It is a two part equation. PART 1: You are reducing the air flow thru the radiator which also reduces the air flow around the engine block causing the engine to heat up sooner and therefore encouraging the thermostat to open quicker thereby providing hotter water to the heater core quicker and heating up the cab. PART 2: Once the thermostat is open and the coolant is circulating, the lower air flow thru the radiator helps keep the coolant temp at a higher level than if there was no restriction in air flow. the quicker you can get the engine up to operating temperature, the quicker you will receive heat to the cab, and the more efficient your engine will be. That's how I see it anyway.

 

Living in Montana, we're of course prone to temperatures well below zero, so cold that the engine on some vehicles can't get up to full operating temperature and as a result the heater in the vehicle doesn't put out near as much heat as normal, hence the card board in front of the radiator to reduce the heat loss of the engine to where full operating temperature can be achieved. This is something I've done since I've owned vehicles, and pretty much everyone else I know does also. Of course this can vary from vehicle to vehicle, for example, some vehicles, it may become an issue at -10, others not until -25 or colder. it just depends on the vehicle. I had an old Buick once that if it got below -10 you had to do this otherwise you'd freeze to death because of the lack of heat from the heater, on the other hand, I had an old Ford van where I never had to use card board, even at -40. Ether way, no matter what means you use to block the radiator, you need to know when to pull the card board, i.e. at a certain outside temperature or by monitoring the engine temperature other wise if you forget, your engine can overheat.

A few years ago I came across a car load of girls pulled off the side of the highway with their car overheating, being a gentleman I thought I'd pull over and help them out, the fact that it was a car full of girls had some influence on my decision to help also. Anyway I asked them to pop the hood on their car (had to show them how to do it), and what do you know, a piece of card board in front of their radiator with an outside temperature of +35. They said their dad put it in and they forgot about it, and of course being the type of girls had no idea what they're looking at under a hood let alone even knowing how to open the hood never would have figured it out on their own. Luckily they didn't fry the engine in the process, so after pulling the card board, filling them up with some antifreeze they where back on their way,

 

I tried the cardboard in front of the radiator today. I'm not sure it made any difference.

Truck was in a heated shop bay all night, around 70 degrees. Still took around 10 minutes to get to operating temp, its 24 outside here.

 

24 is getting a little warm to be doing the card board trick, this usually isn't something that's needed until its well below zero, having card board in front of the radiator at 24 degs, you start running the risk of overheating. At 24 degs your temperature should be coming up to normal operating temperature just fine, sure it's going to take longer than it would at say 40 degs I don't think 10 minutes to warm up is to far out of line, especially if you have a diesel, which are built quite a bit stouter than a gasoline engines, which means there's quite a bit more metal to heat up.

 

^^ yes.

Even the block heater is only mentioned at less than 0°F.

:

 

Here is the picture of my install of the Ford winter front for the 2010 F-350 Super Duty.