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MG Midget and Sprite Technical - 3D Ignition map for 1275

Two questions.

I have decided to Megasquirt my naturally aspirated, early '67 1275 (high compression motor, per the VIN and British Leyland manual specs). First question is whether people who have tried this agree that doing fuel-only initially is easier than ignition-only. Seems like lots of DIY websites (including DIY auto tune) think so.

Second, I wonder if anyone has a 3D ignition map I could use as a starting point.

The car now runs HS2s on a stock manifold, which I was about to replace with a custom aluminum job discussed on this forum months ago. Last week, bogging rich at 10,000 feet 40 min. from my 5,000 foot-high home yet again, I realized, heck, I'll just can the HS2s in favor of BMW K100 35mm throttle bodies (don't like the later models with integral injector ports, which won't fit preferred injectors). Their rubber mounting bushes slide right onto the 6061 aluminum tube I'm using for intake runners, and I've designed very simple billet mounts for side-feed Subaru injectors (infinite variety of flow rates available and no overhead fuel rails).

Anyway, I read a 2008 string elsewhere involving an 'Anthony' (Cutler?), who said he had some sample maps for 1275s, presumably in various states of tune. However, I'm guessing from strings I've read here that more than one person may have developed maps from which I could extract something workable for a stock 1275.

Ideally, a fuel map or two would be great, too. However, it looks to me like tuning the fuel maps using a wideband 02 sensor is easier without a RR than dialing in ignition timing. (I need the bread for the MegaSquirt-II itself.) So, any help with 3D ignition mapping would really help me get the thing functioning. She's my daily driver.

I am happy to share whatever final maps I ultimately come up with, in case anyone wants to plug-and-play an early 1275.

Total cost of the DIY intake setup is about $150 U.S. including rebuilt injectors and GM-style TPS attached where the BMW dual-rate model was, perhaps $200 if farming out welding as I will. Add $85 for Rick Patton's billet IAC manifold, which I believe uses an Autozone/Duralast AC106 stepper motor, $53 w/lifetime warranty. I use the AC133 which is $18 cheaper but puts the pigtail downward, so you have to mount the manifold on the edge of something or remove the slit pins and rotate the motor. ;-) So, we're talking out-the-door cost equivalent to a rebuilt HIF6/44 from Minimania.

Any 3D mappers out there?

JM Young

I know Onno has some extensive testing with EFI and I kbow trevor jessie has done some ground breaking those are the 2 id talk to with extra attention

Prop and the Blackhole Midget

Hi Joel

I have a 3D map for a 1.8 KSeries ... it's used on an Emerald, and uses throttle position and revs for timing (not manifold vacuum).

I'd be happy to send it in a txt or XLS should it be relevant; it would be a reasonable starting point for your map, eg. max mech advance is around 32 degrees; max advance (mac & mech) is around 50 degrees (at max revs with closed throttle).

If you wanted to just take a look at it, I could send a JPG of the map using the Emerald app.

If you're using manifold vac, then someone with an Omex system may be able to help.

Anthony Cutler

this is mine..

load is from throttle pot not manifold vacuum.

285 cam weber45 etc.

d cusworth

EFI on an A-series is not as easy as you'd think.
The siamese port design gives some charge robbing issues.
If you want to know all about it (certainly useing the mega squirt) go o the turbominis forum as some guys there have put alot of work in it.
You can also check out the ecu.

For progamable ignition I think the csi ignition or 123 tune are the easiest solutions.
And that combined with my delorto 45 is what I'll use with my new engine.
If the result is not as I want it I might go the EFI route I have some suzuki GRX TB's around..
Onno K

Thanks very much for the help, everyone.

Onno - I've now checked out the write-ups on charge-robbing, and the Canems, Specialist Components, and Emerald solutions. Thanks. Matt at DIY Auto Tune tells me that the MegaSquirt clan has a staged, semi-sequential solution, too:

First, I'm going to go 'wet manifold' for simplicity. The injectors are nearly 5" away from the ports, driven at moderately high pressure for a TBI system (18~20 psi) to take advantage of the excellent atomization of the Subaru injectors, and emptying into runners made of 6061 drawn tubing which should produce good vaporization, due to radiated heat from the MG Metro, dual-outlet exhaust manifold (which really does fit without hitting the footwell; the downpipes simply arc forward a bit, while the slot next to the rail gets extended back a few inches).

Second--and here's the heretic peeking over my spectacles--I plan to use a single 02 sensor in the downpipe for the Metro manifold's branch serving the outer cylinders. Number 1 and 4 are the most vulnerable on the long, steep climbs in NM (maintin 75+ mph or say hello to Mr. Peterbilt), so if the ECU is tuned just rich enough to keep from melting a piston in 1 and 4, I figure 2 and 3 will run a bit richer... and who cares about that?

At any rate, I'm curious to see how well it works in tandem form, versus Emerald's cyclops, wet-manifold experiment:

It seems like Specialist Components' TBI setup is similar to Emerald's solution. SC claims very even temps in the cylinders.

Anthony: yes, please do send me your map. The form in which D. Cusworth sent his would be great, if you can manage that. Is that the JPG version you mentioned? My system will use a TPS for load, which I believe is possible with MegaSquirt-II, and I think it has cells I can fill in from this sort of table. If a spread sheet is easier for you to generate, that's fine.

One question, though. I didn't realize that the curve in these engines goes up to 50 degrees; I thought it stopped far short of that. Could you explain that, please? Sorry for my ignorance.

I forgot to mention that I will not be running a crank trigger, but using a GM HEI set up instead. I've got Rick Patton's Lucas conversion kit, which is very easy to install, and MegaSquirt has an option for the Wells/Autozone DR140 8-pin ignition module. Inexpensive, simple option for mappable ignition.

JM Young

Intrested in that trigger any more details?
Onno K

Hi 23 degrees seems to be the max advance normally seen.
My map was mainly set up on the rollers on full throttle

It ran well on part throttle, idles very well and starts fantastically in all conditions.

I last played with it in 2009 and what surprised me was the load sites.
I thought I had the sites at 500,900,1200 2000,2500,3000 etc

I had the no load 500 set low for ease of cranking, high advance @900 low @ 1200. All to help idle

Hope that helps.
d cusworth

yep, got it all. Run a 1293 supercharged with EFI en ign control by Megasquirt I (extra) and a Crane Fireball XR700 optical ignition.

Runs great, few issues:

- you need a swirl tank
- innovative wide band O2 sensor

I'll sent the maps asap (as I am behind my PC)
a.o. arnold

there you go, fuel and spark.

If you need any help, we better communicate on the MSEFI.COM site

a.o. arnold

Hi Joel, I was just looking at what you said about doing a turbo. I've done it to my 1275 engined BugEye and it's great fun. If you are going to do it properly it needs to be EFI'd not a carby through a turbo. This gives all the controls you need... I use a Microtec computer and there is quite a bit of material in the recent archives of this forum. You wanted mapping, look at . There are also some YouTube addresses... first one on that page wasn't correct... look further down.

Have a great Christmas

Bruce Roberts Australia
Bruce Roberts

Dear A.O. Arnold - thanks for the ignition map. That is very helpful. I don't see a fuel map, if you had intended to post one.

Bruce - Sorry for the confusion. I am not presently considering a turbo. Instead, I am trying to configure a workable, dual throttle body setup. I may eventually end up with a single, larger throttle body, since several people seem to have reduced charge robbing that way without descending into staged or semi-sequential timing.

Why? I guess that I like the aesthetics of dual TBs, the fact that the K100 TBs are just 1mm larger than the HS2s and will use my existing air cleaner housing design and MED Engineering stub stacks with minor dremmeling,, or maybe it's something even less rational, like nostalgia for the original designer's decision to mount dual carbs.

Everyone - pardon my ignorance, but I'm still vexed by how it is that the advance figures on the maps posted exceed 'max' mechanical advance, and the purpose of those higher figures in engine management. A.O. Arnold's supercharged map and D. Cusworth's naturally aspirated map both do this. Anthony seems to suggest above that this is intended to simulate what occurs with an old vacuum-assisted distributor under certain conditions.

One of the reasons this confuses me, is that it happens at the upper R/H corner of the maps. That is, exaggerated advance makes sense under *heavy* load while accelerating, when an ECU's acceleration enrichment feature would spike and then (for some period of time) maintain an expanded pulse duration. More fuel to burn. However, on *trailing* throttle--Anthony's extreme example of max revs with closed throttle--I was under the impression that many ECUs cut fuel or dramatically lean out the mixture. Doesn't that mean there'd be less fuel to burn, and that one wouldn't need to start the burn so far in advance of TDC?

Just trying to understand the upper R/H upper corner of the maps, which seems counter-intuitive to me. I would think you'd have maximum (or exaggerated advance) at the *lower* R/H corner of the map. Perhaps this will become more clear if I had a fuel map to use as an overlay...?

JM Young

Sorry, I forgot. I'll also add a few pics of my home made manifold. My throtle is in front of the blower...

a.o. arnold

A.O. Arnold - thanks for sending the 3D ignition map. Anyone else have one they'd like to share?

Thanks, also for the image of your throttle body. Wow--four injectors. No problems with lean mixture, and you could have staged and/or semi-sequential injection to boot (if you wanted). Neat. It makes sense that you'd put the TB upstream of the blower; that's what Moss does with their carbureted setup, and I think that most supercharged OEM and racing designs do the same.

Regarding the MSEFI website, I'm assuming you are just 'Arnold'; that person has a blown 1380 Midget, so it sounds like it might be you.

Everyone - Given that my aim is getting as close to a plug-and-play setup as possible, which might interest other 1275 Midget owners who face wide altitude and temperature variations, I'll post a new string showing my manifold design once it's welded up. That will probably take a few months, given the way that work, home life, etc., seem to interfere with messing about under the bonnet.

Essentially, though, the manifold is a pair of 6061 drawn aluminum tubes (1.625 O.D. x 1.375 I.D. - available cheap at with flanges at the cylinder head of similar thickness to the MG Metro exhaust manifold flanges (6061 T6511 aluminum extruded rectangle .375 x 2.0). The BMW TB rubber bushings simply slip over the other ends (after breaking the outer edges by hand with sandpaper). The I.D. of the tubes (34.925mm) is so close to the 35mm BMW TB bore that light feathering with a Dremmel sanding drum is all that is needed to match things up nicely.

The fuel injector mounts will attach to the runners similarly to A.O. Arnold's mounts (steep angle), and are 6061 T6511 aluminum extruded rectangle with a 1" well that houses the side-feed Subaru injectors and of course supplies them with fuel without the need for an overhead/outboard fuel rail. A smaller hole at the bottom seals the nozzle's O-ring, and a small step at the top end of the 1" well lets the injector sit flush, so as to use the stock Subaru retainer. Nice little hooded piece affixed by two machine screws.

I got, and almost used, a pair of stock Subaru fuel rails for $25 with (useless) injectors and retainers attached at the junkyard. The spread was wrong, and after I realized how simple the design concept was (1" wells, etc., as described above), I discarded the idea of sawing through the pipe that connects the mounts and reattaching with fuel hose.

Instead, there will simply be a feed line entering the forward injector mount; a second line connecting the two mounts; and a third line going to an Aeromotive adjustable regulator, which of course connects to a pressure gauge and a return line to a surge tank or the fuel tank. Haven't yet figured out whether the regulator should remain keyed to atmospheric pressure or the manifold pressure, but most TBI systems appear to do the former, whereas port injected and forced induction systems seem to do the latter.

JM Young

This thread was discussed between 20/12/2012 and 29/12/2012

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