29

I've noticed that engines are rarely exactly 1.5 liters or exactly 4.0 liters, and I'm okay with that. It makes sense that engines would have some arbitrarily ideal size for specific applications, so I would expect exact engine displacement to be all over the place.

But it doesn't usually seem to be. Nissan's VQ35DE is 3,498cc, Toyota's 4U-GSE is 1,998cc, and BMW's N52B30 is 2,996cc.

Why aren't they just 3,500 cc, 2,000 cc, and 3,000 cc respectively?

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    I'm not asking to be patronized. It's something that I noticed so I asked if there's a reason for it. – Trevor D Nov 17 '16 at 16:30
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    What about the Kawasaki Z1 that was sold as a 900 but was actually 903cc very frustrating as under 900cc was a whole lot cheaper to insure. – Iain Nov 17 '16 at 22:23
  • @Iain I think there's been a paradigm shift since 1972 regarding doing that. In cars and motorcycles. It's very rare to have that type of a displacement situation where the engine is actually larger than what was advertised. – DucatiKiller Nov 17 '16 at 23:38
  • Have you noticed that noone uses cubic inches to measure displacement for their modern engines? A 202 is still a 202, but the modern version is 3300 cc. – Criggie Nov 18 '16 at 0:55
  • @Criggie I would estimate that is largely due to 1) how much larger the Japanese and European market share is today compared to decades past. And 2) the globalization of cars in so many markets where inches are not used. – Trevor D Nov 18 '16 at 1:03
46

There are regional taxation and tariff issues associated with displacement

Manufacturers will intentionally keep their displacement just under a limit that may increase their local/regional taxation as well import/export tariff regulations in accordance with trade agreements, etc.

It's easy enough to make an engine exactly 3 Liters. Math is exact if you want it to be.

Manufacturers will intentionally 'fudge' the numbers and round up for advertisement/marketing. 2 CC is a nominal fudge and a barely measurable performance increase and IMO not a misrepresentation.

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    @RossMillikan Indeed it is. TY for pointing out my nonsense. – DucatiKiller Nov 17 '16 at 21:46
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    @RossMillikan thanks, now I have to go into the edit history to see a rare error from ducatikiller. – Criggie Nov 18 '16 at 0:52
  • @Criggie oh the humanities. – DucatiKiller Nov 18 '16 at 1:06
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    @Criggie Moderators don't make mistakes. – JoErNanO Nov 19 '16 at 7:23
  • That pretty much is it. You want to look as high as possible, while avoiding a specific tax cutoff level. For safety you add a little more (2-3 CC instead of 1). – TomTom Nov 19 '16 at 10:20
16

Until relatively recently road tax (in the UK at least and presumably in many other countries as well) was based on engine displacement. As this worked in bands and engine which was slightly less than 2 litres would be a lot cheaper to tax than one which was slightly more. So nominal engine size made a big difference to the cost of ownership of a vehicle.

Equally, engine size is one of the headline numbers which determines the desirability of a vehicle ie a 2l engine sounds a lot better than a 1.5l engine on paper. So from a marketing perspective it makes sense to set your engine sizes close to the respective tax brackets.

However when you design the basic concept for a particular engine the capacity will be one of the first things you decide so you want to leave a bit of margin, If would be unfortunate if you spent a year designing the block and then a small tweak to the cylinder or crankshaft tip it over into the wrong category for no measurable gain in performance.

After all the difference between 1998cc and 2000cc is only 0.1% and you really really don't want to have to redesign and retool your engine plant to make everything 0.1% smaller if the government takes a different view to you on what is an acceptable tolerance, especially as an engine design may last for decades.

Also displacement is the sum of a lot of different parts, even disposables like spark plugs and head gaskets can make a measurable difference so even with pretty tight manufacturing tolerances you will see some measurable variation from one nominally identical engine to another.

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    I think you mean vehicle excise duty, not 'road tax' 8) – nekomatic Nov 18 '16 at 9:55
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    UK 'Road tax' went out in 1937, there's not much recent about it. – Separatrix Nov 18 '16 at 11:32
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    Germany still bases tax on multiples of 100ccm displacement. – DevSolar Nov 18 '16 at 12:29
5

From a more historical view I will put out the idea of "Dimensional Lumber"
Dimensional lumber is where a 2 X 4 is not actually 2 inches by 4 inches when you go to use it.
It varies by wood type and processing methods.

With that said way back in the cubic inches displacements (302, 327, 460 and such) the values were not as posted. 1940's 50's and 60's Chevy 216 = 216.48, 235 = 235.49, and 261 = 260.9. This was not for taxation.

If a manufacture wanted to make a finished engine with a precise displacement of a fixed value then they could start with the finished value and design backwards calculating in all the design and machining variables to find the starting values. But that is not the process.
But from the drawing board to the working model adjustments are made. Piston domes change, bores are changed for ring design, valve placement is adjusted, stroke is varied by crank and piston rod mods and machining, deck height changes, gasket types and even thickness adjusted. Some of which factor into displacement.
In addition factors relating to expansion and contraction of engine parts, allowance for oil lubrication change dimensions.

In the end you're close to a value, but it is not a perfect cylinder with perfect matched flat tops.

Last some of the odd values are based on parts exchange or squeezing out that extra bit of displacement. If a manufacturer can build one block but make two different engine sizes into it (one for economy, one for power) so much the better. Take engine "A" adjust stroke and make engine "B", (for instance 350 to 383 chevy) push the stroke to the maximum and you have a odd number of CID. Same goes for piston upsizing or down sizing.

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    FWIW there are only three design parameters which determine an engine's displacement: bore, stroke (or throw), and number of cylinders. All sorts of considerations may influence the final selection of bore and stroke values for a given engine, but displacement is simply the volume swept by the piston as it travels from bottom dead center to top dead center (or vice-versa), multiplied by the number of cylinders. Size or shape of the combustion chamber or shape of the piston top is irrelevant to displacement because they are fixed. – Anthony X Nov 19 '16 at 4:51
  • @AnthonyX Yes I do so understand still the same formula but as noted the other factors can change one of those parameters in final design. – spicetraders Nov 19 '16 at 14:58
3

Slightly different to the other answers but with motorcycles, there are often laws that restrict the usage (with your license) by the capacity in CC.

Sometimes these laws state that the size of the engine can not be greater than 125cc for example.

However, in other countries the same law may state that the engine size must be less than 125cc

Therefore, manufacturers like to maximize their sales by producing 124cc engines and marketing them as 125cc motorcycles

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    The law cares about the actual displacement of the engine, not the marketing value. If what you're saying were true, they'd be able to put a 750cc engine on the bike, as long as they marketed it as 125cc. Or did you mean to say that they produce a 124cc engine and market it as 125? – David Richerby Nov 20 '16 at 21:05
  • Or did you mean to say that they produce a 124cc engine and market it as 125 – Jalapeno Nov 21 '16 at 9:23
  • that is exactly what i meant and I had that written orginally, I think my post was edited – Jalapeno Nov 21 '16 at 9:23
  • Yeah, the edit history shows that somebody switched them around. I strongly recommend that you click the "edit" link and put it back how it should have been! – David Richerby Nov 21 '16 at 9:58
2

The size you are referring to is a measurement of volume of a cylinder.
V = pi * radius^2 * height
When you are multiplying by pi, it is hard to get an even number. Also, they care more about how it works than making the numbers nice and even.

In more an engine calculation Displacement = pi * (bore / 2)^2 * stroke * #cylinders

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    "When you are multiplying by pi, it is hard to get an even number." – I can do that very easily: (1/π)*π = 1. See? Not really that hard. – Jörg W Mittag Nov 17 '16 at 22:09
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    @JörgWMittag To nitpick, 1 is not an even number... It's an integer, but it's not a divisible-by-two integer :) – ArtOfCode Nov 18 '16 at 1:22
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    This is surely wrong. Surely the tolerances on engine manufacturing are tight enough that they could make the cylinders slightly wider/narrower/longer/shorter to achieve any displacement they wanted, at least to the nearest cubic centimetre. – David Richerby Nov 18 '16 at 10:52
  • You're clearly coming from a mathematical perspective rather than a mechanical one. In the real world things are more complicated than you describe, and the engineering tolerances and calculations are far more accurate than you imply. – Darren H Nov 18 '16 at 22:34
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    With all respect, Darren, we do not live in the middle ages where things where done by hand. Tolerances of 1/100 of a mm are not exactly tight - and they would result in hitting a target volume for a cilinder to VERY tight margins. Tight enoug that you round to the exact volume wanted, likely even with 1-2 digits to spare. Your answer assumes that things are so loose you would hear the engine rattling. – TomTom Nov 19 '16 at 10:16
1

I think a better approach of looking at this is: Why, when the engine displaces 2,996cc's, do they call it a 3L engine?

While displacement of the engine is primarily what we're looking at, it all boils down to marketing. As @rpmerf stated, it's hard to get an even number. From an engineering standpoint, why even worry about it. It won't make that big of a deal in performance and would cost more money (in most cases) to come up with the alternative. So it boils down to it being a marketing standpoint. Saying an engine displaces 3L is a lot sexier than saying it displaces 2,996cc's. It will ultimately help sell cars.

This plays out particularly well when you consider what Ford did for decades with their 5.0L designation on the venerable Windsor small block 302ci engine. It actually only displaced 4,942cc, which if rounded correctly, should have been called a "4.9" and not a "5.0". 5.0 is a lot sexier than 4.9 from a performance standpoint. It wasn't until the new Coyote engine did the "5.0" actually become correct @ 4951cc's (rounding correctly this time, of course).

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    Ah, come-on a 4.951L badge on the back of a Ford Mustang would just be to wild! – spicetraders Nov 17 '16 at 22:09
  • " As @rpmerf stated, it's hard to get an even number." - that is likely because he (and you) failed basic high school. For anyone with high school math it is trivial to get the numbers down to hit a given CC target down to 1/1000 of a CC. And given manufacturing tolerances way better than 1/100 of a mm.... your argument seems to assume medieval age "wing it by hand" manufacturing. – TomTom Nov 19 '16 at 10:18
  • @TomTom - First of all, it's not the tolerances which have to be better than 1/100 of a mm, it's the dimensions. Manufacturers don't get that anal, and why should they. If you plug it into a handy dandy calculator, you'd discover you'd need to have a bore of 85.01mm and a stroke of 88.092mm to get exactly 3000cc displacement. Second of all, you completely missed my point. It's easier and costs less to upmarket than it is to have displacement exact. – Pᴀᴜʟsᴛᴇʀ2 Nov 19 '16 at 12:04
  • "Manufacturers don't get that anal, and why should they" - actually they do. Otherwise you get ridiculous wear and tear and vibrations at high RPM. – TomTom Nov 19 '16 at 12:36
  • @TomTom - You are getting in the weeds and missing the point. You are talking about tolerances and I'm talking about dimensions. Two separate things. – Pᴀᴜʟsᴛᴇʀ2 Nov 19 '16 at 12:43
1

Rounding up is common and considered good marketing. Governments may base vehicle taxes, by whatever name, on round numbered values, 100cc steps, 850cc and below, 3.0 liters and above, etc. Should you race your vehicle, racing classes are almost always set by round number displacements.

But this is the best answer, because pi's irrationality (it goes as many digits as you care to enumerate) and non-roundness make a 3.000 litre or 1.600 or any other round displacement difficult to hit within .5 CC of a target value. (0.5 / 1500 = 1/3000 = 0.000333...) Round number displacements mean making piston bores, piston diameters and/or crankshaft strokes with 5 digit precision, non-round, values. If you're making zillions of something, any dimension is as good as any other, although that +/-0.0001 part tolerance is going to cost real money.

As the orders fall from millions to thousands, getting suppliers to make (3.14159 X 25.4)mm parts, +/-0.01% gets expensive too. Training people setting up the machines, ordering stock material, is easier, if you're making 76.2mm pistons, compared to 79.796mm. US built engines used to have 3.000 or 4.000 inch pistons, +/- more than 0.001". Far easier to make an 1149cc motor and sell it as a "1200". That's a 0.5% difference. No big thing. Unless you're trying to hit 1200 +/ 0.5, where you need 1/2400, 0.0417%.

  • That would be "But this is the best answer, because pi's irrationality" combined with incompetence to have any style of modern production tolearance and hiring engineers failing high school math? Really? – TomTom Nov 19 '16 at 10:17
  • Hi TomTom, have a look at piston sizes, both stock and overbored sizes, connecting rod lengths and crank throws. Here's Fords: performanceparts.ford.com/download/pdfs/EngineDimensions.pdf And here's for Toyota. The Toyoda is particularly interesting. The pistons are mostly whole numbers of mm, while displacements are ugly looking 4 digit numbers: For example, 75.00mm pistons, on several pages, are found in 1988, 1077, 1166, 2253, 1290, and 1298 cc motors. – Bill IV Dec 19 '16 at 9:28
  • Here's a Toyota piston dimension chart, I apparently failed to paste it last year: grantpistonrings.com/_Catalog_PDFs/TOYOTA.pdf Entries are Vehicle name, Engine name, piston diameter, mm, engine displacement, CC, model year for this engine in this vehicle: Landcruiser F 90.00 3878 72-75; - Landcruiser 2UZFE 94.00 4663 98-08; - Landcruiser 3URFE 94.00 5663 08-15; - Landcruiser 2F 94.00 4230 74-87; - Landcruiser 3F 94.00 3995 85-89; - I appreciate TomTom's enthusiasm but the plain facts are that pistons have simple dimensions and displacements do not. – Bill IV Nov 7 '17 at 20:32
1

I would guess that the reason is a combination of the desired performance, "real world" factors like taxation, and practicalities – what size pistons do you already have, how many parts can you gang at one time on your machines, what does your shop floor transport system look like, etc. Then you give it a designation that makes the marketing people happy and start building… Keeping in mind that while we think "4.951 x 10^3 mL" might look awesome on the back of a Mustang, somebody in sales might dread having to explain it.

I don't think the math/rounding argument explains it at all, with just close tolerances (±0.01 mm) you could have displacement errors well under a 0.1 cc. I think the train of thought goes something like "how close can we get to X liters (or cubic inches) without going over (taxes/insurance) and without having to redesign everything – and keeping in mind this big pile of other constraints."

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