Lowering the compression ratio Xj550

Hello,

I have to take my head of the engine coz it developed an oil leak around the oil return Orings, so i intend also to change the cylinder head gasket. Since i come from europe were we have 95/98 octane fuel as standard i was wandering how much i could lower the cylinder head?

The XJ400 back in 91 had a compression ratio of 10.7, (xj550 9.5), so i calculated if i would lower it to 10,5 i would have to take of 0.5mm (1/64inch).

Does anybody know how much can be taken off? Can maybe somebody tell me how much clearance i need for the linear expansion of the part?

 

I can't answer about the modification, but I can say this about the octane ratings (quoted from French and English Wiki):

Two values of octane exist:

octane "research" (RON, Research Octane Number) characterizes the behavior of a fuel at low speeds or when accelerating;
octane "engine" (MON, Motor Octane Number) measures the resistance of a fuel at high rpm rattling.
Three different types of motor gasolines available in France in 2010 exist:

premium unleaded gasoline 95 or SP95 (RON 95, MON 85);
premium unleaded gasoline 98 or SP98 (RON 98, MON 87);
premium gasoline without lead or 95-E10 SP95-E10 (RON 95, MON 85).


Anti-Knock Index (AKI) or (R+M)/2
In most countries, including Australia, New Zealand and all of those in Europe, the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2). It may also sometimes be called the Posted Octane Number (PON).
​

So (98+87)/2 = 92.5 AKI
and (95+85)/2 = 90 AKI

I've seen the AKI of SP98/95 fuels differ by a couple of points but I think they are similar to US options. In Midwest US, I'm using 91 AKI because it's the only option without ethanol, and I haven't had any issues.

I'm certainly not an expert, but I thought it might be helpful information for you.

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TNX, i did study engine design @University but this fact newer came to my attention, so im very glad to know now something more

Nevertheless the gas quality was improved over the course of 30 years, consequently the compression ratios / efficiency of the engines went up. (if we exclude the direct injection engines)

 

My Seca 750 runs just fine on 87 Octane ( 10% basted Ethonal) You should be just fine as is .

 

You do not need to alter the compression ratio. Lowering the compression ratio will do two things. You will have an engine that develops less power per CC, but you will be able to run an even lower octane fuel. Since you do not live in a poor far-off land you do not need to worry about only having access to really low-octane fuel.

Also, by shaving (lowering) the head you will be increasing the compression ratio (squeezing the air into a smaller volume), not reducing it.

 

Sorry my bed, i ment lowering the cylinder head not the compression ratio. So does anybody know how much can be taken off? The workshop manual says 0.25mm, so that would then be a CR of 10:1

 

I think we need to discuss your goals.
If you are just decking the head for reassembly then take off the minimum amount needed for it to be flat.