924Board.org

[Rick MacLaren]

Peter: Excellent. So the harder surface means that the heat remains in the chamber instead of the piston.

Dwak: I'm not sure exactly. There might be two questions there. I can speak to only one of them.

Why up the compression? I want to up the compression to raise my performance at the low end of the RPM curve. I also want to reduce the time required to get up to the threshold speed (2500 RPM) where the turbo kicks in. I don't know if that answers it or not, but they're my reasons. Finally, and probably most importantly, I'm acting like a chimp: Monkey see monkey do. I read a neat article from the mid 80's (I've gathered a collection of anything dealing with this car since high school) on the GT and it said basically three factors distinguished it from the normal run of the mill turbo: High compression pistons, intercooler and digital electronic ignition. So I'm just copying that strategy.

Why does a turbo fail when installed on the NA motor? I'm not sure. It might be that the turbo doesn't work with the NA motors given the shape of the NA pistons and the resultant shape of the combustion chamber. Or maybe they're running too much boost. I'd imagine given a 9:1 compression ratio you wouldn't need much boost to blow it up.

_________________
1980 Porsche 931 Bitched.
1979 Porsche 924 Tweaked.

[ This Message was edited by: Rick MacLaren on 2002-01-15 02:44 ]

[924RACR]

Yeah, I'd suspect the shape of the combustion chamber as the source of the detonation. You really need a hemi head to make power, and that's more or less the kind of thing the 931 has.

I'd be a little suspicious of a coating you put on yourself, because the longevity/durability may be highly dependent on surface prep. However, the idea is fine, I'd just feel more comfortable having a shop prep and coat them instead, under controlled conditions. Keep in mind, anodizing is just another type of coating.

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

[Rick MacLaren]

Can this coating come off under stress? 'Cause I intend to subject them to considerable stress.

[Peter]

I became interested in thermal coatings for piston tops after reading various articles or on engine rebuilding forums on the net --whether its Porsche, VW or Chevy engines, the principals are the same. Various coatings for engine parts have been accepted by both race engine builders and general production engine builders (Ford, BMW, etc).
If you do a search for "thermal coatings" the company techline coatings comes up a lot. There are numerous shops that boast using their products --so I went to their site and some of their products are for a DIYer. They list shops that will do it for you but they also list simple instructs for a DIYer. Now as to applying it incorrectly, that is always a problem with anything we do, sometimes you just have to ask a lot of questions and just give it a try. BTW, places charges about $20 per piston top for thermal barrier coating.
As for to flaking or coming off during stress, I assume that if incorrectly done, it could come off. I am awaiting the return of my pistons and other engine parts from the machine shop that is balancing my engine. After I get them back I will email techline with some questions. I will ask them for step-step instructions on (1) preparing the piston top for the coating, (2) applying the coating, (3) heating/cooling the piston, and (4) if improperly done, what would be the results (flaking, etc) and the possible damage if any to the engine. If you have any particular questions, post them here and I will send them 1 email with all are inquires.
After I coat my pistons I will inform the Board. Of course if my engine blows-up, I will be too embarrassed to write, but since Vince Ponzo leaves near me, I am sure he will be laughing out load on the board.
-Peter A. Holiat

[Rick MacLaren]

Ok thanks. Now assuming only 12 PSI boost, is the thermal coating 'needed' or just 'nice to have'? Or is PSI even relevant?

[Peter]

Hey Rick,
My own opinion is to have the pistons coated because I believe it works and it is worth the money for any engine. Read this article for some more info: http://www.automotiverebuilder.com/ar/ar29836.htm
You can also go to their home page http://www.automotiverebuilder.com and search through thier archives.
-Peter A. Holiat

[dwak]

Peter,
Wouldn't you have to rebalance your pistons after coating them?
dwak

[Zuffen]

technically yes but it won't make a difference...

On turbo if you did the turbine in ceramic and compressor in teflon you would need to balance them due to the rotation tolerances, very critical on something spining that fast.

generally these coating guys know how to get an even coat and a mil or two won't throw the weight off.

we did the piston tops and the skirts, pins, crank and rods in low friction and the exhaust chamber and header in ceramic. extrude honed the intake and mathced the intake ports and o-ringed the head on the 951's.

with a chip and a S turbo and these mods 500 HP isn't hard to reach.

I think my buddy just gteched his engine on his 951S at 300 HP with just a chip change

_________________
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

[924RACR]

Yeah, but G-Tech's are junk, seriously over-estimate the power output. Well known fact. Our measurement guys have checked them here at work (Bosch), and they're not good enough. We use acceleration sensors that cost thousands, piped into measurement systems that cost tens of thousands, in each and every test car.

Now, a GPS-based system can be fairly accurate (though still not as good as some)...

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

[Rick MacLaren]

Vaughan, how much overestimation from the GTek? I'm not interested in getting one, as Police radar is, for me, the gauge that most commonly confronts me in daily life. But I'm interested for a friend of mine who is considering getting a GTek.

Re: GPS

On the GPS measuring the speed of an automobile, you're sampling through an extremely small window of time. About 12 to 14 seconds on a 1/4 mile stretch. Yet GPS accuracy increases with sampling time. If there's an error of, say, 1 metre for every 100 metres, we're still out 1%...do the GPS account for this error?

I guess where I'm going is this: Isn't there always a "probability of error" in any measurement? I know that regarding anything measured, measurement error is a simple fact. I suppose it boils down to 'how much error' is acceptable. Is a fraction of a meter/centimeter accuracy possible with inexpensive consumer GPS receivers?
_______________
1980 Porsche 931 Bitched.
1979 Porsche 924 Tweaked.


[ This Message was edited by: Rick MacLaren on 2002-01-17 02:16 ]