The misfire problem in the Prius is fixed, the car runs nice and smoothly but mpg seems to be down by 5%. One of the things I did to try to fix it was out in new plugs. They were Bosch iridium rather than the OEM Denso ones, and there’s a lot of talk about Bosch being poor.

So how much difference do spark plugs make? Could the noti eable drop in economy be due to some kind of mismatch in spec?

Sometimes cars do like the OEM plugs more than other ones, even fancier ones. Did you check the gaps on the new plugs before installing?

It either sparks or it does not

If it was designed to run with them then it will

More likely a plugs gap issue

It wasn’t designed to run with the Bosch ones, that’s the point. They are not full iridium, some kind of hybrid affair.

I did check the gaps, yes.

It’s either the plugs or the misfire has knackered something else.

I was told not to check the gaps as some plugs are now coated with something on the surface, and the feeler gauge can scratch them. Dunno if thats true but I chucked my new feeler gauge in the toolbox and fired up the car, no issues so left it.

I am thinking of buying a particular Volvo and the wisdom online in the Volvo forums says the engines they use do prefer a certain OEM plug to work at their best, even really fancy ones can cause problems fitting in an otherwise standard engine.
I don’t think you’d be able to gain any performance with spark plugs alone so using the OEM ones is probably the best choice in most cars.