What it cost to own a 500$ car part 23

"StudyStudyStudy" (jesterjin)
09/11/2015 at 16:05 • Filed to: None

So if it wasn’t as if I had enough projects already. I sent the log to my tuner, and he said that the bogging issue is probably just under the acceleration enrichment. He told me to take a look and that way I could drive it around until he could put a real good tune on it.

This took me a little while to find, so this post is going to go into tech detail so others can find it if needed.

For megasquirt you enter your engine parameters and it will actually calculate a base table for you. Enter a spark table off the internet and you should be in the ball park. However just like carburetors, when you give the car sudden throttle input, it takes a while for the injectors to scale to the correct value so it bogs. Thus it has acceleration enrichment, which in terms of carb’s is similar to the acceleration pump.

The table looks something like this. Unless the car comes with a TPS, most people run the MAP driven acceleration enrichment in the beginning (chart on the left with the strategy turned to 0). That uses the rate of change in the manifold pressure to determine how much longer the injectors are held open during rapid changes in kPa.

I was borrowing a tune from a car without a TPS so I had my values for MAP threshold set running 0% TPS strategy. This can be tuned using the active chart readout below. You play with the throttle at different rates of acceleration until you can generate a nice curve.

However, while this is ok, using TPS based acceleration is much better. I imagine the ECU can read the TPS faster and calculate a rate of change faster then having to sample the pressure and going backwards. So switching to 100% TPS is what usually happens once you install a TPS.

Since I was running full MAP based AE, I didn’t have any values for the TPS based AE so I referred to google to get my values.

I copied this:

The symptoms were a wicked bog (no power, then a huge burst of it), but rolling into it it would be fine. My problem was that the values I had copied were really not good. Most curves end up looking like the curve in the first picture at the top a fast rise then a gentle slope. This one did not. Combined that with the TPS threshold value of 200%/s meant that unless I was snapping the throttle open at crazy speed, the graph would not come into play, I think even stomping on it I can barely get it to register above 150%/s. After looking up more stats I found out that the TPSdot Threshold value should be just above background, most being around 50 and some going down to 10-20. To check this you can either log your TPS% with the car running, or you can run the real time and read the red display and see what it is peaking at during idle.

Basically what was happening was that when I rolled into the power I was rolling in slow enough that the ECU did not need to engage the acceleration enrichment graph and just ran the fuel VE table. When I stepped harder on the throttle, it would engage the acceleration enrichment, but given the high threshold it would not release any additional fuel.

To do this the correct megasquirt endorsed way. You start with a rough curve high slope value tapering off. Then you turn your car on and look for the TPS%/s change at idle. It should be below 10 and at that point you can run the TPSdot Threshold at 20 or 50 if you don’t want to over fuel. That tells the ECU that under that rate of change to not engage acceleration enrichment. You step on the throttle, run a log and determine at what slope you are running lean or rich and add or detract fuel on your curve to compensate.

The easier way is to generate the graph and look at where the real time tracker dot sits on the graph. Floor it as quickly and harshly as you can and make sure the dot does not move off of your curve. Once that is determined just stare at your AFR and the dot location while you play with the throttle until you are sure it does not run extremely rich or extremely lean on throttle manipulation. You can actually do all this sitting still and just have to tweak it under load.

So that is how the car sits.

It has a fresh acceleration curve and boost controller installed, ready to put some real power down (was pushing 9-10lbs, currently have it set to push 14lbs). Just trying to figure out my axle problems. Going to have to find a way to adapt the output flange of the 240-280z to the input flange of a 95 infiniti J30. There are some adapters out there, and I think a custom axle shaft is also in my future.

Costs: 0, yea! Finally!

Total spent: 19670

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DISCUSSION (9)

deekster_caddy > StudyStudyStudy
09/11/2015 at 16:39

Wow, I’ve got a lot of reading to catch up on! How do you like Megasquirt overall? I’m using Calcom on a very old Speed Pro (pre-FAST) controller. The hardest part is keeping a computer around that’s old enough to read it. I’m about to move to a VM and get a USB-Serial adapter.

StudyStudyStudy > deekster_caddy
09/11/2015 at 17:54

I hope it will be a pleasant read!

I am growing more and more fond of it, I bought it slightly more out of necessity originally for the price point, but it really is open box in what you can control. I’ve deleted a few luxuries like idle control, but it is fully capable of running that, and I love the output options. Can’t wait to mount my tablet permanently.

DocWalt > StudyStudyStudy
09/11/2015 at 18:41

I ran into this exact issue on my newest MegaSquirt setup (well, same setup, new engine) and it took me a while to figure out what I was doing wrong, haha. After making the curve look correct it runs way better than it ever did on the old engine.

StudyStudyStudy > DocWalt
09/11/2015 at 19:16

Yea hope this might help some others, I really should open up my tune file and grab values and how I got to them, that would probably be another good post to do at some time.

Curious, what engine and setup are you running? Always nice to know what other oppo’s favor

DocWalt > StudyStudyStudy
09/11/2015 at 20:34

I’ve got a ‘92 VW GTI 16V. Massive cams, stroked, ported and polished head, some other odds and ends. I just had it dynoed at 162whp, but pulling 1.5inHg of vacuum while at WOT... Serious intake restriction in my factory airbox, haha. I’m using a v3.57 board with some mods done to it for more stable idle, using MS Extra firmware v3.4

What setup do you have MegaSquirt on? I’m guessing a Nissan lol

StudyStudyStudy > DocWalt
09/13/2015 at 19:23

Car sounds good, quite a bit of power out of an NA 4 banger.

Mine is running a 7mgte out of an 89 supra in the datsun.

DocWalt > StudyStudyStudy
09/14/2015 at 12:28

Thanks! I should be up about 20-30hp from where I’m at. Really conservative tune and I have an issue with an intake restriction, I’m pulling a ton of vacuum at WOT.

7mgte... Lovely engine! What turbo setup are you running and what’s done to the engine? Got pics of the car? Sounds like a pretty badass setup.

StudyStudyStudy > DocWalt
09/14/2015 at 13:30

That would be crazy to see almost 200, out of an old 4 cylinder. Is it the 1.6, 1.8, 2.0? That’s over 100hp a liter depending on the displacement.

There are pictures in the last post part 22, the whole build up is on here if you are interested. 7mgte running stock ct26 turbo limited to 0.8 bars of boost (12-13lbs?), arp head studs, MLS head gasket. I just drove it yesterday to shoot a go pro video, it has some rattles and squeaks, but goodness does it scoot.

DocWalt > StudyStudyStudy
09/14/2015 at 22:10

It’s a 2.1. Started as a 2.0 though :) It should be right around 190crank hp right now, so I should be able to make 100/liter without much effort.

I’ll go check out the build! Sounds like a hell of a lot of fun!