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Old 06-14-2011, 09:41 AM
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BBC Total Timing

I've been playing around with DynoSim5 (Cheetos powder tastes better than 10W30)...and the ignition timing it suggests for a 496 BBC are a bit surprising.

Almost 43 degrees total advance at 6500 rpm.

Since I've never tuned on a BBC before, and I often hear "38 degrees total timing" on this board, I was just wondering. Is it just the big bore, open chamber, and dome that cause more advance requirements?

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Old 06-14-2011, 10:10 AM
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Quote:
Originally Posted by nofearengineer
I've been playing around with DynoSim5 (Cheetos powder tastes better than 10W30)...and the ignition timing it suggests for a 496 BBC are a bit surprising.

Almost 43 degrees total advance at 6500 rpm.

Since I've never tuned on a BBC before, and I often hear "38 degrees total timing" on this board, I was just wondering. Is it just the big bore, open chamber, and dome that cause more advance requirements?
The 43 degrees total advance at 6500 RPM probably is including the Base (initial) + the mechanical (centrifugal) + the vacuum to = the 43 degrees.

Here is an example:

initial=13 mechanical =20 vacuum=10 Total advance =43

The 6500 RPM is unrealistic as the Total timing should be "all in" around 2500-3000 RPM for most street engines.

When you read or hear "38 degrees total timing" this is usually referring only to the Total Mechanical Timing with no vacuum advance added. The reason the vacuum advance is not added is because #1, it is a variable figure based on the vacuum at varying engine loads and RPM's. #2 is because when going to WOT (wide open throttle) the vacuum drops to 0. This is true whether the vacuum advance is connected to a ported source or a full manifold source. All manufacturers instructions state that the vacuum advance should be dis-connected and plugged when checking/setting the timing.
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Old 06-14-2011, 11:20 AM
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I have seen BBC make max power with relatively high timing numbers. (40 - 43 degrees).

This is due to a wide piston and a slow burn rate (inefficient cylinder heads and domed pistons). The flame has more ground to cover and moves slower so more timing head start is required to finish the burn.

furthermore, there can be errors in the timing marks, +/- 2 degrees is common. So a "set" of 43 can easily be 41 actual.

But, no doubt BBC's like more timing. This is why they need to be set up with a little less compression as compared similar setup sbc.

20 initial, 40 total mechanical, plus another 10 from the vacuum advance (50 total) works well on my mild BBC. I also use the vacuum advance on the manifold source, so it idles at 30 (20 initial + 10 vacuum advance).
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