924Board.org

samivaan · Joined: 24 Sep 2003 Posts: 35

Hi,
is there any differences between Head gasket heights? I found from here:
http://www.924.org/parts/parts.htm
that "Cylinder Head Gasket I measure about 1.5mm thick (new) "
so if somewhere is a little bit higher Gasket available, it might be easy way to decrease compression from euro version?

With this: http://www.fordmuscle.com/calculators/compression.shtml
I just simulated, that 0.5mm higher gasket, would be enough for me. It drops compression from 9.3 to 8.6

Anyone sure from that, what really is the size of piston volume in euro, and US version? Is compression decreased from euro to US version, with different pistons, or just with the highger gasket?

Lizard · Posted: Thu Mar 11, 2004 4:28 am Post subject:

why, the more compression the better, 9.3:1 is a good compression ratio
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3 928s,

samivaan · Joined: 24 Sep 2003 Posts: 35

I can't find enough high fuel octane from standard stations to use turbo or supercharger. So 8-8.5 would be fine for low boost. Higher gasket should be a fine way for low boost, if I can find higher gasket.

kidporsche · Posted: Fri Mar 12, 2004 10:20 am Post subject:

Someone had copper gaskets made back in the rennlist days I think, I can't remember for sure but I think it was possible to get thicker ones.
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Chris
1981 924 - RoW (Australian) spec

leadfoot · Posted: Fri Mar 12, 2004 11:25 am Post subject:

It all depends on what boost your running...
4-6 would be considered safe at 9.3 to 1 compression, even up to eight psi would be the limit...
you would have to have an intercooler though running at least 85% efficiency and make sure you have less than 1 psi drop in pressure across the entire system to see that boost not go to waste.
I bought corky bells' supercharged book, a must for anyone contemplating a supercharged system. There are many factors involved and the only one not able to be calculated is the effiency of heat dissapation and combustion efficiency. These can only be adjust for after installation and may need timing retardation with higher boost and temperatures.
The turbo head is a completely different chamber design and this may be a telling factor into running higher boost.
After speaking to member on this board and reading supercharged and the more i think about strapping on a supercharger, even though my engine would be considered better than average on a leakdown test, i am still leaning towards a freshening up the block, honing bores and using more suited pistons and possibly cylinder head. This would only benefit cooling of the block by having it tanked and it would mean more boost could be run reliably. Attention could the be placed on shot peening rods, balancing flywheel etc.... You would also have more time to enjoy your engine before needing another rebuild anyway... Leadfoot
Good luck with your plans...

Lizard · Posted: Fri Mar 12, 2004 12:12 pm Post subject:

on an NA 2.0 L w/ the combustion chamber at 9.3:1 cr I would not run more than 8 PSI but that would require a IC,
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3 928s,

samivaan · Joined: 24 Sep 2003 Posts: 35

I don't have a charger yet, but I've been planned to use Eaton M62 charger, which is "planned" to use 5-7psi. (at least Mercedes use it with 5-7psi, if I have a correct info)
Maximum fuel octane here, is 99 RON octane. In MON it's 87, and R+M/2 it's 93.
I've been found very different kind conversions for effective compression ratio. I know, that 99 RON, is enough for 10.5-11 compression ratio, but not more. Here is one chart, which says, that 9.3 compression ratio with 6psi boost, is same than compression ratio 13, which is way too much for 99 RON.

http://home.dmv.com/~carlb/sc/ecr-chart.html

But different page gives very different value
http://www.autocomponenti.com/boosted_tech/boosted_tech.htm

There 9.3 with 6psi, is same than 11 compression. So, it might work without decreasing compression ratio ?!

1982Porsche924 · Joined: 14 Nov 2002 Posts: 679 Location: Cupertino, CA

The problem with this thicker head gaskets is that if they get too big, the distance between the cam and the crank increases, and you face the problem of finding a new timing belt that will work.
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1979 Porsche 924 "S"
Engine: Euro pistons, Light Flywheel, Weber TB, Bursch Header, Cam Wheel
Suspension: Front coil-overs, adj sways, Solid bush. Thicker tors. bars, 5-bolt, BBS rims
Body: Fiberglass widebody kit, 931 Nose

CBass · Joined: 03 Nov 2002 Posts: 2807 Location: Vancouver, Canada

If you plan on rebuilding, using lower compression pistons from a North American 924 might be a good idea, the NA compression ratio was 8.5:1 here.

You could probably convince a board member over here to directly trade you the 8.5:1 pistons for your 9.3:1 pistons, and have them pay for the shipping.
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'81 931 in various states of assembly

Lizard · Posted: Tue Mar 16, 2004 10:46 am Post subject:

good thought Cbass,

I would be up for that, I have a spare set of 8.5:1 pistons I would be willing to trade you, I can pull them and measure them for yah too
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3 928s,

leadfoot · Posted: Thu Mar 18, 2004 8:04 am Post subject:

ok i'm going to take a shot in the dark here so anyone can feel free to correct me if i'm wrong....
i would suggest that static compression is not such an issue depending on what the intake charge temperature has increased to.
Can anyone make comment upon the ron ratings... i understand that the higher the rating, the greater the ability to resist detonation, but what are the temperature figures for the various fuels??
The resource i have says that detonation occurs around 1075 degrees... although ron is not specified.

CBass · Joined: 03 Nov 2002 Posts: 2807 Location: Vancouver, Canada

Detonation dependent on many variables, and combustion chamber design plays a large part in it. The 924 head has a no real combustion chamber, making for a flat cap on the end of the cylinder, with most of the volume being inside the piston dish. By going to the larger dish of the 8.5:1 piston, you should be able to considerably raise the detonation threshold.

Also, the engine is running using CIS, which is not the best method of fuel delivery for a forced induction engine. The higher you can get the detonation threshold, the better.

Simons supercharged 924 is a whole different kettle of fish. He runs EFI, a 931 head, and custom pistons, IIRC. Definately a better way of going, but also much more pricey.

Lower the compression, get a 931 ignition distributor, and the only thing you should have to worry about is detonation. Many custom turbo conversions use a "rising rate" fuel pressure regulator. As the boost level increases, the fuel pressure increases. This would tend you make you run a little rich at certain spots, but with the steady climbing boost of a supercharger, it wouldn't be all that bad, I would imagine.
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'81 931 in various states of assembly

leadfoot · Posted: Wed Mar 24, 2004 8:30 am Post subject:

Is the vacuum advance profile different on the 931 ignition distributor??
Is there a retard on the dizzy as well, did this vary with year or are they all designed the same.
Leadfoot

Lizard · Posted: Wed Mar 24, 2004 8:31 am Post subject:

the 931 used a different ICM,
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3 928s,

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS