hello all i know this has been covered in other threads, somewhere out there deep in the fourm, but is a twin engined car road legal ? i know there are people building twin engined cars
forum.retro-rides.org
Road tax on our cars is based on CO2 emissions..... so in theory sticking in a 2nd engine, you have doubled its official output.. IIRC The Micra DIG-S only *just* qualified for £0. Might be worth discussing with the DVSA on your plans. There is a question mark on the legality of a twin engine car on the road, but it seems likely it will need to be SVA'ed at minimum.
The other thing to note in one the the threads is the ability to corner the car with twin engine in a safe manor. Essentially what you will have built us a car with 4 wheel drive system without any control over torque/power distribution. The Micra has a short wheel based, and with extra weight to the rear could make it a handful to drive. Its something to consider as 4WD systems which are designed to aid cornering or power delivery will push/pull power from specific ends or even corners depending on the situation.
With your twin engine setup this wont happen and it will be more or less a 50/50 split full time. If you stick with open diff's then that could make the car rather unpredictable!!
Not wanting to put you off this project as it would be quirk and interesting, but throwing an engine in the the rear of a car is one thing.... making it functional for anything else other than straight line grins is another! You would probably gain more from seeing how much power could be gained from any upgrades of the existing single engine. From a simple supercharger pully change, alternative supercharger, turbo conversion (however as the car runs a miller cycle, Nissan claimed there was not enough exhaust energy to make a turbo viable, but I'm wondering how much of that could be tuned out)...... I dare say a gain of around 40hp would be all that's needed to net the same performance of the 2nd engine without the weight.
However, is the engine/gearbox strong enough to be played with like that as despite its age, there seem to be very little 'modification development' being done.