silverscoobydan

I apologise if this is a really daft question, but I am going to ask it anyway!!!!

All engines have a cubic capacity that is lower than the stated figures. For example my old Alfa was 1980cc (I think) however it is a 2 litre engine. I understand the principles of rounding up to keep things simple. Manufactures are able to produce accurate capacities; however they appear to be replicating historical engine sizes.

Does anyone know why a 2 litre engine is not actually 2000cc??

(Could not sleep last night and its amazing what enters my head while lying there!!!!)


Dan

ru'

Capacity = cylinder qty x area of cylinder x stroke

Cylinder qty and stroke are likely to be nice round-ish numbers.

Area of cylinder is pi x (radius x radius).

Again, radius could be a nice round-ish number but pi is certainly not.

Have a go at getting a combination to make 2000cc (not tried it myself, but I guess it's not that easy - I'm guessaing you'd end up with a bore and/or stroke which had to be made to numerous decimal places of a mm).

Dracoro

There are, or at least were, many tax breaks, racing categories etc. that the capacity affected. To make a 2000cc car would have to be very precise so as to not be anything over 2000cc. Making it 1995 or so gives you margin of error/manufacturing tolerances etc. so that, if checked, the car will still be at or under 2000cc.

silverscoobydan

Thats great guys.

Therefore I presume that the different engine manufactures stick with what they have done in the past (Alfa 1980cc, Ford
1998cc etc).
However with the high level of computer based manufacturing I though they would have got closer or updated the figures. Would such a small increase in cc hold any economy or performance improvements?


(Hopefully I can get some sleep tonight!!!!)

Dan

TopBanana

No!

Dream Weaver

They can round down as well, the Pug 1.9's are 1905cc, though the 1.6's are 1580cc

Dracoro

Most engines now are pretty much the same as engines 10/20 years ago in simple design terms. Various other bits and ancillaries may be new, camshafts, variable valves, piston materials etc. The block however and the piston 'shafts' are simple things so to re-engineer that just to up the capacity by a few CC's isn't worth the trouble for your typical car.

I stand to be corrected though, I'm just thinking logically (i.e. why re-invent the wheel etc.).

alcazar

iTrader: (2)

I asked my dad this question, years ago, and the answer HE gave was that the slightly lower capacity was to allow rebores if and when necessary, without going OVER the original stated capacity.

Makes sense, I suppose, and is the reason why it's always slightly UNDER, and never OVER.

Alcazar

SideShowBob

Not sure if the wheel was actually "invented", or more "discovered", like a caveman rolling a rock down a hill cause of its round shape didnt invent the effect of rolling a circular object, but rather utilised it, and wheel shapes were about before man.

An invention is more like the telephone or something, so I recon that whole "invention of the wheel" thing is bull****.

Anyhoo, just thought id add that interesting morsel, its boredoms fault.

fast bloke

A round rolling rock isn't a wheel - its a round rolling rock - he could equally have based his wheel design on a square not rolling rock, in which case life would have been much less comfortable today

STi wanna Subaru

iTrader: (1)

I've seen the Flintstones too

Mungo

Seems that "developed" may indeed better cover the way that the wheel came about, especially in its modern form!

Leslie

What Alcazar says makes sense since taxation used to be based on engine size in previous times,

Les

Diablo

Arguably because, for example, Ford Escort RS 1998 or Peugeot 205 GTI 1.905 would just look wierd??

I'm convinced its simply in the rounding for badging purposes.

As for rounding down, look no further than the aforementioned pug, not to mention 318 BMW's that were actually 1.9's (and maybe even 2.0).

Going the other way, all 4 dr C class 4 cylinder mercs are 1.8L (or 1,798 ish) bit badged as 180, 200, 230....

NotoriousREV

In the early days of motor taxation, the RAC set the tax limits based on bore size which led to old engines being narrow bore and long stroke (torquey but couldn't rev). Once this was relaxed we started to see oversquare engines.

The internal combustion engine is so developed now that there's a sweet-spot of bore x stroke that manufacturers are comfortable with. For example, you don't see many 4 cylinder petrol engines over 2.5 litres because the pumping and frictional losses plus the weight of the pistons start to cancel out the benefits of going bigger.

Dracoro

My 740 is a 4.4 so should be a 744 New shape ones are 745 though.
Also I think the 323 bmws are actually 2.5s.

Richard_P

BMW do some strange badging!

The 323 is surely badged so as to make the 328 more appealing as the difference between 2.5 and 2.8 does not sound as good as 323 and 328!

NWMark

on the last 3 series didnt BMW do a 323, 325 and 328, with the first 2 both being 2.5's and the other 2.8?

what do they do on the current 3 series, is it just the 320, 325 and 330 now?

Mark

midget1500

loads of car manufacturers do it, generally for historical purposes, e.g.

pre 2001 merc, a 240 was a 2.4 petrol, then the facelifted car came out and the 240 was a 2.6 v6 petrol.