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I posted the following questions (only relevant ones repeated below) , in this thread:


One of the Calgary Opel Co-op guys (Jason Selby) has a similar "problem" with excessive compression ratio created by domed pistons. They are ostensibly 11.5:1. When I measured compression pressures, I saw between 190 and 220 psi. Yikes. The gauge might be reading a bit high, so I am going to re-do with a new digital gauge. But it is not out by more then 15 psi based on a recent comparison on my engine. Atmospheric pressure here at 3000' ASL is at most 14 psi. I think the domed piston CR is more like 13.5:1. This is the engine I mentioned in a previous post that has suffered cam lobe and lifter failure. So it is coming out and apart for some kind of rebuild in any event.

The engine was built by a known Previous Owner, but much of the details are unknown to Jason. Gil Wesson is compiling a list of what the PO purchased through OGTS, but I suspect the pistons did not come from them. We have been told that the cam is an Isky OR66H (aka OGTS "Combo" Hydraulic) with 0.420" valve lift and 268 degree duration.

At present, the engine has a Weber 32/36DGEV, an OGTS header, and 2" exhaust. I believe it has 2.0 valves (waiting on Gil to confirm) but no significant intake modifications. I have suggested Jason install a surplus 38DGES that the Co-op has, and I have acquired a fair bit of experience in jetting one of those in my car.

Jason wants a "street-able" engine, but with decent power, say a bit over 100 HP. But of course he is restricted by a budget. I have suggested the following:

1) Mill the piston tops to reduce compression ratio. We can do that "in-house" as one of the co-op members has done that while building his RallyBob-stroker 2.4. Aiming for ~10:1 CR. De-glaze and install new rings (assuming the bores look OK, and they should be, as this engine has less than 5000 miles on it)

3) New hydraulic Combo cam and lifters

4) Modify the cam gear for a Comp Cams offset bushing. Several of us here are running the Combo Cam with the cam timing advanced 4 degrees, with quite good results.

So the questions I need answered, if you would be so kind:

1) What is the minimum piston top thickness to leave?

3) if we can't get the CR down to under 11:1, what cam spec might you suggest? OGTS has some longer duration cams (such as their Max Comp, 0.430 lift and 284 degrees duration, but solid lifter only). Getting a cam custom ground is a bit problematic here, and likely won't fit Jason's budget.

4) Can cam timing be used to offset some of the dynamic CR? I am uncertain, but I think advancing the cam can do that. The Comp Cams bushing kit comes with 0, 2, 4, 6, and 8 degree bushings. We would "degree" the cam to confirm. Any suggestion as to what to start with, assuming the Combo Cam, and 11:1 static CR?

5) Any other suggestions or comments?

Thanks in advance.
The problem with this engine initially was that the CR was SO high that high octane fuel AND octane booster was required to minimize detonation. And it was bloody hard to crank, and virtually impossible when hot. And simply wasn't all that powerful, considering that the original owner had spent many thousands of $ on the engine.

Then, it started running poorly and make a lot of lifter noise. When I tried to adjust the hydraulic lifters, the cam and lifters were obviously damaged. As in worn down, and so much that the bottom of the lifters were mushroomed.

RallyBob graciously provided several suggestions, including milling some or all of the piston domes off, to achieve a more tolerable compression ratio. He suggested that a piston top thickness of 0.125" (an eighth of an inch) should be acceptable. And that the Max Comp, 0.430 lift and 284 degrees duration cam might be a good fit if the CR could not be easily reduced.

So, this past weekend, we dropped the engine out, and commenced disassembly and parts ID. Gil Wesson has been preoccupied (some of you may know the story) and so hasn't got back to me with a parts list of what went into this engine. As it turns out, I suspect that many of the engine parts did not come from OGTS. More on that in a moment...

So, what did we find?

A re-test of compression with a digital gauge confirmed VERY high compression. Averaged 180 psi dry, 200 psi wet. That suggested a CR of approximately 15:1. Wow.

Six of eight lifters were so beaten and mushroomed that they refused to be lifted out. I had to lift them away from the cam, remove the cam, then drop the lifters in the cam area, removing them out the open ends. I have never had to do that before, but good to know it can be done. Forcing the lifters up and out would surely have damaged the lifter bores, junking the head.

The cam is (was) a Norris 290H. I believe that it had similar specs to the Isky OR-66 (OGTS Combo Cam) with 0.435" lift, 230 degrees duration @.050" lift, 110 lobe separation. Seven of the eight cam lobes were scrubbed. If we get the CR down to 10:1, I like the Combo Cam in my engine, and I think we have a brand new spare in the Calgary Opel Co-op (the subject of a future post regarding a stroker 2.4 cam)

Valves are Chevy 11/32 non-tulip stems, 1.72" (44 mm) intake, 1.50" (38 mm) exhaust (stock is 40 mm intake, 34 mm exhaust). Hmm, the builder was a Chevy muscle car/hot rod guy, so I shouldn't have been surprised. That and the Chevy Orange engine block paint. Lovely proper umbrella valve stem seals all around. Some serious head work done.

Hmm, no sign of hardened exhaust seats, but a lovely three-angle grind. F'ing hot rodder engine builders....

Valve springs also appear to be Chevy; dual springs, with matching retainers and keepers (note to self: do not pull a Charles and mix Chevy retainers and Opel keepers.... Gordo should be very dissatisfied) . Very likely VERY high spring pressures. We measured installed heights, and will perform some spring pressure measurements using a digital pressure scale and a digital milling machine for spring displacement. I suspect the cam/lifter failure is a result of excessively high spring pressures, like + 150 lbs closed, +300 lbs open. Or more than double the factory spec. Muscle car guys love high spring pressures, but they all run roller lifters and rockers these days. Not good with flat tappets.

We noticed more than a bit of oil sludge in the radiator cap area. Oil had LOTS of metal in it, but no indication of coolant contamination. Yikes. Indicates a leak between the high pressure oil system and coolant. But the answer is easy. Older style 10-bolt head gasket with a cork gasket at the front. Chain case is the flush 12-bolt style. No room for the cork gasket, so it was squeezed out, and the head didn't seal well at the front where the oil passage is. Head gasket showed clear indication of leakage past the copper crush ring into the coolant passage at the front. Grrr....

All the bearings (std size, not undersize) plasti-gauged at between 1 and 1.5 thou, so very good. But two of the main bearings had clear evidence of scoring into the copper.. Crank journals look very good, except a couple of nicks. Hmm, maybe they were damaged at installation (or before) and that scored the bearings. Or a bit of cam and lifter debris. Rod journals and bearings were perfect. Looks like a minor crank journal clean and polish and new main bearings and all is good.

So, onto the main topic: the high compression domed pistons.

Cylinders look almost perfect. Should be so, with so few miles. Measured almost exactly 94 mm, so halfway to 2.0 litres (93 mm stock 1.9, 95 mm for 2.0). Rings were clearly sealing very well, so a gentle de-glaze and all should be good.

Pistons are substantially domed. Made by "Jahns". No marks indicating piston specifics, so any insight would be appreciated. Nice thin rings. Google suggests that almost all Jahns pistons were "sand cast", and not likely forged. But piston-to-cylinder clearances measured at a very large 0.005" to 0.006". High even for forged pistons, but apparently common for Jahns pistons. Jahns was around for many decades, and eventually became JE Pistons, a venerable piston manufacturer. Jahns had a reputation for being able to supply many types of pistons, due to the low-volume nature of sand casting, but had a reputation for problems with quality control and breakage.

Pins are tapered internally and floating, with spiral locks. Helpful in easily removing the rods, so long as we don't damage a lock ring. Probably hard to replace.

The good news is that the piston tops have lots of meat. Over 0.200" thick even at the flat edge, no internal dome, so the entire dome can apparently be milled off if desired.

Which is the "plan", subject to leaving a portion of the piston dome to provide a bit higher (say a true 10:1) CR. We will attempt to calculate how much dome to mill off (and to leave on) in order to achieve that.

I would REALLY appreciate any knowledgeable comments and suggestions, especially from RallyBob and other experienced engine builders.
 

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Discussion Starter · #2 ·
A few more photos... (last photos to compare these Jahns pistons to a stock Opel flat top piston)
 

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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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Discussion Starter · #4 ·
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.
Thanks Bob. We'll get quite precise values for dome volume and combustion chamber volumes, which should help verify the "as found" compression ratio.

But how do you arrive at a CR under 12:1? Simple math (perhaps too simple?) suggests that 200 psig (214 psia) divided by 14 psi (our atmospheric pressure) equals 15.28:1. I am a gas compression guy, and while air isn't an Ideal Gas, the trusted and true P1*V1=P2*V2 suggests I am not far off. Any pressure drop due to restrictions in the intake would only reduce the measured compression.

Irrespective of the precise ACTUAL CR, it is too high for this street engine in practice. If a flat top piston provides a CR of 9.0:1 (sure, closer to 8.5:1 when considering the inaccuracy of Opel's numbers from 1969), and some reduction of combustion chamber due to the big Chevy valves, how much piston dome volume should we consider milling off? If the combustion chamber is notionally 50 cc, and the piston dome is 30 cc (all values to be determined by plasticine and fluid displacement), then might we mill off half the dome volume?

Do you have any experience with these Jahns pistons? Is the six thou clearance a big concern? Should I even bother re-using them? Jason is on a budget, and new pistons and rings will hurt that. But he was expecting to have to buy bigger valves, and now doesn't (in my opinion) so a few hundred $ won't break the bank.

I just noticed that the cam gear has been modified with a Comp Cams off-set bushing. We couldn't degree the cam, what with it being worn so badly, but we will on re-assembly. I still have most of my off-set bushing kit, so we'll have a careful look then.
 

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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.
Gas Automotive wheel system Rim Circle Plastic

Nickel Gas Composite material Auto part Aluminium

Automotive lighting Gas Circle Auto part Rim




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.
Kitchen appliance Home appliance Cylinder Cookware and bakeware Gas
 

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Discussion Starter · #7 ·
Nitpicking some low hanging fruit, but...

Atmo isn't 14 even, it's 14.7.

That'll account for 5% right there. 14.56:1
Atmospheric pressure is only theoretically 14.7 psia at sea level, Matt. Atmospheric (aka barometric) pressure changes constantly, and is consistently is lower at higher elevation. Up here in Calgary at 3,000 ft above sea level, the pressure is typically at closer to 14.2 psia ("a" for absolute). This morning, with the skies clear and cold, the barometric pressure is actually higher, at 102.9 kPa, or 14.9 psia.
 

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The cranking pressures reported above indicate a static CR of around 12 to 12.5, with a typical cam, not 14 or 15. So it is not quite as bad as you think. Dividing standard psi atmospheric into the cranking readings is not how it is computed... you have to convert you gauge reading to absolute psi then account for the intake closing angle on the compression stroke...

But that would be with a normal cam.... you don't have a normal cam any more.... just a hunk o' junk LOL. This damaged lobes with shortened durations, and thus an early intake closing angle, will raise the cranking pressures considerably, probably at least 20 psi, and it will throw off normal comparisons. So I would agree that these are more like 11 to 11.5: SCR pistons.

That piston to bore clearance is pretty common for the older materials in forged pistons. It will have plenty of blowby and the pistons may rattle a bit at cold start-up. The rings will not live as long either. So those are not a great recipe for a long lived street engine. I used some old forged units that I had for years in an engine about 1 year back, and it has those characteristics.

If you keep the pistons, then 10:1 true SCR is a good place to shoot for with a mild street cam. But realize that your end cranking numbers are a combination of cam and SCR. It'll take some modest attention to timing and tuning to make it run without detonation but that is my standard street SCR target, so that I end up with around a 155-160 psi cranking pressure with a modest duration cam that helps the low RPM torque (as much as you can on a 1.9L).

Here is a pix of how that last pistons looked in the block with the domes massaged a bit to get to the desired SCR; this was with larger valves and a Felpro head gasket. These pistons stick about 0.012" above the block deck in the quench side (the large flat area) so this 'dome' looks lower.
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Actual cranking pressures on this engine are 170-175 psig at 2400' elevation with the Isky OR-4H cam. The slow opening and closing ramps on this cam profile make the cam durations a bit on the short side so that raised the cranking compression numbers above what I had planned. But it runs without detonation.

BTW, when you get the crank set up and do an initial re-assembly, but before you have milled the domes, put together one piston-rod assembly and assemble it in, and measure your flat quench area relative to the block at TDC. This is important to push up CR with while avoiding detonation. Take that clearance and the head gasket thickness and work out the quench/squish gap and see if you have something like .035"-.045". Setting that gap in that range (between the flat quench area of the piston top and the flat area on the head) is a known good method to fight detonation. Closer is better, but too close and you risk a piston kissing the head. You can massage the block's deck height and the gasket thickness and the quench are on the piston head to dial this in. This may be getting in deeper than you want, or more than the budget allows, but I wanted you to be aware of this gap and its beneficial effects.

BTW, your local barometer readings are not the actual absolute air pressures in your local atmosphere. Barometric readings are normalized to sea level pressure.
 

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And you can always open up the combustion chambers in the head to lower compression ratio.... I have purposely dropped 1 full point in CR that way.

And FWIW.... 150 closed and 300 open presures are doable with flat tappets. It's done all the time. Try to find what oil was used and determine the ZDDP content. That is the most likely cause of the tappet/cam failures.... too many people are not up to speed on the need for adequate ZDDP when spring pressures are increased. Soft, stock pressure setups seem to survive with the newer lower ZDDP oils, but higher spring pressures will cause an almost 100% guaranteed fail of the lifters/cam without the older, higher ZDDP levels in tyhe oil.
 

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But how do you arrive at a CR under 12:1? Simple math (perhaps too simple?) suggests that 200 psig (214 psia) divided by 14 psi (our atmospheric pressure) equals 15.28:1. I am a gas compression guy, and while air isn't an Ideal Gas, the trusted and true P1*V1=P2*V2 suggests I am not far off.
Keith, it is called adiabatic compression. Wikipedia has a good website on it. As the gas is being compressed it heats up and increases the pressure beyond what the ratio of volumes would give. The P*V=const relationship is only valid for slow compression, where the added heat can be dissipated into the cylinder walls and the gas temperature stays constant.
For a diatomic gas (nitrogen, oxygen) for fast compression the formula P*V^(7/5) applies. For a CR=10, this would give a whopping 25x increase in pressure, but due to factors mentioned before (valve overlap, leaks, incomplete cylinder filling) the actual increase is much smaller.
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Kwil... are you familiar with computing DCR.... dynamic compression ratio? That is what you really want to know and relates directly to cranking compression. You set the SCR to work with the cam duration and timing to achieve the desired DCR number. For street running on premium gas, you ought to shoot for around 8:1 DCR. That invokes some reasonable degree of tuning, timing, and carb mixture control, but not an inordate amount. Most gearheads can work with that DCR on premium and avoid detonation.

If you want to use a cam with an advertised duration of around 268, then for a DCR of 8.0 then the SCR needs to be right at 10.0:1. If the deck height is 0.000" (i.e., the top of the piston's flat is 0.000" relative to the deck at TDC, then the net volume of (dome volume - valve reliefs volume) needs to be 2 cc's. This assumes a head gasket thickness of .040" and a chamber volume of 50.0 cc. Once these things are known (deck height, head gasket thickness, and chamber volume), then the net dome volume can be dialed in.

Please let me know if you want further help in this. The DCR calculator that I used is the Pat Kelley calculator that can be downloaded here: Dynamic CR

Some knowledge of how to use this is helpful; let me know if you need assistance.
 

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If you are serious about reducing compression, ask Gordon how to do it.
 

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To decrease the CR, or more accurately dynamic cylinder pressures, you either create more room in the cylinder for air or you decrease how much air stays in the cylinder, or a combination of both.

-Thicker head gasket
-Different piston design
-More room in the cylinder head
-Increase piston ring gap (more blow by, not recommended)
-Less boost
-Retard the cam timing
-Different cam
-Higher viscosity oil (more blow by, not recommended)
 

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Discussion Starter · #14 ·
Thanks for all of your replies.

I have benefited from a few learnings, such as I didn't know that "barometric pressure" as reported by the weather folks, is corrected to sea level wrt to the ground elevation at the locale being reported for. It occured to me that the loss of atmospheric pressure due to elevation was significant, and it seemed odd that reported barometric pressures didn't seem low enough. It turns out there are "standards" for mean (average) altitude-corrected atmospheric pressure. At our 3,000 ASL, the mean atmospheric pressure is MUCH less than 14.7 psia, at 13.2 psia.

Which, since I WAS using "absolute" pressures (hey, I am a Mechanical Engineer for crying out loud!), my perceived CR by compression test was even higher than I thought: the compression test value of 200 psig (g for gauge, for the non-scientific and non-engineering folks in the audience) equates to 213.2 psia, divided by 13.2 psia, equals 16.15:1. But even I didn't think that these pistons would provide THAT high a CR.

I hadn't contemplated the effect of the damaged cam on Dynamic Compression Ratio (DCR), which, to some extent, a compression test exhibits. It makes perfect sense that the early intake valve closure and late exhaust opening would increase the DCR, and hence compression test readings. It is surprising to me how much it affects it.

A few comments.

Adiabatic means "without heat loss", or, in other words, theoretical. I am very familiar with the term and practice. It does not mean "constant temperature". In fact, quite the opposite. In many cases, the assumption of adiabatic compression results in significant errors. But for approximations, and in the case of a few compression-test strokes, it is likely a valid, if not precisely accurate, assumption.

One other small finding. The cam gear had been machined to accept a Comp Cams off-set bushing. It was set to 2 degrees advanced. That would also slightly increase the DCR.

We are meeting tomorrow at John's machine shop to accurately determine a few key elements:

1) installed and open spring pressures using a digital scale and digital
2) accurate piston dome/valve relief volume
3) accurate combustion chamber volume

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.

Our current goal is to build this engine with an SCR of ~10.0:1, and a DCR of 8:1 seems reasonable. We're set up to mill the piston domes, and once we get a precise determination of combustion chamber and piston dome volumes, that is relatively easy.

More details to follow, including the spring pressures. When we pulled the cam and lifters, 7 of the 8 lobes and lifters were badly scrubbed. The owner of the car claims than ONLY high-ZDDP oil has been used, but he did not build nor break-in this engine. That person, Jim, is out on a big pipeline spread (he is a B-ticket pressure-welder) and is an experienced car guy who builds hot rods and drag racers, and should be very familiar with flat-tappet cams and ZDDP. But this was his first and only Opel CIH build, and therefore gets a hall pass.

Here is Jim's personal Opel GT. Full tube frame, Dakota digital dash, blown Big Block Chevy 454. Nice car, great guy. But likely didn't contemplate that his hot rod experience wouldn't translate to an Opel CIH. Dare to dream...
 

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Keith, just so it’s clear, dynamic CR never looks at the cylinder pressures compared to atmospheric pressure. That’s not how it’s calculated. Wallace Racing has a good DCR calculator and it will give a rough idea on dynamic cylinder pressures.
 

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Keith, just so it’s clear, dynamic CR never looks at the cylinder pressures compared to atmospheric pressure. That’s not how it’s calculated. Wallace Racing has a good DCR calculator and it will give a rough idea on dynamic cylinder pressures.
While atmospheric pressure does not enter into SCR or DCR computations, DCR is a step to reach cranking compression estimates, which can follow on with average local atmospheric pressure to compute actual cranking pressures... which reflects the loss in cylinder pressures in general as altitude increase in a normally aspirated engine.

I start with The Pat Kelley DCR calculator as it is so easy to enter measured parameters from engine builds directly, and things like dome volumes and eyebrow volumes (or the net of the 2) can be entered directly from piston catalog data to get SCR and DCR, plus it gets you a good view on cam timing events. If I want to know altitude effects, then I jump to the Wallace calculator with the Pat Kelley outputs.

Good feedback on the ZDDP.... hope it is all true..... seems like he would know but that sure is suspicious. Look forward to hearing the spring pressures... and an examination of the head and the oil pump to see if just a plain ole head oiling failure could have been the culprit. 7 of 8 is a big failure.
 

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Discussion Starter · #17 ·
Keith, just so it’s clear, dynamic CR never looks at the cylinder pressures compared to atmospheric pressure. That’s not how it’s calculated. Wallace Racing has a good DCR calculator and it will give a rough idea on dynamic cylinder pressures.
Zeppie, ratio's are generally independent of the units, and in this case, of the dimension of the units, so long as they are consistent.

But dynamic cylinder PRESSURES are absolutely dependent on atmospheric pressure, or more correctly, absolute cylinder intake pressure. Otherwise the caculator wouldn't have altitude as an input. It also allows for a boost pressure.

That calculator requires a SCR, so not as useful as the one that John Warga provided from Port City Engines. I have an HTML file that performs the calculation, but I don't see how to attach it using my phone.
 

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I hadn't contemplated the effect of the damaged cam on Dynamic Compression Ratio (DCR), which, to some extent, a compression test exhibits. It makes perfect sense that the early intake valve closure and late exhaust opening would increase the DCR, and hence compression test readings. It is surprising to me how much it affects it.
Which is why smaller cams are used for street engines or in any applicatoin where lower RPM 'umph' is desried.... it creates more cylinder pressures at low RPM's (when the exhaust scavenging has not yet kicked in) and thus more low RPM torque. You just have too much of a good thing now LOL
 

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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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