Opti elimination

[N20LT4Bird]

I have read Injuneers article on the SDI install, looks pretty good. But, I have seen guys run a conventional distributor. I assume they are still using the optical sensor in the OS for crank reference. Has anyone tried this? How do you do it? What are the potential pitfalls?

[Injuneer]

You need to keep the Opti for "cam reference". The stock PCM requires the low resolution pulse to know when to fire the sequential injectors. The high resolution pulse is used for high accuracy spark timing, so its not as important if you're using a "conventional" distributor. If the PCM doesn't see the low res pulse, it shuts down the fuel pump and injectors.

There are several schemes for altering the ignition. There have been two systems that deleted the OptiSpark in its entirety. One was the Electromotive SDI/Opti-Eliminator. They only made about 20 of those, then stopped. It wouldn't work with the OBD-II setups, because it only used the crank position wheel for crank reference. It synthesized the high and low resolution pulses that would normally come from the Opti, to keep the stock PCM happy. Problem is, it lacked a true cam position and on 1/2 the startups it would "synch" the low res pulse incorrectly, and the injectors would be 180-deg out of phase. That was not a problem at all on the batch-fire 93's, a minor problem on the sequential fire OBD-I's, and a major problem on the OBD-II's because it upset the crank velocity profile and caused a misfire indication.

The second system was a home brew by a guy at New England Dyno.... A crank trigger wheel, and a magnet on the cam sprocket with a sensor tapped into the timing cover for cam position. That fed a FAST computer, with an eDIS 8-channel driver. The system has never actually been commercialized, and someone who tried to buy the parts and duplicate it was unsuccessful.

The current "best" approach is the "LTCC" of LT Coil-per-Cylinder setup, sold for about $400. It uses the high and low res pulses from the Opti to feed the stock PCM, and then drives 8 LS1 coils in direct fire mode. In effect, you remove the high voltage function from the Opti, and some of the problems go away. Couple the LTCC with the new Dynasprk billet Opti, and it would seem you've got a fairly bulletproof system - but there is only limited experience with both units, and you are looking at a minimum of $1,200.

I've been running the 8-coil direct fire system for more than 3 years, and its the same system George Baxter used to run 9.04@155 in his 3,900# 30th SS Convert. Opti feeds a MoTeC engine management computer, with their IEX 8-channel ignition driver and 8 direct fire coils.

[N20LT4Bird]

That system looks awesome. I would imagine you can use the stock opti for your low and high res pulses, but from what I understand they have a substantial number of problems with the optical sensor, or primary side of the opti. Is there a web site for that setup? Thanks for the info.

[Injuneer]

Once you get the high voltage function out of the Opti, it seems to be a lot more reliable. On the other hand, those of us who have been using that type of system for several years are typically people who seldom drive the cars in bad weather, and don't put a lot of miles on them, so it may not be a fair comparison.

There are two main "culprits" in the Opti... 1) is moisture.... and the big problem for moisture leaks is the harness connector seal. 2) ozone induced corrosion.... the bearing and the retainer plate rust real bad. The rust blinds the optical sensor. GM added the "vent" system in 1995 to get rid of ozone.

But if you eliminate the high voltage spark from the Opti, you eliminate the ozone, and corrosion seems to disappear. I did convert to the "vented" design on my 94.

Bailey LTCC

[N20LT4Bird]

what did the 8 LS1 coils run you?

[Injuneer]

The coil/computer setup was done as part of the complete stroker buildup. For some reason, I never could find the coils listed on the itemized bill. I think a set of 8 goes for around $200. But you could probably use salvaged sets from LS1's for a lot less... you figure even if they are used, they only fire 1/8th as often as a single coil setup .