Talk:Engine displacement

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Page naming conventions[edit]

Probably better to discuss the linking to this page here than on the 1 E-3 m³ page, to keep it where it's relevant. Which part of Wikipedia:Naming conventions or Wikipedia:Naming conventions (capitalization) do you feel applies on that page? I think the third paragraph in the latter page says the 'e' shouldn't be capitalized when it's used in a phrase, as in automobile engine displacement. You wouldn't write "automobile Engine displacement" normally, after all. -- JohnOwens 02:44 Mar 27, 2003 (UTC)

Haven't you noticed that all pages in Wikipedia have the first letter capitalized? iMac leads you to a page titled "IPac." The first letter of the title doesn't matter so IMac and iMac go to the same place. The convention say to lowercase "second and sebsequent words". --dan
That's the difference between what the link says and what it actually takes you to. See Wikipedia:How to edit a page, section 3, for a bit about how that works. And, it definitely reads better without the capital letter in there. "all pages" != "all links" -- JohnOwens 02:57 Mar 27, 2003 (UTC)

Removed incorrect statement[edit]

that 'octane ratings are higher' in the modern day. They are not. Octane ratings available slowly climbed during the 50s and early 60s, and many cars from the mid 1960s through the early 1970s were designed to run on 100 octane leaded gas, which is unavailable these days except as aviation fuel. If it says "Premium fuel" on a car of that era, 100 octane is what it means, not the 92-95 available today.

However, modern engine designs (both in physical design and electronic engine control) permit higher compression ratios to be used with ordinary gasoline than was available then.


The Japanese is similar to the European, except that automobiles are inspected after three years very harshly.

This sounds weird. Car inspections are usually for car safety, not for power measurements such as displacement, which depend solely on the engine model. There is no reason to inspect a car after 3 years for displacement issues.

I suggest to remove this remark unless there's a good explanation for it. David.Monniaux 08:33, 29 August 2005 (UTC)

history is a bit messed up[edit]

I'm not comfortable editing the page, but American cars had small engines prior to the 70's. For example the Ford Falcon had a 144 cubic inch-2.4ish liter inline 6 engine when it came out in 1960. Ford Falcon (North America) -- 11:22, 11 August 2006 (UTC)

Sorry! Wrong on this one! 409, 390, 427, 428, 426 Hemi, 440 Magnum all are before this date. Many were first produced as early as 1964. 1958 Had the 300 HP , 352 Police Interceptor; with a 2.72:1 rear end ratio it was about a 145 MPH Car!

Long list of auto engines, spare us![edit]

Boring, irrelevant and parochial. Isn't there a car-fanciers page where this lot can go off to? Engine Displacement should really be about the engineering concept primarily, with a mere sentence or two about its use in equipment marketing. Matt Whyndham (talk) 06:44, 24 October 2008 (UTC) 06:43, 24 October 2008 (UTC)

Chevy Engine Displacement[edit]

The article says that Chevy engines got to be as large as 632CI. I have never heard of a production Chevy gas V8 that was larger than the 454, besides the Vortec 8100 496. This should be changed to say either 454 or 496. The 632 is not even a GMPP crate engine; it has nothing to do with GM's engine offerings for any year.

I agree, i was under the impression the 454 big block was only ever stroked and bored to become larger, but was always the same actual prodcution 454 block...

429 inconsistency[edit]

"7 litres equates to approximately 429 cubic inches...", no, 7 litres ~ 427.166209 cubic inches. maybe 429 comes from somewhere else, or 7 litres is inaccurate? —Preceding unsigned comment added by (talk) 13:05, 12 June 2008 (UTC)


at the top it says it is the volume from top dead center to bdc but isnt that a 180* turn (a quarter of two strokes) so the grand total of volume expelled from TDC to BDC is one cyl volume on a 4cyl motor. i am assumming that the Engine displacement is the volume expelled from two whole strokes. —Preceding unsigned comment added by (talk) 01:04, 26 December 2010 (UTC)

So what does it matter?[edit]

This page is useful for knowing how ED is calculated, but fails to describe what impact ED has on the performance or output of an engine. I would change it, but that's what I came here to find out, so yeah... (talk) 07:40, 11 December 2011 (UTC)

Good point—so far, the article content says "what", but not "why". The simple version of the answer, to give you at least some immediate satisfaction on the question that you came here to answer, is that the bigger the engine (that is, the bigger the displacement), the more torque and power it can produce. Now, it's worth mentioning in the next breath that there are hundreds of variables that can modify that general principle. For just one example, there are Ferrari engines with around 4 to 6 liters of displacement that can put out way more torque and power than old truck and tractor engines with 10 or 12 liters of displacement. But of course, the trade-offs (there are always trade-offs in engineering) include cost and complexity to build or maintain. This should be discussed in the article. I'll keep it in mind for my to-do queue of content development. Thanks for communicating on this point. — ¾-10 17:11, 11 December 2011 (UTC)

Precision vs accuracy[edit]

The calculation near the top of the article : 3.1416/4 * 4.3122 * 3.65 * 8 = 426.41380142592 CID. is incorrect from an engineering and scientific point of view. The answer is given to 14 significant figures, which implies you know the bore and stroke to a similar degree of precision. 14 digits means you are measuring the bore almost down to the level of the atom. Richard Koyd. — Preceding unsigned comment added by (talk) 05:36, 14 February 2012 (UTC)

You're right, of course, and in fact the sig figs were already correct for most of the article's history, before someone messed with them on 2011-11-06, probably just as vandalism (or maybe out of innocent ignorance of the realities of practically applied math versus pure math). I just fixed it. Thanks for helping out. — ¾-10 01:57, 15 February 2012 (UTC)

Fix issue[edit]

PLEASE NOTE Somewhere in your article: This is only Mechanical displacement i.e. an engine that scavenge also moves or displaces the combustion chamber, which isn't counted by the pistons mechanical displacement

Engine displacement should be AT LEAST introduced as how much air an engine actually moves, not how much the piston moves, that is only the piston's mechanical displacement. an engine is more than the piston

I have too many people trying to tell me a supercharger doesn't displace air because of this article, or that the pressurized air magically disappears before entering the cylinder

N/A engines can effectively displace up to 125% of it's mechanical displacement, with the right intake runners, momentum can make pressure. Also with scavenging the combustion chamber

a supercharger does displace air but they are only rated for one turn, but most times superchargers turn faster than the engine to create more boost

Atmosphere is ≈14.7psia @sea level so if i add 15psi boost to any engine, we get twice as much air in the chamber ≈30psia meaning twice as much air is displaced. 15psia is NOT equal to 30psia

5L @30psia is actually 10L of atmosphere(15psia) being displaced.

some displacement is lost to thermal inefficiency BUT is made up with scavenging. most supercharged engines have no valve overlap but a supercharged engines doesn't need this to scavenge, the exhaust valves open to atmosphere(14.7psia) leaving less pressure in the chamber. when the intake valve opens the chamber will half fill with 15psi(30psia) before the piston even moves away from TDC, effectively scavenging half of the chamber.

I am trying to explain to some kids how a supercharger works, but your article only alludes that piston mechanical displacement is the same as effective engine displacement.

They even believe this equation breaks the laws of physics: 5L @30psia = 10L @15psia when it's only 8th grade math with a little algebra, ugh ford fans

the reason why 15psi boost about double Horse Power because it doubles displacement — Preceding unsigned comment added by XXXVaporXXX (talkcontribs) 05:06, 2 September 2013 (UTC)

Effective displacement is inapprpriate for this section[edit]

This is incorrect. The automotive industry defines engine displacement using spatial volume displacement. This is exactly what the very first sentance of this Wiki section descibes, "Engine displacement is the volume swept by all the pistons inside the cylinders of a reciprocating engine in a single movement from top dead centre (TDC) to bottom dead centre (BDC)." Your explanation of exhaust scavenging and superchargers adding more spatial volume is incorrect.

Firstly, there are two kinds of displacement that you are failing to see. What you are talking about is mass movement displacement, which is air mass moved. The other is spatial displacement, which is what engine displacement is. An engine of a certain spatial displacement will move or displace (or move) a variable amount of air mass. Very succinctly, spatial displacement is not the same as mass movemnet displacement.

For example, a 5L engine idling at 1000 rpm will move or displace a certain weight of air mass, but at 5000 rpm, the engine will move or displace 5 times weight of air mass. Its spacial displacement is still 5L, even though it displaced 5 times more air mass at 5000 rpm.

Now, with spatial displacement and mass displacement established, we can consider supercharging. Superchargers too have a fixed spatial displacement. And just like the engine, they will move or displace a variable weight of air mass depending on its rpm. Superchargers move more air weight into the engine's intake plenum faster than the engine can use it, so a pressure is created. All that means is more air weight is shoved (i.e movement displacement) into the intake plenum and more air mass rushes into the cylinders when the cylinders induct the next air charge. Even though the engine is moving (movement displaceing) more air mass, it is still doing it with only 5L of cylinder spatial displacement.

Secondly, your addition of effective displcement is inappropriate for this page because you are incorrectly saying that air mass displacement (movement displacement) is a spatial displacement (a volume) when you say the engine's spatial displacement doubles.

Thirdly, there are other significant reasons why effective displacement by supercharging is an inaccurate concept. Engines can vary in power not just by supercharging, they can vary in power because of 1. Compression ratio 2. Volumetric Efficiency 3. Various other factors including ambient temperatures, air pressure, humidity, and many more.

To say that supercharging increases an engine's displacement is just as faulty as saying that changing the compression ratio changes the displacement.

There just are too many factors to make the effective displacement concept practical. — Preceding unsigned comment added by Thomas McP (talkcontribs) 01:59, 7 September 2013 (UTC)


a supercharger spatially displaces air TOO, it must be counted for a 2stage ENGINE(supercharger) if a 2.3L supercharger turns 5 times faster than the crankshaft then it must be counted 5 TIMES

unless you can figure this out, then use MATH

DO NOT REMOVE THINGS JUST BECAUSE THEY ARE OVER YOUR HEAD — Preceding unsigned comment added by XXXVaporXXX (talkcontribs) 06:18, 8 September 2013 (UTC)


Is engine displacement used also on steam engines and stirling engine?-- (talk) 10:30, 25 May 2014 (UTC)

Not for steam engines. They're direct drive, rather than geared, so the convention is to describe cylinders by diameter and stroke separately. Stirling engines use displacement, but owing to the differences in their designs its not terribly useful for any direct comparison. Andy Dingley (talk) 11:04, 25 May 2014 (UTC)