correct compression ratio

[ggg1102]

Hi, I have a doubt about correct compression ratio of my 4.2, 2+2 (made on march 1966 for US).
A specialist remade the engine of my car and they used 8:1 pistons.
Now I read probably my car uses 9:1 compression ratio but on request 8:1.
I have the original pistons (on pics) and on they are stamped number C28087 and S14.
I checked on jaguar bulletin but I was not able to find information about number C28087.
On service bulletion I found only these number: C24086 for 9:1 pistons and C24753 for 8:1 pistons.
Do you know something about C28087?
Thanks
Giovanni

[PeterCrespin]

What is your engine number? That will show the original ratio as -8 or -9.

I’m guessing your car number is around 1E75250?

[ggg1102]

Thanks Peter,
I checked on the plate, as you suggested, and the correct compression ratio is 9:1.
Unfortunately the engine was rebuilt with 8: 1.
Discussing with the mechanics they told me that it is better because the engine (and in particular the head) is less stressed and works at a lower temperature.
What do you think about it?

[Philk]

That's interesting. A 9:1 ratio engine has a tendency towards wanting higher octane fuels and a 8:1 ratio engine will reduce peak pressure in the head by around 10-15%. However, unless your head had weaknesses in it, I'm surprised at their advice to convert the engine. Many of us have had their 9:1 engines overhauled with no issues arising from that.

[christopher storey]

The problem with 9:1 pistons is, as I think Andrew Bowie mentioned , that by the time these engines are 55+ years old, many have been rebuilt including shaving of the head and sometimes decking of the block as well . THis can raise the CR to an unacceptable degree e.g. 10 or even 11 to 1 . In any event, these engines even when new were always a bit marginal on detonation, both in the form of easily heard pinking in the medium speed range, and much more damagingly unheard detonation at high speed. This was with the 100+ octane fuels then available, and the situation is now much worse with the lower grade fuels available . Even 8:1 engines as in Mark VIII / IX , 3.4 etc were regular pinkers.

For those who ask why this does not happen with modern engines, the answer is knock sensors which instantly retard ignition, alter mixture of electronic fuel injection etc

I have both a 9:1 and an 8:1 E type ( actually 7.8 with XJ6 pistons ) and the 8:1 is both sweeter and more powerful than the - admittedly older - 9:1 car

[abowie]

The problem with 9:1 pistons is, as I think Andrew Bowie mentioned , that by the time these engines are 55+ years old, many have been rebuilt including shaving of the head and sometimes decking of the block as well . THis can raise the CR to an unacceptable degree e.g. 10 or even 11 to 1

Exactly. We only use 8:1 pistons on our rebuilds for exactly this reason.

I had a 4.2 with compression around 185 psi and I wasted ages trying to stop it pinging without success.

Eventually I took the head off and installed a custom head gasket around 100 thou thick ( I can't remember the exact dimension any more) and brought the pressures down to around 155. Problem solved.