I have an '89 L98 car. I was told highway mode will get me a couple more MPG. How do I go about finding or making a chip for a stock motor that enables this mode??

 

It's already there in the stock software.

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1982 Z-28

See http://www.mycar.net/mafb/registry/detail.cfm?id=276 for details

 

Yes, the Highway Mode "logic" is already in the eprom. I have worked on both SD and MAF, and they seem to be very similar. All you have to do is "invoke it" is lower the "Minimum Enable temp" and the "Minimum MPH". On the stock eprom, GM purposely disabled it by setting both of those values to their max limit, which will never allow them to work. Resetting those to more reasonable numbers gets the Highway Mode working.

The amount of increase you get will depend on how lean you go. There is a point where going any leaner does not have any positive effect on the gas mileage. You have to play with this yourself. It also is best to have the overall fuel mixture at 128/128 BEFORE you even attempt to invoke Highway Mode.

When I first tried to invoke Highway Mode, my gas mileage got WORST because I was now relying solely on the tables and not the O2 sensor for correction. Even though I was attempting something like 15.7:1 at the time, because my BLM/INTs were too low, I actually was running richer than I would have if I had just left it not invoked and use the O2 sensor to correct me to 14.7:1.

So after working on my Fuel Tables and then invoking it, I then got some positive results. However, when you go leaner, I have found my throttle response seems "spongier". To restore my throttle response, I had to increase my Spark Advance. Again, you can only go so far until you'll notice a "surging".

On SD, invoking the "Highway Mode Spark Advance" is difficult. I've made a post in the past on how to "tweak the bits" to alter the programming so the Spark Advance part of Highway Mode will work.

I have not attempted to invoke Highway Mode Spark Advance on a MAF car, so I cannot comment whether you have to do the same "bit twiddling" as you do with SD. If your lucky, it just might work "as is" by setting the temperature and minimum RPMs.

 

When you do invoke Highway mode and say, set your A:F ratio at 15.5, the ECM is using what constants for that basis? I assume Injector Flow Rate (which is bad for me, because I am using a small injector with the FP higher...) but is there anything else? -Matt-

 

OK so bottom line, what do I have to do to get this to work on my car? I dont have any burning equipment yet, and i dont have a clue what to do.

 

Than buy and aftermarket chip with this enabled


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1989 Iroc-Z 5.7 350, Blue, T-Tops

Lightly Moded:
Edelbrock Headers, Rebuilt B&M 700R-4 Tranny, Higher Stall Torque Converter, Airfoil, 3.4 w\posi, K&N's, Ported & Polished Plenum
-- Blue LED Bakelite Dash & Climate Control
- I am just some 17 Year old spending every dollar I make on my car.

 

Start here ---> https://www.thirdgen.org/newdesign/tech/promintro.shtml

There is also a number of great posts that further describe various "tuning techniques" within this Board. But start with the link I posted above, and then start reading some of the older posts and read to the most current. Yes, its a fair amount of reading, but it will show you the "mechanics" of prom burning.

After that, you just have to "do it".