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Author: Subject: A Series SPI Fuel Injection Speeduino

posted on 8/6/24 at 10:36 PM Reply With Quote
A Series SPI Fuel Injection Speeduino

Hi all, hoping to access some knowledge here to see if what I want to do is possible. The SPI system in my 1995 Mini SPI is in need of updating, components are aging, hard to get hold of or expensive and with modern ECU’s there’s the potential to improve the state of tune of the 1275cc lump and improve reliability hopefully in the process.

My plan is to ditch the SPI injection manifold and injector in favour of a new throttle body and 630cc injector from along with some custom adaptors to mate it to the 1275 cylinder head. I will need to relocate some sensors as part of this but no massive problem.

I want to use a Speeduino ecu to control the engine as these are cheap and relatively easy to build and there is a tuner close by me who can support. I want to integrate this into the existing Engine wiring loom with a patch cable of my own design so the existing wiring loom is in use and if needs be I can swap the old ecu back in.

Is this a reasonable plan of attack and is there anyone who has done something similar either on the A series engine or engines of a similar early fuel injection vintage?

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rusty nuts

posted on 9/6/24 at 08:27 AM Reply With Quote
The biggest problem with the A Series engine apparently is charge robbing where the outer cylinders end up getting more fuel than the inner cylinders which is one of the reasons why 8 port heads were designed. A couple of books well worth reading if you haven’t are David Walkers book on Fuel injection which IIRC has a section devoted to injecting the A Series? The second book is David Vizards Tuning the A series
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posted on 9/6/24 at 01:47 PM Reply With Quote
I may be wrong, but I'm sure the charge robbing is only a problem with the MPI , the SPI is closer in operation to a carb so much less affected.

There is quite a bit of intentional pooling of fuel in the SPI manifold (aka wet manifold), without this the charge robbing is more of an issue than standard, certainly removing the webbing that allows this makes the engine run poorly, so using a different manifold you would need to account for this effect.

Megasquirt MS2 (but not MS1 or MS3) has A/B series semi sequential siamese code for the MPI setup, this would need the manifold/injectors/throttle body setup to match.
I don't think the Speeduino has any siamese head specific code, so only SPI would be an option unless someone ports over the siamese code, this might not be possible anyway due to copyright.
The Megasquirt can also do full sequential siamese injection with a cam sensor and 4 injectors, to allow accurate per cylinder fueling with no charge robbing at all once tuned.

It might be worth looking on the Megasquirt forums to see if there is anyone who has done similar to your intended plan, iirc Canems ecus also have siamese support so may be another avenue to look for similar solutions.

This is on Jean Bélanger's website, he developed the siamese code for the Megasquirt, and this is a overview of that development.

Basic setup and modifications required for sequential operation including the siamese modes.


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posted on 9/6/24 at 01:55 PM Reply With Quote
Might be worth checking on the speeduino forum as there's a few people who appear to have done TBI/ wet manifold setups on the same engine or the MG-B engine too for reference.


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posted on 9/6/24 at 03:55 PM Reply With Quote
Join the speeduino forum and discord server. People have done this before. I've written the decoder for the rover trigger patterns. The mini has (iirc) a 17-1 pattern. This means in 360 degrees you have two missing teeth opposite each other. The downside of this is no ECU can't know within 360 degrees when TDC is.

You've got two options, keep a distributor so the sparks can be dished out to the correct cylinder or start modifying things. For modifying you've got another two options, add a different trigger pattern (eg 36-1 - one tooth missing out of 36) or add something that occurs once per 720 degrees.

Honestly, the best thing is to join the forums and read, read, read, read some more, then start asking questions.

If UK based DIY-EFI is a good source of parts. You've also got someone on ebay ( who's very helpful / reliable - he makes ECU's and also tries to figure out what has gone wrong with them.

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posted on 9/6/24 at 07:21 PM Reply With Quote
Where in the UK are you Gusrus?
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