LT1 Sequential Fuel Injection

[Danno74Z]

I'm trying to figure out and understand how the 93-96 LT1 engines can run the fuel injection system sequentially without a cam sensor. I know all about the Optispark (nothing more then an optical distributor) and its functions, but I'm at a loss on how the computer knows when to "squirt" the individual injectors. Many FIS systems are bank to bank or phased type FI but true sequential requires precision timing with the cam - correct?

 

Thanks,

Danno74Z

[Guest Anonymous]

I could be wrong, would'nt be first time but I believe the lt-1 has batch fire injectors. Not sequential. Mark

[Mudge]

There are 360 timing holes in the Optispark, sounds like enough to me.

[Pop N Wood]

I was gonna say. If the computer knows when to trigger the plugs, it should know enough to fire the injectors.

 

Didn't realize the LT1 injectors are batch fire....

[Danno74Z]

Well, I got it figured out after some study. The OptiSpark (OS) is much more then your run-of-the-mill distributor. Too bad it's located under the water pump assembly. Before I forget, the LT1 is a sequential fuel injected system. The Ignition Control system comprises of the (2) CAMSHAFT position sensor (CPS), ignition coil, ignition coil module and PCM. The CPS is part of the OS distributor. (A dowel pin off of the front of the camshaft drives the Optispark.) The OS contains 5 circuits: Low resolution signal reference, high-resolution signal reference, distributor reference low signal, distributor ignition feed, and IC (ignition coil). Briefly, in order. LRSR - provides the PCM with reference signals, firing order, and CAMSHAFT position information (this was the key information I was looking for). HRSR- Provides PCM with detailed reference signals and CRANKSHAFT position information. DRLS - A redundant ground for the distributor. DIF - Provides power to the distributor. IC - This circuit controls the ignition coil on and off time.

 

The Optispark contains not one but two optical camshaft position (CMP) sensor assemblies that provide signals to the PCM for timing control. One CMP senses 360 equally spaced slots to provide 720 signals per camshaft revolution for high-resolution timing control. The other CMP sensor senses eight disk slots of variable length, providing a low-resolution signal that tells the PCM which spark plug is being fired.

 

Well there you have the low-down on the OptiSpark Distributor System. It's too bad that GM did not use a distributor-less system that was used on the 3.4L engine or at least use hall–effect sensors within the Optispark. I think 90% of the issues with the OS would have been eliminated. Optical sensors on an engine – crazy, it probably worked great in the lab!

 

Danno74Z

[Mudge]

The only true CPS is on the OBD II cars, 1996 and 1997. These have a different timing cover to make room for the CPS. These cars also have missfire detect, OBD I cars 1992-95 Vette, 93-95 Fbody dont.

 

We have to accept that the LT1 was a short term, interim engine. It lasted for but a handfull of years, it was not going to be some perfect project. It had a few improvements over the L98 (95 optispark setup is fine), but it was only a temporary thing, and thats just the breaks.

[Danno74Z]

Mudge,

 

I own a 95 LT1 engine and it's going into my 74 260Z. This engine is a major improvement (some might not agree) over the original Datsun engine that I am ready to extract! My only complaint is that with just a little more engineering work in the optispark area it could have been better package – short or long term. The Optispark ignition is definitely the Achilles Heel of the LT1 engine and I will have to learn to live with it. Maybe one day, I will trash the entire system and put an Electromotive Tech3 system on – who knows. Through my research I now understand how the system works and hopefully others learned a little too.

 

Danno74Z

95 LT1 /T56