Blowdown is the amount of time/distance/degrees between exhaust port opening and transfer port opening.

The absolute most accurate description of blowdown would be... the amount of TIME between the exhaust and transfer port openings. This "measurement" would of course only be accurate at one rpm, so blowdown is commonly referred to in degrees of crank rotation.

If you're referring to engines of all the same stroke and rod length, you can of course, refer to it in "thousanths of an inch", which is commonly done in something like a Yamaha (a typical number might be around .380). This can be measured using a long-travel dial indicator, and simply measuring the distance from the point where the exhaust ports just begin to open to the point where the transfer ports just begin to open.

(Realize though that on an engine with say -- .380 blowdown, the actual blowdown (in time and degrees) would change slightly if you were to raise or lower the cylinder. Distance between ports represents degrees/time only at a given spot in the stroke.)

Blowdown can change the way the engine runs throughout the rpm range. There is no set magic number, though on kart motors, it's typically between 25 and 30 degrees of crank rotation.

Usually (but not always) running more blowdown results in an engine that has the potential to be tuned for more peak power. Peak power not necessarily meaning "top rpm", but rather a bigger HP number at the price of having it over a narrower rpm range.

Direct drive kart motors (like what's run in Europe) typically have quite a bit less blowdown than what gets run in this country with a clutch. That is achieved by lower exhaust and/or higher transfers.

Information provided by Muller Machine