It depends on the stiffiness of the springs and the weight of the weights. On re curve kits they usually have a set of lighter weights and also different springs with difference stiffness rates and that allows for different timing curves. On my Hei distributor for example I bought a kit that has a set of light, medium and super stiff springs. I use a medium set of springs with the weights that came with my distributor since they were better quality then what came in the kit
Mine starts to advance at about 1200 rpm and will not be all in till around 3000 plus prm and is all in by 3500 rpm at the most. I have tried light springs but it makes my timing come in to fast and caused issues with my initial timing and would make my engine buck and surge because it would come in to fast. It can vary on the kits on what springs and the weights will do what. On a bone stock unit from the 70's they would not start to advance till way up in the rpm and not fully advance at all unless you were reviving it up so high it would blow the motor up from what I have read.
Stated on the old smogger days they would not full advance till over 5000 plus rpm but I can't say for sure as I was just born and everything I ever came across has been aftermarket and they always have a mild performance timing curve to them and it varies on the brand.
This is one of the better write ups on HEI advance curves. The bone I have to pick with it is the usual addition of base, centrifugal, and vacuum advances equal some super huge total advance. While the capability of having everything at max advance is mechanically present, typically engine operation prevents centrifugal and vacuum from being maxed out at the same time.
As the throttle opens manifold vacuum falls taking the vacuum advance downward with it. This happens whether the advancing throttle nets an RPM increase or not. So vacuum advance is not linear to RPM but to throttle position and thus loosely to load on the crankshaft.
Centrifugal advance is strictly controlled by engine RPM as that affects the force ratio between the counter weights and their spring force. This can be impacted by changing the weights and or the spring force. Not all HEI’s are equal in either or these as delivered from the factory so just changing springs may not net the change your looking for by either too little or too much.
The big headache is getting the timing of the vacuum going down to the centrifugal coming up such that you don’t meet places where the combination is too much, a common cause of mid throttle ping. This suggests the validity of using an adjustable vacuum can as well as fudging around with the centrifugal system.
Big cams want more base advance, getting beyond 10-12 degrees of static base timing will require an equal amount be removed from the centrifugal by way of bushings or screw stops depending on your HEI’s exact advance system design.
When you set base timing, lower the idle to about 600 so you ensure there is no mechanical, and then set it to about 12 degrees. You might even be fine with up to 16 initial with a stock engine. Then i suggest connecting the distributor advance to full vacuum to get better idle speed.
I would also take the carburetor off and check the transfer slot exposure. Adjust the idle screw to get the right exposure (usually the slot looks like a square). Then reinstall the carburetor and with the vacuum advance disconnected and plugged, use the mixture screws to set best idle.
If the transfer slot exposure is way out of whack it can throw the mixture off during the transition from idle to cruise, which may cause a bog or stalling.
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