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

[jd74914]

Pulses of ~10 ms aren't too bad.

But...

I don't see where you add an engine pumping efficiency (VE) into the air flow calculation. I didn't check your calcs from a PV=nRT sense so I'm assuming those are correct in terms of air/fuel mass flows with perfect cylinder filling.

The engine is only going to be pumping maybe 10-30%* 'efficiently' at idle making your actual fuel flow requirements much less. Pulses of 0.96 ms are too short. Most injectors have 1-1.5 ms latency (voltage and fuel pressure dependent).



*Edit: Found a post showing Falcor76 has ~20% VE @ idle so I think these numbers are somewhat reasonable.

[bbrock]

Pulses of ~10 ms aren't too bad.

But...

I don't see where you add an engine pumping efficiency (VE) into the air flow calculation. I didn't check your calcs from a PV=nRT sense so I'm assuming those are correct in terms of air/fuel mass flows with perfect cylinder filling.

The engine is only going to be pumping maybe 10-30%* 'efficiently' at idle making your actual fuel flow requirements much less. Pulses of 0.96 ms are too short. Most injectors have 1-1.5 ms latency (voltage and fuel pressure dependent).



*Edit: Found a post showing Falcor76 has ~20% VE @ idle so I think these numbers are somewhat reasonable.

Excellent! Yes, there's a little obscure note in the chart that says VE is calculated at 100%. This is all new to me so I was going to refine the values to include more realistic VE numbers later. Any guidance on some standard VE values to plug in? So far I've only found typical values at max HP.

This is obviously critical to the analysis. So assuming 10% VE, the idle pulse width should be around 1.3 ms with 26 lb injectors and, as you said, 0.96 ms with 36 lb. That puts the nail in the coffin for the larger injectors it seems. It seems even the 26 lb injectors might be pushing the limit. Assuming 20% VE at idle, I get a 2.7 ms pulse width with the 26s (1.9 ms with 36s).

Playing with VE a little and assuming a static 85% through the range above idle stretches out the theoretic band where sequential could potentially benefit. I've added curves for % of charge sprayed into open valves below with a reference line at 50% to show the point where sequential is no different from batch. Once I have more realistic guestimate numbers for VE, I'll post updated tables.