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]

This is all great info everyone! Looks like I'll be adding a crank sensor which was the original plan anyway, but seeing that his new cam sensor will have a trigger wheel (I assume missing tooth) got me thinking.

Thanks @JamesM for pointing me toward those injectors. I wasn't very clear before but I had already decided to go with new injectors. The question was whether to upgrade the fuel rails which I'd rather not do to keep the stockish appearance. I agree that going to all this trouble and running old injectors doesn't make much sense. I figured I'd need to replace the stock pressure regulator too.

You raise an interesting question about injector size though. I read somewhere that for running full sequential injection, it is better to err on the larger size for injectors. The rationale was that larger injectors allow for a shorter pulse width that allows for more of the squirt to go into an open intake valve and therefore could spread the benefit of sequential a little higher up the rpm range. Made sense to me but wonder what others think.

The vanagon distributor is an interesting idea. Didn't know that was an option.

[JamesM]

You raise an interesting question about injector size though. I read somewhere that for running full sequential injection, it is better to err on the larger size for injectors. The rationale was that larger injectors allow for a shorter pulse width that allows for more of the squirt to go into an open intake valve and therefore could spread the benefit of sequential a little higher up the rpm range. Made sense to me but wonder what others think.

I think the theory is sound, shorter pulse width will buy you more time firing on an open valve. You could probably run some math to figure out what the exact rpm difference would be between a larger and smaller injector. Looking at the math before though was what made me loose interest in sequential in the first place, at least for my autox car. Only gains are in emissions/economy at idle/lower RPM and once past a certain injector duty cycle and depending on your cam duration you are firing on a closed valve either way. Given how my autox car is normally driven, wouldnt have had any advantages for me.
Also with these motors usually having to be tuned on the rich side everywhere, including idle anyways i don't know if it would wind up making any measurable difference at all.