11 replies to this topic

[podifold]

Hi all,

 

I currently have a 998cc Metro with what I believe is the high compression engine, quoted at being 10.3:1.

 

However, using my compression ratio calculator ( https://jscalc.io/ca...RNEglVkKLQir11T ) with 64.58mm pistons, a 76.2mm stroke crank, 2.8cc head gasket, flat top pistons, 25.5cc in a CAM4810 head and the pistons level with block deck height I still only get around 9.72:1

 

Anyone know what makes up the difference?

 

Page 28 of this document...

 

http://metroownerscl...-1984_24_41.pdf

 

...states: "MODIFICATIONS TO THE PISTON HEAD REDUCED THE SIZE OF THE COMBUSTION CHAMBER"

 

Any ideas?

[nicklouse]

What is the engine number?

[podifold]

99H A08P 100235

[nicklouse]

I would go back to basics and work it out. There may be something wrong in that calculator. 
 

I agree with your assumption of it being a high compression build from that engine number.

[ACDodd]

That’s what you usually get, but you must actually measure your components are they do vary. This high ratio is usually achieved by smaller chamber volume either in the head or deck volume.

Ac

[GraemeC]

I wonder if the 'marketing speak' of MODIFICATIONS TO THE PISTON HEAD REDUCED THE SIZE OF THE COMBUSTION CHAMBER actually means the head was skimmed, bringing the chamber volume down?

 

Otherwise it would have to be piston tops out of the bore or raised crowns

Edited by GraemeC, 04 December 2020 - 12:27 PM.

[Cooperman]

I wonder if the 'marketing speak' of MODIFICATIONS TO THE PISTON HEAD REDUCED THE SIZE OF THE COMBUSTION CHAMBER actually means the head was skimmed, bringing the chamber volume down?

 

Otherwise it would have to be piston tops out of the bore or raised crowns

 

Yes, you will recall that the 998 Coopers had raised D-Top pistons to increase the CR.

[Spider]

I would go back to basics and work it out. There may be something wrong in that calculator.

 

The internet is a wash with these Calculators for all manner of things. Those I've looked at and checked by old school means have never been right.
 

[Turbo Phil]

For anybody not familiar with what’s involved in calculating static compression ratio, I made a video for my YouTube channel explaining it,
Super simple!

Phil. 

Edited by Turbo Phil, 04 December 2020 - 08:40 PM.

[podifold]

Just to answer my own question for anyone else searching for this answer:

 

When I took my cylinder head off recently and took some measurements it looks like they skimmed off 40 thou at the factory to increase the compression ratio to the quoted 10.3:1.

 

This would take the normal CAM4810 head chamber volumes from 25.5cc to roughly around 23cc.

[Steam]

Have you tried this.

 

https://www.theminif...tio-calculator/

[podifold]

Yes. I wrote it.