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I’m not arguing here at all, enjoying a lively discussion….
As Steve says, it’s very vague !!
The guidance also states….
Acceptable changes
It does not count as a ‘substantial change’ if….axles and running gear have been changed to improve efficiency, safety or environmental performance
Does a more modern fuel injected engine give better environmental performance or is it more efficient than a Sprint engine ? I suspect yes, so the engine swap isn’t substantial.
The first thing I’m going to do when my car is ready is to get an MOT, even if I categorically don’t need one…
Just to come back to this, I think when they say “substantial change” that they mean “replaced with different to stock”. I think they mean more along the lines of modification to the stock running gear/axles that deviates from how it left the factory (i.e. like-for-like replacement of parts for maintenance or repairs)
For example, replacing carbs with electronic injection or changing the final drive ratio would be a “substantial change”. Replacing a bad sprint engine with a better sprint engine wouldn’t be that substantial but replacing a bad sprint engine with a modern engine would be too substantial to argue the benefits. It likely could be argued for some other historic cars like pre-war cars/pre-war replicas where the engine isnt easy to come by and therefore it’s more cost effective/ efficient to fit an alternative engine