Corry

Alright, I have this posted on some local forums, but I figured I'd get a bigger audience

Ok, As of right now, there are to big rocks in my path to a fbasically free LT1....I was hoping you could help me out.

First is ODBCI vs ODBCII. Obviously I'd like to get a newer lower milage motor, but 3rd gens are ODBCI so would there be a problem putting an ODBCII computer in there? I still need to be able to run my digital dash.....
Also, I heard VATS is a problem, ODBCI and II what do I have to do to disable it?

Any other problems? I know about the motor mount one....

Thanks
Corry

Vader

Corry,

First, I'll preface this by stating that I've never done the LT1 engine swap, so take this for what it is worth. What I do know about the possible issues may be of some help to you, however. Having two LT1s, and having rebuilt one of them, as well as "a couple" older Gen I small block Chevys, I can tell you what the differences are and offer some possible solutions to potential problems.

The significant mechanical differences between the LT1 and Gen I SBCs are:

• Lighter, stronger, and better balanced powdered metal connecting rods in the LT1;
  
• Better overall rotating assembly balancing right from the factory;
  
• Reverse-flow cooling that pumps coolant to the heads first, allowing more timing/compression with less tendency for detonation;
  
• Revised timing cover to accomodate the reverse-flow water pump and Opti-Spark distributor;
  
• Camshaft gear driven, close-coupled water pump;
  
• Roller camshaft with stub shaft to drive the oil pump and replace the older style distributor-to-case seal for the camshaft oil gallery;
  
• Revised (extra) coolant passages in the front of the case, cylinder decks, and heads;
  
• Revised oil pan to accomodate the timing cover changes (and seal better);
  
• Improved port designs on the cylinder heads;
  
• Externally supplied EGR gas;
  
• Revised harmonic balancer design;
  
• Improved starter motor design;
  
• External OIL-FLOW cooler for engine oil;
  
• A Banjo-bolt coolant bleed line set from the rear of both cylinder heads to the return tank;
  
• The timing drive is a single row roller chain, with a spur gear cut on the rear side of the cam sprocket to drive the water pump gear/shaft;
  
• the camshaft includes a larger blind hole in its front end and longer dowel pin to accept and drive the shaft of the OptiSpark distributor.

Electrically, the differences are as follows:

• Ignition is timed via the encoder pulses generated by the OptiSpark, then fed to the PCM, whereupon the PCM drives the ignition coil based on input sensors;
  
• The eight fuel injectors are individually and sequentially driven by the PCM based on OptiSpark encoder pulse inputs;
  
• The PCM also shifts the transmission electrically;
  
• PCM data is shared with the instrument cluster and cruise control (among other systems) for various functions, like AC compressor drive, TCC, instrument displays, warning lamp control, etc.;

First - your car. Presuming that you want to keep the digital display, using the newer PCM would be an electronic nightmare. Signals from the PCM probably won't integrate with the display cluster. You'd have to design and build interface circuits to convert and scale the signals to even get it to work at all.

You don't need any electrical signals for your transmission other than to lockup the TCC.

Your indicator lamps, fuel pump, and AC compressor are all driven by local control outside the ECM,

The PCM from a stock LT1 car would expect inputs that you don't have, requiring reprogramming.

In summary, you would have a much easier time retaining you ECM than retrofitting a newer PCM on your car.

One possible solution to the integration problems is to change the LT1 control scheme to the older ECM that you have already installed in your car. There are a few items that would change as a result or have to change to allow this:

• The fuel injectors would be batch fired instead of sequentially fired. The injectors really don't care, and the real advantage to sequential firing is for low RPM and part throttle fuel economy and emissions control. At high RPM and full throttle, the sequentially fired systems basically go "batch mode" anyway. The same thing happens if an OptiSpark reference pulse fault is detected by the PCM, and the engines still run fine;
  
• The OptiSpark would be thrown in the trash (where it probably belongs, anyway). This would require only a seal plug on the front of the timing cover;
  
• The intake would have to be machined to accept and older style distributor (your current one) so ignition control would be returned to your ECM, and timing would be more adjustable by you;
  
• A fitting would have to be adapted to your current exhaust manifold to provide EGR gasses to the EGR valve. If you retained the original LT1 manifolds that come with the engine, this would be already in place;
  
• Your engine compartment would have to be fitted with the reverse-flow expansion tank, and the radiator would be plumbed slightly differently.
  
• You would have to provide some sort of timing indicator for the harmonic balancer to set the ignition timing;
  
• Your ECM may have to be reprogrammed to optimize the spark and fuel curves for the better breathing LT1 intake and heads.

If you search the archive for all those who are installing LT1 intakes on their cars, you'll find that using that intake with a conventional distributor isn't too difficult, and you wouldn't even need to have the coolant crossover modifications, since the intake is designed for the heads. Just machine a hole for the distributor, remove the oil pump stub shaft while the intake is off, and install your older distributor and wiring.

Corry

Well, I have seen that I can make the speedo work, and can probably convert the tach pulses.

I guess the hard part would be the whole driver information center....it checks the lights, doors, coolant, etc, and below that has a MPG, Odometer, Tripometer, and some other stuff. I suppose I'd need to track all that down and see where they get their data from....be it the computer has all those numbers already and/or a several 7 segment decoder build in, or if the dash reads fuel consumption/time and MPH and then divdes out the the time leaving only miles and gallons lol. That would be the nice easy way....except for the fact that changing the FP, Injectors or entire intake would throw that off tremendously unless it had something to recalibrate on it....
I wonder where the odo number is stored though? I thought that would have to stay with the car. Now mine is already certified to be inaccurate, which apparently at least in PA was legal after 10 years. I also would like to know how it tested the coolant/etc....

Most of that is irrelevent however, because that lower screen w/ MPG, Odometer, etc is cracked and unreadable. The top one lies about my coolant lol, but other than that is cool. I suppose I should just break out the Helm (which I just noticed is 1 letter away from being help lol) and look this all up...
If anyone knows for sure, post. I really would like to keep it an LT1, and not do any drilling/etc. I can't really say why....I know the optispark is a POS. I'd just like to keep it LT1. Most of the problems have been fixed by modders, like the reverse cooling, etc.
Again, if you know anything for sure about how that digital dash works, let me know. One of these days I plan to have the screen fixed....probably by replacing the entire dash...

Corry

Ok, posted here for posterity.

Digital Dash Information:
If you do a motor/computer swap everything is fine. Well maybe not everything, but as long as it uses the same oil pressure sender values, coolant temp sender values, and VSS values. For everyone doing the t-56 swap we know that trans doesn't use the same vss, but Dakota Digital makes a device that splices in before the VSS buffer that corrects for the differences. As for the senders, worst case you wont have oil pressure and coolant temp guages on your dash....I personally can live without those until I can design a box to correct them, but knowing GM, they are probably very close or even identical. So as far as the dash goes, ODBII shouldnt be a problem. Now I have heard some stuff about like 4 O2 sensors on an ODBII car, I havent verified any of that, so I don't know. I guess thats my next item of research. If I can make the updated computer work, I will probably get one. Otherwise I'll just have to stick w/ the older computer...

Once again, if anyone knows anything about ODBII, could you please post. It will probably help me out a great deal. I have a few other problems, but will post seperately or on different forums...