Bore and Stroke Basics

**JonnyGreenwood** · 06-10-2022, 14:10

I've just joined the forum, so nice to be here.

I'm a student interested in engine building and this is a very steep learning curve for me but I'm enjoying it. I'm one of the few people my age who think that the ICE is the future, not the past.

OK, so here's my very basic question:

If we take a basic Cosworth YB engine with a bore and stroke of 90.8 x 77: Simple maths (using pi at 3.14159) gives us the area of each 'cylinder' as 498cc x 4 gives us the overall capcity at 1994cc.

I get that, but here's where I am scratching my head....

The 'cylinder' isn't just the bore and stroke because that would assume the piston hitting the head at TDC, but obviosuly it doesn't. So there has to be an allowance for the compression area.

If the compression ratio is 10:1 then the head/piston clearance at TDC is 7.7mm

So at BDC the actual chamber above is 77mm + 7.7mm compression area. So as the valves shut and the compression cycle starts, the cylinder capacity is 548 making it a 2.2 engine.

It's not as though the compression area is not equally filled with air and fuel, so why is this element not included in cc calculations?

I know this is going to be one of those duh! answers, but I'm only learning...

JG

**katana** · 06-10-2022, 16:40

You can't simplify it to pure maths (bore / stroke / CR) You need to know a) the swept volume of the cylinder and b) the actual compressed volume of the combustion chamber to get static CR. The combustion chamber shape isn't uniform, the piston may have a dome or a dish which will alter chamber volume. All are usually measured during a dry build to check / establish likely CR.

**JonnyGreenwood** · 06-10-2022, 20:01

Originally Posted by katana

You can't simplify it to pure maths (bore / stroke / CR) You need to know a) the swept volume of the cylinder and b) the actual compressed volume of the combustion chamber to get static CR. The combustion chamber shape isn't uniform, the piston may have a dome or a dish which will alter chamber volume. All are usually measured during a dry build to check / establish likely CR.

This is my point: So there's an unknown element between piston top and head at TDC, which is greater than the mere bore and stroke of the piston. Whether it's 5% or 7% makes no difference as an example.

What I'm curious to know is why this unknown element is not factored into a car's CC. All we use is bore and stroke.

**Miniliteman** · 06-10-2022, 21:05

Because that's the definition: cc's of an engine is the displacement. Explanation here:

https://en.wikipedia.org/wiki/Engine_displacement

**beeRS** · 07-10-2022, 04:39

It’s simply convention. We use, and quote, the swept volume of combustion engines. It tells us something about the engine, but by no means everything. If we include the combustion chamber we have a different (larger) figure to quote, but this has never been the convention. These norms were probably established by steam engine engineers centuries ago, as it is the swept volume that is used in calculations and governs how much work (energy) a steam engine may produce.

Good to see a young person getting involved with combustion engines.

**Graham** · 07-10-2022, 06:40

Originally Posted by JonnyGreenwood

This is my point: So there's an unknown element between piston top and head at TDC, which is greater than the mere bore and stroke of the piston. Whether it's 5% or 7% makes no difference as an example.

What I'm curious to know is why this unknown element is not factored into a car's CC. All we use is bore and stroke.

one answer is because thats how its been done since the internal combustion engine was invented.

the compression has no effect on the displacement as its actually the swept area which draws the air/fuel charge into the cylinders, the "unknown" area has no bearing on that, you way of thinking would mean the engine capacity would vary with different compression ratios. no one would call a low compression transit engine bigger than the same engine from a sierra. if you were to follow your logic where do you stop? volume down the side of the piston to the first ring? that usually about 1 cc, do you try and take into account ring blow by, or valve timing such as when looking at dynamic compression ratio. Heaven only knows about rotaries and two strokes!

At the end of the day the way the capacity is arrived at is simple easy and suits everyone- almost!

**JonnyGreenwood** · 07-10-2022, 09:35

Originally Posted by beeRS

It’s simply convention. We use, and quote, the swept volume of combustion engines. It tells us something about the engine, but by no means everything. If we include the combustion chamber we have a different (larger) figure to quote, but this has never been the convention. These norms were probably established by steam engine engineers centuries ago, as it is the swept volume that is used in calculations and governs how much work (energy) a steam engine may produce.

Good to see a young person getting involved with combustion engines.

Thank you. I have always been a bit inquisitive and I probably ask too many questions for some. My Great Grandad used to say 'don't contradict your elders' but it never stopped me from being curious!

I suppose I could have simplified my question to ask "Does engine displacement include clearance volume?", which would have resulted in lots of people just saying NO, but i'm trying to figure out the reason why. Sometimes knowing the answer is to one question is just the start...

**JonnyGreenwood** · 07-10-2022, 09:40

Originally Posted by Graham

one answer is because thats how its been done since the internal combustion engine was invented.

At the end of the day the way the capacity is arrived at is simple easy and suits everyone- almost!

Sorry for asking the question, I'm sure you were young once...

**Miniliteman** · 07-10-2022, 09:42

https://www.youtube.com/watch?v=cg1a5QlCdYo

**Graham** · 07-10-2022, 10:28

Originally Posted by JonnyGreenwood

Sorry for asking the question, I'm sure you were young once...

Sorry i wasnt knocking you, im 56 and still learning

**rallyrob** · 07-10-2022, 11:31

Originally Posted by Graham

Sorry i wasnt knocking you, im 56 and still learning

Certainly, every day is a school day.

**JonnyGreenwood** · 07-10-2022, 11:56

Originally Posted by Miniliteman

https://www.youtube.com/watch?v=cg1a5QlCdYo

Love it

**stevieturbo** · 09-10-2022, 13:21

Originally Posted by JonnyGreenwood

This is my point: So there's an unknown element between piston top and head at TDC, which is greater than the mere bore and stroke of the piston. Whether it's 5% or 7% makes no difference as an example.

What I'm curious to know is why this unknown element is not factored into a car's CC. All we use is bore and stroke.

No it isn't..unless your rod beaks and the piston now moves into that "dead" area....but then it might struggle to recover.

bore/stroke is the only motion that relates to engine capacity/swept volume. Dead space is irrelevant in this regard.