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Discussion Starter · #1 ·
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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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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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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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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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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Discussion Starter · #29 ·
While I generally enjoy the trip down the off-topic journey of head gasket materials, compressed thicknesses and such, this station will now return to the original programming...

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.
We have now been able to get accurate measurements and volumes from this engine "as built", and here are the results, as summarized in the attached table (input and calculated results from the html calculation file provided by Port City Engines) and related photos:

The stroke measured at the stock 2.75" (69.8 mm).

The bore is 94 mm (a 1.0 mm overbore) or 3.700".

The piston small ends had been bored to fit the floating pins on the Jahns pistons, and the effective rod length is now 5.02"

We used plasticine to measure a combustion chamber, with a spark plug in place, and determined using fluid displacement that the chambers are almost exactly 48 cc. That is with Chevy 1.72" intake valves, and Chevy 1.5" exhaust valves. It turns out that there are hardened exhaust seat inserts installed.

The net piston dome volume was determined by directly measuring the net non-dome volume (the volume above the piston "flat", below the dome crown) using a ring compressor at the crown height and plasticine to fill the void. That was then verified by repeating the process of the dome piston with the top at the block deck height and subtracting that from the volume of a flat-top piston at the same depth. Both resulted in a net dome volume of 12.5 cc. That does not include the head gasket volume, but does include the piston deck net height.

The PCE calculator allows the input of top ring land volume, which measured 0.20" below the deck.

The head gasket that came off this engine was measured at 0.034" thick, compressed, at the sealing rings.

And the envelope please.....

Static compression ratio was 12.36:1

Dynamic compression ratio (with the 2 degrees cam advance) calculated at 9.96:1


So while some folks might aspire to have a CR of 12.4:1, that is simply too high for this street engine.

We are still evaluating how much piston dome to mill off, but the target SCR is ~10.0:1 and a DCR of ~8.0:1. That seems achievable by milling off approximately 4.5 mm of the 6.27 mm (0.247") piston dome. The valve reliefs measured at 1.5 cc. That is all illustrated in the attached PCE table.

Oh, and as an aside, when the cam duration is reduced from 268 degrees to a bizarre but perhaps illustrative 134 degrees (with the cam lobes worn down), the SCR stays at 12.36:1, but the DCR climbs to 12.34:1. Hence the seemingly huge compression test pressures.

Finally, we tested the "installed" and "closed" spring pressures on the dual springs in this engine. They measured at:

Intake (installed/open): 72 lbs / 158 lbs
Exhaust (installed/open): 75 lbs / 155 lbs


The spring rates at open (0.420") for both springs measured at 205 lbs/inch and 190 lbs/inch respectively. I suspect that they are actually the same if we had used more precise measurement equipment.

The exhaust springs have shims installed, which explains the slightly higher installed pressure. The intake springs did not, so we will add a shim to get installed pressures closer to 90 lbs. Otherwise, the springs are no longer considered complicit in the cam/lifter failure, so that is being ascribed to poor break in, and insufficient ZDDP in the oil.

Any further comments (on-topic, if you please)?
 

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Discussion Starter · #31 ·
I think you might want to include a different gasket as part of your solution, unless you make the pistons flat tops.
Why (aside from using the later non-cork version)? We are milling off most of the piston domes off, which achieves the net combustion chamber volume (and SCR) desired.
 
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Discussion Starter · #33 ·
A standard 1.9 gasket usually specs out to .039” new, and .031” compressed with a 94.5 mm ID.
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.
 
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