I am tuning the black car, and right around 5800 rpm - 6200 rpm, I hit what is equivilent of a spark cut rev limiter. I thought it was the tune, because the VE table looks like it is getting into a higher cell than I planned for, and it may be dumping 20% more fuel right when it gets above 160kPa. So, that is a possible problem.
What else could cause this? juicedz4 (Dan) said that in his cavy, he had the exact same problem, and it was the ignition. By gapping his plugs down and changing out his coils, he got rid of this problem. I am hesitant to beleive this is my problem though, because the plugs are one step colder, gapped to .035" and it is only 8 psi right now. I suppose it could be a bad coil, but it is strange to have happen right around the same rpm range like that every time.
I am going to troubleshoot in a few ways. First, I am going to go to 6k rpm and higher at a low engine load, so that I am not in boost, and see if it still does it. That will tell me if it is a tuning cell problem, or if it is something else.
I guess next step, gap the plugs lower and try again?
Does anyone see anything I am missing here? What else could cause something like a machine gun pop pop pop at around 5800 rpm in boost, in any gear?
I have a few, I just want to make sure I am not overlooking something stupid like “oh mark, that means the motor is about to give out, it is running 18:1” or something dumb. I get my head so wrapped around figuring out the problem sometimes, I overlook the obvious.
I am pretty sure that a cut like this can only be caused by spark or fuel cut, not just a lean condition, but I want to verify. Lean pops are more like sparatic pops, versus a complete cut, correct?
This has to be ignition related, it is just aggrivating me to all end.
Not even close. It’s a 1:1 regulator and only using like 38% duty cycle.
In half an hour, I am going to re-gap the plugs, and try this new tune I just made, leaning out the cells around the cells I am currently in. Hopefully that will get it. If not, I will be back!
.035 is a pretty large gap. gap them down and try again. if your hitting a new load cell make sure you dont have any kinda overboost protection on. also make sure your surounding load cells are similar and flow with the load cell being used. whats your total timing at 160kpa?
What would you gap them down too? I can go down to .025" maybe?
Good call on the overboost protection! I thought maybe that is what it was, but I just went in and it is set up to 225kPa for the current overboost, so it’s not that. Timing is conservative, since I am using the billet steel Ross pistons, which are a flat top design. At 6k rpm and 8 psi, it is running right around 19 degrees of timing. When I would run the domed pistons, I would run closer to 26 degrees.
I smoothed up the VE table this morning a lot, and I am going to reflash it here on my lunch break. I am pretty confident that is what was at least helping to contribute to the issue, but we will see!
I gapped mine to .020" for 15-20lbs so…yeah u def want to be in the .020-.025 range at least. .03x is too big for boost. It shouldn’t be blowing out the spark at such low boost pressure tho, so most likely, its prolly not the plug gap…
I mean, if you are getting 20% extra fuel, and you haven’t touched the timing…well, that explains your popping/hesitation/cutoff. Too much fuel, not enough time for it all to burn in the cylinder, hence shitty efficiency and it finishes combusting in the exhaust ports and ex. mani.
Either take away the extra fuel(w/o leaning out) or bump the timing, or a combination of both, depending on ur A/F, load, blah blah blah.
I’ve had lean “pops” (~13.5 a/f) in medium-high load mid-powerband, where it was exactly like a fuel cut…except the rpm kept climbing (eventually). Just a very unsmooth pull, with hesitiation, popping, etc.
Wait, so you are using flat pistons (lower compression) and you are running 19*…vs when you had domed pistons (HIGHER comp) and you were running 26* of timing??? Both at 8psi??
seems like you figured out you had too much fuel. if its still doing it, i’d highly recommend bumping the timing back up into the 20’s…19* on 8psi is :meh::mamoru:
Rule of thumb: Take your ign values @ 0 vacuum/0psi, and take away 1 degree per pound of boost.
I’ve found that rule of thumb is for pussies, and I’ve since modified it to my personal rule of thumb: Take your ign values @ 0 vacuum/0psi, and take away 0.5 degrees per lb of boost to 10 psi. Take away 0.75 degrees per lb of boost from 10psi +.
If you are scared of pinging/knocking, fill up on delta 100. You should always tune on a higher octane gas so you have the margin of error to play w/ ignition timing.
It’s not knocking, but it wasn’t the gap of the plug either. I went out after re-flashing the megasquirt, and did a first gear slow climb to 6k rpm. As soon as it got to 6k, it did the same thing, instant spark cut, and the car pretty much dies. That confirms to me that it’s not fuel related, it has to be something ignition.
I just did plugs last week, and the gap wouldn’t matter at that low engine load, so it has to be the coil, wires, crank sensor, or the megasquirt itself. The coil and wires are easy, from there, it gets more difficult. I may end up switching spark back over to the stock computer if it does not fix it with coil and wires, to see if that would fix it. That would tell me if it is the megasquirt or not.
Always something huh!
The good news is, that it pulls like a deamon at 8 psi up to 6k. I imagine that the new owner is going to have fun once we work out all of the bugs, and get it up to 30 or so psi
The flat pistons are 9.2:1 afr, and the domed pistons are 8.1:1 compression. From what I’ve read and experienced though, despite the compression difference, domed pistons (or is it dished, not sure if there is a difference…) handle timing better, and allow for more of it, due to the shape of the combustion chamber.
Here is a post I just did on neons.org in the megasquirt section, this may clear up any questions people have
The 98 r/t 20g hahn car, full MS fuel and spark control is going into the machine-gun sounding spark cut at 6k rpm. Sometimes it is 5800 rpm, sometimes up to 6200 rpm. It was happening initially at the top of 2nd and 3rd gear (8 psi), and I was looking at the VE table, so I was just assuming that it was creeping up into the next set of cells, getting like 20% more fuel, and losing spark because of the fuel. I proved this theory wrong when I revved it in 1st gear with a slow rev, while driving, and at 6k, it hit cut again.
This HAS to be spark related, so I am thinking it has to be coil, wires, crank sensor, or the MS itself. Terry is going to try a different coil and wires today.
My question is, what do you think could be causing a consistent rpm spark cut like that? The MS rev limit is set at 7500 soft and 7600 hard, and the overboost protection is set to 225 kPa, so I don’t know what else could be causing it. If some of the ignition modules in the MS were fried, I don’t think it would be consistent, it just wouldn’t run.
Ideas? I can post up logs if necessary, but they look clean, it just looks like the car hits a rev limit, but it isn’t being told to do so.
is this car auto? if not, does it happen when u rev it up in neutral?
yah thats what i said earlier, its most likely not the plug gap. its never THAT simple.
I think you meant dished…domed gives you less space in the chamber, and more compression…
Pistons play a part in how they “handle timing” but its mostly 1) Fuel Octane 2) Combustion Chamber design as a whole, not JUST the piston. If your head sucks, then the shape of the piston dome wont make a difference & 3) Compression.
Sounds like u just need to go thru & test individual components, Its hard to internet tune/diagnose tho.
I didn’t get a chance to do it in neutral, but it did it in 1st gear, with almost no load at all, since I was SLOWLY accelerating, but I would assume it would have done it in neutral as well.
I posted it up on one of the MS specific forums, and someone seems to remember a bug in one of the firmwares where this would happen if one of the enrichment tables wasn’t set to 100%, so I will check that when I get home. That would be slick, if it was just some stupid software setting.
