Going to Megasquirt, More questions Options

[bbrock]

After about 1,700 miles of driving my freshly restored car with rebuilt engine, I’ve decided I’ve had my fun with Weber carbs and distributor-based ignition and am ready to step up to modern EFI and coil on plug ignition. Even with the jetting that came out of the box leaving the carbs running rich, the performance has been fantastic. What is not fantastic is the garage stinking of gasoline, no compensation for altitude, and being generally too fiddly to set up for my taste. My trials with the ignition are documented in another thread. Yes, I know a 1-2-3 would solve those problems but for the money, I’d rather invest in modern COP ignition as part of an EFI upgrade.

My goal is a smooth, efficient, and reliable street machine that I can drive from sea level to 11,000 ft. without starving or choking on fuel. Efficiency is at least as important as performance. As long as I can get at least the stockish 100 hp, I’ll be happy and beyond that, I’d like to wring as many mpg out as possible.

The engine is a mostly stock euro-spec 2L engine. The only mod is a fairly mild Elgin 6048 camshaft with 256 duration for the carbs. A source of pride of this build is this custom 911/914-6 inspired air cleaner I made which I think looks cool and really silences the carbs.



Now for the questions:

• Single throttle body or ITB? I think I’ve made a decision but still interested in thoughts. I was thinking about welding injector bungs onto the carb manifolds and using my carbs as throttle bodies. The main appeal is that I would keep my cool air cleaner to make the other kids jealous. However, it seems the stock throttle body would greatly simplify the conversion. Also, even though the custom intake is designed to allow access for servicing and easy air filter replacement, it does crowd an already crowded engine bay and makes working in there just that much more of a challenge. My stock TB needs some TLC and might have to be sent for professional refurbishing. I think I could sell my carb setup to cover that cost but not sure. The upshot is that I’ve all but decided to go back to the stock TB, but curious what others think.

• N Alpha, Speed Density, or MAF? I’ve been reading up on this and think I understand pros and cons, but still a little confused about sensors needed. With my efficiency goal, I think MAF is the way to go. It looks to me that cutting off the tube connecting the stock air cleaner to the TB and replacing it with a MAF could be a really slick way to add MAF in stealth fashion. Has anyone done this? If not, how does one find the right MAF to use? Other than dimensions, what else needs to be considered?

Another question is about MAP + MAF vs MAF only. I’m a little confused about advantages or when a MAP sensor is needed if you have a MAF.

• Barometric correction – this is an important feature for my location, but the hardware needed to implement it is a little confusing. It seems like if you are running a MAP, then barometric correction is obtained by adding a second pressure sensor (another MAP?) to read reference atmospheric pressure to make corrections to the fuel mixture. How does it work with MAF? Do you only need one pressure sensor to read atmosphere? Or do you still need to reference it against manifold pressure? I assume a lot of this is done in the software but I haven’t looked to far into the tuning part yet. I’m more trying to figure out a shopping list for parts at this point.

• Anyone running a CAM sync and sequential spark and injection? Again with the efficiency goal, this is appealing. Looks like Mario is working on a new version which isn’t available yet, are there alternatives available? It seems people say you still need a crank position sensor even with a cam sensor in the mix. It isn’t entirely clear why though. Lastly, and this is mostly just curiosity, but is it correct to think that the lifespan (in miles) of spark plugs are cut in half with wasted spark?

I have many more questions but this is already too long so will save them for later.
TIA

[Frank S]

Fuel Injector Size:
https://thedubshop.com/pages.php?pageid=44
I'm running 21lb/hr and they are fine for a 120 HP Engine.
Don't go to large as this will be generate problems at idle and in overun conditions with to low pulswidth (out of the linear range of the injectors).
MS3 can deal much better with small puls width as the Dead Time Voltage Corretcion is not linear and MS3 is addressing that problem (which alone is a clear advantage if you compare with MS2 or Mircrosquirt).

Again Crank and/or Cam sensors:
A Crank sensor alone can only be good for Batch injection and Wasted Spark ignition.
Why? Because a engine cycle is 720° Crank rotation, so the ECU does not know if the engine is at #1 or #3 TDC. If you add a single tooth Cam Sensor in addition the ECU will be enabled to define #1 TDC, so you could run sequential fuel and spark.

If you run a Toothed Wheel Cam/Distibutor Sensor, this sensor allone will enable the ECU to define #1 TDC. This is because 720° Crank rotation = 360° Cam/Distributor rotation.

Sequential Modification:
You don't need to modify the MS2V3 board for sequential Fuel and Spark if you use the MS3. The MS3 daughtercard has all the outputs available, you just need to connect them to the JPB Board you are planning to use.

[bbrock]

Fuel Injector Size:
https://thedubshop.com/pages.php?pageid=44
I'm running 21lb/hr and they are fine for a 120 HP Engine.
Don't go to large as this will be generate problems at idle and in overun conditions with to low pulswidth (out of the linear range of the injectors).
MS3 can deal much better with small puls width as the Dead Time Voltage Corretcion is not linear and MS3 is addressing that problem (which alone is a clear advantage if you compare with MS2 or Mircrosquirt).

I think I need to look a little more into MS3 or maybe the MAX EFI option @crash914 mentioned to see if they would let me run a smaller pulse width. MS3 would add about $200 to the project ($100 more than MS2 for the board kit plus $100 for the MS3X board to get the sequential output channels.

I've looked at Mario's and a couple other injector calculators and they all followed the same equation. Looks to me like they are just sizing the injector to be able to provide enough charge per cycle for the HP to be generated. Makes sense but it looks like squeezing efficiency our of sequential injection really depends on the timing of when the charge is sprayed to get as much as possible into the open valve for better atomization. Hence, the obsession over the larger injectors.

Again Crank and/or Cam sensors:
A Crank sensor alone can only be good for Batch injection and Wasted Spark ignition.
Why? Because a engine cycle is 720° Crank rotation, so the ECU does not know if the engine is at #1 or #3 TDC. If you add a single tooth Cam Sensor in addition the ECU will be enabled to define #1 TDC, so you could run sequential fuel and spark.

If you run a Toothed Wheel Cam/Distibutor Sensor, this sensor allone will enable the ECU to define #1 TDC. This is because 720° Crank rotation = 360° Cam/Distributor rotation.

Good info!

Sequential Modification:
You don't need to modify the MS2V3 board for sequential Fuel and Spark if you use the MS3. The MS3 daughtercard has all the outputs available, you just need to connect them to the JPB Board you are planning to use.

I looked into the daughter board and you still have to add the MS3X board to get the sequential output channels. Seems kind of weird MS3 doesn't have at least 4 injector channels built in, but I haven't found anything to indicate otherwise. The MS2 mod is really just removing a few components and adding some jumpers to repurpose unused features as output pins for injectors. If you are building the board from kit, the mod is more just skipping a few steps on assembly. Still, if MS3 can handle short pulse widths, that would be a good reason to use it.