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Compression ratio looks to be in the realm of 11:1 or 11.5:1 or so. Certainly less than 12:1.

And actually, 180-200 psi…though much higher than stock, is not that high.

I suspect the cam choice and valve spring pressures have a lot to do with the difficulty cranking.
 

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I have no experience with the Jahn’s pistons, other than from other people telling me about them. Roger Wilson used to talk about them, since they made pistons for LOTS of oddball cars over the years.

My understanding is they used one basic casting design per engine type, then milled the domes down to the customer’s desired compression ratio.

As far as how I know the compression ratio is less than 15:1…short stroke engines are very difficult to get high compression out of. The 1.9 Opel is one of the worst engines to get compression out of.

Static compression is all about area at BDC compared to area at TDC, nothing else. Atmospheric pressure and cranking compression have nothing to do with it.

Here is a theoretical maximum dome for a racing 1.9. Yes, a lot of valve relief reduces the compression. But it is truly a max dome, made exactly .060” (1.5 mm) smaller than the actual chamber. It calculates to a true 11.8:1 compression ratio.
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This piston is one of mine. It has a measured 11.3:1 compression ratio after I deburred it, added flame travel slots, and corrected the valve reliefs. It started at 11.5:1. Note the shallower valve reliefs than the all out racing piston design, and note the dome height.
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We already have a preliminary estimate of piston dome/valve relief volume, using plasticine and a ring compressor to model the non-domed volume above the piston crown. That calculated at ~13 cc. Subtracting that from a typical 1.9 combustion chamber of ~48 cc, results in a SCR of ~12.74:1 and a DCR of 9.94:1. Clearly higher than what a pump-gas street engine calls for.
Definitely get all your hard numbers in place first.

-calculated swept volume (actual bore x stroke)
-combustion chamber volume (using the actual plugs run in the engine)
-valve relief
-piston dome
-deck height (positive or negative)
-top ring land volume (usually about 1 CC but easily calculated)
-compressed head gasket ID and thickness

This is really the only accurate way to get the true compression ratio.
 

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As noted above, and as I was thinking a bit more about this, the head gasket is a big deal too. You typically have 4-6 cc's in just that.
Yup, usually in the realm of 5.4 to 5.9 cc’s on a CIH engine I’ve found.
 

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Bob, is Cometic Gasket's part listings for the head gaskets the uncompressed height or compressed height?
Uncompressed.

I’m not sure how much an MLS gasket compresses.

A standard 1.9 gasket usually specs out to .039” new, and .031” compressed with a 94.5 mm ID.
 
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Bob, on the topic of head gaskets, could you please provide advice as to what size of head gasket to purchase?

The cylinder bores on the engine are 94 mm, so exactly in the middle of the 1.9 (93 mm) and 2.0/2.2/2.4 (95 mm) cylinder size. The head gasket that was in this engine was a Fel-Pro, with a measured ID of 95.1 mm. Visually, it appears that the sealing ring is JUST barely larger than the cylinders. It was the 10-bolt style with the cork spacer gasket at the front, which was incorrect, as this engine has a 12-bolt style chain case, negating the cork gasket. Clearly it sealed the cylinders, even at the elevated compression pressures this engine created. Perhaps not so much the high pressure oil passage, due to the cork gasket, as evidenced by the oil found in the coolant.

My inclination is to purchase a 2.0 (95 mm ID) head gasket, to ensure sufficient diameter for the 94 mm cylinders. Or is a 12-bolt (non-cork) 1.9 head gasket a better choice, as it provides less lost volume around the edges of the cylinder?

I will also ask Gil what head gasket he suggests, based on the actual ID of his head gaskets.

TIA.
You don’t need a 2.0 head gasket for a 94 mm bore.
 
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