I have a 1991 Z28, LB9 and T-5. And I can't get over about 110 mph. I don't run out of power, it pulls hard all the way to 110. And I'm only doing about 3000 rpm. So why can't I go any faster? Whenever I get up to that speed the car jolts back and forth pretty hard, but won't accelerate anymore.

Is it computer related? I thought that Z28's didn't have electronic limiters?

 

If it happened closer to 5000 rpm, I would have said "that's TPI". But you say it happens at 3,000 rpm - which is where TPI has it's strongest effect.

There are speed limiters inside the eprom (in the memcal which is inside the ECM). It all depended on the tires that originally came with the car. Frankly, I would have thought your car would not have a speed limiter (1991 LB9, TPI & 5 speed), but that doesn't mean that is original equipment either. It may be that your car might not be a G92 and your car came out with non-Z rated tires. The F-bodies that came with Z-rated tires in 1991 had no limit in the eprom.

If you tell me the BCC of your memcal, I will see if I have a version and check up for you.

The other possibility - you have something mechanically wrong.

 

yes something mechanicaly wrong... like a stock 305 cant do a 110MPH in the 1/4... so why do it on the highway...

 

come on now guys my lo3 cranks well into the 130's on the highway dont put him down so bad...car jerks back and forth eh...sounds like a speed limiter thats exaclty what it will do..kill the power until back into the running range and then let it go to town again till you get to 115 or 112 i forgot what the chip at.

 

yes my LO3 would too doesnt mean you should...just get tired of seeing these cars with some teenager smushed into a tree...

 

It's definitely not the TPI. Mechanically I can't think of anything that could cause this, and so consistently.

My car came with 245 r15 z rated tires on it, right now it has z-rated tires too, (w-rated to be exact). It's not a G92, but I still don't think it had a limiter stock.



It really sounds like an electronic speed limiter. I did have my ecu replaced after a fuel injector shorted it out but we used the same eprom I believe. Could this be the problem? I think the replacement ecu was an oem replacement but i need to ask my mechanic.

 

nope i believe you have a speed limiter with the 15 in rims... it used to say it in the info pages but they have been playing with them lately
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wait a minute.. you couldnt get a 15 IN rim on a Z28 in 1991... but still you probably didnt have the Z rated tires to begin with as that was an option...lots of places will sell you a Z tire because it is more expensive doesnt mean it came on the car

 

If it's TBI, then you have a speed limiter.

Go to your the Regional Board nearest you and try and find someone with the eprom burning equipment (preferably someone who is active on DIY Prom). It is not a hard process, but there is a plastic holder in the ECM that holds the eprom and it becomes brittle with age, and easily breaks...


If that plastlic holder breaks then you will really have a problem as the plastic holder is tough to find and I don't believe is available from GM. You will be forced to buy a used TBI ECM (and hopefully not break the plastic hoder...again) So make sure they know what they are doing.

 

theres a very simple way to bypass your vss if your looking for more speed. the way it sounds some guys dont suggest this so im not gonna either just gonna throw it out there. i did it. and its awsome. and i can go really fast. and my car does too. and its alot of fun. but im not sayin you should..

 

yeah the 305's are definitely capable of 130mph easily. Had my 83 LG4 at about 130 yesterday. My 90 Bird w/L03 also pulled hard all the way to the limiter at about 118mph. You can simply unplug the sensor and go as fast as you want but wont know since speedo wont work, or I've heard of some who install a switch on the sensor wire to do the same thing. I dont recomend these practices. I have also heard of someone installing some sort of signal splitter that could be used to shut off input to pcm and still send it to speedo, but I have no clue if thats actually possible or advisable.

The best way obviously is to get that Prom burned or replaced with an unlimited one. If anyone tells you that you can bounce of the limiter 6 or 7 times and burn the circuit out, ignore them, they are full of crap. I know I've bounced mine at least 50 times and it never burnt out.

 

My wheels are stock... maybe they're 16 in and I remember wrong? I can check the original documents to see if I had Z rated's standard.

To Grim Reaper- It's TPI, not TBI. When I said "not TPI" I meant that the TPI wasn't causing it to run out of breath/power at high speed.

And to others- I've heard that dissconnecting the vss disables the limiter, but I thought that sensor was only on the auto transmission?

 

No, the VSS is on manual & auto. Only time a VSS isnt used is on cable driven speedo cars, which yours isnt.

Yes it disables the limiter by not telling the ecm how fast your going, but like I said before the bad part about that is your speedo wont work either.

 

hey guys, go to TBICHIPS.COM and get a new one. they are pretty inexpensive, and they take out the limiter. mine even gave me about 15hp and a few miles per gallon. he leans out the afr under 3k and then makes it a little richer above 3k and 60% throttle or so. mine has been workin great for the past 3 years. I have a 91RS 5.0 with a 700R4.

 

While you do have a standard, the VSS controls a number of features which affect driveability; as well, you have an electronic speedo, so you won't see your speed as well. As well, it will cause the ECM to go into "SAFE MODE" which affects the ECM calibration and thus will not have all the performance possible. With automatics, this causes problems with the locking of your Torque Convertor which means your tranny cooler won't work and eventually you burn out your tranny.

Bottom line - forget trying to trick the ECM. People did this 15-20 years ago when we didn't know how to get inside the ECM. But now that we know how and know what happens when you do - we do not advocate doing this at all. Especially when the real fix is simple and realitively inexpensive.

The only problem with using a "mail order prom" (which is better than "off-the-shelf proms like Hypertech), versus getting someone to do it "hands-on" (such as finding someone in your local Regional Board to do it); is getting the TBI eprom out of the plastic holder. I will assume the person that does this on the Regional Board will know how to do it OR work with you to install the Moates adapter so you can use a modern day Flash Prom and avoid all that silly non-sense. Also, at that point, you can do some other tweaking to help get a little more HP.

As I stated earlier, the big risk with these old cars, the plastic holder gets brittle with age and easily snaps - then you are in doo-doo as they are not readily available anymore. I personally won't attempt getting the eprom out of that plastic holder anymore if I am helping a guy because of that very problem.

As well, you may be able to arrane get it tuned "hand-on" at a dyno with a Wide Band. On a basically stock engine, I find that you need to lean the fuel more often than add fuel - and rework the spark tables.

Even on basically idential engines, I have found great variance in how much spark each engine can handle. And, to get the most power out of an engine, I find taking the spark to the point of "almost detonation - but not quite" is where you get the most power. And this can be just a fraction of a degree.

But, when you get into burning your own chips; you can do this. And when you don't want to burn your own chips, then "hands-on tuning" is the next best thing.

 

Thanks. I've wanted to do burn my own chips and do some tuning for a while but don't have the money. I know it's not a lot, but I'm only 17 right now and really don't have any money. Someday though. Thanks for the help and I'll probably leave it like it is for now.