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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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Just Some Dude in Jersey
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They're all steel or aluminum, so not much compression. They're made from multiple layers of sheet metal and nothing else. The thicker you buy them, the more layers of metal they have. I'm not sold on them, they look like they would leak water between passageways. But they probably seal good enough to keep coolant out of the cylinders. Never used them, the one I had I gave to Charlie.
 

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Pedal Smasher
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MLS head gaskets use steel that has been stamped to create a spring for each seal. This head gasket was originally created by a Japanese company so it could expand a little bit when the cylinder head is pushed upward by combustion. This prevents high cylinder pressures from forcing the head gasket to fail. It is now one of the most common head gaskets used on production vehicles.

That's what my internet research had to say about MLS. Sounds like a head gasket I'd want to use. One result said the compression height was 75% roughly of the thickness, but I can't verify it yet.
 

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Opel Rallier since 1977
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Bob, is Cometic Gasket's part listings for the head gaskets the uncompressed height or compressed height?
Everything I have ever read, and every number I have used for Cometics, is that their specified thickness IS the compressed thickness. The uncompressed thickness all over the map due the multi-layer make up of these gaskets, plus the embossed sealing ridges. One example of an actual measurement done in the field:

Be aware that Cometic spec's a finer head and block milling finish for their gaskets. They have softened their insistence on this in the last few years, but it is something to be aware of. From what I have learned in using them, the older flat grinder methods of finishing blocks and heads (like Blanchard grinders) typically produced a rougher finish that might give the Cometics some sealing issues. Newer milling methods tend to produce a finer surface finish that does not give any problems.
 

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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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Opel Rallier since 1977
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Good deal on the springs.... it's unlikely that those pressures would damage the cam IF the ZDDP was maintained with good break-in practice. Make sure you check out the whole head oiling system thoroughly to make sure it is all good. My last spring install was:
  • At an installed height of 1.60 to 1.55, the closed pressure is 120 lbs + or -. (Stock numbers that I have are 85 lb IN & 74 lb EX.) For my .407 lift cam, open pressures are going to be around 210-220 lbs, vs the stock 150-160 lbs.
  • This works out to 235 lb per in spring rate.
It's all surviving well so far.... the lifter were re-conditioned units from OGTS... I trust the old metalurgy more than new. (And I think Gill feels the same.) Break in was with Gibbs oil and I have switched to Mobil1 15W-50 to get a better cold start viscosity to keep down some of the leakdown and start-up lifter rattle. (The higher spring pressures cause more leakdown issues.)

The piston milling oughta work well. Is the deck height above or below the deck? Just be sure of the sign convention used in the calculator.... ditto for the sign convention used for the valve reliefs and domes. This is one spot where folks go astray. Sounds like the piston to head clearance on the piston flat will be 32 to 36 cc (depends on the deck height being above or below block deck ). Either would do well for quench/squish effect to help combat detonation so that is good. I just get a bit nervous down around 0.030"; my last was .028", but so far, no evidence of pistons kissing the head!

BTW, the the cam duration of 268 that you used is the right number for DCR computations if you got it out of the OGTS ads, and would be called 'advertised duration' by US cam companies. Just be aware that the slow ramps of the old Isky hydraulic cam designs will make the duration effectively shorter than 268 in its effect on DCR (and raising the cranking compression numbers). I'd like to hear your cranking compression numbers when it is broken in. A real DCR of 8 at 3000' computes to a cranking compression of around 150 psi.
 

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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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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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Opel Rallier since 1977
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FWIW, I noticed that the fire rings were sealing well on the pix of the head. As long as they seal, there is no need to go bigger. It is only to make sure that the rings register well around each cylinder.

When I have selected Cometic's in the past for Mopar V-8's I have typically gone to holes .080" (2 mm) larger than the actual bore, but that extra is just to insure that any bore mis-registration is not going to slip under the edge of the fire rings. Sometimes bores DO get registered off-center a bit when cylinders get bored. (And on occasion, that is intentional...)
 
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