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> Going to Megasquirt, More questions
bbrock
post Sep 11 2021, 10:46 AM
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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.

Attached Image Attached Image

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
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bbrock
post Oct 10 2021, 07:03 PM
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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?
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JamesM
post Oct 11 2021, 02:21 AM
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QUOTE(bbrock @ Oct 10 2021, 05:03 PM) *

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.
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bbrock
post Oct 11 2021, 11:59 AM
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QUOTE(JamesM @ Oct 11 2021, 02:21 AM) *

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.

QUOTE
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.

QUOTE
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.
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Posts in this topic
bbrock   Going to Megasquirt   Sep 11 2021, 10:46 AM
BeatNavy   Brent, you'll enjoy the challenge and learning...   Sep 11 2021, 01:43 PM
Morph914   Brent, I will be watching this closely as I may sw...   Sep 11 2021, 02:13 PM
Chris914n6   I'd run Speeduino over MS as it's a newer ...   Sep 11 2021, 02:44 PM
JamesM   I'd run Speeduino over MS as it's a newer...   Sep 11 2021, 04:53 PM
Superhawk996   Just sayin' . . . . https://thedubshop.com/du...   Sep 11 2021, 03:10 PM
r_towle   Buy ITBs that look like carbs Buy manifolds with i...   Sep 11 2021, 03:22 PM
JamesM   Throttle Body? Given you already have the cool ai...   Sep 11 2021, 04:30 PM
930cabman   Throttle Body? Given you already have the cool a...   Sep 11 2021, 04:44 PM
JamesM   Looks awfully confusing. Is the Weber setup that ...   Sep 11 2021, 04:56 PM
Montreal914   Will follow this thread with great interest. :pop...   Sep 11 2021, 05:02 PM
GregAmy   I go to all the trouble to write this stuff up... ...   Sep 11 2021, 06:03 PM
bbrock   I go to all the trouble to write this stuff up......   Sep 11 2021, 06:41 PM
JamesM   Mario at the dubshop makes some awesome parts and ...   Sep 12 2021, 12:18 AM
GregAmy   And I have read ALL of it! In fact, I blame y...   Sep 12 2021, 09:57 AM
bkrantz   Brent, welcome to the club. You may make progress...   Sep 11 2021, 07:21 PM
bbrock   Brent, welcome to the club. You may make progres...   Sep 11 2021, 08:15 PM
Montreal914   In case you haven't seen/read my partial adven...   Sep 11 2021, 08:59 PM
moto914   Hi. Running Microsquirt with ITBs here. The kno...   Sep 12 2021, 08:47 AM
bbrock   @[url=http://www.914world.com/bbs2/index.php?showu...   Sep 12 2021, 10:49 AM
JamesM   [b]@[url=http://www.914world.com/bbs2/index.php?s...   Sep 13 2021, 04:46 PM
bbrock   I still think you are adding needless complexity ...   Sep 13 2021, 07:51 PM
JamesM   Sadly, the more complicated it looks, the more I...   Sep 13 2021, 11:07 PM
Superhawk996   Sadly, the more complicated it looks, the more I...   Sep 14 2021, 05:38 AM
bbrock   @[url=http://www.914world.com/bbs2/index.php?show...   Sep 15 2021, 08:08 AM
Superhawk996   I tried, but can't let this slide. . . . ...   Sep 15 2021, 08:41 AM
Montreal914   For ITBs, you have probably read that some people ...   Sep 12 2021, 12:25 PM
bbrock   Good to know about the MAP sensors. Looks like mo...   Sep 13 2021, 08:26 AM
GregAmy   Good to know about the MAP sensors. Looks like m...   Sep 13 2021, 10:28 AM
falcor75   I went the ITB route for my 2.3 but my car isnt bu...   Sep 13 2021, 11:14 PM
GregAmy   What I'm not happy about is the low idle at st...   Sep 14 2021, 06:00 AM
jd74914   What I'm not happy about is the low idle at s...   Sep 14 2021, 07:10 AM
ClayPerrine   If you don't have manifold vacuum at idle, but...   Sep 14 2021, 06:54 AM
VaccaRabite   James is the man, and I'd take his thoughts as...   Sep 14 2021, 08:13 AM
bbrock   @[url=http://www.914world.com/bbs2/index.php?showu...   Sep 14 2021, 12:05 PM
JamesM   [b]@[url=http://www.914world.com/bbs2/index.php?s...   Sep 14 2021, 01:08 PM
Montreal914   [b]@[url=http://www.914world.com/bbs2/index.php?s...   Sep 14 2021, 09:09 PM
bbrock   WOW! Really impressive! :worship: ...T...   Sep 14 2021, 10:36 PM
ClayPerrine   But... yeah. I think I'm going ahead with se...   Sep 16 2021, 06:21 AM
Superhawk996   Check out the MS3 Pro Module. Clay :yikes: ...   Sep 16 2021, 07:39 AM
bbrock   [quote name='ClayPerrine' post='2945887' date='Se...   Sep 16 2021, 08:24 AM
jd74914   But... yeah. I think I'm going ahead with se...   Sep 16 2021, 09:08 AM
Mike D.   Ahhh, yes! The beginning of another 7 year thr...   Sep 15 2021, 10:31 PM
jd74914   They're certainly right in terms of the poor s...   Sep 16 2021, 08:52 AM
Superhawk996   Modern injectors are objectively better. . . . I...   Sep 16 2021, 11:38 AM
bbrock   [quote name='jd74914' post='2945908' date='Sep 16...   Sep 16 2021, 05:00 PM
bbrock   An additional 800 miles of seat time in my car has...   Oct 9 2021, 08:44 AM
Superhawk996   - Do I need a crank sensor? To go full sequentia...   Oct 9 2021, 09:07 AM
JamesM   - Do I need a crank sensor? If you want to run...   Oct 9 2021, 04:53 PM
nditiz1   I have only seen the crankfire setup that McMark w...   Oct 9 2021, 01:15 PM
bbrock   I have only seen the crankfire setup that McMark ...   Oct 9 2021, 01:51 PM
Frank S   I have only seen the crankfire setup that McMark...   Oct 9 2021, 03:56 PM
ClayPerrine   Add a 36-1 wheel behind the fan replacing the spac...   Oct 9 2021, 04:56 PM
bbrock   This is all great info everyone! Looks like I...   Oct 9 2021, 06:17 PM
JamesM   You raise an interesting question about injector...   Oct 10 2021, 01:15 AM
bbrock   Okay, here's some fun with math. I calculated ...   Oct 10 2021, 12:20 PM
Superhawk996   I didn't check your math. I'm impressed th...   Oct 10 2021, 04:15 PM
bbrock   I didn't check your math. I'm impressed t...   Oct 10 2021, 04:18 PM
bbrock   Found a major error in my calculations. I accident...   Oct 10 2021, 07:03 PM
JamesM   Found a major error in my calculations. I acciden...   Oct 11 2021, 02:21 AM
bbrock   With larger injectors you lose some level of prec...   Oct 11 2021, 11:59 AM
jd74914   With larger injectors you lose some level of pre...   Oct 11 2021, 12:38 PM
bbrock   I'm not sure this is really a 'code' ...   Oct 11 2021, 02:08 PM
JamesM   Im just going to leave this here to give you more ...   Oct 12 2021, 12:33 PM
jd74914   Pulses of ~10 ms aren't too bad. But... I d...   Oct 11 2021, 03:00 PM
bbrock   Pulses of ~10 ms aren't too bad. But... I ...   Oct 11 2021, 04:04 PM
Frank S   Fuel Injector Size: https://thedubshop.com/pages....   Oct 11 2021, 03:49 PM
bbrock   Fuel Injector Size: https://thedubshop.com/pages...   Oct 11 2021, 07:45 PM
Frank S   I looked into the daughter board and you still ha...   Oct 11 2021, 10:55 PM
JamesM   It sounds like you really want to go with the 36lb...   Oct 12 2021, 01:17 AM
jd74914   I've looked at Mario's and a couple other...   Oct 12 2021, 07:18 AM
jd74914   Good info! I actually have a toothed cam w...   Oct 12 2021, 07:08 AM
crash914   Call Mario. Ask about the MAX EFI. will integrat...   Oct 11 2021, 04:06 PM
bbrock   Well no reason to stop this insanity now, so here...   Oct 11 2021, 07:24 PM
Superhawk996   You're becoming a real Geek. :lol: I've...   Oct 12 2021, 08:13 AM
bbrock   You guys are awesome! :trophy: I was reading...   Oct 12 2021, 08:38 AM
VaccaRabite   I worry that the larger picture of building an EFI...   Oct 12 2021, 10:22 AM
bbrock   I worry that the larger picture of building an EF...   Oct 12 2021, 11:10 AM
Superhawk996   Here's the view from my window today so I...   Oct 12 2021, 01:54 PM
JamesM   I worry that the larger picture of building an EF...   Oct 12 2021, 12:04 PM
bbrock   I worry that the larger picture of building an E...   Oct 12 2021, 12:35 PM
bbrock   Im just going to leave this here to give you more...   Oct 12 2021, 12:36 PM
KELTY360   Obviously, while the snow is on the ground you nee...   Oct 12 2021, 01:36 PM
bbrock   Obviously, while the snow is on the ground you ne...   Oct 12 2021, 01:57 PM
KELTY360   Obviously, while the snow is on the ground you n...   Oct 12 2021, 02:00 PM
bbrock   :chair: Don't be such a cynic. With all th...   Oct 12 2021, 02:31 PM
Superhawk996   BTW... :idea: see PM   Oct 12 2021, 02:37 PM
jd74914   That question was for the collective. I expected ...   Oct 13 2021, 07:24 AM
ClayPerrine   If you want distributorless ignition, there is an ...   Oct 13 2021, 06:28 AM
Frank S   Had a chance to check some of my data. With the 21...   Oct 14 2021, 09:58 AM
bbrock   Had a chance to check some of my data. With the 2...   Oct 14 2021, 05:49 PM
JamesM   Had a chance to check some of my data. With the ...   Oct 15 2021, 10:15 AM
bbrock   [quote name='bbrock' post='2952564' date='Oct 14 ...   Oct 15 2021, 06:21 PM
Frank S   Still not sure how much precision is being sacrif...   Oct 15 2021, 11:58 PM
JamesM   Still not sure how much precision is being sacrif...   Oct 16 2021, 10:56 PM
Frank S   And here how it looks like if you integrate the MS...   Oct 14 2021, 10:03 AM
bbrock   And here how it looks like if you integrate the M...   Oct 14 2021, 05:36 PM
Frank S   And here how it looks like if you integrate the ...   Oct 15 2021, 01:57 AM
bbrock   I've been wondering if there is an advantage i...   Oct 16 2021, 10:42 PM
JamesM   I've been wondering if there is an advantage ...   Oct 16 2021, 11:04 PM
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