After hearing and balking at the $35 price to replace my cabin filter (sure the first change was cheaper) I heard about people using furnace filters cut to size to replace the cabin air filters. Good idea and easy to do, well worth the savings. My question is can it be taken a step further and used for the engine air filter as well? Personally I'm not cheap especially with the Silver Bullet but moneys money. Any cautions or thoughts? Cheers

 

I wouldn't. The proper engine air filter has many more pleats than a furnace filter, and so a lot more surface area and presumably less of a flow restriction, particularly after it gets a little dirty. (Furnace air filters need changing every couple of months or so in furnace use, and I suspect the engine air intake is typically dirtier than a furnace air return.) It may also filter more finely than many furnace filters, and be more immune to dampness, etc.

 

I'd advise against building either cabin or air filter with furnace air filters. Especially the air filter.

Aftermarket cabin air filter from a place like Napa can be either $15 basic or $35 charcoal. Last a year or more. It won't let dust, bugs, and moisture blow past and ruin your HVAC system and blower motor.

Air filter? A car consumes more than 72,000L of air per hour. That's a lot of air travelling through a filter. I would not leave this to a furnace filter. It would collapse and you could have some very expensive repairs down the line like MAF sensor and throttle body.

 

I buy the finest chinesium ones on eBay. They're under $10 with shipping. It's a 30 second job to swap them out.

 

Damage the HVAC? OK, no. I have cut up furnace filters for years for all my cars. I replace them as often as I want for pennies on the dollar and never have to feel compelled to leave an overpriced store bought filter in too long because the store charges me $10+ for something that costs 25 cents to make. The only possible damage you could cause by using too flimsy of a cabin air filter would be if it collapses down and rubs against the fan blades. Use your OEM cabin filter to mark and cut up your new cabin air filters from furnace filters.
As for using them as a substitute for your engine air filter, that is the equivalent of pouring Wesson oil into your crank case to lube engine parts. Don't skimp on anything directly involving a combustion engine.