924Board.org

[Rick MacLaren]

Is it worth getting these 8.5:1 compression pistons and putting them in an old '80 931?

Thanks.

[turbo93one]

Yes, I´ve got 8.5 pistons in my car and it gets more low end power as well as more hp. Just be careful with the boost as this setup will blow your head gasket and pistons if you have too much boost with stock 1980 ignition and no intercooler. I ran .9 bar and blew 2 sets of pistons but now with the stock .7 bar it runs with no problems.

Erik
1980 931

[Rick MacLaren]

Shoot! So I MUST get the intercooler then if I'm gonna run 10 PSI boost.

[Zuffen]

Turbo93one

when you say blew the pistons what do you mean?

I bet you were pinging and blew holes in them.

Or did you lean out to much a burn them up?

Real curious.



_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

[turbo93one]

The pistons broke between rings and the headgasket blew as well. The first time I got water in the combustion chamber when the car was going like hell, (probably the result from a blown headgasket) all of a sudden I just saw white smoke behind the car and then the car died completely. The second time wasn´t as dramatic but after breaking in the engine I started giving full gas and revving it, I clearly heard detonation so I smoothed off but it was too late..... The car ran for another month but kept getting harder and harder to start and of course got excessive blow by and I could see the charismatic bubbles in the coolant tank. I tried changing the head gasket but it didn´t help since the pistons had again broken between rings...

[Nick Neves]

test

[Nick Neves]

Hi guys, I haven't been on the board in a while. I have the head off my 81' 931 right now because it was really hard to start when cold. A compression test at opperating temperature showed ~150psi accross the board, but finally I checked it cold and one cylinder was down(125psi). The expansion tank was also, well, expanding as I cranked the engine for long periods trying to start it. I figured blown head gasket. How can I tell if the pistons are broken between the rings without taking them out of the block? There is absolutely no scoring on the walls of the cylinders, but something metal came through the engine at one point and got smacked up against the head in one cylinder badly pitting the combustion surface. I figure I will have to replace the head-- the little pits would probably cause serious hot spots/detonation. If anyone has one available, please let me know. Also, are the 8.5 comp pistons you are talking about available? If so, where and for how much? I assumed they were Carrera items and therefore very rare(=$$$). Any feed back, as always, is very appreciated.
nick<>

[turbo93one]

The 8.5 comp pistons are standard 931 pistons on the euro 81-82 (83) cars as well as the Carrera GT. I think I´ve seen them advertised from "Engine builders supply co" in Excellence. Of course they are readily available in Europe at your Mahle dealer.....

Erik, 1980 931. Had it for 10 years.

[Rick MacLaren]

Yes, I've ordered the 8.5:1 pistons.

$750 including rings, clips.

[John H]

If you're going to use 8.5 pistons try to get the KS piston's as per the GT. These are forged rather than cast and tend to be a bit stronger - i.e can take a bit over over boosting more than the cast units. I've had these in my GT an dits done 260,000 miles running boost up to 1.2 bar boost and have not melted or damaged them.

[Rick MacLaren]

Thanks, but, unfortunately, they only had the Mahle.



I've got to try NOT blowing these ones up!

[924RACR]

You could also see, once you get them, about having them hard anodized at a place locally that does anodizing and other metal surface treatments (like chrome, powder coating)...

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Rick MacLaren]

That sounds like a great idea. Is there an article out there that discusses this process?

[Peter]

I am not sure about powder coating piston tops, but take a look at: http://www.techlinecoatings.com/Engine.htm
They sell a product called "CBX POWERKOTE (Cermet Coating Thermal barrier)
Part # 3 oz. 2253
Designed for Performance engines running; Nitrous oxide, Supercharged or Turbocharged. For all other applications use CBC2
Creates a hard surface specifically designed for engines running High volumes of fuel and/or high compression ratios
Thermal barrier coating
Reduces part temperature. Keeps heat in combustion chamber longer through the power stroke
Increase torque and H.P. Increase combustion chamber efficiency
Reduce fuel consumption
Water based no hazardous fumes, non flammable
Requires baking. Cures at 350f for 1 hour.
Must be oven cured"
I am looking into this myself --about $24.00 and coats about 24 piston tops. Does not increase compression.
-Peter A. Holait

[dwak]

Peter,
That sounds interesting. Let us know for sure if your engine blows up or anything.
Now another point of issue that needs clarifying for my small 'log hewers' brain; if standard NA/8.5 compression ratio motors don't do well with a turbo, why are we boosting the 931 motor up to 8.5/1 compr. ratio?