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]

Found a major error in my calculations. I accidentally included a wrong column in the pulse width calculation. The corrected results are more interesting. It looks like with 26 lb injectors, it would start spraying part of the charge onto the backs of closed valves starting at about 1,500 rpm but with 36 lb injectors, that wouldn't begin until about 2,200 rpm. At 5k rpm about 29% of the charge from a 26 lb injector would spray into an open valve and that increases to about 40% with a 36 lb. injector.

Based on that, it seems like maybe larger injectors would be the way to go, but there must be a down side. Thoughts?

[JamesM]

Found a major error in my calculations. I accidentally included a wrong column in the pulse width calculation. The corrected results are more interesting. It looks like with 26 lb injectors, it would start spraying part of the charge onto the backs of closed valves starting at about 1,500 rpm but with 36 lb injectors, that wouldn't begin until about 2,200 rpm. At 5k rpm about 29% of the charge from a 26 lb injector would spray into an open valve and that increases to about 40% with a 36 lb. injector.

Based on that, it seems like maybe larger injectors would be the way to go, but there must be a down side. Thoughts?

With larger injectors you lose some level of precision in fuel metering. I experienced this first hand when a early version of the MS1 code was unable to control the larger injector well enough for a decent idle.

At higher duty cycles both batch and sequential are putting the same amount of fuel though the open valve just as a result of how long the injector is firing for, so the only real difference is going to be seen below the point where the sequential starts firing on the closed valve.

So when comparing 26lb to 36lb injectors with sequential injection we are basically looking at a trade off between fueling precision across the entire operating range vs possible improvement in the 1500-2200 RPM range due to firing 100% on an open valve, and honestly how much time do you spend driving in that range? I think I would go with the smaller injector either way.

You really took me seriously on the math there! I cheated when i looked into it and just used recorded duty cycles across the RPM range for my setup from my datalogs, wound up with a similar result though, if I recall in my case the the cutoff was going to be somewhere around 1800rpm which for my purposes wasn't worth the effort/cost.

You give up the potential for some features going with Microsquirt but i think that platform has some advantages as well and in my mind it is the better compromise.

[bbrock]

With larger injectors you lose some level of precision in fuel metering. I experienced this first hand when a early version of the MS1 code was unable to control the larger injector well enough for a decent idle.

I was reading about that last night. It sounded like this has been improved in later code versions. One discussion indicated it becomes a problem with pulse widths of 2 ms. Seems like it would be worth doing some more research.

At higher duty cycles both batch and sequential are putting the same amount of fuel though the open valve just as a result of how long the injector is firing for, so the only real difference is going to be seen below the point where the sequential starts firing on the closed valve.

So when comparing 26lb to 36lb injectors with sequential injection we are basically looking at a trade off between fueling precision across the entire operating range vs possible improvement in the 1500-2200 RPM range due to firing 100% on an open valve, and honestly how much time do you spend driving in that range? I think I would go with the smaller injector either way.

If my logic is right, batch firing delivers a charge in two squirts per cycle so a minimum of 50% of the charge will be sprayed onto a closed valve. That suggests some potential benefit up to 2000-3000 rpm depending on injector size because you'd have at least 25% more of the fuel charge spraying into open valves. Batch and sequential wouldn't be fully equivalent until at least 50% of the sequential charge goes on closed valves.

Engine speeds up to 3000 rpm covers pretty much all city driving, which has been about a third of the driving I've done in the car so far. Most of that is speeding up or slowing down and there isn't much chance to cruise a 3K rpm. That's exactly where I expect the most potential to improve fuel economy. Looking at my old mileage records for this car when driving on pure leaded gas, I consistently got mid to upper 30s on tanks of pure highway driving (remember it was 55 mph speed limit then) and don't expect much room for improvement there. But I got low to mid teens in town. Pretty horrible for such a small car really. I don't know if it is realistic, but low to mid 20s in town would be nice.

You really took me seriously on the math there! I cheated when i looked into it and just used recorded duty cycles across the RPM range for my setup from my datalogs, wound up with a similar result though, if I recall in my case the the cutoff was going to be somewhere around 1800rpm which for my purposes wasn't worth the effort/cost.

Ha ha. It was a fun little exercise. Glad to hear we came up with similar results. I wasn't sure how reliable this process for calculating would be.