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

[JamesM]

Throttle Body? Given you already have the cool airbox setup and the manifolds and linkage, and not knowing how your cam will react with a shared plenum intake, I would just pick up a set of ITBs.

Fueling Algorithm? This is going to depend on if you decide to go with a single throttle body or ITBs as well as well as what sort of vacuum your cam produces. Also the MSExtra firmware (that you will want to run) has additional fueling algorithms that I would recommend over the above mentioned. Single throttle body you are most likely going to want to use "% Baro" which is basically speed density scaled to your current altitude. ITBs if your vacuum signature allows I would recommend "% baro" but with ITBs it usually wont, in which case I would run "hybrid Alpha-n" which is somewhat more difficult to initially tune but gives you the best of both worlds.

Barometric correction is always applied however i believe what you are looking for is "Real time Barometric correction" which requires a 2nd MAP sensor if you are using the 1st MAP sensor for your fueling. With a single MAP sensor the MS takes a barometric reading before you start your engine and uses that for the entire run time so if you have a drastic change in altitude over your drive your fueling will be off. Adding a 2nd sensor allows it to get a constant baro metric reading. This is a really easy thing to add when building your harness.


My thoughts on CAM sync and sequential injection are that it is really overkill for these cars. The impact of it is extremally minimal and only makes a slight difference to emissions at lower RPMs as at higher RPMs the injection time required means you will always be squirting on to a closed valve anyways. It does add a lot more complexity to the build and tune though, which is already going to be a steep learning curve if you have never done this before.


Plug life with wasted spark? That all depends on what you are referring to as wasted spark. An actual wasted spark coil pack where 1/2 of your cylinders get fired with reversed polarity has the potential to more than double the wear on the 2 plugs that get fired in reverse, especially if you run stock plugs. This was one of the reasons for some of the exotic metal plantings they stared putting on the electrodes. You can run coil per plug or a 4 coil pack in 2 channel batch fire which is the same as wasted spark in that you have a wasted spark, however all 4 coils still fire with standard polarity so you dont get that extra plug erosion.