Which way does the coolant run when cooling down the engine?

Dear Car Talk

Dear Car Talk | Jul 01, 1998

Dear Tom and Ray:

I need some help. My roommate and I have a serious bet going on. He thinks he
knows everything about cars, and I DO know everything about cars (just kidding).
He says that the cooling system runs water from the engine to the lower radiator
hose, then up through the radiator and out the top radiator hose. I say it's the
other way around; that the water runs into the radiator at the top, and out the
bottom. We're talking about good old American V8s here, not a little Subaru.
What's the answer? If he's wrong, he has to wash my truck once a week for a
year. -- Tim

RAY: We're enclosing a good set of rubber gloves for your roommate, Tim, because
he's going to have some serious dishpan hands.

TOM: For traditional, old, American cars, you're right, Tim. For ages and ages,
the hot coolant came out of the top of the engine, through the open thermostat,
down through the radiator, and then out the bottom of the radiator where the
cooled water went back into the bottom of the engine.

RAY: But now, on a lot of cars, the flow HAS been reversed. The hot coolant
comes in the bottom of the radiator, gets pumped to the top, and then goes back
into the top of the engine. And for a very good reason.

TOM: Engineers realized that the top of the engine really needs the most
cooling. That's where the cylinder head is, and, more important, where the head
gasket is. So they turned the flow around, so that the coolest water would come
out the top of the radiator and flow right into the cylinder head where it's
most needed. And since they've done this, the number of blown head gaskets has
decreased dramatically.

RAY: So if you want to cut your roommate a little slack, you might mention that
if you were talking about modern cars, you'd both be right. But why do I suspect
you're going to wait about 11 months before bringing this up, Tim?

Get the Car Talk Newsletter