In one fell swoop Crane has caused us to re-evaluate our conventional distributor vocabulary. With their new unit, the terms 'centrifugal' and 'mechanical' advance have lost their meaning. Why? Because the design, which simplifies distributor re-curving almost beyond belief, has no centrifugal or mechanical advance in it. Instead, all the engine rpm and vacuum related advance is handled by 12 different dial-selectable built-in electronic programs. For this reason-you won't see it in the Jegs Engine Masers Challenge, but we wanted you to see it. Nine of these are rpm related while the other three are vacuum related. Before we look at this unit's ability to meet curve requirements, let us first go through the factors which most affect the curve required.
Cylinder Pressure: The Number One Influence
The reason an engine requires an ignition advance curve is that the speed of combustion varies greatly with the prevailing circumstances. As rpm increases, there is less time for the charge to burn. This, for optimum results, calls for the combustion process to be started earlier. Countering this is the effect of mixture agitation, which increases with rpm. These two factors leave us with the basic advance curve concept. In essence, the timing has to advance as rpm goes up but this levels off as the effect on increased mixture agitation speeds the rate of combustion.
The overriding influence on ignition timing requirements at the prevailing rpm is the cylinder compression pressure. To get a handle on what might be needed for an advance curve in a normally-aspirated engine, we need only look at the two biggest factors affecting the compression pressure. These are the compression ratio and the cam timing, and here's how they affect the burn rate.
The higher the pressure of compression, the faster the charge burns. Higher burn rates produce optimum results with less advance. The primary factor influencing compression pressure is the engine's static compression ratio. The influence the compression ratio has is also closely allied to the cam duration, or more accurately, the point of intake valve closure (Fig. 1).
If the compression ratio goes up while all else remains constant, the amount of advance for maximum output will come down at all points in the rpm range. If a short cam is used, increased compression can have a substantial effect on the advance curve at low speed. If the compression ratio is increased and the engine uses a short duration cam then the initial advance needs to come on much slower as well as total advance being less.
If the cam's duration is increased the compression pressure will drop. This slows the burn, so calling for more advance is the preferred move (until pressure wave and ram filling of the cylinder at higher rpm cause the compression pressure to increase). The effect of the longer cam is to bring about the need for the initial advance to come on quicker but total advance may not change that much.
All the aforementioned applies to an engine operating at wide open throttle (WOT) with the advance curve being totally rpm related. (That's what we would have called mechanical or centrifugal in a regular distributor.) At less than WOT (idle, cruise etc.), the amount of air entering the cylinder is reduced which in turn reduces the compression pressure. As manifold vacuum increases the amount of advance needs to increase. At idle and low-speed operation, the amount of advance required to most effectively utilize the air and fuel entering the engine can be as much as 50 to 55 degrees. This is handled by the vacuum advance; a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen up and listen up good. A functional vacuum advance is the single most effective camshaft tamer you can get. By taking the time to hook up the vacuum advance to a manifold vacuum source you can get a big cam to idle as if it were about 20 degrees less than it really is. Conversely, if you are looking for a decent idle the use of vacuum advance will allow you to use a cam of, at the very least, 5 degrees more duration/overlap than would otherwise be the case.
Okay, with some idea of what might be called for in the way of an advance curve, let us take a look at what Crane's electronic distributor supplies. Basically, we can break down the advance curves available into three groups. In group one we have moderate to high performance street; group two, mild to all-out race; and group three, vacuum.
Fig. 2 shows the rpm referenced curves of group one. An experienced eye on these indicates that these curves, when factored in with a static advance of between 5-10 degrees, will deliver a total timing of 29 - 34 degrees along with a close-to-ideal curve for something in the order of 90-95 percent of pump gas burning street motors.
Fig. 3 shows the curves most likely to be used on a big-cam high-compression race motor. With 8-15 degrees initial, these will result in a total timing of between 28 and 41 degrees.
Fig. 4 shows the vacuum advance curves and good advice here is to try this on your racer--you will almost certainly like the results. The three selections available are likely to cover almost every situation because the window of timing accuracy under vacuum operating conditions is quite large. If the system can get within about 5 degrees of the optimum timing then you will reap about 95 percent of the benefits.
Allowing one degree increments between five and ten degrees on initial, nine rpm-related advance curves, and three vacuum curves, Crane's electronic distributor delivers 216 possible advance situations. It is going to be difficult not to find something that works. Also, finding which one works best should be a breeze. Seat-of-the-pants driving will tell you most of what you want to know and a session on a chassis dyno will tell all.
So what are our thoughts on Crane's electronic distributor? It's a winner!