PLEASE BARE WITH ME ... I'm new at this. I am trying to convince myself to purchase a PROM burning setup, the dilemma is ... will I be able to do the things I need to do? I have read the arcticles I could find concerning PROM burning and this site has made me eager to learn, But ... one thing that I do not understand very well is just exactly what can be changed. Speedo Ratio for Elect. Speedo? Auto to Man Trans? Fan on/off Temps? Idle RPM? How big of Miagrain will result from attempting A/F mixture changes? How about timing curve? These are the things I need to adjust. The wire harness/computer/chip is for a 92 Z28 350 TPI Auto. The car is a 92 rs 350 TPI man Trans Originally it was a 305 TBI *LOTS of runability probs* ... Also... Just bought a scan tool program and 02 voltage seems to fluctuate A LOT from one extreme to the other at steady speed and throttle position and mostly in closed loop, is this normal? Thanks for any HELP!



[This message has been edited by ptc92rs (edited January 26, 2001).]

 

Dont tell me this one has everyone stumped.

 

Well, you have asked a lot of questions and many of them require a long discussion in their own right.

I am going to just answer a few, and then let others answer some of the rest, or I may get back to them later.

The Speed ratio (pulses/mile) has no effect on the display of the electric speedo. It DOES affect how a scan tool displays and how the ecm interprets it, but it has no direct effect on the display of the speedo. Sorry, I personally wish it did so I wouldn't have to change my speedo gears when I swap in my 3.73 gears that I have in storage.

The Auto/Manual allows you to select the transmission the bin is for. It will not disable the TC settings in the BIN; they'll still display, but the eprom will not use them and work on a manual car without triggering an SES code of some type. It will use the "shift light" instead of the TC values. Useful if you swap in a standard behind an L98 which never existed in a 3rd Gen F-body.

The Fans On/Off temp controls the temperature that the fans will come on and turn off. Useful if you want to run a cooler T-stat or (as I do) to have the engine run at a very consistent temperature. Fan #1 is the one to use as the temperature for Fan #2 will only come into effect if the A/C is on and you have the "high pressure" switch kick in. I am currently looking into the "hack" to figure out how I can bypass the "high pressure" requirement and make it work with just when the A/C is on for Fan #2 and the temperature requirement. With the A/C off, you'll never need the second fan...only when the A/C itself is on.

Idle RPM works as it names imply. You select the desired RPM for various coolant temperatures and the ecm will then try and run the engine at those rpm levels. If your IAC is operating properly, it will work. I have used this to make my engine idle smoother at certain rpm levels. Should be looked at if you want to run a cooler T-stat as your engine will rev too high.

The A/F mixture is not actually touched (other than the overall ratio in closed loop). If you are saying how to adjust the VE tables... the changes are easy...determining the proper amount to change them is another story. Ditto with the timing curve.

With Speed Density, I use a scan tool (Diacom+) to capture a lot of info while I drive around. I then review this captured data to determine how close I am to 128/128 on my BLM/INT. I actually "lock" my BLM "min and max' to 128 and then rely on the INT reading. If the values are above 128, I will "richen" the mixture. If the values are lower, I will "lean" the mixture.

If the car needs a "global change" because it is rich or lean through out the entire rpm/map range, I use the Injector Flow Constant to adjust it. If I only have a specific area of the RPM/MAP area to deal with, then I do "fine tunning" within the VE table.

Before you start playing with your VE tables, I recommend that you lower your spark readings first, ESPECIALLY if you are leaning the mixture. This will avoid blowing a hole in your piston. Once you are confident that you have the tables fairly close to what you want (ie.. the injector pulse widths yield close to 128/128 throughout the rpm/map range; then play with the spark to the point where knock retard is occurring.

And after that, go back and play with the VE table again, as they BOTH effect each other.

It sounds like a LOT OF WORK... and it is. But remember, you are making a custom eprom SPECIFICALLY FOR YOUR CAR. If you purchased an aftermarket eprom, it wouldn't even be close to what you probably adjusted it to on the "first pass".

The first time I started playing with my injector pulse widths, I was TOTALLY RICH..so I made a global change by adjusting my Injector Flow Constant..specifically tuning for a smooth idle with a reading close to 128/128 (though do not get hung up on 128/128...smoothness is far more important IMO).

This reduced my richness problem through out my VE tables.. I then just pluncked away at the various RPM/MAP readings to where I now have almost all my readings within 122-135. This is FAR CLOSER than my GM eprom and aftermarket POS ever was. But I am a perfectionist and I want (no, I demand) a reading within 126-130. That will be as close as you can ever get it as the VE Tables have an adjustment variance of only .3-.4 minimum. (If your original VE value was 64.6... you can only adjust it +/- .3 or .4 to either 64.2 or 70.0 .. NOT .1 increments)...thus 128/128 is unattainable....but readings from 126-130 are.

My fingers are tired now, so I am going to stop here. We REALLY do need some more tuning tips. There are a variety of methods and I personally like to use "every method possible" because they all have advantages. There is NO ONE WAY to tune...eventually you need other methods to quantify and verify your adjustments.

Lastly, I like scan tools, especially for the 8192 baud ecm...but don't be a slave to one, always verify your changes with other methods....the ultimate being a time slip at the track.

[This message has been edited by Glenn91L98GTA (edited January 26, 2001).]

 

Thanks!! Finally ... of everything I have read, that one was in english! If anyone has more answers, Bring It On. Oh by the way, what is the alternative for adjusting the speedo after a gear change? Mine does not appear to have a gear, only an electromagnetic pickup.

 

Actually you DO have speedo gear and it is the only way to correct your speedo for a gear change. The gears are unique for VSS cars but that is how you adjust the speedo.

For 3.73s with an automatic, the gears are 15 tooth drive and 45 tooth driven.

 

I dont doubt your word but I'm getting conflicting info. I called a man today here in Atlanta that calibrates speedos. He told me the price was 3 Franklins to do mine because it could not be fixed with gears because it was an electro pulse pickup from a rotor. Looks like he may be trying to pull one over. If you know, could you explain how it really works? Are the gears internal to the tranny?

 

There are two methods to correct the speedo for a VSS car (which yours and mine are): 1) Install a Cyberdyne box that alters the signal for about $90 (last time I looked) and 2) swap the speedo gears.

Swapping the speedo gears on an automatic involves replacing the gear on the output shaft of the transmission (this is about 1 to 1.5 hours of shop time max). This is the drive gear.

Then there is the driven gear which has the VSS sensor connected to it. It takes less than 15 minutes once the car is lifted.

Go down to your local GM dealer and ask them for the time and labour quote. And then use that to negotiate with some of the smaller shops. As for that guy, find someone else.

It is similar for a manual tranny but they use different gear combos and I am not sure if they are available.

You should act fast in getting the combo from GM. I understand that the cable version is no longer available and probably the VSS version might get discontinued soon. I picked up my speed gear combo a year ago as I was planning to install 3.73 gears. I now have a complete 3.73 posi rear end...just have to wait for spring to do it.

But most assuredly, you can only correct for a gear change with either speedo gears or a Cyberdyne box.

 

Good info! Thanks! Do you know where to find the cyberdyne adapter? (I have a manual Tranny) Can anyone comment on the other ? Just bought a scan tool program and 02 voltage seems to fluctuate A LOT from one extreme to the other at steady speed and throttle position and mostly in closed loop, is this normal? Thanks for any HELP!

 

Re the O2 sensor...that's normal. It oscillates above and below 450 mV as part of its normal operation. If it remains steady (high or low) while in closed loop for more than 3 or 5 minutes (can't remember which), it'll throw an SES code.

In WOT Mode, it will remain steady somewhere between 800-950 mVs. But don't rely on the stock O2 sensor @ WOT for tuning. Its too inaccurate.

 

I found the cyberdyne box at http://www.racenet.net/f-body/electrical.php3

 

That sounds good but when I say it fluctuates I mean back and forth from @ 40 to 800 or what ever the high temp may be. Is this norm or something breaking down?

 

It can vary that wide. The bottom line is that as long as it is constantly varying, then everything is normal.