V2Rocket

Porsche themselves said the ceramic 951 head liners were purely for emissions reasons, to help light off the catalytic converter faster despite the low CR and the looong exhaust path.

Guys have been using NA heads without ceramic or other coatings for big HP turbos for a long time...John Milledge was doing it at least back into the 1990s.

Smudo

Great info. Didn't know that. I thought that it as to keep the head cooler.

Smudo

Small update on my cylinder head. It was in the machine shop to make the head’s surface straight for the install of Cometic head gasket. I was informed that there is no need for any shaving of the head, thus they just cleaned the remains of the OEM HG, which is good news, as the head was shaved/grinded already two times (last time 3 years ago when I did engine rebuild). I measured the cylinder head + piston dish volume and it came out slightly below 79 cm3 (assuming the dish volume of my pistons is 25cm3 (there are also 20cm3 versions), my head volume is slightly below 54 cm3). Standard head volume is around 56 cm3, so each shaving reduced the head volume by around 1 cm3. This means that if I use 0.04 inch HG, the CR will be around 8.55 : 1. Thus, probably will reuse the head and will need to take in consideration the increased CR when retuning it.

V2Rocket

the stock HG is 0.039" and goes on easier than the cometic 0.040.

a lot of sanding/smoothing work on your block in the future for that 0.001".

Smudo

There is a lot of contradicting info about OEM standard HG thickness. The one that came out of my car measured at 1.25mm (0.049'') and in a compressed state it supposedly is around 1.1mm (0.043"). I am putting the Cometic HG due to having APR head bolts. Based on some theories due to reason that they do not stretch as much as OEM ones, when the block and the head expand due to the heat, it compresses the OEM HG more (APR bolts do not stretch as much as OEM) and due to such heat cycles at one moment OEM HG fails - in my case after 3 years and 7000 miles. Almost all cars (many of them are diesels) for the last decade or so now come with metal HGs as for those diesel engines now OEM boost is around 2 bars! Thus, there is a lot of experience in installing metal HGs and preparing the block should not be big issue.

V2Rocket

Let me be clear I have nothing against metal HG's, I just put the OEM MLS onto my Outback in February (they are something like 1.7mm thick!).
My general preference is for the older fiber/composite type gasket like the 944 uses, because it's more forgiving in installation...

With a MLS preparation is everything, both the block and head surface need to be super flat and near-surgically clean.
Then there's the "RA factor". RA, roughness average sounds complicated but it's just a measurement of how bumpy the surface is...it is translatable into "grit numbers" like sandpaper.
So you have two options for prep...
- get a giant sanding block and progressive grades of paper and go to town, carefully ensuring not to take off too much material in any one spot, or do like the Subaru guys do and get a sheet of plate glass (basically perfectly flat) with sandpaper glued to it and slide the block/head around on that.
- find a machine shop that actually knows what "RA" even means, and also has the tooling capable of cutting that smooth. Here in go-fast heaven (SoCal) all the machine shops in a 15 mile radius were like deer in headlights when asked about RA. Maybe your area or luck will be better for that.

With a fiber/composite gasket all you need for prep is a scraper, rags, brake cleaner, and maybe green pots/pans scotch brite. Scrape down the big stuff, scotchbrite the small stuff to clean aluminum, spray with brake cleaner and wipe down with rags...something you could do on the side of the road if you had to.

Luckily, my machinist of choice was recommended by a local Subaru shop for head work so I gave him a shot. The Subaru gaskets have features built in (type of coating) to be more forgiving than a Cometic, also, so my block was super-scrubbed with scotch brite to be near perfectly clean aluminum and checked for flatness.

Smudo

Thanks, I will take that in to consideration. I will post in couple of weeks whether the installation was successful and what was done!P.S. Peep today confirmed that 8.55 CR should be totally fine. He said that on his old 924 race car he left the standard 9.3:1 CR and run it with 1 bar. Of corse, with higher CR less ignition timing is required (especially on track and hotter days).