Why is the secondary injector circuit so complicated?

[Matt B]

Recently I grabbed a beer and studied some wiring diagrams of my ZR. Just out of curiosity...
Looking at the fuel delivery section a question came to my beer-drenched mind: why did they do it that way?!?

My point is: each primary injector is controlled directly and individually by the ECM.
In the secondary path two injectors are controlled as a pair and ultimately energized by one of the two well known secondary injector relays.

To my limited understanding this doesn't make any sense...
I thought about a potentially higher energy demand for the secondary path. The FSM says that in full power mode the fuel delivery is equally split between both injectors. Also technically primary and secondary injectors seem to be identical. So, a higher energy demand can't be the reason to add a relay - which potentially may fail one day...

Does anybody know why GM/Lotus didn't apply a direct (no relay) and individual control to each secondary injector but chose the two-as-a-group and relay layout?

[XfireZ51]

Basically, GM didn?t want to or could not develop another ECM for a limited run of units like the ZR-1. There are many similarities between the L98 programming and the LT5, although the LT5 uses Speed Density instead of MAF for some obvious reasons, ie a new MAF would have been required. Controlling the secondaries using the ECM would have required additional injector drivers, again necessitating a significantly modified or new ECM. So the injector relays are a less expensive alternative while using the primary injector as simply the ?trigger?. Then u cut the PW by 50% whenever the secondaries are engaged. Voila!

[Paul Workman]

Originally Posted by XfireZ51

Basically, GM didn?t want to or could not develop another ECM for a limited run of units like the ZR-1. There are many similarities between the L98 programming and the LT5, although the LT5 uses Speed Density instead of MAF for some obvious reasons, ie a new MAF would have been required. Controlling the secondaries using the ECM would have required additional injector drivers, again necessitating a significantly modified or new ECM. So the injector relays are a less expensive alternative while using the primary injector as simply the ?trigger?. Then u cut the PW by 50% whenever the secondaries are engaged. Voila!

Well put!

[Matt B]

Thanks, very interesting. I didn't consider adding a few control channels to be that much of an issue but yes, back in 1988 it for sure was a bigger thing.
So in the end it's a piece of "efficient engineering". Good to know! Thanks for the insight!