Have a '12 Civic EX with the 51R size battery and am looking at using a more common and more powerful battery. Is it possible to put a group size 35 battery in these ? I know other models (i.e. Accord) have the room to do this but from what I've searched, I've found nothing indicating it's feasible or simply, no one has tried.

 

The 12 SI I bought had a group 34 installed. It was tight but it fit.

 

I removed the battery and measured the space and there isn't much. There's a reason Honda puts this 51R battery in there ! For this model with the 1.8L engine, I'm not sure how well a group 34 would fit. It might though.... I didn't look at every possible battery group size either because then you run into how common or available a (slightly) larger one is.