Recurved dizy question

[CorAce]

So I've heard the term "recurved dizy" on here before but I don't know what it means. can any one shed some light on this for me.

[datzenmike]

The distributor has two types of advance... three if you include the static timing done with a timing light. In addition there is a vacuum advance and a mechanical advance.

 

1/ Static timing never changes. It is set at idle speed and is about 12 degrees on an L series engine and overall timing may increase by added vacuum and mechanical advance but will never go below 12 degrees.

 

2/ Vacuum advance is regulated by manifold vacuum and is load dependent. Light throttle has high vacuum and advance while heavy throttle is low vacuum with very little advance.

 

3/ Mechanical advance increases with RPMs

 

To get the most energy out of the hot expanding gasses in the cylinder the peak pressure should be reached at about 17 degrees after the piston passes TDC and is just beginning to drop down the bore. Because the burning fuel/air takes a finite time to expand to it's peak pressure it has to be started burning before TDC. As RPMs go up the time shortens so the ignition point must be moved forward more and more. A set of weights is spinning inside the distributor. The faster it spins the harder it tries to throw them outward from the spinning shaft. This outward movement can be used to move the advance forward and is directly related to RPM.

 

If only it were that simple we could stop here..... but it isn't. When operating at part or closed throttle the cylinder does not get filled. In fact there is often just a whisper of air and fuel getting past the closed throttle plate at idle. How does this affect the burn time? Well when the cylinder is full from full throttle and the air compressed tightly, the molecules are closely packed and when ignited, the flame quickly jumps from one to the next across the combustion chamber. At idle or light throttle the molecules are very loosely packed and the flame takes longer to jump from one to the next. A longer burn time means that the ignition timing must be advanced in a direct manner depending on cylinder filling. Light throttle... more advance...heavy or full throttle less or no extra advance. Well luckily the manifold vacuum follows exactly how much the throttle is open or closed. Vacuum from the manifold is used to vary the ignition timing in direct response to throttle and cylinder filling.

 

The ignition advance is not always linear. To make matters worse proper ignition timing tends to create pollution so distributors are 'curved' and tend to be retarded slightly to reduce emissions. In fact all carbs made since the '60s have no vacuum advance at idle. Remember what was said about low cylinder filling needing more advance? Well idle would be much better if there was about 4-8 degrees of extra advance. Next time you have your engine timed, turn the distributor clockwise in the advanced direction. You will note that the idle speeds up and runs much smoother because this is what the motor wants but isn't getting. Smog carbs retard at idle. Engines want MORE advance at idle/low and part throttle when the cylinder is not filled.

 

Now to your question... The distributor weights and springs that are part of the mechanical advance can be changed or modified to bring in more advance earlier giving the engine the advance it wants and making more power and torque. The vacuum advance can be changed to run directly off the intake manifold and it's 'curve' tailored to dovetail with the mechanical advance. After all you don't want the two to add up to too much advance and cause pinging at part throttle. At full throttle there is little or no vacuum advance and only mechanical advance.

[CorAce]

And there you go! You make it all sound so simple Mike. If a person was going to remove the vac advance because of mods, like lets say a turbo or supercharger or even bike carb with no vac port, could the dizy mech advance be changed to compensate? And if so who can do this type of mod?

[ggzilla]

You can't compensate for removing the vacuum advance, if removed you will lose a bit of power on a naturally aspirated engine. The mechanical advance is for a different purpose.

 

A boosted engine can't handle as much advance, so you have to restrict it. That is also called recurving, but the purpose is to retard the spark compared to a non-boosted engine.

 

Normally when people talk of recuriving, it is about getting more advance more quickly, for maximum HP at any throttle opening or RPM

[datzenmike]

The vacuum advance could be left alone. For one thing it only affects light or part throttle. During acceleration into boost you will have full throttle (with very low intake vacuum building up into positive pressure) as the revs and boost build. The same with mechanical advance it also is pretty much 'all in' before 3K and again, if boosting you will likely be way way above this.

 

 

 

A boosted engine can't handle as much advance, so you have to restrict it. That is called recurving too, but the purpose is to retard the spark compared to a non-boosted engine.

 

Exactly, remember that the tighter the gas and air molecules are packed into the combustion chamber (whether by compression ratio, turbo or super charger) the faster it will burn. A faster burn time means that you can start the ignition process much later (retard) in order to have the peak cylinder pressure on or about that 17 degree ATDC 'sweet spot' to push down on the piston.

[CorAce]

Ultimately I'm interested in adding a draw through SC. And from what I'm gathering the stock dizy should work fine as is, even without the vac advance. All though I suppose I could reference vacuum from the intake side of the SC depending on what carbs I use.

[ggzilla]

nah, you need vacuum from the intake valve side.

 

Any engine runs fine without vacuum advance, but runs a bit better with it. For supercharging most just leave it disconnected.