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

[Frank S]

Had a chance to check some of my data.
With the 21lb/hr injectors I run with 5ms PW at idle and down to 2.5ms in some areas in overrun, which is the smalest PW I have.
Linear range of my injectors ends at 1.5ms and if you add deadtime this is getting close to the none linear region.
This is meassured data, not calculated...

[bbrock]

Had a chance to check some of my data.
With the 21lb/hr injectors I run with 5ms PW at idle and down to 2.5ms in some areas in overrun, which is the smalest PW I have.
Linear range of my injectors ends at 1.5ms and if you add deadtime this is getting close to the none linear region.
This is meassured data, not calculated...

I plugged 21 lbs/hr into my spreadsheet and played with the VE at idle until I got glose to 5ms PW. It hit at about 29%. I used 25% as a lower VE for a margin of safety and recalculated the VE curve. With that adjustment, calculated PW for 36 lb injectors at 850 rpm is 2.5ms (3.5ms for 26 lb.).

[JamesM]

Had a chance to check some of my data.
With the 21lb/hr injectors I run with 5ms PW at idle and down to 2.5ms in some areas in overrun, which is the smalest PW I have.
Linear range of my injectors ends at 1.5ms and if you add deadtime this is getting close to the none linear region.
This is meassured data, not calculated...

I plugged 21 lbs/hr into my spreadsheet and played with the VE at idle until I got glose to 5ms PW. It hit at about 29%. I used 25% as a lower VE for a margin of safety and recalculated the VE curve. With that adjustment, calculated PW for 36 lb injectors at 850 rpm is 2.5ms (3.5ms for 26 lb.).

Well your math is good as that is roughly what i was seeing when running the stock 36lb injectors.

Now stop playing with spreadsheets and build the damn thing (IMG:style_emoticons/default/poke.gif)

[bbrock]

Had a chance to check some of my data.
With the 21lb/hr injectors I run with 5ms PW at idle and down to 2.5ms in some areas in overrun, which is the smalest PW I have.
Linear range of my injectors ends at 1.5ms and if you add deadtime this is getting close to the none linear region.
This is meassured data, not calculated...

I plugged 21 lbs/hr into my spreadsheet and played with the VE at idle until I got glose to 5ms PW. It hit at about 29%. I used 25% as a lower VE for a margin of safety and recalculated the VE curve. With that adjustment, calculated PW for 36 lb injectors at 850 rpm is 2.5ms (3.5ms for 26 lb.).

Well your math is good as that is roughly what i was seeing when running the stock 36lb injectors.

Oh wow! There are several school teachers who would likely keel over in shock to here that if they are still alive.

It's a great relief to have real measurements that agree with my chicken scratch! Thank you.

Now stop playing with spreadsheets and build the damn thing (IMG:style_emoticons/default/poke.gif)

Shit. I thought this was just a mental exercise. You mean I actually have to build this? (IMG:style_emoticons/default/biggrin.gif)

Got a few smaller projects I need to finish before I can dedicate serious time, but I'm ready to pull the trigger on ordering the MS3 kit. Think I finally figured out how the daughter boards figure in. If I've got it right, the v3 board is just a base "non denominational" board and the daughter board determines which version of MS. That was a little muddy until I looked at the assembly instructions. Think I'm going to go with MS3Extra as it will be easier for only about an extra $50 overall.

I sent Mario an email to get an idea of when his goodies will be available for purchase again, and also when the v2 of his CAM SYNC might be ready. That would also greatly simplify things. He has this note on the description: "It will incorporate a small trigger wheel in stead of the flying magnet. For basic applications this can be used without a crank trigger. In performance applications both are still recommended."

Still not sure how much precision is being sacrificed using a gear driven distributor to send tach signal vs. a crank sensor.

[JamesM]

Still not sure how much precision is being sacrificed using a gear driven distributor to send tach signal vs. a crank sensor.

This is entirely observational but "some"

I went from picking up my tach signal from a pertronix on a locked dizzy to Marios 36 tooth crank wheel and the difference under a timing light was visibly noticeable. couldn't tell you in a number what the difference was and im sure its very minor, like less than 1 deg, but with the dizzy trigger there was a bit of jitter under the timing light, the timing mark had a slight haze to it. After moving to the 36 tooth crank wheel the mark under a timing light was rock solid, like i was looking at a laser pointer.