All very fair replies, I get it now. Maybe the key is not to ‘overdo it’, seems that the more ambitious projects get a bit unstuck.
From the humble standpoint of owning a Mk3 1.8SE, my ‘next step’ would probably by the 2.0 litre car with the factory Bilstein shocks. But then again, I get so much joy just driving at 40-50mph in this car that I wonder if I would even appreciate the increased power…
I believe that the ND Skyactive G engine has a system called “Ion Current Sensing” built into the coil packs. The function of this system is to monitor the combustion and detect the occurrence of knock and other combustion parameters.
Can anybody on here confirm if this is so, and also what effect changing the spark plug gaps would have on operation and response of the system, particularly in relation to knock detection?
the skyactive stuff is an engine mode that uses charge compression to ignite the fuel. It is more fuel efficient than spark ignition but it is limited on load as well as due to knock. It is likely that mazda has used this technology. if they did then my assumption is that changing the gap will change the ionization detection as the gap will be a factor into the system.
If you were to modify an nd engine for power my gut feeling would be to deactivate all skyactive stuff from the ecu (or use an aftermarket ecu) as they are primarily for fuel efficiency and economy rather than performance.
Isn’t the ND’s engine a Skyactiv-G engine which operates a normal spark ignition cycle? I thought it was the Skyactiv-X engine (used in some Mazdas like the Mazda 3) that operated a hybrid cycle that changes from compression ignition at low rpm to spark ignition at high rpm?
I was with the impression that all the skyactive family of engines operate on the same principle but it may seems that you are more versed in the different skyactive variants than I. If that is the case and its a normal otto cycle engine with high compression ratio (which makes sense in a sports car) I suppose it makes “life easier” if you wanted to mess with it. If you have very high compression ratio you are very dependant on the fuel used for knock so it is possible to have used the technology you mentioned to try mitigate possible issues due to various fuels being used in the car.
Exactly this. It’s spark ignition on Skyactiv-G.
It isn’t totally conventional however. The mathematical compression ratio is 13, which is managed by exhaust design to minimise back pressures from other cylinders, piston-top cavities that improve combustion, and a pseudo-Atkinson cycle under specific conditions which is achieved by changing the valve timing. If that sounds vague it’s because that’s where my understanding is:)
When all else is said, it’s just Suck, Squeeze, Bang, Blow 
Thanks George and John. You learn something every day!
See the link below which partially answers my original question regarding “Ion Sensing” anti-knock sensing on the Skyactiv-G engine. Personally, I think that altering the spark plug gap must impact on this system and consequently sensing of knocking/detonation. How much that would be and in what sense I have no idea but if anybody on here does please educate us.
https://www.hexorcism.com/16ND/sh13084/
Cheers.
Nothing in the page you posted says the plug gap is vital for knock detection. The in service gap can be
1.1 to 1.4 mm so that would seem to suggest that the gap isn’t important.
I appreciate the spark plug gap is allowed to vary from the 1.10 mm (new spark plug) gap to up to 1.40 mm gap maximum for a used spark plug before replacement. Does it also not matter if the gap is smaller than specified e.g., 0.8 mm. I would have thought that there must be limits to the variation in spark plug gap before impact on the measures taken and the correct functioning of the knock detection.
I really wish it was!
yea i think that’s the case as well.
It’s a burn, not a bang 
If it goes bang then something’s gone badly wrong and that’s when damage occurs.
There are some very good clips on YT which I can find that illustrate this very well.
One thing I haven’t seen mentioned here about the Skyactive spec is DI (direct injection), and it’s this which is the key to it’s performance. It’s been a total game changer in the engine world and has given the petrol engine what it was lacking that the diesel engine had.
… without wanting to be drawn to a long discussion GDI exists for donkey years…
The Skyactive G efficiency benefit is from the high compression ratio and the electrotrickery they used to prevent knock.
It’s DI which makes this possible.
Yes, SKYACTIV-G engines run conventional spark ignition but can switch to Atkinson cycle which is essentially a short compression stoke for a long expansion stroke.
SKYACTIV-X uses SPCCI, Spark Controlled Compression Ignition, using a small “fireball” to rapidly increase cylinder pressure and temperature and can run as a conventional spark ignition engine too. SKYACTIV-X is not used in the ND.
It’s crazy that a my 2-litre petrol Mazda6, with Skyactiv-G, which is a big car - it’s a couple of inches longer than my old 5-series Beemer - does more than 10mpg better than my NB. More on a run. That’s progress!
Yes the ethos of the SKYACTIV engines is efficiency. Rather than go down the route of smaller capacity turbo charged engines, Mazda worked on efficient larger capacity ones. “Challenge Convention” and all that.
Di carries its own issues though. Mini had a terrible implementation with EGR which ultimately meant that slowly but surely your inlet and valves would get covered in ■■■■, because no fuel was flowing over them to keep them shiny. Eventual rough running and EML were likely. DI still has issues to work out, that will affect owners of higher mile cars.
