Author Topic: 403 Ignition Timing  (Read 4155 times)

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Offline Jefe

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403 Ignition Timing
« on: September 24, 2020, 06:41:47 PM »
A couple of weeks ago, I finally fired up the new 403 in my Formula. Followed the factory spec for setting timing and found out a couple of things in the process, most of you probably already know these things, but it was new to me. Base timing per spec is 18 deg BTDC with vac can disconnected and idle at 1100rpm, which is where I set it. I then checked the mechanical advance up to 3000 rpm and found that it is offering about 10-11 degrees advance at that engine speed (it's a stock distributor that I rebuilt). Then, when I reconnected the vac can, lowered the idle to spec and checked the timing again, I found that the vac can was giving an immediate 24 added degrees of timing at idle, which indicates the vac source is definitely manifold vacuum. So my base timing with vac advance connected, showed 42 degrees advanced. I freaked out. Everything I've read says 36-38 max total timing by 2800-3000 rpm. I'm literally running 52 degrees advanced at 3000 rpm with no load. After spending a couple of hours searching around, I finally found an article that addressed this situation. When there is little load on the engine, such as when the car is sitting still or even cruising out on a flat highway, there is vacuum available and that much timing is absolutely normal if you're running a vac advance on your distributor. When you step in to it or begin climbing a hill, load is placed on the engine which decreases available vacuum and the vac advance drops out as the available vacuum drops off.
Anyway, just wanted to post this here for anyone else like me who might not have been aware of how this stuff works...
« Last Edit: September 24, 2020, 06:44:29 PM by Jefe »
Current project
1979 Firebird Formula / in the family since new
Previous projects
68 Chevelle
72 GTO
78 DKM Trans Am #127

Offline b_hill_86

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Re: 403 Ignition Timing
« Reply #1 on: September 24, 2020, 07:18:23 PM »
Good info for someone who may be trying to get a handle on ignition timing

1977 Trans Am 400 4 speed hardtop

Offline Wallington

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Re: 403 Ignition Timing
« Reply #2 on: October 09, 2020, 07:02:37 AM »
who sticky's posts on here?