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WEASEL149
Joined: 19 Aug 2005
Posts: 595
Location: UK, Sheffield
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 Posted: Wed Sep 26, 2012 2:22 am Post subject: Measured 931 head gasket thickness |
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Has anyone ever tried to measure the exact fully crushed thickness of the stock 931 gasket when it's been torqued down?
If not, what is the accepted crushed thickness?
Actual specs listed by parts places list the Reinz at 1.75mm thick with 88.5mm bore.
Goetze is listed at 1.79mm and 88.5mm bore.
I take it these thicknesses are un-crushed, as-new?
The Goetze I removed from my own car measures ~1.6mm on the crushed parts but could be a little thinner on the car at full clamping torque.
1.6mm crushed gasket thickness with 21.5cc head chamber, 86.5mm bore and 45cc S1 piston dish (Omega measurement) would give 7.5:1 CR.
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Sep 26, 2012 3:32 am Post subject: Re: Measured 931 head gasket thickness |
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| WEASEL149 wrote: | | 1.6mm crushed gasket thickness with 21.5cc head chamber, 86.5mm bore and 45cc S1 piston dish (Omega measurement) would give 7.5:1 CR. |
Am I understanding that 45cc is a measured volume on a piston sample that you sent to Omega?
CR will depend significantly on the total deck height of the block, as measured from the bearing cap surface to the deck. I've always read that 931 pistons were designed to be flush with the block, but I'm not convinced this is the case. Using stock dimensions for compression height (45mm measured), 1/2 the stroke (84.4mm / 2 = 42.2mm; published spec), and rod length (144mm, published spec), the computed deck height at zero piston crown clearance should be 231.2mm. However, I have personally measured several used blocks that had deck heights that were greater than this number.
I'm also not sure on the 21.5cc figure for the head. This was a number postulated by CBass some time ago, and has been used by me and others for some time. However, RC and I both had actual heads measured recently, and both came in at somewhere around 19cc. Now granted, those heads had both been skimmed, but I have yet to be able to derive a calculatable figure between material removed from the head and the actual chamber volume.
All of that said, there are some really old posts here on the board that indicated compressed stock thickness for 931 gaskets is .046", which is 1.168mm. This is the figure I have been using in all of my calcs, but I'm not convinced that it is accurate either.
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Sep 26, 2012 3:41 am Post subject: |
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Just plugged in some numbers, for your consideration, using some assumed values of what a factory fresh motor may have started with:
head volume = 19.5cc (assumed based on actual measurements)
dish volume = 45cc (measured by Omega?)
gasket thickness = .046" / 1.168mm (calculated guess)
compression height = 45mm (measured)
stroke = 84.4mm (published spec)
rod length = 144mm (published spec)
deck clearance = 232mm (calculated guess based on several measurements)
crown-to-deck clearance = .8mm (calculated)
total combustion chamber volume = 76.308cc (calculated)
Compression Ratio = exactly 7.500:1
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WEASEL149
Joined: 19 Aug 2005
Posts: 595
Location: UK, Sheffield
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| Posted: Sun Sep 30, 2012 4:16 am Post subject: |
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I think the problem is that the original 931 gasket was thinner than the current Goetze/Victor Reinz head gaskets. 0.046" could well be an accurate figure but I'm pretty sure the design has changed.
I took a micrometer and digital vernier calipers home from work and the current Victor Reinz gasket measures 0.069"-0.070" thick (1.753mm-1.778mm) at the composite/fibrous parts and 0.075" at the metal fire rings.
The gasket bores measure 88.5mm across all of them.
The Victor Reinz has an extra bead of black sealant around the smaller oil-way holes and also down the full length of the gasket near the edges.
All these measurements are from a brand new un-used gasket.
The Goetze I removed from my car measures between 0.068"-0.070" thick at various points on the crushed composite/fibrous parts.
The fire-rings were slightly thicker at 0.072" and were brass coloured rather than the silver coloured fire rings of the Victor Reinz.
The bores of the gasket were not round. They measured 88.3mm at smallest point and up to 88.8mm at the largest. This would suggest that the nominal figure of 88.5mm listed at various parts places is correct.
The Goetze did not have any black sealant bead anywhere like the Victor Reinz.
Various sources and much reading suggest the gasket crushes 0.008"/0.2mm and this does seem correct - I do not believe the fire rings or indeed the rest of the gasket crush much beyond 0.008".
This opens the question of whether the squish/quench distance is too large with current OE replacement head gaskets. 0.062" may shrink when piston expansion is taken into account and bring the distance back to an acceptable figure.
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WEASEL149
Joined: 19 Aug 2005
Posts: 595
Location: UK, Sheffield
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| Posted: Tue Mar 26, 2013 10:12 am Post subject: Re: Measured 931 head gasket thickness |
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| ideola wrote: | | All of that said, there are some really old posts here on the board that indicated compressed stock thickness for 931 gaskets is .046", which is 1.168mm. This is the figure I have been using in all of my calcs, but I'm not convinced that it is accurate either. |
I've recently done a few searches and I think I found the old posts regarding 931 head gasket thickness that you mention.
There's no definitive proof in the threads but it would seem that the original 931 gasket used to be thinner, until they decided to have one gasket for both NA and 931.
As far as quench/squish is concerned or 'mechanical octane' this could be bad news with some builds unless relevant action is taken regarding blue-printing/machining etc.
I've read plenty of heavyweight books on the subject and I believe it's in the Ricardo book (Sir Harry Ricardo) that he states there is a 'no man's land' between 0.050" and 0.180" regarding quench distance (at operating temp) that you need to stay out of.
This would make the 0.046" thickness you mentioned pretty much perfect all things being ideal, and definitely makes sense for the original 931 gasket and piston /head chamber design.
Interestingly Porsche managed to stay out of 'no man's land' with both 931 and NA (if the 0.046" figure is correct).
I would say the distance is pretty much critical if you use OEM style quench design and greater than 8.5:1 static CR.
I have read that steel rods can stretch up to 0.030" at sustained high revs so going lower than 0.040" cold clearance is probably risky 
The action of squish is to significantly reduce the octane requirement. Ideal squish clearance is defined by C. F. Taylor as a maximum of .005” per inch of bore diameter in operating conditions of speed and temperature. The tighter the squish clearance and the greater percentage of bore area dedicated to squish, the less octane is required for normal combustion at a given power output. In other words, as the action of squish goes up, the octane requirement goes down, all else equal, load, boost, RPM, temperature, etc.
Two of the factors of greatest influence in the effect of increasing resistance to detonation are turbulence and a more compact combustion space at TDC. Each of these act in different ways but bring about the same result, which is combustion of the same mass of working fluid (A/F) in less time, therefore fewer crank degrees. When the combustion is accomplished in fewer crank degrees less ignition advance is required.
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