing stops advancing and get the sharpie line to match up with the zero on the timing tab while using the timing light. Lock the distributor down. Install a heavy set of springs in the advance mechanisim. Set the curb idle speed and read your current timing at idle. This is your baseline initial timing spec with no modifications.

A typical performance curve for a 3.75" stroke SBC seems to usualy end up at around 34°-36° as a total of initial plus all the centrifugal. You can increase the initial by reducing the travel inside the distributor centrifugal advance mechanisim. There is a slot in the autocam plate that we typicaly need to shorten to reduce the travel and that will increase the initial. If this seems like a strange way to increase the initial, you are not alone. This is how it is done though. The difference between the new initial and the 36°total is the new centrifugal. If you can get the centrifugal reduced down to around 20°-24° you will have an initial of around 12°-16° and this is a good base line.

Now you want to work on the curve. Start with the lightest set of springs but make sure the initial has not changed at all. If your initial setting has gone up with the lightest springs, you need to go one step heavyer until that goes away so you are not "idling on the curve". The curve needs to start advancing couple hundred RPM higher than the idle speed, say about 1200 RPM or so.

Now, if you hear ping at WOT, add one stiffer spring till it goes away. Once it is gone, you are done. Hook up the vacuum advance to the ported tap and drive it around. If the vacuum advance causes any ping or drivabillity issues, you may need to back off the distibutor a degree or two but any more than that and I would be looking at changing out the vacuum canister but that is realy just the "icing on the cake".

I personaly like to hook up the vacuum advance to manifold vacuum but let's get your distributor curved up right first.

-Mark.
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