924Board.org

TroyDest · Guest

Apple' is right; you can't really tell much about static compression ratio from a compression test. My '82 931 with 8.0 compression ratio has 150 psi compression. With that in mind, the 9.3 ratio should have higher than 156 psi if you could actually tell anything with just a compression test.

Jon, do you have a turbo gasket yet? How thick is it? Who makes it? What's the part number?

I found out that my local Kragen auto parts can get an OE Victor Reinz turbo head gasket for $50. Since nobody seems to have a caliper or know how thick the 924 vs. 931 gasket is, I guess I'm just gonna buy both so I can measure them myself and let everybody know once and for all.

AppleBit · Joined: 16 Nov 2002 Posts: 1516 Location: Minneapolis, MN

I have one on the way... should be here this weekend... I'll measure um both and let everyone know the difference...

I bought it from Auto Atlanta

1-800-P924-944

_________________
Classic British Sports Car Restoration v6 + v8 Engine Conversion Swaps

Porsche 924 Wide Body LS1 Corvette 500 Horsepower Engine

-nick · Guest

you're absolutely right about measuring compression. i thought i had some old numbers still laying around from when i measured it once before. no luck in finding them though.

i'm curious to find out how this goes. i changed out headers and the bigger valve head at the same time so i can't make any seat of the pants comparisons.

regards,
-nick

Cbass · Guest

So what would readings of 175 and 180 be?

AppleBit · Joined: 16 Nov 2002 Posts: 1516 Location: Minneapolis, MN

It doesn't mean anything. If it is consistant across all 4 cyl that is good.

The pressure you read is from a cyl leak down test. When you have a stock cam in a car you might get a reading of 150 psi, but if you put a wild cam in your car, you might only get 100 psi... The reason: The cam is open for a longer time and allows air to escape at lower rpms....

So, if you have 175 psi, I would hope you have at least 11.0:1 pistons and a cam that matches, otherwise your engine is not breathing as well as it could for more upper end HP....

Cbass · Guest

It does have 11:1 pistons.

Needs a good cam though. I'm looking at prospective cams right now, I just want a well tuned streetable motor. Not more than 200hp, with a smooth idle.

AppleBit · Joined: 16 Nov 2002 Posts: 1516 Location: Minneapolis, MN

CBass, dyno your car when you have it ready... I'd love to see the dyno

I am more than likely somewhere around 200 hp...

it's nice having that much

- Jon

And yes, I will be dyno'ing my car this spring....

Richard · Guest

If someone could mike a used head gasket from each we could do a quick calculation to see if it is worth it. It looks like you would need to lose 3.97 cc of clearance volume to raise the CR from 9.0:1 to 9.5:1.
With a bore of 8.65cm, it would take .017cm or .0067" to get rid of one cc. That would be a .027" thinner head gasket to raise the CR to 9.5:1. Another way to look at it is .1 CR ratio change is about .0054". So if the head gasket is around 5 thou thinner, you get 1/10th point. Depending on the head gasket material, it may be difficult to get an accurate reading. Applebit is correct. Compression readings are really "effective compression ratios" and have everything to do with the cam and not the actual CR. It is a good tool to compare the change in an individual engine before and after, but not so good to extrapolate to some one else's.

TroyDest · Guest

I just received a 931 (supposedly) gasket from AutoAtlanta that cost $45. The thickness is .073" compared to my used 931 gasket of .066". After being compressed, they are probably the same thickness. The new gasket is a better quality gasket though. It is Goetze (gurts) 30-024335-10 and has the Porsche tag 931 104 337 06. My used gasket is Victor Reinz 43RNZ96 (stamped on gasket) and AFM10 1.75 8 1 (printed on gasket). All of the water passages are the same except the new gasket has slightly larger water holes. The new Goetze gasket has wider steel fire rings, wider section between #4 and the rear water passage where the old one started to blow out, and has a nice solid sealant bead around the entire perimeter top and bottom.
I still have not been able to find an '03 gasket which should be thinner. I don't think there any around any more. -Troy

Cbass · Guest

I'd like to dyno it, but I'd have to find a dyno first, and pay for the car too.

So the skinny is that adding a 931 head gasket will not noticeably raise the compression ratio.

AppleBit · Joined: 16 Nov 2002 Posts: 1516 Location: Minneapolis, MN

I measured the 924 and the 931 gasket last night.... if the 931 gasket was truly a 931 gasket then:

the 931 gasket is thinner, but it is only .03" thinner

TroyDest · Guest

Hey Jon, do you mean .003"? Was your gasket from AutoAtlanta a Goetze? -Troy

AppleBit · Joined: 16 Nov 2002 Posts: 1516 Location: Minneapolis, MN

AutoAtlanta

Richard · Guest

If my math is correct, and I welcome cross checks, .03" is enough to raise the compression a significant amount. This is assuming the actual crushed thickness stays in proportion. Any takers out there to double check? I was thinking of putting one on my N.A. also.

Richard · Guest

I forgot to mention it, but the relationship between clearance volume and compression is not linear. It takes less reduction in volume at higher initial compressions to net a higher ratio e.g. you would need a reduction of 20cc to get from 5.0:1 to 6.0:1, but only 5cc to get from 10:1 to 11:1. This is based on a 500 cc stroke. The Porsches are 497 cc/cylinder.