Tag Archives: 18-71

Blowers by the numbers


Most of our website fans and followers are familiar with the term “Blower” (supercharger) on an engine.   But have you ever questioned where the numbering system came from?  4-71, 6-71, 8-71, 12-71, 14-71, 18-71?  This blower thing goes way back to 1885 when Gottlieb Daimler got a patent on the thing and it probably goes back even further but he’s the one who finally received the original patent and then it goes from there.  Several different types and styles have come along since and constant improvements are being made even today.

There are centrifugal blowers (from the word centrifuge) which are somewhat of a bowl shaped aluminum housing usually sitting above the engine (but not necessarily) with a grooved circular plate revolving around a center shaft.  It is generally gear driven off of the cam gear, crank shaft or other means of a positive drive.  Air is usually ducted into the intake manifold or directly into the carburetor or fuel injector.

Then there is the “turbo” charger (from the word, turbine).  Similar in appearance to the centrifugal, the turbo charger is driven off of exhaust pressure and has no mechanical connection to the engine.  As exhaust gas exits the engine it first travels through the vanes of the turbo charger and creates “boost” in the incoming air to the carburetor or fuel injector, sometimes referred to as “free air”, “free boost” or “free horsepower”.

The numbering system mentioned earlier is strictly for the “roots” type blowers used on most supercharged racing engines.  It is the big aluminum “thing” sitting on top of a racing engine with either carburetors or fuel injection bolted to the top of it.  On street rods it’s usually polished or chrome plated.  Briefly, it is belt driven off of the crankshaft and has two counter-rotating helical shaped rotors (or “screws”) inside that increase the volume of air to the engine.  Thus, you are able to add more fuel.  More air, more fuel, more horsepower.   If you ask an owner or read about an engine having a 6-71 or 8-71 blower and wonder, “what in the world are they talking about?”, here it is:

The Roots type blower numbering system of today originated from the 2 cycle, GM series diesel engines.  The engine design requires the blower to blow fresh air in as the piston is coming up and blow it out as it goes down.  A series of oblong holes towards the bottom of the cylinder lets the blower blow the exhaust gas out as the piston passes these slots and blow a fresh charge of air in as it comes up and is ready to fire again, every stroke is a power stroke, thus the 2 cycle, only a different concept.

6-71 Blower

A 4-71 GM Diesel has 4 cylinders and 71 cubic inches per cylinder, thus the size of the engine is 4 x 71 or 284 cu in.  The blower would be a 4-71 blower.  6, 8, and 12 cylinder GM 2 cycle diesels are 6-71, 8-71 and 12-71.  The aftermarket has seen fit to continue the numbering on up to 18-71 but there are no 18-71 GM Diesels.

Some enterprising racer of the day took the blower off of a GM 2 cycle and strapped it on an engine and wa-lah! – a blown racing engine and the beginning of an era for sure.  As a note, back in ’58, the Bender and Hier Lee’s Speed Shop small block Chevy dragster had one of the first Potvin “crank-drive” blowers (mounted in front and driven off of the crankshaft) and set two Chevy “firsts”, both an elapsed time and speed record at the old Lions drag strip in Long Beach, CA.

Thanks for visiting and…

See you at the races!

Ronnie Hier

Home Of Nostalgia Drag Racing


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