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]

An additional 800 miles of seat time in my car hasn't lessened my desire to ditch the carbs and dizzy. I continue researching how I want to approach this and have more questions.

- Do I need a crank sensor? To go full sequential I need a cam position sensor for sure. I've read that cam sensors are not as accurate as crank sensors but that is because of chain or timing belt slack. Type 4 cams are gear driven. Is there enough gear lash in distributor drive mechanism to affect position accuracy? I have no issue with adding a crank sensor, but it is the only part that will require dropping the engine so the question of whether it is necessary if I have a cam sensor has to be asked.

- I'd love to hear thoughts on whether modern injectors would be worth the cost if I don't upgrade the fuel rails. Still not sure what I want to do, but curious why modern fuel rails would make a difference.

- Anyone know when Mario plans to open online ordering on The Dub Shop site? It's been closed since June so he can catch up on orders and restock parts. Sure, I could reach out directly but seems counterproductive to pester a guy who is already swamped. I know some of you are in touch with him so wondered if you've heard anything.

That's it for now. There will be more.

[Superhawk996]

- Do I need a crank sensor? To go full sequential I need a cam position sensor for sure.

I’m gonna get flak but who cares.

No -- you don’t need a crank sensor in the technical sense.

Yes - agree you need a cam sensor for full sequential. If you know where the cam is, then you know when to fire the spark. You can infer where the crank is if you know where the cam is, but, the opposite is not true.

Case example. Early 90’s Miata NA 1.6L engines (timing belt driven cams) have a cam angle sensor only and they run great! Full sequential. However that crank angle sensor on a miata is actually sort of absolute position encoder. It is a $200+ sensor. That car uses to fire up literally in 1 crank shaft revolution or less because of that sensor. Most FI systems need a couple crank revolutions to work out the signal timing between the seperate crank and cam sensors to figure out where to put the spark.

Found a decent link on the Miata crank angle sensor signal if you care:
http://www.rivercityroad.com/garage/cas.htm

Eventually Mazda went to a more conventional Hall Effect crank sensor + Hall Effect cam sensor, probably due to cost and to become more consistent with OBD2 regulations.

The problem with cam only is if someone sells a sophisticated enough cam sensor off-the-shelf and who has the software to run cam only for a T4.

I’d talk with Mario if you get serious about it to understand what his cam sensor was, what he's doing with the signal, and what software he's running with it.

In the limited digging I've done, he seems to have what I'd consider the minimum for easy DIY success:

1) He's done it before with T4
2) He has a dyno
3) He offers a complete kit.

Not negating the great work done by the guys on this site either. Lots of good information here too but the further off the rails you get trying to do non-standard MegaSquirt, the more you're on your own


On your other question

- I'd love to hear thoughts on whether modern injectors would be worth the cost if I don't upgrade the fuel rails. Still not sure what I want to do, but curious why modern fuel rails would make a difference.

I don't think it will net you any performance. I haven't seen anyone saying the OEM fuel rails are a flow bottleneck. Personally, I'm just not a fan of how the weight of the fuel rails, and associated fuel hose are not well supported. Basically just cantilevered off the injectors. It works, but, it wouldn't fly by modern engineering standards and road load testing. That's my opinion.

@bbrock