Crane's Racer Friendly Electronic Distributor - how good?

[DavidVizard-GFN]

Crane’s Racer Friendly Electronic Distributor.

Total ease of use can spell more torque, power and mileage from this innovative distributor.

Text, Photos and Drawings,

By

David Vizard

Caption: In the GFN shop we build engines to meet a wide variety of applications. Most of these engines are putting aftermarket performance parts under a spotlight. That means calibration of fuel and ignition to show precisely what these test parts do is important. The use of Crane’s electronic re-programmable distributor has changed this job from a chore to near total simplicity.


A typical hot rodder build scenario for a Detroit V8 goes like this – heads, cam, induction and exhaust system, either as a group or individually, are first decided on. Usually as an afterthought, the question of the ignitions systems compatibility is considered. Most hot rodders are aware the advance characteristics of both the rpm related advance and the manifold vacuum advance need to be tailored to suit the engines inevitably changed breathing and burning characteristics. Just how the advance curves need to change in relation to the engines new parameters proves a stumbling block to many. On top of this, for a conventional distributor, it takes an expensive piece of shop equipment to be able to check that what ever changes are actually required are, in fact, delivered. The usual technique is to leave curving of the distributor to some expert and accept that they have made the right choice. But if they haven’t are you ever going to know?

With the introduction of Cranes electronic distributor during ’05 the difficulty of re-curving a distributor ceases to be a problem. If you know roughly what curve characteristics are needed and can turn a selector screw you are in business – it’s that simple. If you have no idea what curve is needed just read on – all you need to know comes later.

Caption: Crane have both small and big cap (shown here) variants of their electronic advance distributors for small and big block Chevy’s and small block 302/351 Fords.


Just before Crane’s electronic distributor hit the market they sent a pre-production sample for testing. Since then GFN has used a Crane unit on about half a dozen Ford and Chevy project engines – and always too good effect. Here’s what was seen with our test units over the 18 months or so they have been used.


Physical Performance Results.

Caption: The Crane distributor must be teamed up with an ignition box and a coil. Although it can be used with a variety of top aftermarket ignition systems the simplest option is to use matching Crane components.


Crane claim accurate ignition timing to close limits due to the use of precision bearings top and bottom of the units billet aluminum body. Additionally this accuracy of firing is also claimed to be a result of electronics as apposed to mechanical means to trigger the ignition. Our experience to date has shown very steady timing on all the engines that these units have been used on. This distributor is designed to be compatible with a number of leading ignition manufactures ignition boxes. For our tests it was used with one of Cranes ignition boxes. Excellent results, comparable to anything else on the market, were seen.

Caption: The GFN tests with most of the Crane distributor installations tested used Crane’s most basic ignition box Part # 6000-6440. This is a capacitive discharge design with among other, stage and overall rev limiting capabilities built in.

The rpm related advance curves (we can’t call it mechanical advance now because, in this distributor, there are no mechanical parts related to the advance produced) programmed into the distributors module provide a good range of curves for any normally aspirated engine. With the appropriate ignition box and accessories supercharged engines can also be catered for but we have not as yet checked out such an application. For most practical purposes the programmed curves are more than acceptably close to what will be needed for probably 98% of engines built. To date we have been able to successfully utilize one or other of the rpm related curves to get the job done well. The vacuum advance is a boon. It has allowed far better tuning of the idle and part throttle cruise than a distributor with no vacuum advance.

Advance Requirements.

In order to have a good working knowledge of what rpm and vacuum advance curves are likely to be required it will be necessary to understand how various factors affect the situation. The first step here is to appreciate that the mixtre in the cylinder does not explode – it burns, and does so at speeds far and away less than an explosion (check out our ‘Turbulence and Combustion Dynamics story). At 6000 rpm and Wide Open Throttle (WOT) the charge burn speed, in a typical 10/1 CR V8, is about 100 - 150 mph. The speed for dynamite 17,500 mph! At idle the burn rate in an engine is only about 8-10 mph. The key to making the most from the combustion cycle is to light off the mixture so that it exerts the highest Mean Effective Pressure on the piston. This usually happens when the burn is such as to reach peak pressure about 15 degrees after TDC – give or take 5 degrees.

The factors that affect the speed of the burn are the charge pressure at or near TDC and mixture motion/agitation. The easiest to appreciate is the change in burn speed brought about by pressure. If the throttle is closed, the intake manifold pressure drops. In other words a partial vacuum is created. Typically a 10/1 motor with a street cam will show about 200 psi when the manifold vacuum is non existent (as it would be at WOT). However at idle the highly restricted flow due to the throttle being near closed would mean the cylinder pressure around TDC would only be about 75 psi. It will also be polluted with a lot of exhaust from the previous exhaust stroke.

All this causes the speed of combustion to severely slow. At 6000 rpm the majority of the charge is burned in a little over two thousandths of a second. At idle it takes 15 times as long for that same process to occur. At 6000 rpm and WOT the timing advance required will typically be about 34 degrees before TDC. For the minimum rate of fuel consumption at idle the timing will need to be about 45 or so degrees before TDC. The same goes for cruise conditions. This is when the vehicle is going down the highway at part throttle and the manifold vacuum could be anywhere between 10 and 20 inches of mercury. (one atmosphere at sea level is 29.9 inches of mercury or 14.7 psi).

From what has just been said we can see that as manifold vacuum increases so the amount of advance needs to follow suit. Optimal use of fuel (minimum to get the job done) for a big cammed engine during idle and low speed operation can call for as much as 50 degrees of advance to be present under those conditions. The only way to get this is by means of vacuum advance. Often the irony here is that many hot rodders believe vacuum advance is not needed because their favorite drag racer does not use it. If you want to use the biggest cam possible on the street and still have acceptable street manners now is the time to realize that the vacuum advance is the most important tool toward achieving that goal. In other words it can be considered as the single most effective camshaft tamer you can get. Time taken to simply hook up the vacuum advance to a manifold vacuum source can make big cam to idle as if it were some 15 degrees less than it really is. Conversely if you are looking for a decent street quality idle, the use of vacuum advance will allow a cam of about 5 degrees more duration/overlap than would otherwise be the case.

We can see from what we have just discussed how manifold vacuum (or Manifold Absolute Pressure (MAP) – which is 29.9 minus the reading on the vacuum gauge) affects the final compression pressure just prior to the ignition taking place. But throttle position is not the only factor affecting the compression pressure. If we raise the CR of our engine the compression pressure will go up and the speed of combustion will increase. This will call for less advance than a lower compression would need. Installing a cam with more duration does the reverse. A short cam may close the intake valve when the piston is only a short way up the bore where as a long duration cam may not close the intake until the piston is fully a third of the way up the bore. This means until ramming from dynamic effects in the intake and exhaust cause the filling process to considerably increase, the timing will need to be more than for a short cam.

Caption: This chart demonstrates the differences in advance curves for a short (blue curve – comes on slowly with increasing rpm) and a long cam (red curve – comes in much faster with increasing rpm). Unless the longer duration cam is given the right curve all the negative aspects of the longer cam, such as poor low speed output, lumpy idle and increased around town fuel consumption will all be accentuated.

When a long cam is used the ignition advance needs to come in quicker although it is likely not to top out at any significantly higher value (Check out the drawing above). For instance let’s assume we have two engines identical in every way other than cams. One has a short cam the other a long cam. The short cam may have the rpm related ignition advance start at 1000 and reach a total of 34 degrees at 4000 rpm. The long cam on the other hand would need the timing to come in quicker. Starting at 1000 rpm the total of 34 degrees would need to be reached by about 3000 rpm. The business of achieving the right timing curve at low speed when a bigger cam is used is important to the generation of low speed output. So often a disappointing low speed output is blamed on too much cam when in reality is too little timing! Get the timing curve right and that big street cam will turn out to have better low speed drivability that you may have previously supposed.

As far as vacuum advance goes do not follow the trend of deleting the vacuum advance as seen on race engines for your street driven machine. If a fairly big cam (280 -290 degrees at seat) is used and the engine not provided with vacuum advance it can cut the cruise mileage by at least 20%!

Dialing It In.

At this point we should have a good idea what is needed in the way of timing characteristics. Now let’s look and see how to get the desired curves and what the Crane distributor has to offer in this department.

First selecting the desire pre-programmed curve. Take a look at the photo of the unit below.

Caption: The left hand arrow indicates the vacuum port. This is usually best connected to manifold vacuum not the ported vacuum on the carb. The middle arrow indicates the selector for the rpm related advance. There are ten options here. The right hand arrow indicated the selector for the vacuum. This has 3 options.

The left hand arrow indicates the vacuum port. This is usually best connected to manifold vacuum port on the carb or to a direct tapping into the manifold itself not to the ported vacuum on the carb. The middle arrow indicates the selector for the rpm related advance. There are ten options here with zero as an advance lockout. The right hand arrow indicates the selector for the vacuum. This has 4 options with zero as a lockout.

RPM Advance Curves

As far as the available curves are concerned we will deal with the RPM related (what would be called mechanical advance on an old style mechanical distributor) first. The nine curves (the tenth is the advance lockout) can be sub-divide into two groups. One group mainly for street style engines and the other for race engines. To see which is likely to suit what refer to this next graph for street style engines and the one after that for more race style engines.

Caption:This chart shows the rpm related advance curves which will primarily suit most of any sensible street applications.
Curve #1 Blue. Suits applications using a moderate CR (about 9.0/1) and a street cam of about 270 degrees of off-the-seat duration.
Curve # 2 Green. Suits bigger street cam of around 285 degrees of off-the-seat duration and CR’s in the region of 10.5/1
Curve #3 Black. Suits cams around 275-280 degrees of off-the-seat duration and CR around 10.0/1.
Curve #4 Red. Suits bigger cams in the range of about 275 to 285 with fast burn style heads and a CR of around 10.0/1.
Curve #9 Yellow. Suits short cam lower compression engines such as might be used in trucks and RV’s.

Caption:The curves seen in this graph will be mostly suitable for race engines.
Curve #5 Red. Suits really big race cam and CR in the range of 12.5 – 14.0/1 or more.
Curve # 6 Green. Suits race cams a little less aggressive than #5 setting.
Curve # 7 Blue. Suits moderate race cams with CR in the range of 11.0 to about 12.5/1
Curve #8 Yellow. Suits engines with big race cams, high compression and known fast burn characteristics.

Notice all the graphs start the advance at ten degrees. This is just a matter of convenience. In reality the static or initial timing (timing set at idle rpm with vacuum disconnected) can be anywhere from about 8 to 15 degrees depending on the cam and compression ratio used. Assuming one degree increments in initial timing this gives, with 8 -15 initial, a range of total advance between 28 and 41 degrees. Couple this with 3 vacuum curves and we have 216 combinations, which, as was said earlier, should cover at least 98% of likely applications.

Vacuum Advance Curves.

For vacuum curves check out the following drawing. These curves cover most of everything likely to be needed for a normally aspirated or supercharged engine.

Caption: # 9 Fig 4 Shown here are the vacuum advance curves the Crane distributor will deliver.
Curve # 1 Red. Most likely needed curve for typical moderate high performance street spec engine. Cams around 270 to 280 and CR’s of 9.0-10.0/1 or big race cams and high compression.
Curve #2 Blue. Use with relatively fast burn engine specs such as those with higher compression ratio’s (10.0 11.5/1) and moderate cams.
Curve #3 Yellow. Street with moderate cams and fast burn heads or high compression.


It should be noted that all the recommendations with the advance graphs are starting points. Remember the real asset of this Crane unit is that changing the advance curve to find the most appropriate is easy. It takes no more than the turn of the selector and the resetting of the initial advance and you are good to go. On page 7 of the 8 page manual that comes with the distributor, Crane does a good job of detailing the procedure to optimize the curve settings so I won’t repeat it all here. It will be easy to find a curve that works well. 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.


First Attempt 'Start and Run' at First Install.

It’s nice to install a distributor such that the timing is sufficiently close for the engine to start and run at the very first attempt. This really impresses an audience should you have one at that time! To do this the position that the trigger is for #1 cylinder needs to be known. Just to check this out a cap was cut away and the timing light used to ‘flash’ the rotor to establish this position. The shot is not as good as we would have liked primarily due to the Crane systems very high spark producing capability. Although all but #1 cylinders were disconnected the timing light would trigger from the giant stray sparks produced hence the multiple exposure you see in the nearby photo. However the principle one i.e. #1 cylinder, was captured so the info needed was obtained.

Caption:What you see here are the relative positions of the rotor and chopper blades at the point of firng #1 cylinder 12 degrees before TDC with zero vacuum advance.
This is a counter clockwise rotating Crane small block Ford distributor. When using the info here for a Chevy remember it rotates in a clockwise direction.
The green arrow indicates the post for #1 cylinder plug lead. As can be seen the counter clockwise rotating rotor arm is about 10 degrees past the post. As the timing advances it will get nearer straight on alignment with the post.
The red arrows show both the chopper blade on the rotating assembly and the sensor. Although this position looks like the chopper has not yet reached the pickup to trigger the system the reality is that the chopper blade further on around did the triggering and the spark was then retarded from there to the position shown here by the electronics. Set up your distributor as shown here (a mirror image for Chevy) and it will light off and run at the attempt.

Conclusions.

The reaction to the Crane distributor among those dyno testing here at GFN is basically one of ‘why didn’t someone do this long ago'. Not an unreasonable question since the technology has been available for at least ten years to do so’. Top marks definitely go to Crane for taking this concept and successfully bringing it to market. If it were not for a couple of problems we had with the wiring of the pre-production prototype supplied (zero problems with the subsequent 5 units tested) for the first batch of testing this unit would have been given a solid 5 star GFN rating. Even with that minor mar on the record the Crane distributor (and associated system) gets a 4.7 star rating which basically means it’s very highly recommended.

A search was done for the best price on the parts tested and, of all the big name suppliers, Summit had the best price by a small margin.

[Ron Golden]

David,
For the last 25 years I've been recurving distributors here in Kansas City, MO on a Sun Model 404 distributor machine. Originally designed to handle points only distributors I've modified it to handle all types of electronic distributors. I haven't kept count, but I'm sure I've recurved hundreds of distributors during those 25 years.

Before I curve a distributor I need to know the CR, cam duration, torque converter stall rpm, gear ratio and on and on. Then the problem becomes one of getting the weights and springs correct to achieve the right advance curve. Sometimes I have to grind and/or weld on the advance mechanism. On Ford and Chrysler distributors that means taking the distributor completely apart to make each change then spin it on the distributor machine to see if the change was correct. Often times I'll have the distributor apart 6-10 times before the advance curve is right.

Crane has taken the work out of curving a distributor and made life much easier for me. Now a hot rodder can buy the Crane distributor, read the instructions and tune it themself.

Now I can take more naps in the afternoon.

Ron

[MadBill]

Very informative article, David!
It reminds me though of an issue I encountered some years ago with a point-triggered CD conversion on my 495 c.i. big block Chev (single 850 Holley, factory 12.5:1 CR and ZL1 cam (~ 265° @ 0.050") I discovered that with the CD system switched on, the engine would not run smoothly at cruise unless the vacuum advance (~15° @ 12"Hg.) was disconnected.

The best explanation I could come up with was that at the high cruise vacuum (didn't have a vac. gage) of a stout-engined '70 Camaro with 4.10:1 gears (3,000 RPM at 55 MPH), the total advance of perhaps 45° meant that at the ignition point the cylinder pressure was so low that literally there weren't enough oxygen and hydrocarbons molecules present in or near the arc to initiate combustion.

This preamble is a long wind-up to the question: Does the relatively short spark duration of a CD ignition like this Crane unit imply any similar risk of misfire if one twists the knobs too enthusiastically?

[Ron Golden]

Madbill,
I ran into the same misfire situation with the local Cobra club cars. They ran overdrive trannys with too big a cam and carb. They were trying to cruise at 1800-2000 RPM with high manifold vacuum, lots of ignition advance and very light throttle opening. If they gently pushed on the throttle (still with pretty high manifold vacuum) the misfire would go away. When they disconnected the vacuum advance the problem would go away and the engine would no longer misfire. (Vacuum advance ~10-12* above 12" Hg)

I felt it was a situation where with the high manifold vacuum and lots of advance the throttle wasn't open very far and the depression across the boost venturi's wasn't high enough to start the fuel flowing. The engine was basicly running on the idle circuit and was going lean. When the vacuum advance was disconnected more throttle opening was required to maintain cruise power, and the boost venturi's would flow fuel, and the misfire would go away.

I called Holley's Tech Line and got someone that had seen this same situation several times and he agreed with my theory. I helped the situation by raising the float level so tip-over came in sooner.

I remember that David Vizard did some testing on carb venturi's and found that he could modify the boosters and drasticly increase the depression across the boosters. That might be the best way to cure this type of problem. Maybe David could publish the article again on this forum.

I suggested the Cobra owners could completely eliminate the problem by running a Rochester Q-jet carb. I bet you can guess how that suggestion was received.

Ron

[DavidVizard-GFN]

Ron,

I feel for you on the distributor deal. I have done a little of this myself and after a while it wears real thin. You must have patience beyond belief (mind I suspected that when I was over visiting a few years ago).

Bill,

I have not experienced the problem you speak of but I think I can see a possible situation along the lines you describe contributing to the end reult. First an advance of 45 degrees at criuse with say 8-10 inches of vacuum is not by any means unreasonable. That is about where I would expect the timing to be. I have had engines that required as much as 52 degrees to get the job of best mileage at criuse done.

That said the miss could be that when the timing is right and the throttle is further closed the carb goes onto a part of the tranfer circuit that is leaner (needs slightly bigger idle jets). This might well be the case if the carb had 4 corner idle. When that happens the system goes into a lean mis-fire.

In addition to that you might want to check that the plugs may need a bigger gap. The amount of spark energy deliverd to the gap drops as the gap gets smaller or the vacuum goes up. Try a 0.055 gap.

Let's here how this fits in with your further thoughts on the subject.
DV

[MadBill]

Some excellent points David! I'll keep them in mind when this "Rip Van Winkle" project next hits the street. That particular CD unit is long gone, but I'll be trying the Jacobs unit that contributed to my 60 + MPG Metro's economy for many years. Based on how it responded to ever-larger spark gaps (up to 0.080"!) I suspect my 0.030" gaps were the main problem.

[MadBill]

Ron: Sorry, I missed your post until today! The Cobra experience sounds exactly like mine, less any OD effects. I now have high-signal annular discharge boost venturis, which may indeed help (but for the fact that there will be eight of them next time out...) and probably more importantly will have available a FAST 2 channel AFR logger.
Likely won't be on the road until next summer, but will update this thread when it is.

[jacb]

Originally Posted by DavidVizard-GFN

Crane have both small and big cap variants of their electronic advance distributors for small and big block Chevy’s and small block 302/351 Fords.

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