Using Datamaster, what should the normal timing show at idle, driving and WOT for a stock 6 speed '92 LT1? The car has a MSD Probillet distributor on it and someone messed with the manual timing screw. Screw has been reset to 2 turns out as shipped (? GM factory opti setting). Car seems to have lost some power. Injectors are new.

My friend had a mobile diagnostics tech put his computer on it and he said the timing seems low (36-38*) but would not let me see the data stream and said he could not print it out. For $165.00, he should provide the data log. He only does diagnostics, no tuning.

How should, can the screw be adjusted to correct the timing? Car not running, while running and at what rpm? Or data log it and have a new chip burned?

My '94 LT1 A4 (also MSD distributor, modded and tuned) shows about 20*~ at idle and 40-42*~ driving and at WOT.

 

BAFL Prom, 92-93LT1 with 6 speed.
I would think the best thing to do is datalog it and adjust the MSD timing screw.

 

Can I not attach a Datamaster scan file to my post for someone to look at and advise?
I can attach pics.

 

I don't know much about Opti's, but if the timing is adjustable on it, then the timing shown in the Datamaster won't be accurate. It would be like on a distributor car. It assumes certain base timing, and adds to that. But if the base is off, it can't know that.

Is there some way you can check the base timing with a timing gun?

 

No, there is no timing plate or marks on the harmonic balancer as is on a regular distributor driven car.

I have scanned the car with Datamaster while turning the screw on the MSD opti and it does not register any change on the scan.

The scans that I made while drivng the car show the timing advance dropping at spirited acceleration almost down to the same level as at idle. Sometimes it shows a 'Advance Retard' and some 'Knocks" but does not show any retard or knocks when driving at a steady speed and the timing is showing around 38-40* as it should.

I'm thinking knock sensors.

 

Like I'd mentioned, I can't imagine how the ECM could possibly know where you adjusted the timing mechanically to. So the spark it is showing could be wildly off depending on how far off your base timing is. If you can't physically measure the base timing, an adjustable opti sounds like a pretty bad idea to me.

I don't know why you'd suspect the knock sensors are working improperly if in fact you have no idea what your WOT timing actually is. It seems to me it could certainly be real knock events.

 

I only changed the MSD screw, assuming MSD claims +/-5 degrees adjustment, about +/-1 or 2 degrees each way with no noticable difference in the sound of the engine at 2000 rpm's, the rpm suggested to make the change.

This is why I suspect the knock sensors. Since at WOT (100% throttle), the knocks show up in the scan, but not otherwise. Hence it retards back to about 25* at WOT.
Still the car runs very strong at about 38-42* when less than WOT and seems to shut down (retard) at about 100 mph and loss of power.

Actually the base timing is built into the engine components, mechanically, at 4*BTDC and is not adjustible from what I have been told by mechanics, etc. The running timing, idle and above, is in the PCM programmig and should not be retarded by the PCM unless there is a problem. This is why MSD designed their distributor with an adjustment that can add more/less base timing of the 4* built in. Its there before the PCM comes into the picture from what I understand. For what, I don't know. Perhaps the bad idea you mention.
Perhaps a bad MSD.

My friend is about ready to sell the car since he has changed the Opti, injectors, P/R, plugs and wires twice, water pump, the fuel pump and fuel/air filters with no joy. Before the problem started the car would run above 150 mph and still pulling (6 spd).

Thanks

 

Hey, it's your car. I've neither seen it nor am some kind of expert. If you want to change the knock sensor go for it.

However, knock is most likely to happen at WOT when the cylinders are the most full, the temps are highest, etc. Knock retard only at WOT would not make me suspect a faulty knock sensor.

I don't know anything about MSD's opti, but how is it only adjustable 1-2 degrees each way if you say they claim it can be adjusted +/- 5 degrees?

As far as "base" timing, I meant the timing difference from the ECM sending no advance and the actual advance that will happen in that event. Perhaps I used the wrong term, as possibly the ECM gets some kind of crank signal that it uses as the "base". I don't own an Opti car and am not familiar with the specifics.

Whatever term you use, it seems pretty obvious that the overall timing can be mechanically adjusted without the ECM being aware of it, meaning your overall timing is a guess if you don't measure it. Otherwise, what does turning the **** on the opti do?

 

Aurora40 is correct.

if your knock sensors are reading they are fine.

if your timing is too far advance it will retard timing due to knock. your datamaster will show knock counts but there is a filter that seperates engine noise from true knock.. when the suspected knock frequency from the sensor sends its signal to the ECM, the filter interperets the frequency and if it falls in the factory frequency range for detonation it will react by retarding timing. if it is not in the suspected detonation frequency range it does nothing but will show that knock counts were received so that the ECM knows the sensor is still functioning properly.


your "BASE TIMING" is always used by the ECM. all of the internal timing tables are add on's to this very vital engine position locator just as Aurora40 stated. Having an MSD Billet Distributor and not knowing where your true base timing is can be a bad thing for performance and the health of the engine.

Here is the correct way to fix your issue. Whether you choose to do so or not is solely dependent on how stubborn you are or how lazy and unwilling to get dirty to complete the job.

You need to remove the number one spark plug, use a spark plug hole piston stop and turn the motor until it stops against the piston stop. . at this point you need to add some form of stationary pointer to your engine near the balancer that will be visible with a timing light from above the motor. Doesn't matter where the pointer goes as long as it is sturdy and will not move on you!

Once the pointer is installed, make a mark on the blanacer where it is pointing too. now turn the engine over the opposite direction until it again hits the piston stop. again make another mark on the balancer where the pointer is pointing to on the outer rim.

now the center position between those two marks you just made is true TDC for the #1 cylinder and is what everything in your ECM is calculated off of including ignition timing, fuel injector timing etc etc.

Measure the diamter of your balancer and order yourself a timing tape sticker and put it on with the zero at the center TDC mark you found.. now you can use a timing light and verify your base timing or at least adjust the MSD distributor using a timing light until the real flashing value on the balancer matches the value in the data stream.. now your ECM and the mechanical timing are synchronized.

This is the only way to do this (with the engine together and in running condition) there is no other way, there isn't a second way to skin this cat and it is supposed to be done when installing the MSD Billet Distributor or any other ignition system that has an adjustment to set base timing and a computer controller timing advance.

So you can either try throwing parts at it, and find it isn't helping or get the setup synched as it is intended to be used and then you can put the car on a dyno and datalog it for knock and find what advance makes the best power and torque for your application without detonation with the fuel you will primarily use and set it!

the adjustment on the MSD is primarily used to re-adjust the optispark if a camshaft is used with a different then stock advance or retard adjusted in. it "can" be used to do minor archaic tuning that will change the entire timing curve across the board, but I would recommend using a ECM editor such as Moates software so that your part throttle tune can be adjusted indirectly to your WOT timing curves.

Chris (I am an expert )

 

Perhaps I didn't make myself clear. Actually the MSD unit can adjust +/-7* as some have measured. MSD only advertises the +/-5* for some reason. What I said is I only turned the adjusting screw about 2* each way, not the whole +/-5 or 7* it is capable of moving the mechanical internal components of the opti, to see if it changed the data scan. I just wanted to see if it would affect the scan.

That was the base timing I was referring to, the mechanically built-in 4* advance the LT1 was designed to have, without any electronic advance being sent from the PCM. The '92 through '94 LT1 is a strange beast in that it does not have a crank position sensor as I believe the '95-'96 LT1 does. Not really sure of the '95. The opti is driven by a splined shaft with one spline missing, and is the keyway, running off the end/center of the cam shaft and is the only way it knows where everything is positioned. It does not even have a keyway in the crank shaft, just timing marks on the 2 gears to position the crank and cam letting the timing chain keep everything where it belongs. Get the 2 marks on the timing gears aligned correctly and nothing else can affect the timing, not even possible to get it 180* out and the reason it did not need a pointer and timing marks. Yes, it is a beast by design.

If you haven't seen or worked on a LT1 opti, it would blow your mind wondering how it works. Just don't look inside of one because you would then lose your mind. It has 2 disks, one with holes and one with slots in them, of different sizes that an optical (hence the name opti) sensor reads as they spin and sends signals to the PCM and even a rotor. One is, lets just say, measures the rough timing and the other is the fine tuned timing. The designer is a genius, but should have figured how to put it somewhere other than the front of the engine where you have to tear everything off to get to it....especially since it fails so often if sprinkled with water and especially coolant, a killer of opti's. GM didn't want anyone screwing with the mechanical base timing since the PCM programming could do all it needed and very accurate....much more than a vacuum advance.

I really don't suspect the knock sensors for the reasons you and Icvette mentioned, but I have seen stranger things happen on these engines to cause them to go nuts.

 

Icvette, I learned about the knock sensors on my own '94 LT1 when my opti went out at 82k miles. I replaced mine before I learned how a LT1 worked (my first Vette) and didn't know about the infamous Opti Spark. Didn't take me long to read up on it though and buy a set of FSM's. The early knock sensors are too sensitive, filter or not. You need to replace them with the '96 ones that are not so sensitive. Even the good ones can cause you a headache by picking up other external noises besides the usual engine noises.

You couldn't even turn the opti shaft by hand, but it would run like a scalded ape until it got up to operating temperature. The shaft and bearing was rusted so bad I don't know how it even ran at all. The inside was full of coolant and grime.

The MSD opti adjustment is for use on racing engines driven by a opti and need that +/-* tweak on the mechanical side. But what do you do when an idiot desides to turn the screw because he had 2 plug wires crossed, injectors and P/R both leaking. He was told NOT TO TOUCH THE SCREW, it is pre-set by MSD and is not used on a stock engine.

MSD only says the screw is 2 turns out (equivalent to the factory setting for a GM opti). I have heard some say that they could tell the difference by the seat-of-the-pants adjusting it. Bull.

I'm not sure if the MSD opti can be returned to its original shipped setting without putting it on MSD's machine. I don't think your suggestions for setting the base timing would would work since there is no way to reset the adjustment screw to it's proper location. If the screw had not been turned, there would not be a need to locate base timing. Your way would only give you the designed (built-in) 4* base timing. It would not correct the error in the location of the rotor caused by the opti adjustment. Perhaps on a dyno it could be corrected if the operator knows what he is doing.

By the way, this was not done by my friend but by the mechanic that installed the opti. I was not available to install it for him at the time and the damage was already done, just not owned up to until later when I questioned him.

Thanks guys. I think it is time to go to the opti store.

 

OK, guys, I just got educated by a friend on how to do what you said above to reset the Base Timing and get the MSD reset to the 'shipped setting'. Hope that fixes my other friend's car.

I hope this fixes the fiasco the mechanic caused. If my friend doesn't sell the car before I can get it fixed, he wants to put the LT4 hot cam in it.

Just ordered the timing strip for the harmonic balancer. Now to make a pointer that will stay positioned properly. Wish someone made a pointer for the LT1's that could be used with the 0-360* tape.

Thanks

PS: I would still like to find someone that is willing to review the 2 Datamaster data scans I made on the car. I would have to send them by email since I don't have a web site anymore to upload them to.

 

1996 OBDII knock sensors will not work with a 1994 OBDI PCM. Are you refering to the 1996 LT4 knock module?

 

the 96 knock module as stl94lt1 mentioned was designed with a higher noise threshold to contend with the roller rockers installed from the fdactory on LT4 motors.. they are fairly reasonable but I have heard some say it works well in LT1's and some say it doesn't jive with them.

I say if you are going to replace the module you should accompany it with the same year knock sensor.

Chris

 

Are the '96 LT1 and LT4 knock sensors different? I'd say the LT4's w/ hot cam. I've only heard they would work and are not as sensitive.

I don't need them for my '94 since I have already replaced them about 3 years ago with the stock LT1's and don't have any trouble with them even though I have the LT4 Hot Cam w/ 1.6 roller rockers, GM/ HD Hyd. roller lifters, ported heads and headers.

 

Go to my thread for this:
http://forums.corvetteforum.com/c4-t...post1581974899