Hello Everyone!
On Thursday I got my 1986 RX7 race car delivered to my house. Over the last couple of days, I have been doing the standard check-up on the car/engine. The 13b had a narly vacuum leak on the brake booster which I fixed, and I plan on doing a smoke test on it to make sure there are no other leaks that I can't hear. I also changed the plugs and they were very crispy, but I didn't expect anything else for a rotary lol.

Over the weekend I took it for a spin around the block and under any type of throttle input above 2.5k it would stutter and hesitate. It also has an above-normal smell of fuel. After this, I decided to plug in tunerstudio into the MEGASQUIRT and took a look at the readings:
1. The AFR is bouncy and hovers around 14.1 - 15.3
2. The AFR consistently pings up to 18.1 for a millisecond and comes back down to the numbers above
3. I included pictures of the target numbers for the spark advance and volumetric efficiency which all look normal for a rotary but the car refuses to follow these targets
4. Car idles at 1500 RPM not ported, any change to throttle cable tension/spark advance does nothing


The car is just dumping unburnt fuel causing a gas smell and a lot of backfiring to the point that it sounds like it has a popcorn tune lol. Aside from a potential vacuum leak, I'm stumped, could this be a software issue? Any info would help a lot!
Thanks, everyone!

 

First off, I would check the req_fuel setting. It's under Basic / Load, Engine and Sequential. The numbers in the VE table are arbitrary, so they don't need to look exactly like mine, but mine idles around 45 and tops out around 92. Yours is scaled differently, so making sure required fuel is correct is a good place to start. You might find that it's fine.

Secondly, I would look at the injector dead-times and make sure they're correct. I think I'm using 1.0ms and getting good results.

That's a misfire. The AFR gauge reads air in the exhaust. A misfire means no air was consumed, so it reads as a lean spike. It could be ignition related, but read the rest of my post and try these suggestions because it is probably fuel related.

The VE values are not normal. The scale doesn't always matter, but you have a lot of bins in the center (right around 2500 where you start to have issues) that are really lean for some reason. For example, at 40kpa and 2000rpm the VE value is 35.5 and then at 2500 it's 29.9. So that's about a 20% decrease in fuel from one bin to the other.

While that area normally is leaner generally (since it's low load cruising RPM), that's a really dramatic change. I would try smoothing out all those low spots and then trying again. If you are reading lean consistently in a given bin, raise the value and see if it helps. The nice thing about standalones is that you can always just load up a prior tune if you mess something up.

4. Car idles at 1500 RPM not ported, any change to throttle cable tension/spark advance does nothing

Check whether the idle-valve idle control is enabled under idle settings, or the spark-advance idle control. Either will try and compensate for changes in idle. The base idle on these engines is set by turning the small set-screw on top of the intake manifold. Since it isn't ported, you should be able to achieve a smooth 900rpm idle without the air pump stuff, or even lower with it.

As an aside, people here seem to run a lot of advance at idle. The smoothest idle is where Mazda put it at -5 degrees, but I am running 10 degrees with good results (makes the transition into the rest of the table smoother). I get a lot of popping idling above 10 degrees advance.

 

Thank you for the reply, gives me a lot to go off on and make the FC run better. I was able to check on the required fuel and injector dead-times, I will work on the VE table to smooth it our tonight.

I took a look at the required fuel and it does look correct here's a picture from tunerstudio:



I was also able to check on this and right now it's at .8ms. I will change it to 1.0ms and see what the difference is. Here's a screenshot of that:





This does make sense, I will check for spark on all plugs just in case to rule that possibility out.

I will smooth everything out later today as well as play around with the advance, when I was attempting this yesterday there seemed to be no reaction from the car. It could be that I was being too cautious with it and not changing a whole lot.


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Before you change the fuel settings, required fuel doesn't look correct to me. Changing the units to metric makes it a bit easier, but a 13B has 1308cc of displacement and the stock injectors are 460cc / min (assuming you have stock injectors, that is). If you put those values into the calculator it should provide you a different number.

My required fuel ends up around 9.5.

Also, it's a good idea to make sure the firmware is updated on the Megasquirt. I think they changed the way required fuel was calculated awhile back. This is probably why your engine displacement is shown at 2600cc in the settings vs 1300cc. Injector size is also set to 555cc, which is close to the stock Turbo II 550cc injectors. I would verify that all the injectors are the same, especially since they are set to simultaneous staging. Then if they all look identical, check the 2nd gen FAQ to find out which injectors you have for sure.

 

Alright so I have some interesting developments, first off the injectors are indeed 555cc! When I went to start the car with it set to 1300cc it wouldn't even start. I might have not updated the firmware.

Anyway I went to test the injectors to make sure they are working properly and when I went to inspect the O-rings I found this:




I have no clue what this part is called but it was in the injection port and the one was completely snapped in half. This brings up new issues such as metal in the engine but regardless in my mind the flow from the injectors should be uninterrupted no? Does anyone know what this part is called and its use? Would this be the cause of my fueling issue?

 

Those are injector diffusers. Old injectors like the ones used on FCs didn't atomize the fuel as well as modern ones, so that little screen helps it to atomize.

All things equal they do help, but they get super brittle. You can buy replacements, or alternatively you can break off the legs of the remaining diffusers and just use the upper donut piece. If you do the latter, it will impact fuel economy and power. I don't know by how much as I've never done it myself.

One thing you can't do is install injectors without the diffusers, since the upper donut piece acts as a spacer.

 

Ahh never would have guessed, another thing to add to the Book of Discovery

Do you think that would be causing the lean spikes? I changed up the settings on tunerstudio for 1300cc and 550cc injectors and the engine wouldn't even start. I didn't have a chance to fully test the injectors but that is on the list. I stopped when I saw the broken diffusor. Also these injectors are brand new, non OEM.

 

If I'm guessing, this is a combination of a few issues.

1. The fuel settings in the MS just aren't right. I would update it first so that you're using the most recent firmware. Then you should be able to set displacement to 1308cc in the main setting screen. I'm 99% sure that it's set to 2600cc because that's how older firmwares did it. Keep injector size at 555cc if you're sure that's correct. When you go to calculate required fuel, put in 1308cc displacement and 555cc injectors and see what happens. You'll probably get a wildly different required fuel amount.

Required fuel is the basis for how the MS does fueling. It can vary a bit (basically you can use it to adjust the fuel up and down globally for all of the VE table at a time) but to keep things simple it's best to start from a known point.

Then find the dead-time for your injectors. If they're 555cc they aren't stock Rx7 injectors, so I would try to find any data you can on them. The stock injectors are 1ms, so if you can't find the dead-time for your particular injectors you can start there.

While we're here, check your ignition timing. With the car warmed up lock the timing (Ignition Decoder, change the setting on the top right to Fixed Advance, -5 degrees) and use a timing light on the L1 or L2 plug wire. It may run poorly while doing this, which is expected. It should fire right when the red mark on the crank pulley crosses the pointer. If it doesn't you can adjust the CAS (or remove and re-stab it, but avoid that if possible). Then make sure to set the timing back to "Use table" so it uses the Ignition table.

This is to make sure that when the MS3 commands a particular timing (say -5, which is stock idle timing) that is actually when the coils are firing.

2. Once the above is done, all of the fuel settings and timing should be correct. Then you can try (starting at idle) increasing the values in the VE table at that bin until it runs well. Once that is stable, start holding the engine at various rpm and tuning those VE bins. Smooth out the surrounding bins as you go.

Before you proceed to the next step, make sure to smooth out those low-spots we talked about.

Then start a log and go for a short drive on a back-road, and when you get back review the log and add or subtract from the bins.

Keep in mind that it's always easier to start from a rich tune and go leaner than it is to go in reverse. So if every bin is lean, just select the whole table and add some until it's in the ballpark. You can always lean it out later.

EDIT: Also, I just noticed that "Squirts per engine cycle" in your settings should be set to 2, not 1.

 

A cool way to characterize the injector dead time involves running with AFR incorporated, getting a smooth idle with the reported ratio exactly what it is in the AFR table, then change the AFR table by one point richer. The engine should run at exactly that AFR. If not, your dead time is wrong, and you are incorporating that wrong deadtime into your fuel calcs. When it is correct, the computer will correctly add the right amount of fuel.

This is CRITICAL for things like air temperature compensation, since that is a fixed percentage modifying the calculated fuel because the air is more or less dense.

Charts are a good starting point but the deadtime is also dependent on the circuitry in the computer as far as how fast the injector can open and more importantly how fast it closes. I had to change my deadtime when I went from PWM to resistor block and had to change it again when I went from two injectors per driver to one injector per driver.

Make sure you are running at the fuel pressure that you think you are! Not sure about FC but the GSL-SE regulator is 2.5 bar, not 3 bar, so injector flow will be different than spec because spec is generally at 3 bar, but sometimes 4 bar.

 

Brain is trying to wrap around this, if you set the AFR one point richer, and it is not, what is the general rule to adjust deadtime based on this?

Set 1 point off, and it is richer, means deadtimes are too low, meaning the commanded opening too long and leaner, it is too long and not letting it use needed fuel to maintain?

Or do I have those backwards?

 

This is one of those things that I always seem to have to work off of first principles

If your deadtime is too high, then your "real" pulsewidth is short to get the correct fueling. So making it richer will be (richer * too short) + wrong deadtime = not enough injector opening = not rich enough. Conversely, if your deadtime is too low, then your "real" pulsewidth is long to get correct fueling. So making it richer would be (richer * too long) + wrong deadtime = too much injector opening = richer than desired.

So yes, I believe you had it correct.

In practice, I usually just go one direction, if that doesn't do what I want, I go the other way, until it works.

Of course, after you change the deadtime, you have to redo your VE table to get it right again. Which is kinda why you need to get the injector characteristics down first.