When you put a load on the engine the extra advance will go away. If you are simply cruising the additional advane will improve your economy. If you floor the pedal you will likely have 36* total.

 

Humm if true than I would be right where I want it to be. How and why does timing drop with increased load?

Does my centrifugal advance seem about where it should be?

 

If your engine is happy with your mechanical + initial of 36, I would limit total with vacuum to 50. Thats usually the happy spot. The reason why your timing decreases with load is because your vacuum decreases, wot vacuum should drop to 0.

 

WOT vac is normally 0" or very close to 0"Hg.
vacuum goes up as the throttle blades close. Kinda like putting your hand over a vacuum cleaner hose. (Your hand is the throttle blades)

 

Oldcutlass, I believe you answered my main question. "What is a good limit on the total advance with Vacuum". Your saying 50°. I am ~ 42° so this is a good place to start with. If I want to improve power and MPG than I can slowly add in advance to a limit of just before she pings when lugged around.

It I believe also answers my second question indirectly "what is a typical centrifugal advance"? I think I am hearing what I got is "typical" even though there is no such thing as every engine is different on custom builds.

Gaining confidence here thanks again!:hurray:

 

Vacuum advance has more to do with fuel economy at cruise. As most HEI dist have about 18-21 degrees of mechanical advance. You generally can get away with 16-21 degrees of initial. Most V8's like 34-38 total mechanical advance without vacuum with it all in between 22-3000 rpm. With vacuum they like 48 - 56. This has been my experience.

Better to err on the conservative lower end.

 

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1st is " All ref to timing i make here is with motor running with timing light at the crank."

Well 36 deg total at crank when at WOT where vac adv drops out due to lower int vac at WOT is just fine.

Then when you back out of throttle to low load part throttle at cruise where vac adv becomes operational again anywhere from 48-50 (even 52 deg if not detonation) is ok .

Then to get what timing should be when motors at idle just subtract the 21 deg mech in dist from what timing is at low load part throttle cruise to get that as long as vac adv is being run on full int vacuum.

But a better timing curve for your setup would be to use an 18 deg stop bushing in dist and setup to be all in by 2500-2600rpm max.

Then run 18 deg base timing so you would have 36 deg total all in by 2500-2600rpm which gives you a little better idle quality with the additional 3 deg base timing along with better throttle response coming off idle and when at lower rpm too where you have mech/total coming in kind late at 3k rpm making motor a bit lazier.

Then add vac adv of 10-12 deg max to the 36 deg total for somewhat conservative 46-48 deg when all timing is in (18 base + 18 deg mech + 10-12 deg vAC ADV ) and your good to go.

Some setups run 50-52 deg with all timing in at low load part throttle cruise using 93 octane pump fuel.

At idle my rec timing curve should be 18 deg base + 10-12 deg from vac adv hooked to full int vacuum = approx 28-30 deg at idle which is fine with the perf cam your motor has .

But the motor only has 18 deg timing for startup where vac adv isnt functioning yet till motor actually starts producing vacuum after that where vac adv will quickly add 10-12 deg to the timing at idle.

Just ensure there's a proper macth between idle vac and vac adv can so vac adv is fully deployed at idle to ensure motor has a stable idle .

Thats esp an issue if running auto trans when you put it in gear where idle speed is reduced a bit along with idle vac being reduced a bit too at the same time which could in some cases issues if vacuum cut off lvl/spec of the vac adv is higher then motors actual int vac at idle in gear with auto trans which would result in an unstable idle in gear vs no problem in park with higher idle speed and higher int vac.(PHEW!!/LOL!)

Dont get me wrong, your current setup should run ok ,but your motors current timing curve is a bit lazy and the timing curve i rec will not only improve your motors idle quality it will also improve throttle response due to the additional 3* base timing along with mech adv coming in considerably quicker at 2500-2600rpm vs at 3k rpm too collectively together improving perf a bit.

My rec setup could also improve fuel economy a bit and reduce engine temp at idle in traffic on a hot day too.

Use 93 octane fuel with the timing curve i rec ,but if detonation is heard back off/retard base timing by 2 deg to see if that stops the detonation.

Make sure to set base timing with motor at max idle speed of 550-600rpm to ensure mech adv isn't adding timing to base setting,also remove & plug vac line for vac adv when checking or setting base and or total timing too.

Scott