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

[GregAmy]

I go to all the trouble to write this stuff up...

https://tgadrivel.blogspot.com/2020/03/on-m...914-part-1.html

[bbrock]

I go to all the trouble to write this stuff up...

https://tgadrivel.blogspot.com/2020/03/on-m...914-part-1.html

And I have read ALL of it! In fact, I blame you and your blog for getting me into this trouble (IMG:style_emoticons/default/happy11.gif)

Thanks everyone for all the feedback so far. A lot of great stuff to ponder.

I'm surprised by the discussion on ITBs. I really though the overwhelming advice would be to ditch the fancy air cleaner and use the stock TB. So let's talk about this a little more. My second stop on this journey (after Greg's blog) was looking at the Dub Shop kits. It didn't take long to rule those out. Yes, the ITBs are sweet but are as expensive as a set of carbs and more than I think I could sell my carbs for. Also, I cannot assume they would be compatible with my air cleaner. Yes, the ITBs are the same dimensions as 40IDFs so no issue there, but what about the manifolds? If they are as much as a half inch different and total height than mine, then the geometry of the cleaner intakes will be off and they won't fit correctly. To me, if I went the ITB route, it makes more sense to just use the carbs as throttle bodies. Others have done it. Why would that be a problem. It would eliminate a major expense.

As for the cleaner. To be honest, that was a personal challenge. I'm very proud of that cleaner, but now I've done it and won't cry if I pass it on to someone else to enjoy. Working in the engine bay will be a little easier with the stock TB and cleaner setup. Then again, the point of doing this is to greatly reduce the need to work in the engine bay.

WRT vacuum signal. I guess I should measure what I have. All I can say now is that I'm getting good enough signal from a single barrel to actuate my vacuum advance (port vacuum) and retard (manifold vacuum). My cam is pretty mild. Basically stock lift with a bit longer duration. Enough that it would run rough with D-Jet which is why I'm not just refurbing the D-Jet system. I don't have an appetite to split the case on a freshly rebuilt and running well engine to swap cams.

So the real question WRT ITBs is, what advantage do they bring other than looking cool and letting me keep the cool air cleaner? Honestly, I need more than that. I imagine they provide significant advantages in big displacement engines, but would they do anything meaningful for my little hundred pony power plant?

Re: barometric correction - Yes, I'm talking about real time correction. Where I live, I can easily drive through 8000 ft. of elevation change on a single tank of gas. I'd like to be able to cruise over high mountain passes without missing a beat.

WRT if the Weber setup is that bad. In a word - yes. I would say I hate them. Just the stench of gasoline in the garage they cause is enough to get rid of them. I have 3 fuel injected cars dating back to MY 1991 with a combined total of nearly a million miles on them. All of them have performed flawlessly through those miles. Starting in all conditions, running well at all elevations and temperatures, and all that with zero tune-ups among the 3 of them. I want that for the 914.

[GregAmy]

And I have read ALL of it! In fact, I blame you and your blog for getting me into this trouble (IMG:style_emoticons/default/happy11.gif)

(IMG:style_emoticons/default/drunk.gif)

ITBs are airflow. You'll flow more air with four 40mm(?) bores than with a single 55mm(?), though one can legit argue only one hole is pulling through the stock TB at a time...and as noted above the shorter run gives you throttle response.

I will also note that I'm not seeing a significant drop in manifold pressure as the car revs to redline, so I'm pretty sure the stock setup is not a big restriction. FAT 2056cc with a FAT fuel injection cam.

I've been mulling the idea of replacing my 40DRLAs on the historics race car with Mario's setup, but $$$ and time have allowed me to procrastinate. Plus I'd have to re-do my fuel system (it's single feed line, not recirc) and I'd have to install a voltage manager (I don't run an alternator). But I think it would make for a better race car.

I would not be surprised if you can use your air cleaner with Mario's system; he's using CB Performance throttle bodies which I'm pretty sure are based on Weber IDF carb bodies. Uses the same manifolds, IIRC.

GA