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Old 07-15-2013, 03:51 PM
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Problems with timing curve

My HEI advance seems to be a light switch, on or off. No gradual advance.

I had been using 2 medium Mr. Gasket springs. With those advance began at 2,000 RPM's and was all in by 2,500 RPM's.

Next I bought the Crane adjustable vacuum advance can (not hooked up yet), but I am using the springs in that kit. The recommended starting point was 1 light spring and 1 medium spring. This seems even worse. Advance starts at 1,500 RPM's and is all in by 1,800 RPM's.

What is the deal? Are the weights too light?

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Old 07-17-2013, 12:40 PM
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Originally Posted by Silver Surfer View Post
My HEI advance seems to be a light switch, on or off. No gradual advance.

I had been using 2 medium Mr. Gasket springs. With those advance began at 2,000 RPM's and was all in by 2,500 RPM's.

Next I bought the Crane adjustable vacuum advance can (not hooked up yet), but I am using the springs in that kit. The recommended starting point was 1 light spring and 1 medium spring. This seems even worse. Advance starts at 1,500 RPM's and is all in by 1,800 RPM's.

What is the deal? Are the weights too light?
The springs control and counter the centrifugal weight of the fly weights. For a given fly weight's weight a softer spring allows it fly outward (which takes the advance plate with it) sooner and further. A heavier spring slows this action.

The use of dual spring allows a careful trimming of the amount and rate of advance.

The use of vacuum advance adds to the complexity as it is designed to add advance when cruising at moderate RPMs with little throttle opening. The throttle position of nearly closed results in a low density mixture which burns slowly, so that demand for more burn time is provided by starting the burn earlier. One needs to be careful of where the centrifugal advance starts coming in and by how much as the addition of vacuum advance can push the engine into detonation.

For a race engine more advance sooner is usually desired, but a street engine is a more complex problem where too much vacuum advance with too much centrifigual has to be avoided. So you will need to tune around their characteristics to get the best advance for economy and power at partial throttle cruise while achieving a smooth transition as the throttle is opened and the manifold vacuum thus this advance decreases all the while the RPMs are coming up so the centrifugal is comming in.

You can't really trim this till the vacuum advance is installed. For springs you will need to go with how the engine is reacting rather than the spring makers recommendations as those are just a starting point. Lacking the vacuum advance unless the engine detonates, you would want the centrifugal advance to come all in by 2000 RPM for best economy and torque since there is no vacuum advance right now to cover the cruise RPM and light throttle condition.

Bogie
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Old 07-17-2013, 02:38 PM
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Originally Posted by oldbogie View Post
The springs control and counter the centrifugal weight of the fly weights. For a given fly weight's weight a softer spring allows it fly outward (which takes the advance plate with it) sooner and further. A heavier spring slows this action.

The use of dual spring allows a careful trimming of the amount and rate of advance.

The use of vacuum advance adds to the complexity as it is designed to add advance when cruising at moderate RPMs with little throttle opening. The throttle position of nearly closed results in a low density mixture which burns slowly, so that demand for more burn time is provided by starting the burn earlier. One needs to be careful of where the centrifugal advance starts coming in and by how much as the addition of vacuum advance can push the engine into detonation.

For a race engine more advance sooner is usually desired, but a street engine is a more complex problem where too much vacuum advance with too much centrifigual has to be avoided. So you will need to tune around their characteristics to get the best advance for economy and power at partial throttle cruise while achieving a smooth transition as the throttle is opened and the manifold vacuum thus this advance decreases all the while the RPMs are coming up so the centrifugal is comming in.

You can't really trim this till the vacuum advance is installed. For springs you will need to go with how the engine is reacting rather than the spring makers recommendations as those are just a starting point. Lacking the vacuum advance unless the engine detonates, you would want the centrifugal advance to come all in by 2000 RPM for best economy and torque since there is no vacuum advance right now to cover the cruise RPM and light throttle condition.

Bogie
So do all HEI's go to full centrifugal advance all at once? I was under the impression they would gradually advance.

Until I get the mechanical advance figured out I am holding off the vacuum can...one change at a time.
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Old 07-18-2013, 09:15 AM
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So do all HEI's go to full centrifugal advance all at once? I was under the impression they would gradually advance.

Until I get the mechanical advance figured out I am holding off the vacuum can...one change at a time.
No at least for a street application the OEM HEI brings the advance in pretty slowly starting about 1000 RPM with everything in by 3000.

To trim this with the Mr. Gasket kit, (assuming the 3 spring kit) you will have to try combinations of springs as not all HEIs use the same weight of counterweights (there should be a part number stamped on them).

The fast and short curve your seeing would be for a large cammed engine where there is a lot of base timing combined with a high idle. The remaining curve is then timed to come in quickly and be complete by 2000 RPM.

To slow this down and spread the timing out will take heavier springs or lighter counter-weights.

You also need to check the advance cam and counter-weights for cleanliness, burrs on the parts, and condition of the posts, bushings, and wear pads to inside smooth operation that doesn't get hung up.

Bogie
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