Autosport Labs

Ok,I can't believe I'm asking this since I have almost come to the complete end on my installation. The car the Megajolt and EDIS-4 is installed on,is a Sierra 1,8 pinto,here she is

all standard parts with a carb. I've had a look through the archives and don't see anything close to my question. So what is the advantage of controlling the ignition curve?
The only answer I've seen is on google,and it was mentioned that if an engine has been tuned,then mapping the curve would improve the timing of the spark.
thanks
Paul

Hello,

That is a big question! However, your conclusion is basically correct.

1) to make the existing engine run better for the application you are seeking: tune for more power or greater fuel economy.
2) to compensate for engine modifications. For example, forced induction; higher compression engines; higher octane fuel.

I'm sure others will chime in with their perspectives and experience.

I don't know what your ignition was before the change, however if it was distributor based, any slop in the distributor drive, gears, shaft, etc. will cause scatter. Scatter is where the ignition timing varies (or scatters) when it is not supposed to. I found that the rock solid timing from EDIS gave me better driveability, even with the stock distributor curve programmed into the MJ! I have since been playing with the mapping, and have taken care of a tuning problem that I could have never done with a mechanical system.

Right! I didn't call that out specifically since the question was asking why one should change or tune timing. But, the rock-solid precision of crank fired ignition will alone improve engine performance, even if you're running the exact same ignition "curve" as the OEM distributor setup. That is a huge improvement by itself.

brentp wrote:Hello,

That is a big question!

Sorry Brent

brentp wrote:1) to make the existing engine run better for the application you are seeking: tune for more power or greater fuel economy.
2) to compensate for engine modifications. For example, forced induction; higher compression engines; higher octane fuel.

That puts it in a nutshell,thanks

Alexander_Monday wrote:I don't know what your ignition was before the change, however if it was distributor based, any slop in the distributor drive, gears, shaft, etc. will cause scatter. Scatter is where the ignition timing varies (or scatters) when it is not supposed to. I found that the rock solid timing from EDIS gave me better driveability, even with the stock distributor curve programmed into the MJ! I have since been playing with the mapping, and have taken care of a tuning problem that I could have never done with a mechanical system.

Superb!!! Thanks ever so much Alexander. Yes it was a distributor originally. I should have got a curve from the mechanical dizzy beforehand. I would say you summed it up nicely there,"Scatter is where the ignition timing varies (or scatters) when it is not supposed to"
thanks again

brentp wrote: the rock-solid precision of crank fired ignition will alone improve engine performance, even if you're running the exact same ignition "curve" as the OEM distributor setup. That is a huge improvement by itself.

Again,brilliant answer,just what I was looking for.

Here's an explanation in my own words the reasons for and the advantages of Megajolt, and I hope this is a helpful contribution:

Ignition timing is critical to Otto cycle engine performance. Why? For best performance, maximum pressure developed by the burning mixture in a cylinder should occur a little after the piston has passed top dead center (TDC) and is on its way back down the cylinder to take advantage of the mechanical leverage of the position of the piston connecting rods and the crankshaft and optimally develop torque. Developing pressure too early or too late wastes performance potential, and in the extreme can lead to damage. But it takes a finite amount of time for the combustion, initiated by the spark, to propagate throughout the mixture in the cylinder. Pressure, temperature, motion of the mixture and fuel grade all affect combustion speed--pressure being the most important factor. To account for the finite amount of time it takes combustion to occur and for max pressure of combustion to be reached at the optimum position of the piston, the spark needs to be "advanced," typically to occur some number of degrees before the piston reaches TDC. As engine speed increases, the speed of combustion doesn't increase as fast as the engine, so progressive spark advance is needed to keep the max pressure of combustion occuring at the right time. This is true to first order up to a point, because as engine speed (RPM) increases, pre-combustion cylinder pressure (up to the limits of the engine's induction system to flow mixture), mixture motion and fuel atomization increase too, and these things speed-up combustion. Beyond around 3000 to 4000 RPM in most engines, these effects keep pace and so no further advance is needed. Finally, the amount of load on the engine affects optimum spark timing. Less advance is needed when accelerating, with the throttle wide-open and lots of air is cramming into the engine and raising absolute pressures inside it, as opposed to when cruising or decelerating and pressures are reduced. Therefore, at a given RPM, more advance is needed at low engine loads (i.e., low absolute manifold pressures).

Until recently and before the advent of electronics, spark timing on most Otto cycle engines has historically been determined and controlled by the distributor. Static advance is set by the position of the distributor such that the rotor and plug wire electrodes in the distributor cap are aligned relative to piston position at rest as desired (e.g., to set timing at idle). Dynamic advance with a distributor is accomplished via two mechanisms--centrifugal and vacuum. Centrifugal advance adjusts spark timing as a function of engine speed and works this way: little weights constrained by little springs inside the distributor fling outwards more and more as the distributor spins faster and faster with increasing engine speed, up to a limit imposed by pins or some other hard stop. The stiffness of the springs and the mass of the weights controls how far the weights move for a given speed. The outward motion of the little weights rotates a plate that holds the spark triggering mechanism (e.g., a cam actuating contact points, or a multipole magnet interacting with a Hall effect sensor, or a chopper wheel interrupting the light path between a light source and an optical sensor, all of which function as switches to interrupt current flowing in the primary winding of the coil and resulting in magnetic field collapse and induction of high voltage in the secondary winding of the coil). Vacuum advance (or sometimes retard) adjusts spark timing as a function of engine load, where vacuum in the intake system is used as a measure of engine load (e.g., cruising, accelerating or deccelerating). Vacuum can be sensed via a tap or multiple taps on the intake manifold, or a port near the throttle plate. A vacuum line goes from the intake vacuum tap to a vacuum canister on the distributor containing a diaphragm, and the diaphragm is attached to a link that is attached to the plate holding the spark trigger mechanism, so motion of the diaphragm, in response to intake vacuum, can advance (or on some distributors, retard) the spark.

As you progress with engine modifications, the best amount of total ignition advance for a given engine speed and load changes, so to fully reap the benefits of engine performance modifications, it pays to revisit your ignition timing when making performance changes. For example, as you raise an engine's compression, less advance is needed at a given engine speed, and you want to be careful and not have so much advance that you encourage pre-ignition (a.k.a. , pre-detonation, pinging, pinking, knocking). If you go with a "bigger" cam, volumetric efficiency and dynamic compression are reduced at low RPM but increased at high RPM, thereby reducing combustion speed at low RPM but increasing it at high RPM and requiring an ignition map with a different shape and a steeper "slope." So to get the most out of a given camshaft and overall intake setup, ignition timing is crucial to preserving low RPM behavior and maximizing high RPM performance. "More" is not "better" when it comes to advance. Don't be fooled by an ignition map with big advance numbers everywhere. Proper spark timing is about optimizing performance for a given engine configuration.

So, why not stick with a distributor instead of going with a fully-electronic, programmable system for controlling ignition timing like Megajolt? Two main reasons. First, changing the advance map on a distributor is very difficult, time consuming and not really deterministic. It involves adding (welding on) or subtracting (grinding off) mass from the centrifugal weights, changing the centrifugal weight springs, and changing the range of motion that the vacuum/retard mechanism makes or imparts to the distributor. Being able to simply change spark timing incrementally at the click of a mouse, as with Megajolt, is completely deterministic, much faster and allows results to be assessed immediately before environmental conditions change that can affect interpretation of results (like air temperature, humidity, etc.). Moreover, ignition timing maps can be implemented that simply are not physically possible to create with a mechanical system (if such a thing is warranted). Second, fully-electronic systems like Megajolt eliminate several mechanical interfaces involved in communicating piston position and delivering the spark that add uncertainty and reduce precision in spark timing. In a traditional distributor-equipped engine, the crankshaft turns the camshaft through a chain or belt, and the camshaft incorporates a gear that engages and turns the distributor shaft, which actuates the centrifugal advance weights and springs that actuates a mechanism for interrupting primary current to the coil as well as turns a rotor that spins past contacts in the distributor cap to distribute spark to each spark plug. In the case of Megajolt with Ford EDIS components, piston position is sensed directly off the crankshaft by the variable reluctance sensor "looking at" the toothed wheel and all spark triggering, switching and distributing is done purely electronically, and so the timing "slop" contributed by the rest of the mechanical interfaces in a conventional distributor-based ignition system are eliminated, thereby resulting in much more precise timing and virtually eliminating jitter. Lastly, Megajolt senses either manifold pressure (MAP) using a pressure transducer or throttle position (TPS) via a potentiometer to assess and communicate engine load more accurately than a vacuum diaphragm attached to a distributor plate carrying spark triggering components.

So in summary, a Megajolt-based ignition system is more flexible, accurate and precise than a distributor-based one, and enables much faster, simpler and easier ignition tuning.

starman wrote:... In a traditional distributor-equipped engine, the crankshaft turns the camshaft through a chain or belt, and the camshaft incorporates a gear that engages and turns the distributor shaft... In the case of Megajolt with Ford EDIS components, piston position is sensed directly off the crankshaft

Is the piston position on modern cars also sensed off the crankshaft? And cam sensor is for fueling only?

Yvan wrote:

starman wrote:... In a traditional distributor-equipped engine, the crankshaft turns the camshaft through a chain or belt, and the camshaft incorporates a gear that engages and turns the distributor shaft... In the case of Megajolt with Ford EDIS components, piston position is sensed directly off the crankshaft

Is the piston position on modern cars also sensed off the crankshaft? And cam sensor is for fueling only?

Hi Yvan-

Many newer cars are distributorless, and the answer is to your question is "yes." Check out the following:

http://www.aa1car.com/library/dis.htm

http://www.procarcare.com/icarumba/reso ... nition.asp

From the second link:

"The Direct Ignition System (DIS) uses either a magnetic crankshaft sensor, camshaft position sensor, or both, to determine crankshaft position and engine speed."

Why do you need camshaft position sensor to determine crankshaft position and engine speed?

brentp wrote:Right! I didn't call that out specifically since the question was asking why one should change or tune timing. But, the rock-solid precision of crank fired ignition will alone improve engine performance, even if you're running the exact same ignition "curve" as the OEM distributor setup. That is a huge improvement by itself.

That's probably the single biggest advantage - even above being able to re-re-re-map the curve indefinately.
On my old Mini for example the distributor was driven by a seperate shaft, which ran in a gear on the camshaft, that was driven by a stretchy old chain (why on each have manufacturers of multi-cammed engines gone back to chains?!), that was driven by the crankshaft and timed dot-to-dot from the factory. There's room for slack and rattle at every point in that drive route. A sensor pointing sraight at the crank, and so directly linked to the piston position, will give far more accurate spark timing on every single revolution.

Yvan wrote:From the second link:

"The Direct Ignition System (DIS) uses either a magnetic crankshaft sensor, camshaft position sensor, or both, to determine crankshaft position and engine speed."

Why do you need camshaft position sensor to determine crankshaft position and engine speed?

it is a bit awkwardly worded. a crank sensor OR a cam sensor is required to measure engine speed. a crank sensor OR a cam sensor is required to measure crank position, simply in terms of its rotational angle, and that is all that is required for wasted spark ignition or batch fired fuel injection. a crank sensor AND a cam sensor is required for non-wasted spark ignition or sequentially fired fuel injection.

actually thinking about it, a cam sensor alone should suffice for all purposes, but i think a crank sensor is preferred, where it can be used, for its precision, and generally easier access.