4 replies to this topic

[hatchssv8]

There seems to be different trains of thought.
This is how I calculate
Comp ratio-
A=combustion chamber capacity, B=gasket capacity, C=deck capacity, D=cylinder capacity, E=piston dish/dome capacity (+/-)

Add A+B+C+D+E=F. Add of A+B+C+E=G

Calculate F/G= Comp ratio.

This to me is the only method that makes sense.

Now to engine capacity

We can all multiply bore x stroke, but this is where I need clarification. I always thought that capacity was the total volume in a complete engine cylinders' cavity with the piston at BDC multiplied by the number of cylinders.

That would technically mean (A+B+C+D+E) x number of cylinders.

However most capacity figures provided are simply bore x stroke.

Lets say I am selecting a cam for instance. the advertised comp ratio recommended for a particular grind is 9.5 to 10. If A,B and C are not properly accounted for, there will be a significant difference in your calculated comp ratio.This can get even trickier if you are relying on an advertised engine capacity as well. Hope you can follow what I am getting at.

What are your thoughts, experiences. Is there a given 'rule' that engine builders and component manufacturers follow. Cheers Max.

[makka]

your comp ratio one seems right but capacity is the volume of the cylinder only, because this is the volume drawn in by the engine.

[hatchssv8]

Agreed gents.
Is it fair to say then-
Total volume at BDC minus volume at TDC multilplied by the number of cylinders equal total engine capacity.

Edited by hatchssv8, 30 May 2006 - 11:16 AM.

[Dr Terry]

Hi Guys.

It is easier to calculate the capacity of a cylinder (i.e. swept volume) by just multiplying the bore area X stroke. Total engine capacity is just the swept volume of 1 cylinder times the number of cylinders.

That would be bore/2 (bore radius) squared, times pi, times stroke.

I've never seen engine capacity calculated by including the combustion chamber etc.

Dr Terry.