Calculating compression ratio - questions on Payen gasket

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ysmalkie
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#1 Calculating compression ratio - questions on Payen gasket

Post by ysmalkie » Fri Feb 21, 2020 7:36 pm

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
Tadek

e-type S1 3.8 FHC - in restoration phase...
Jaguar XK120 OTS
Austin-Healey 100 BN2

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christopher storey
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#2 Re: Calculating compression ratio - questions on Payen gasket

Post by christopher storey » Sat Feb 22, 2020 10:25 am

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

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PeterCrespin
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#3 Re: Calculating compression ratio - questions on Payen gasket

Post by PeterCrespin » Sat Feb 22, 2020 10:30 am

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.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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malcolm
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#4 Re: Calculating compression ratio - questions on Payen gasket

Post by malcolm » Sat Feb 22, 2020 10:58 am

PeterCrespin wrote:
Sat Feb 22, 2020 10:30 am
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.
Malcolm
I only fit in a 2+2, so got one!
1969 Series 2 2+2
2009 Jaguar XF-S

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#5 Re: Calculating compression ratio - questions on Payen gasket

Post by ysmalkie » Sat Feb 22, 2020 11:22 am

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
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e-type S1 3.8 FHC - in restoration phase...
Jaguar XK120 OTS
Austin-Healey 100 BN2

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#6 Re: Calculating compression ratio - questions on Payen gasket

Post by PeterCrespin » Sat Feb 22, 2020 2:57 pm

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.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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Richardhealey
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#7 Re: Calculating compression ratio - questions on Payen gasket

Post by Richardhealey » Sat Feb 22, 2020 3:33 pm

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

Image
1969 series 2, 2+2... going for a full rebuild.. Gulp...

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christopher storey
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#8 Re: Calculating compression ratio - questions on Payen gasket

Post by christopher storey » Sun Feb 23, 2020 8:30 am

PeterCrespin wrote:
Sat Feb 22, 2020 2:57 pm
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

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PeterCrespin
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#9 Re: Calculating compression ratio - questions on Payen gasket

Post by PeterCrespin » Sun Feb 23, 2020 11:34 am

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... :-)
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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#10 Re: Calculating compression ratio - questions on Payen gasket

Post by ysmalkie » Sun Feb 23, 2020 3:21 pm

Thank you all, very highly philosophical input :-)


I decided to keep the Payen gasket....


Tadek
Tadek

e-type S1 3.8 FHC - in restoration phase...
Jaguar XK120 OTS
Austin-Healey 100 BN2

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