Megasquirt Question: Minimum number of sensors to run.

Hey All,

So, I am considering swapping out the Bosch CIS on my Porsche 928 to megasquirt(probably V3).I installed a wideband, so I already have an O2 sensor and there is an open bung for a coolant temperature sensor. I can also pull tach signal off the coil.

My question is if I provide a O2 sensor, tach signal, and a coolant sensor what else will Megasquirt need to run?

I have seen threads where people has a TPS sensor is not necessary. I want to start simple and build from there. The amin rason I'm thinking V3 is to control spark... I would eventually like to go this route.

There are way more knowledgeable guys than me, I've done one build with a v2.

I'm pretty sure you will need

Map sensor - may be built into the squirt already depending which one you get

Tps - unless they changed a lot for v3 I don't see how you don't need this

Cam/crank sensor

NordicSaab wrote: Hey All, So, I am considering swapping out the Bosch CIS on my Porsche 928 to megasquirt(probably V3).I installed a wideband, so I already have an O2 sensor and there is an open bung for a coolant temperature sensor. I can also pull tach signal off the coil. My question is if I provide a O2 sensor, tach signal, and a coolant sensor what else will Megasquirt need to run? I have seen threads where people has a TPS sensor is not necessary. I want to start simple and build from there. The only reason I'm thinking V3 is to control spark... I would eventually like to go this route.

I believe the Cam/Crank sensor is only used for spark; i'm not changing over from distributor yet.

For now I'm just looking for the minimum sensors to get fuel working.

The reason people say you can run without a TPS (and you can) is because you can use the MAP sensor to detect changes in your vacuum/boost and react accordingly. The downside of this is that you'll always be in a reactive mode, you can never be proactive.

With a TPS, the computer knows that you've just cracked the throttle from 5% to 80% in the last .2 seconds. It's an exact amount of change over an exact amount of time, and it "happens" for the computer before you need the extra fuel. You can set your acceleration enrichment to start giving it more fuel so it's injecting into the (much greater amount of) air right as it's getting there. You don't run lean at all and it has just the right amount of fuel for the volume of air.

With MAP accell/decell, you crack the throttle the same amount. The vacuum amount in the throttle body starts to drop. This is a noisy signal, so there has to be some filtering/buffering built in, but ~.5 seconds or so after you crack the throttle, the computer finally has enough data to conclude that you are, in fact, opening the throttle more as you lose vacuum. Now it can finally start dumping the amount of fuel in, and guess how much it needs. Because air acts as a liquid for all practical purposes, think of it as water in a jug: If you open or close the lid at the top of the jug, all of the water doesn't dump out immediately, it sloshes about in there. Same thing is happening if you force more in, so you're always reacting to the "sloshes" which can be up to a second after you make the action.

Unless this is a pure drag car, I'd think that the annoying-ness of not having a TPS would be much worse than the annoying-ness of installing one...

@WonkoTheSane

Thank you for the explination. It is not so much an "annoyingness issue" as much as a time issue. I would like to be in a position to drive the car into the garage running and pull the car out running(on megasquirt) in a day or less. Bosch CIS is a "reactive" system just like the MAP sensor scenario. I liken the annoying-ness which you are referring, to how the system behaves now. If TPS is better I certainly want to do that (eventually) but I suspect I need to do some fabrication and put on a new throttle body all together (The 16V Porsche 928 TB is a strange setup.

As a follow up question: MAP (Vacuum), O2, Coolant Temp, and Tach singnal will run megasquirt... albeit less than optimal?

From the two builds I've done, I believe that is correct. You will have to trick it to 'see' something realistic for IAT, but you can do this with a resistor, and then use the 'calibrate thermistor tables' to make MS think it's a normal value, like 130f or something. This will also cause some interesting changes in AFR as the intake temperature changes, but coolant temp is responsible for the majority of it.

I've run with no TPS enrichment, using only map* for accel enrichment. It works, but throttle response suffers unless your base map is on the rich side.

Your ignition system is purely mechanical - no ecm control of advance? I'm not familiar with the 928 setup, but if that's the case, you're golden. If not, you might be able to trick the porsche ECM into running spark with MS running fueling.

If you're running a wideband O2, yeah, that'll be enough to run it. Ideally, I'd add a TPS and air intake temp sensors to that as well, and you'll have enough input for it to work very nicely.

NordicSaab wrote: Hey All, So, I am considering swapping out the Bosch CIS on my Porsche 928 to megasquirt(probably V3).I installed a wideband, so I already have an O2 sensor and there is an open bung for a coolant temperature sensor. I can also pull tach signal off the coil. My question is if I provide a O2 sensor, tach signal, and a coolant sensor what else will Megasquirt need to run? I have seen threads where people has a TPS sensor is not necessary. I want to start simple and build from there. The amin rason I'm thinking V3 is to control spark... I would eventually like to go this route.

Beyond the sensors listed, the only other ones that are really needed are the MAP sensor (which would be inside a normal MegaSquirt) and an IAT sensor. While you CAN spoof an IAT sensor, you lose the ability to correct for changes in weather, and I don't see a benefit to doing this.

We've tuned several cars for MAP based acceleration enrichment, mostly for MSPNP development as two of our targets (1.6 Miatas and E30 BMWs) don't have a real TPS. You can get decent results that way, but putting a TPS on there and using TPSdot is a bit easier and can respond a touch faster.

NordicSaab wrote: Thank you for the explination. It is not so much an "annoyingness issue" as much as a time issue. I would like to be in a position to drive the car into the garage running and pull the car out running(on megasquirt) in a day or less. Bosch CIS is a "reactive" system just like the MAP sensor scenario.

Define "running"?

Tuning a megasquirt, while not rocket science, does have a fair number of subtleties. If you haven't done it before, then expecting it to run as well as a factory-turned fuel injection system with one day's work (including hardware installation) starting from scratch is probably ambitious. If your goal is minimal downtime, can you take some of the sensor install work and do it ahead of time, mounting/etc them in the engine, but leaving them disconnected on the wiring end?

I agree, I would try to get the systems running somewhat parallel, IE sensor mounted cleanly, MS mounted and reporting proper values, all wiring cleaned up, injectors mounted to the fuel rail, new fuel system plumbed, etc. Then plan a weekend to pull the CIS components, do the last 10% of the MS install, and derp with it.

Trying to do the whole thing in a weekend will lead to a sloppy and poorly installed system.