What makes a High Compression (10:1) Pre-HE Engine?

[wgsouter]

Okay, new thread but based upon my earlier topic concerning the identification of my Pre-HE Service Replacement Engine ... the background is that I bought a Pre-HE Engine with a Part Number Plate of RTC2691SR that was glued over the top of the Engine Block Part Number - but nobody is able to identify that Part Number (RTC2691SR) or match it to a precise engine description. As this engine is purported to have been fitted in 1991 as a Jaguar Service Replacement into a 1981 XJ12 Series 3, then I was rather hoping to be able to 'prove' that it is a High Compression (10:1) variant Pre-HE ... but I have so far failed.

Consequently, this topic - where I am hoping that somebody can tell me 'what' was done to the 9:1 Pre-HE to turn it into a 10:1 Pre-HE ??

I am hoping that there is some physical difference that is easy to spot to thereby determine 'which' engine I have ... Any clues?

Here's hoping it is easy

Regards, Wolfgang
Westbury, Wilts

[mike020150]

The only physical change was to the pistons. Because the heads are flat, the combustion chambers are contained in the heads of the pistons. Consequently, different compression ratios require different pistons. Fitting normal pre HE fuel injection kit to a 10:1 engine isn't a good idea. It was only the adoption of the later P digital injection system that made it possible to run with the 10:1 ratio without causing piston problems. If you have a 10:1 engine, it would be a good idea to talk with Roger Bywater of AJ6 Engineering to discuss what would be the best ECU to use, assuming you haven't got one from a P digital car. In your other thread, you mention that the compression readings are rather variable. That suggests you probably need to remove a head to take a look at what's wrong. Removing a piston wouldn't be much more work on an engine already out of the car and then you could easily compare the piston with one from a 9:1 engine. Mike.

[wgsouter]

Thanks Mike,
I was expecting that 'different' pistons would likely be the only real way to increase compression ratio without a fundamental change to the engine. However, as I do not have a 9:1 piston to act as comparator, does anybody have a picture of either (or both) a 9:1 and/or 10:1 piston crown that they can send me?
I have an endoscope (camera) that I can insert into the cylinder, but I need to understand what I am looking for and whether the difference in the piston crown is remarkably different (obvious) when viewed this way.
Many thanks again ... had contacted Roger asking this question, but no response yet. Equally, as I intend to fit something like the Emerald System, knowing 'which' engine I have seems pretty important.
Kind Regards
Wolfgang

[mike020150]

Sadly, the piston tops will look the same. It's the height of the piston above the gudgeon pin which is critical. As others have mentioned, in theory, you could measure the maximum piston height through a spark plug hole of your mystery engine and compare it with a known pre HE unit. There should be a small difference in the height measurements in engines with original Jaguar pistons. However, the measurement difference will only be small and you will be relying on both engines' pistons not having deposits on the pistons which could muddy the result. It's a tough one! Mike.