Definition: Boxer and Flat (engines)

[Paul Franssen]

We all know our 911's have a FLAT 6 engine.
Is then also known a BOXER engine?

If so, are ALL "flat" (opposing cylinders horizontally disposed) engines the same as "boxer" engines?

If there be any distinction between them, what is then that difference, would it have anything to do with the varying (sequence/direction of the) movements of the pistons?

i.e. take a flat 4
(1) cyl 1 + 2 compressing and 3 + 4 exhausting simultaneously
(2) cyl 1,3,2,4 firing in say that sequence

Any takers?

[Formerly Steve Wilkinson]

The goofiest reply I ever heard to this question--and it came from a putative automotive journalist--was the they're called boxer engines because they fit very conveniently into a shipping box.

The truth is that yes, flat or horizontally opposed engines are also called boxers, though that's not a term much used in the U.S. (at least not before the advent of the Boxster). They're called boxers because somebody (who apparently didn't do much fistfighting) likened the motion of the pistons to the fists of a boxer punching out to either side.

Stephan

[rvanderpyl]

The term 'boxer' is used to describe a horizontally-opposed engine having a single crankshaft and camshaft in which the lobes of the camshaft are common to the opposing valves.
The origin of the term is unclear.

Other forms of horizontally opposed engines have a crankshaft for each bank of cylinders and are built with a common head instead of a common crankcrase.

The 'boxer' design enjoys a high ratio of common, shared, and interchangeable parts, making it one of the least expensive engines to manufacture and maintain. The simpicity of the design insures good reliability. When designed to use air cooling and when fabricated of light alloy castings, the combination of reliability, simplicity and light weight make it ideal for use in aircraft.

From the Web Site www.askjeeves.com

So a Porcshe motor is actually a flat 6, not actually a boxer (Although it does have the common crankcase) , while most light aircraft motors are boxers.

[rvanderpyl]

The term 'boxer' is used to describe a horizontally-opposed engine having a single crankshaft and camshaft in which the lobes of the camshaft are common to the opposing valves.
The origin of the term is unclear.

Other forms of horizontally opposed engines have a crankshaft for each bank of cylinders and are built with a common head instead of a common crankcrase.

The 'boxer' design enjoys a high ratio of common, shared, and interchangeable parts, making it one of the least expensive engines to manufacture and maintain. The simpicity of the design insures good reliability. When designed to use air cooling and when fabricated of light alloy castings, the combination of reliability, simplicity and light weight make it ideal for use in aircraft.

From the Web Site www.askjeeves.com

So a Porsche 911 motor is actually a flat 6, not a boxer (Although it does have the common crankcase) , while most light aircraft motors and the old VW and 4 cylinder pushrod Porsche motors were boxers.

[jadams]

You need to tell Enzo that he named the Ferrari Boxer wrong - it has an OHC (not common cam) flat 12.

Maybe Enzo should have "asked Jeeves" before naming his car

From the Porsche website:
"1963: The Porsche 911 makes its appearance in the market. The first version of the new six-cylinder boxer engine offers the same output as the Carrera 2 (1991 cc,130 bhp/96 kW)".

[speeder]

Stephan, James, Enzo, and Porsche are correct; Jeeves and R Vander are wrong about the "single camshaft" part. Number of cams is irrelevant, Stephan's "fistfight" answer is "most correct". My BMW motorcycles were all "boxers".

P.S.: Can't you just tell that I missed my calling in life? I am a frustrated school teacher.

[Jack Olsen]

I'll add my ignorance to the fire, here.

As I understand it, there is a commonly-accepted definition of boxer, and a more technically specific one. The Porsche engine fits the former, but not the latter. Then again, most 'boxer' engines don't fit the technical deifinition, including models with 'boxer' in their name.

RoGERK posted this in a thread a while ago:

The Porsche engine is NOT a true boxer. In a true boxer engine the opposing cylinders are on the same cycle and (in theory) cancel out each others lateral force.

The Porsche engine is in fact a V6, but with a 180 degree V. Because of 4 stroke, 6 cylinder equation, 6 cylinder engines are inherantly balanced, leading to a nice smooth feel and (more importantly) very high revving ability free from significant vibration.

The major drawbacks of the flat 6 design include obvious stuff like cost, and width. Less obvious are problems with the centrifugal forces of cornering - oil tends to get thrown into the cylinders and pistons on the outside of a corner, and the inner side of the engine is starved of oil. Porsche get around this by good design of the residual "sump" and the dry sump lubrication system that ensures the bulk of the oil is in a separate tank or circulating around the engine/oil radiator at any given time.

There is obviously a whole lot more to say on this topic, but rest assured, the 911 engine is one of the 20th century's finest...

- roGER

[island911]

Originally posted by JackOlsen
. . .there is a commonly-accepted definition of boxer. . .

. . .And that's all that really needs to be said.

This talk of "sharing the same cylinder bore" . . .that type would be called an "opposed piston" type with two cranks; where the piston faces come together at TDC (suited for diesels)

[Bobboloo]

Did you know that the prototype 911 (901) did not have overhead cams? It had a flat six pushrod engine with two cams in the crankcase. One above the crank for intake timing and one below for exhaust timing.

Bobby

[Doudi]

Paul :

I remember you speak also french. You may find a french definition on my website, in the FAQ :

http://911carrera.free.fr/

I looked in many website, asked engineers in my office (I'm working in automotive company) and finally get a result wich satifies me, and every body looking for details...

Best regards,

doudi

[jluetjen]

Here is how I understand it:

A "Boxer" is a form of a "Flat" motor, but not all "Flat" motors are "Boxers".

Specifically; in a boxer, opposing pistons move opposite to each other. So in our beloved Porsche "Boxer" 6, pistons 1 and 4 both move in their downstrokes at the same time. While one of them is on the "Drive" stroke, the other is sucking air in. Cylinders 2 and 5 also move opposed to each other and Cylinders 3 and 6. You can confirm this by looking at the crank shaft and how those pairs of cylinders are 180 degress apart.

A "flat" motor is any motor where the cylinders are on a flat plane.

A "180 degree" motor has a "flat plane" crankshaft like an inline 4-cylinder. Some V8's are made like this such as the Cosworth DFV and the Ferrari 308/328 motors. In the case of these V8's, think of the engines as 2 four cylinder engines on the same crank shaft. The benefit is that these motors can easily use tuned headers like a 4 cylinder. The trade-off is that they create 2nd order vibrations which can really buzz your teeth if they are not managed correctly. American V8's on the other hand are set up like 4 Harley Davidson 2-cylinder motors on the same crank. In this case the benefit is that the vibrations cancel themselves out and the engine is smoother. The downside is that in order to create a really well tuned header, you need to run some of the pipes over to the other side of the V which makes for a real packaging problem. It can be done though if you remember the "180 degree headers" that some of the NASCAR racers used to use on the short tracks. These headers made the "Amercan V8's" sound like Ferrari's! This style of crankshaft has been used on V6's, V8's, V10's, V12's and I assume V16's. Now some engine designers use the non-180 degree design their crankshaft, but they open up the V to lower the center of gravity. If they open up the V to 180 degrees, they will have a "flat V" engine which is NOT a boxer.

By the way, as I mentioned, the Porsche 911 engine is a real "Boxer" motor. The engine from the Porsche 917 is not a boxer, but really a "180 degree V-12" The 908 motor on the other hand started life with a flat plane crankshaft, but had such an awful vibration that they quickly changed it to a "180 degree V-8" after a couple of races.