What are Bore, Stroke, Porting, and Relieving?

You hear car guys throw around the terms, “bore” and “stroke” all the time.  Most of us don’t know what that means.  I do.  Here’s what they mean, and how to identify them.

Bore:  Cylinder bore is the measurement of the diameter of the cylinders in an engine.

Stroke:  Stroke is the amount of movement a piston can move up and down.

A bored engine happens when you machine out the block.  For example, taking a 400 cubic-inch Chevy V-8 and boring out the cylinder walls to make the engine displace, say 500 cubic inches.

A stroked engine is simple to build.  You just pull out the current internal moving parts, and put in bigger ones.  Because bigger is better.  An infamous example of this is Chevy’s 302 cubic-inch V-8 from 1967-1969.  Chevy took their garden-variety 283 cubic-inch V-8 engine block, and put the bigger 327 cubic-inch V-8 internals in.  Voila.

An easy way to put into perspective just how thirsty an engine can be is to remember that a 302 cubic-inch engine will theoretically inhale and exhale 302 cubic inches of air and fuel in two revolutions of the crankshaft.  If you punch that engine out (slang for stroking it) to 347 cubic inches, then you’re going to have a more powerful engine.  That 347 stroker engine is popular with drifters – it’s based off of Ford’s capable 302 cubic-inch V-8.  A 347 cubic-inch engine is 14.9% larger than a 302.  So, that 350 horse 302 will have 402 horsepower.

Just stroking an engine for more power won’t solve your problems entirely.  A cam (or more nowadays) pushes on the intake and exhaust valves.  If you’ve got a bigger crankshaft and pistons, upgrade the cam!  You’ll have a far more reliable engine.  Upgrading the cylinder heads is also smart.

The nice part about stroking an engine is that the engine remains the same size externally.

Porting:  A ported engine refers to enlarging the intake and exhaust ports of an engine.  This makes for better and more airflow.

Porting is common in drag racing, especially with the use of nitrous oxide (yes, the same stuff that is used to push out whipped cream from the can).  It just makes more room for the air to go in and out.

Relieving:  No, the engine is NOT using the bathroom!  That would be relieving itself.  A valve seat is where the valve is attached to.  It’s usually a small mound of metal, but it doesn’t need to be.  Relieving just removes the extra metal to make more room for bigger valves.

Here are some pictures of the different kinds of engine enlarging.

This is a bored Chevy big-block V-8 engine block.  It’s been bored out from 454 cubic inches to 489 cubic inches.  That’s 35 cubic inches more air and fuel this engine will hork down.

This is a stroked Chevy small-block V-8 engine.  It displaces 383 cubic inches, rather than 350.  Instead of 350 horsepower, it makes 425 horsepower.  You can buy this engine from Summit Racing.

This is a close-up of a beautifully-ported Ford 302 cubic-inch V-8 from a 1987 Mustang GT LX 5.0.  The larger port is the exhaust port.  The small hole is the valve seat.

This is a relieved Ford 221 cubic-inch flathead V-8.