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Well, I didn't want to take up any more of another thread, so here we are. A while back, I installed a northstar in a fiero. In the interest of getting it running, I purchased a Holley commander aftermarket ECM. Luckily it was used, so I didn't waste too much money, but anyways...
Most aftermarket computers are good at one thing - being semi user friendly. As in, they usually come with a book. This is the single largest downfall of the GM ECM. It has a steep learning curve. If you've never tuned anything before, you have to learn *how* to tune, and where to change values in the ECM. Usually you're limited by talking to people who have been there, done that.
So back to my project. Computer install was straightforward, and I got the thing tuned (after lots of driving and several months). 3 things I immediately noticed.
1) When the engine temperature reached 180, and the fan turned on, there was a chance it would stall. It didn't happen all the time, because usually it could recover from the 400 rpm dip, but about 50% of the time, it would stall. This would only happen if you were sitting at a stoplight or driving slowly when the fan kicked on. Holley recommended raising the timing and fuel in the cells where it tried to stall. Did that, still stalled. "Maybe you need to replace your fan motor". Did that, checked and replaced wiring, and fusible links, still stalled. Holley stumped.
2) When I would cruise to a stop, sometimes it would stall. Add fuel, add timing... Still stalled, Holley stumped.
3) It would stall until it was warmed up. Now this one was interesting. Holley recommends changing the throttle stop screw until your IAC counts are about 20 at idle when it's warm. The problem was it needed more air until it warmed up. So it would idle great when warm, but when it was cold, lookout! Any time you lifted off the throttle, it would try to stall. Backing out of the driveway was FUN! Holley wanted me to raise the idle when it was cold to fix it. Uhh, no. I have an auto. Having it idle at 1200 when cold would mean I'd always have to be on the brakes.
So, now I have a custom programmed GM computer. Idles like a DREAM. No longer stalls when the fan kicks on. No longer stalls when coming to a stop. No longer stalls when cold. As an added bonus, I got rid of the dakota digital speedometer interface, because I could now use the ECM to control the speedometer output. (As a side note, dakota digital's interface sucks. Imagine stopping at a stoplight and seeing your speedo jump to 50mph. That happened ALL the time. It picked up all kinds of noise from the ignition and computer. I tried all the DD suggestions to fix it, including using shielded cable, but nothing worked. Thankfully the GM computer fixed it.)
The problem is, most aftermarket computers are crap. GM spends millions of dollars creating the best computer to control the engine under every possible condition imaginable. Most of that is put into the software development. This is where most lack. In the case of the fan-kick-on-stall, the GM computer compensates for the power draw by opening the IAC a little bit before turning on the fan. Brilliant. In the cruise-to-a-stop-stall, the GM computer *slowly* closes the IAC, by watching the car's speed. The Holley doesn't even have a VSS input. For the cold stall, I have no idea. Better IAC logic I guess.
So, still undecided on what to get? Here's some questions about your computer that you need to ask.
Does it have/can it:
-VSS throttle follower?
-Can it control transmissions?
-open IAC for cooling fan kick-on?
-open IAC for A/C, power steering, or user defined (?)....
-divide VSS pulses by (any number) to output to speedo (if not you need something like the dakota digital, +$80)?
-all that emissions crap (important to have if you have e testing)
-knock sensor?
-associated knock retard vs knock event severity
-Adjustable closed loop AFR (i.e. instead of 14.7:1, can run it lean at idle like 15.2:1 for emissions& gas savings)
-Highway mode AFR, like above when highway speed/low load is detected
-Spark advance correction vs load and coolant temp
-control for 2 fans with seperate on and off temps, adjustable IAC kick up for both, as well as vehicle speed "off" control
-idle over/underspeed spark control (with adjustable spark advance/retard to bring rpm back to desired)
All of these are important. Some more important than others. GM has it all. The Holley has 2. I don't know about Megasquirt, but I suspect it doesn't have them all either, since it was basically put together by a couple hobbyists.
Now don't get me wrong - the aftermarket computers have their place - namely in controlling engines that otherwise couldn't be controlled. I think they also help beginners understand the workings of engine management. All good stuff, but in my opinion, it doesn't belong on a daily driver.
Feel free to flame.
edit - one more MAJOR point that I learned about a little while ago. I was talking with an engineer that designed the northstar. During a wide open throttle run, after 20 seconds, the computer lowers the AFR to 10.5:1. That's right, 10.5:1!!! Anybody who knows a thing or two about tuning knows that's a ridiculously rich mixture! Why is this done? After 20 seconds of wide open, the pistons start to overheat. The extra fuel is introduced to cool the pistons and spark plugs. With out that, the engine would self destruct. These guys run the engines on the dyno at wide open throttle for hours at a time.
IF I had decided to be naughty and made a high speed run on a flat road with the holley, I would have blown my engine. Thank God for GM's engine dyno testing.
Most aftermarket computers are good at one thing - being semi user friendly. As in, they usually come with a book. This is the single largest downfall of the GM ECM. It has a steep learning curve. If you've never tuned anything before, you have to learn *how* to tune, and where to change values in the ECM. Usually you're limited by talking to people who have been there, done that.
So back to my project. Computer install was straightforward, and I got the thing tuned (after lots of driving and several months). 3 things I immediately noticed.
1) When the engine temperature reached 180, and the fan turned on, there was a chance it would stall. It didn't happen all the time, because usually it could recover from the 400 rpm dip, but about 50% of the time, it would stall. This would only happen if you were sitting at a stoplight or driving slowly when the fan kicked on. Holley recommended raising the timing and fuel in the cells where it tried to stall. Did that, still stalled. "Maybe you need to replace your fan motor". Did that, checked and replaced wiring, and fusible links, still stalled. Holley stumped.
2) When I would cruise to a stop, sometimes it would stall. Add fuel, add timing... Still stalled, Holley stumped.
3) It would stall until it was warmed up. Now this one was interesting. Holley recommends changing the throttle stop screw until your IAC counts are about 20 at idle when it's warm. The problem was it needed more air until it warmed up. So it would idle great when warm, but when it was cold, lookout! Any time you lifted off the throttle, it would try to stall. Backing out of the driveway was FUN! Holley wanted me to raise the idle when it was cold to fix it. Uhh, no. I have an auto. Having it idle at 1200 when cold would mean I'd always have to be on the brakes.
So, now I have a custom programmed GM computer. Idles like a DREAM. No longer stalls when the fan kicks on. No longer stalls when coming to a stop. No longer stalls when cold. As an added bonus, I got rid of the dakota digital speedometer interface, because I could now use the ECM to control the speedometer output. (As a side note, dakota digital's interface sucks. Imagine stopping at a stoplight and seeing your speedo jump to 50mph. That happened ALL the time. It picked up all kinds of noise from the ignition and computer. I tried all the DD suggestions to fix it, including using shielded cable, but nothing worked. Thankfully the GM computer fixed it.)
The problem is, most aftermarket computers are crap. GM spends millions of dollars creating the best computer to control the engine under every possible condition imaginable. Most of that is put into the software development. This is where most lack. In the case of the fan-kick-on-stall, the GM computer compensates for the power draw by opening the IAC a little bit before turning on the fan. Brilliant. In the cruise-to-a-stop-stall, the GM computer *slowly* closes the IAC, by watching the car's speed. The Holley doesn't even have a VSS input. For the cold stall, I have no idea. Better IAC logic I guess.
So, still undecided on what to get? Here's some questions about your computer that you need to ask.
Does it have/can it:
-VSS throttle follower?
-Can it control transmissions?
-open IAC for cooling fan kick-on?
-open IAC for A/C, power steering, or user defined (?)....
-divide VSS pulses by (any number) to output to speedo (if not you need something like the dakota digital, +$80)?
-all that emissions crap (important to have if you have e testing)
-knock sensor?
-associated knock retard vs knock event severity
-Adjustable closed loop AFR (i.e. instead of 14.7:1, can run it lean at idle like 15.2:1 for emissions& gas savings)
-Highway mode AFR, like above when highway speed/low load is detected
-Spark advance correction vs load and coolant temp
-control for 2 fans with seperate on and off temps, adjustable IAC kick up for both, as well as vehicle speed "off" control
-idle over/underspeed spark control (with adjustable spark advance/retard to bring rpm back to desired)
All of these are important. Some more important than others. GM has it all. The Holley has 2. I don't know about Megasquirt, but I suspect it doesn't have them all either, since it was basically put together by a couple hobbyists.
Now don't get me wrong - the aftermarket computers have their place - namely in controlling engines that otherwise couldn't be controlled. I think they also help beginners understand the workings of engine management. All good stuff, but in my opinion, it doesn't belong on a daily driver.
Feel free to flame.
edit - one more MAJOR point that I learned about a little while ago. I was talking with an engineer that designed the northstar. During a wide open throttle run, after 20 seconds, the computer lowers the AFR to 10.5:1. That's right, 10.5:1!!! Anybody who knows a thing or two about tuning knows that's a ridiculously rich mixture! Why is this done? After 20 seconds of wide open, the pistons start to overheat. The extra fuel is introduced to cool the pistons and spark plugs. With out that, the engine would self destruct. These guys run the engines on the dyno at wide open throttle for hours at a time.
IF I had decided to be naughty and made a high speed run on a flat road with the holley, I would have blown my engine. Thank God for GM's engine dyno testing.
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