AO

As a data point, I replaced my stock HG with the Porsche replacement HG. I believe OE is .25MM and the replacement is 0.40mm. I'm running about 9 to 9.5 psi.

ptuomov

Interesting. I seem to recall from the workshop manuals that the standard gasket is 1.1mm (0.0433 inches) thick and the OEM oversize gasket to be used once the head has been surfaced beyond the tolerance is 1.4mm (0.0511 inches). Perhaps your sizes were when compressed and wsm size when not?

AO

I might be off... I seem to recall a "25" on the OE and a "40" on the replacement. I thought it was mm.

ptuomov

Cool, thanks for the data point.

As far as the other surface parameters go, I don't quite understand the 928 specs. What's 928's RA spec when used with stock head gaskets?

The wsm says that the cylinder head "Max. roughness = 0.015 mm." If I am to interpret this literally, to me this means that the maximum absolute deviation from the plan (that is, deeps valley or tallest peak) can be at most 15 micrometers (591 microinches). How is this useful? I guess it is if there is a scratch and one possesses a precious electronic profilometer. But it doesn't tell me anything about how to finish the surface. For that I would need roughness average, like the RA 50 microinches that Cometic quoted you.

[EDIT: I checked the spec in a different language and it says maximum peak-to-valley = 0.015 mm. So the above is not exactly correct.]

Although Cometic may say RA 50, and it's their gasket so they probably know better than anyone, that sounds larger than what I expected. Ford 4.6L 32v engine is very similar to the 928 32v engine in terms of materials. That Ford engine uses MLS gasket from the factory and has a recommended finish of 8-15 RA (according to this article here: http://www.aa1car.com/library/ar996.htm).

So I am thinking I want to go pretty smooth. And eat noodles.

blown 87

Good luck finding a machine shop that can do that for you.

Mine are a little better than 50, and the machine shop I use was voted the number one machine shop (by other machine shops) a few years ago, and they had to buy new stones to do mine and they look like mirrors.

hacker-pschorr

I think it's a waste of time to stop at 10psi

Blow-by is exaggerated by detonation, not boost. Same goes for the head gasket.

Too many people take knock counts and detonation far too lightly.
It makes my shiver when I see post after post talking about air / fuel wth no mention of ignition timing.

I'll stop now before this post takes up a few pages.

blown 87

Better to be safe than have a bunch of pistons that wont even make ashtrays.

ptuomov

To me, if there's a machine shop that you trust that did the surfacing to a better Ra than what the gasket manufacturers specs require, you're set. And you know what you're doing whereas I am a bit clueless noob, so you'd probably be set even if I said you aren't! ;-)

What I am posting below is just for chits and giggles. Also, the below is a useful reference in case a machine shop says they can do certain level finish, one can ask them what machine they have. Claims may be scaled back a bit in some cases after that! ;-)

http://www.enginebuildermag.com/Arti...surfacing.aspx

Quotes:

...The Sunnen/DCM HB-3820 is CNC controlled with a precision ball screws on both the X- and Z-axis, with accuracy to within .0001" (0.002 mm) on the Z-axis. The HB-3820 has the same size capacity as the HB-3810 but has a programmable drive that can be used for multiple cuts and automatic cycling. This surfacer is popular with racers because of its highly accurate controls and ability to achieve finishes as low as 4 microinches Ra. The price starts at $39,000 and goes up to the mid $40,000 range, depending on options.

...Rottler Manufacturing offers two groups of surfacing machines: the SFA series and SFM series. The SFA machines come with electronic controls and digital readouts while the SFM series are manually operated machines. Both also come in two different sizes, one for surfacing passenger car and light truck engines, and a larger size for heavy-duty diesel work. Rottler's surfacers use electric ball screw feeds for greater precision, and the SFA series machines feature "closed loop" electronic gearbox controls that balance the spindle rotation speed and feed rate to maintain a consistent Ra finish (as low as 2 to 3 Ra is possible). The Rottler SF7M surfacer with manual controls, starts at $17,900 and typically sells for under $20,000 with fixturing. The top-of-the-line SF8A with automated controls sells in the mid to high $30,000 range.

blown 87

I do trust them after almost 20 years, if there is a issue, or thinks there may be he does it him self.

Case in point, on my 928 there was a very small issue and he drove the 20 miles to my shop just to look, picked the parts in question up, fixed them and brought them back the same day.

As far as being set, far from it, my guess is I am almost one 928 engine build in front of you.

Although i have built a few others.



Do not misunderstand that it is more the guy holding the tool, than it is the tool he is holding.
Just saying...................


It also helps to look at the gauge when you pick them up, although Scott's word was good enough for me.


I have to say that is a good advertisement.
Not much of a informative article, to me anyway.

I still want one of those machines though.

Greg

hacker-pschorr

True, but it's a common misconception that low boost = safe.
You can blow a motor apart at any amount of boost (or even no boost) with a bad tune.

blown 87

You been talking to my buddies???????????????????

ptuomov

How is the Cometic MLS holding up?

blown 87

No issues so far.