Hi

If I were to put a bike engine in the back of a car (a mini for instance) would the noice level be to loud to be used as my daily driver ?

...or is it possible to insulate enough so that I don't need to use ear protection every day

If you had a bulkhead between you and the engine it would probably be OK but I wouldn't want my engine in the car with me, even for short journeys, unless I was wearing earplugs and a good helmet. It's noisey enough when it's 6 feet away and covered.

Bulkhead it is ... wouldn't there be a significant difference in noice with the exhaust exiting behind the car instead of "next to the driver"

Cheers

your biggest prob will be induction noise, it can be as loud if not louder than the exhaust

I'll be using a duct and airbox will that help ?

I guess that depends if you take the air from the outside or not, otherwise it doesn't help, does it ?

With some serious sound proofing you could bring the noise level down.

If you want it for a daily driver you'll also need to think about a reverse, as you'll get very p*ssed off parallel parking it. And you will also need to strip the interior out as much as possible to keep the weight to a minimum, so you may not end up with a very comfortable car.

I should think a BEC mini would be great fun (it has more than crossed my mind to build one) - not sure if I'd want it as my daily driver though.

It's also potentially quite an expensive conversion - one I saw for sale on Ebay the chap had spent £9k and still wasn't finished.

Think I'd want a busa or ZX12 in one for the extra torque as well.

Middies can be very noisey, especially from the induction noise.
I've built some for track work which were bare minimum cars and were unpleasantly noisey in the driver's seat.
I've also had a few on the road and have fully enclosed the engine, albeit with removable covers. In one case, I fitted a sealed acrylic window from engine cover to roof which added to the insulation, and allowed me to run modified rear side windows for more air.
In the road cars, I lined the engine side of the bulkheads and engine covers with stick-on sound absorption/heat reflecting panels and also covered the interior side with conventional sound deadening felt and carpet.
That was quite a reasonable car for longish journies. I drove it from Yeovil to Santa Pod on a few occasions without earplugs and it was very enjoyable.