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

[bbrock]

@Montreal914 Thanks for that link! I remember reading it before and thinking it was interesting, but didn't understand most of it. Now I can see the wealth of info there and will be referencing it often!

@JamesM
Thanks again for sharing your knowledge. I do enjoy the crisp throttle response I am getting with the carbs and stock dizzy with vacuum advance. It's been too long since I've driven a 914 with D-Jet to compare though. Definitely something to consider.

I've also done more reading about MAF and unfortunately have concluded it isn't feasible for me. A big plus of heated wire MAF is that it measures air density directly so compensation for temperature and elevation changes are automatically embedded in the measurement. The problem is placement to get laminar flow of filtered air through the sensor. I found a good article that says that ideally you want a length of straight, smooth intake at least 5x the diameter of the sensor on both sides. There are screen and honeycomb diffusers to shorten those requirements but the idea of replacing the connector between the stock air cleaner and TB won't work even with diffusers due to being right next to the throttle plate. Likewise, I could mod the intake snorkel on my air cleaner to accept a MAF, but I think the sensor would get contaminated rather quickly with unfiltered air - especially considering the 3 miles of gravel road between my house and pavement.

I'm not giving up on MAF yet, but it is looking like speed density will be the preferred metering strategy which brings up manifold vacuum signal. Is there a number I should be looking for? Like I said, I get enough manifold signal from a single carb throat to pull the vacuum retard diaphragm in and have the tubes to add ports on the other side or even all 4 throats if needed. I'll measure the vacuum I'm getting now but it would be helpful to have a target.

@moto914
Thanks for the compliment. IFAIK there is no standard for carb manifolds for the 914 but could be wrong. I've seen tall and short manifolds and it looks to me like total height dimensions could vary by manufacturer. Mine are what was sold in the old Weltmeister kits way back in the 20th century. The issue is that the center air box mounting height needs to be matched so the intakes mate squarely with the carb housings. Changing the airbox or carb heights relative to each other but much more than a quarter inch changes the angle and creates gaps. Now it could be that my manifolds are the same height as many that are commonly sold now - I just don't know.

Here's a video I posted some time ago with more detail on the air cleaner. The comment I make in the vid about fitting under the rain tray was foreshadowing. It did NOT fit and required going back in and chopping down the bracket holding the air box about an inch and reworking the intakes to restore the proper fit.


https://www.youtube.com/watch?v=Xm-LOk_IBpg

[JamesM]

@JamesM
Thanks again for sharing your knowledge. I do enjoy the crisp throttle response I am getting with the carbs and stock dizzy with vacuum advance. It's been too long since I've driven a 914 with D-Jet to compare though. Definitely something to consider.

I've also done more reading about MAF and unfortunately have concluded it isn't feasible for me. A big plus of heated wire MAF is that it measures air density directly so compensation for temperature and elevation changes are automatically embedded in the measurement. The problem is placement to get laminar flow of filtered air through the sensor. I found a good article that says that ideally you want a length of straight, smooth intake at least 5x the diameter of the sensor on both sides. There are screen and honeycomb diffusers to shorten those requirements but the idea of replacing the connector between the stock air cleaner and TB won't work even with diffusers due to being right next to the throttle plate. Likewise, I could mod the intake snorkel on my air cleaner to accept a MAF, but I think the sensor would get contaminated rather quickly with unfiltered air - especially considering the 3 miles of gravel road between my house and pavement.

I'm not giving up on MAF yet, but it is looking like speed density will be the preferred metering strategy which brings up manifold vacuum signal. Is there a number I should be looking for? Like I said, I get enough manifold signal from a single carb throat to pull the vacuum retard diaphragm in and have the tubes to add ports on the other side or even all 4 throats if needed. I'll measure the vacuum I'm getting now but it would be helpful to have a target.

If you are going to go with the shared plenum/single throttle body I don't think the vacuum signal for speed density will be a problem. This more becomes an issue when running ITBs. Getting a vacuum reading probably wont tell you to much as it isnt a matter of the engine producing vacuum at idle, in fact you can have poor idle vacuum and still run speed density, the issue with vacuum signals and running speed density is if the manifold vacuum being produced is reflective of the load on the engine at any given operating point. With ITBs (especially larger ITBs) your manifold pressure reaches atmospheric at very low throttle movement which limits the usability of the MAP signal for metering fuel across the entire operating range.

Single throttle body running speed density (preferably %baro) is for sure going to be the easiest place to start. I still think you are adding needless complexity (and cost) going full sequential, but very interested to see the end result.

[bbrock]

I still think you are adding needless complexity (and cost) going full sequential, but very interested to see the end result.

Ha, ha, ha. Boy you aren't kidding about complexity! Digging deeper into what it takes just to get the ecu to support 4 injector channels. Yep - that will be a deep dive. Sadly, the more complicated it looks, the more I'm drawn to it. I'm weird that way.

One happy part is that for the most part, what it adds in cost will mostly be in time because I'll need to build the MS2 myself, then build a peak and hold injector driver board. Combined cost isn't much more than buying a pre-built Microsquirt. One cool thing is that the driver board can run the low impedance stock injectors directly without having to run them through resistors.

I need to step back and think about this part of the project a little more before fully committing, but dammit this looks like my kind of challenge.

[JamesM]

Sadly, the more complicated it looks, the more I'm drawn to it. I'm weird that way.

Believe me, I understand the feeling...

For 15 or so years I ran various Megasquirt builds through a 100% unmolested d-jet wiring harness just to show it could be done. It was a continuous evolution that I kept hacking new features into over the years.

This was the final version I pulled out of the car last year to replace it with a Microsquirt and fully custom harness. There was more I was going to add to it but decided it was time to switch over to something a little more "sane'

You probably wont recognize the board because its not an officially licensed MS board but rather a custom board based on the early 2.2 circuit diagram.

Dont try this at home...

Seriously, after 15 years I can give you all the reasons you dont want to do something like this.



This was how it originally looked before the additional inline hacks to add more functionality.



I wont show you the earlier versions I ran using the official Megasquirt 2.2 board as they were not at all pretty. I was in my 20s, poor, and still had a lot to learn. Think my first running MS conversion cost me somewhere around 100 bucks. Things have changed a lot since then.