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Unread 04-20-2009, 09:50 AM   #1
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Default Engine displacement not exact

Why is it that when a car is for example 2 litre does the actual engine size turn out to be 1961 cc or something odd like that? Can't they just make four half litre cylinders and leave it at that? Can some technical expert shed a shaft of light on this troubling issue, thanks.

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Unread 04-20-2009, 10:48 AM   #2
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My understanding of it is that no two engines are exactly alike, and the machining process will sometimes err to one side or another. (+/- a few cc's)
Even hand finished engines like those found in the GT-R aren't exactly alike.

This is why porting and polishing parts and removing any imperfections left from the factory can lead to more power/efficiency in an otherwise stock engine.
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Unread 04-20-2009, 12:57 PM   #3
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Demon is right. If you have seen any macjining specs before, they always have a tolerance associated with them Fo example, the cylenders in my engine are to be 86mm (+/- .009"), So when you add it all up, the projected 2.5L engine could be anywhere from 2.4-2.6L....
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Unread 04-20-2009, 03:41 PM   #4
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They don't project an engine to be a 2.0L or anything. They'd project it to be 1961cc. It's just easier to say 2.0 rather than a 1961cc for the sake of advertizing. My motor is 81mm x 90mm so it adds up to 2783cc, but they just call it a 2.8, and that is the same for all 12v VR6's (with the exception of the euro 2.9's lol). Nothing to do with machining tolerances.

If you do allow for .009" of leway, it only totals to a deviation of at most 5cc(assuming 86mm bore and 90mm stroke).

Oh, and porting and polishing is to remove the grain left over from castings to improve flow and even heat dissipation in some cases.
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Unread 04-20-2009, 04:50 PM   #5
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It would also be odd to call an engine by its exact engine size, it is just easier to round it.

Mine for instance, instead of being called a "2.4 inline 4" would be a "2.429 inline 4" because it is 2429cc.

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Unread 04-20-2009, 05:56 PM   #6
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Quote:
Originally Posted by apavlov13 View Post
They don't project an engine to be a 2.0L or anything. They'd project it to be 1961cc. It's just easier to say 2.0 rather than a 1961cc for the sake of advertizing. My motor is 81mm x 90mm so it adds up to 2783cc, but they just call it a 2.8, and that is the same for all 12v VR6's (with the exception of the euro 2.9's lol). Nothing to do with machining tolerances.

If you do allow for .009" of leway, it only totals to a deviation of at most 5cc(assuming 86mm bore and 90mm stroke).

Oh, and porting and polishing is to remove the grain left over from castings to improve flow and even heat dissipation in some cases.
With aggressive porting you can take out a lot of material to smooth out the flow of the heads, and increase displacement. When I posted I was actually thinking about how the V8 guys go from 52cc heads to 64cc+ and then have those ported even farther, thereby increasing displacement.

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Why is it that when a car is for example 2 litre does the actual engine size turn out to be 1961 cc or something odd like that? Can't they just make four half litre cylinders and leave it at that?
Anyways, yes it is easier to just round up or down from the factory stated displacement in favor of an even number but I thought the OP was asking about why the engine's displacement is not an even number from the factory, rather than asking about why they are rounded up. Does that make sense?
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Unread 04-20-2009, 08:39 PM   #7
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Machining the head will not affect displacement in any way. Displacement = volume of chamber at BDC (bottom dead center) minus volume at TDC.

The only reason engine displacement is not an even number is because that is the way it was designed. Same exact reason any other part on the car is not necessarily a nice round multiple of 10.

If you go look at specifications for race cars, you'll notice they come much much closer to an even displacement because they want get as much displacement as possible while staying within the rules.
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Unread 04-20-2009, 08:53 PM   #8
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Quote:
Originally Posted by apavlov13 View Post
Machining the head will not affect displacement in any way. Displacement = volume of chamber at BDC (bottom dead center) minus volume at TDC.

The only reason engine displacement is not an even number is because that is the way it was designed. Same exact reason any other part on the car is not necessarily a nice round multiple of 10.
Apparently I have been lied to then...

Funny thing is, I thought that was how it was calculated but was told otherwise.

Oh well, you learn something new every day I guess.
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Unread 04-20-2009, 09:02 PM   #9
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Unread 04-21-2009, 03:25 PM   #10
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I also want to say it comes down to rounding it off to the nearest L. For example the Ford 5.0 is not exactly 5.0L, and infact is below. But 4.9L doesnt sound as good as 5.0L. Something someone told me not too long ago.
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