Some Questions about the 16V 4AGE Trigger

[chrisvanish]

Hello, I'm currently getting ready to install an Atom X into my 16V 4AGE AE86. I plan on using COPs with Sequential Injection + Spark. I currently have a 4+1 tooth distributor and I found this thread from a while back: 

My question is, is there any advantage to using a trigger wheel on the crankshaft for Trigger 1 and using the 1 tooth on the distributor for Trigger 2?

Or should I just wire it like in the thread with Trigger 1 to the 4 tooth and Trigger 2 to the 1 tooth? 

Thanks in advance. 

[Adamw]

The 4 tooth + sync in the distributor will run it fine and still give you sequential and direct spark capability.  But for a number of reasons that I will mention below it is a very low accuracy trigger system so provided you have the engineering and/or financial resources to fit a proper crank trigger system then its well worth doing.

1. 4 teeth on the cam is effectively the same as 2 teeth on the crankshaft.  That means the ECU only gets 1 update on crankshaft position every 180 degs.  Anywhere between those two teeth the ecu can only guess position based on the length of time the last tooth took to arrive.  A lot can change in 180 degs, so that is a lot of room for error.  The main thing you see with a low tooth count trigger is you get lots of timing scatter at low RPM and during transients where the crank is speeding up or slowing down quickly you get lots of timing drift.  The more teeth you have the better but the laws of diminishing returns applies.  Once you get to about 36 crank teeth you only see very small changes by going more.  
2. To make matters worse your trigger is attached to the cam.  It is the crank position that is important for spark control, so this is what the trigger is trying to determine.  Your trigger is attached to the camshaft that is exposed to all the valve train resonance then via a coupling that usually has lots of mechanical backlash, then the cam is connected to the crankshaft via a toothed fiber belt which has a few degrees of elasticity and further backlash in the drive teeth and tensioner system.  So its not a great way to determine the crankshaft position...

I have changed a few classic cars from a distributor to crank trigger ignition in the past and it is well worthwhile gains.  Typically better starting and idle etc, significantly better torque curve through transient conditions and if the engine is high compression or knock limited you can see quite serious gains.   

I have a good dyno chart somewhere of a Peugeot 205 that I converted from distributor to crank trigger a few years ago, I will see if I can find it later.

[chrisvanish]

Ah okay, that makes a lot of sense. I was actually already planning on doing the crankshaft trigger wheel but I started wondering if I was over complicating things after finding and reading that thread. Thanks for the advice!