Calculating compression ratio - questions on Payen gasket

[ysmalkie]

The Payen gasket (I have) for the 3.8 engine has a thickness of 0.4mm Does anyone know how much does it compress to?

Anyone by any chance has calculations for 3.8 engine? It would be great to compare notes..

Tadek

[christopher storey]

Tadek : the amount by which the gasket compresses will not affect the CR to any calculable degree i.e. its effect will be minuscule

[PeterCrespin]

Being a steel gasket, it will not go any thinner when torqued up, the corrugations will merely flatten out under the increased point pressure. It’s this action that provides the important seals around the key apertures. Buretting thin oil at TDC is the best real-world volumetric CR test.

[malcolm]

Buretting thin oil at TDC is the best real-world volumetric CR test.

Excuse the ignorance Peter, but could you explain how this is done?
Thanks.

[ysmalkie]

So, it seems my compression ratio (with 9:1 pistons) is:
9.25 9.20 9.28 9.28 9.25 9.23
across the cylinders.

Should I worry and go for a cometic gasket (ideal would be 0.8mm to get about 9.0) or leave it like it is??..


Tadek

[PeterCrespin]

The volumetric/geometric CR is the ratio of the swept volume (3.142 times bore radius squared, times stroke) divided by the chamber volume at TDC. The volume at TDC is most easily found by assembling the valves and fitting the head with the piston at TDC. A thin oil can then be poured in up to the base of the spark plug threads. The easiest way is to use a burette.

The differences between Tadek's cylinders amount to less than 1 mL and are probably just measuring variability.

Whether to reduce it with a thicker gasket (a normal composite gasket is about twice as thick when crushed) depends on multiple psychological, chemical, mechanical and environmental factors.

[Richardhealey]

And here is the combustion chamber volume being measured for my engine. Very interesting!

[christopher storey]

The volumetric/geometric CR is the ratio of the swept volume (3.142 times bore radius squared, times stroke) divided by the chamber volume at TDC. The volume at TDC is most easily found by assembling the valves and fitting the head with the piston at TDC. A thin oil can then be poured in up to the base of the spark plug threads. The easiest way is to use a burette.

I don't think that's quite right , Peter . The arithmetic static CR is the ratio of cylinder displacement ( as you have stated ) PLUS
combustion chamber volume / combustion chamber volume . If you think about it, the combustion chamber contains air at the bottom of the stroke, which is all part of the air compressed on the compressiion stroke

[PeterCrespin]

True—ish, Chris, and I often sin by choosing/forgetting the wrong stuff to leave out in my failed attempts at succinctness.

You can’t just add the volume of the chamber measured as per Richard’s photo to the swept volume, because the piston dome displacement. also reduces the BDC static volume. Hence you use the TDC chamber volume not BDC chamber volume for the adding/subtracting before you do the multiplication/division bit to get the ratio.

Much of which is arguably b*ll*cks anyway, since unless you prefer your engines cold or as sectioned museum displays, it’s the dynamic CR that matters, which is heavily influenced by inlet cam type and timing.

There’s probably a pukka engineer reading this somewhere and crying into his breakfast cereal as I mangle his discipline and bring chaos where there was order... :-)

[ysmalkie]

Thank you all, very highly philosophical input :-)


I decided to keep the Payen gasket....


Tadek