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-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

I love it when a plan comes together-

21.5cc 931 head combustion chamber, confirmed Cbass' number
11.3cc area in headgasket, headgasket height = 1.9mm
34cc area in JE 8.5:1 pistons, these sit flush to the deck
32.08cc area in combustion chamber using VW pistons, assuming they ]
sit 5.4mm below the deck (and are 87mm diameter)
501cc displacement for 87mm pistons (per cylinder)

So it looks like we only need to take off 2cc out of the combustion chamber in the head to see 8.5:1 with the vw pistons. 6.5-7cc additional out of the head would give us 8:1. These numbers seem to fall pretty closely to what Racing was estimating he has using the vw pistons and some head work.

Next stop is to take my head in to get cleaned up and have about 6.5cc removed from the combustion chamber. There is plenty of material to pull out, so I don't foresee any trouble.

nick

Zuffen · Posted: Wed Jan 12, 2005 2:42 pm Post subject:

why lower the CR if it works out that it raises a few points?

8.7:1 isn't too far out or whatever it comes too

the higher the cr the better the drive in offboost moments
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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

Kenodog · Joined: 25 Jun 2003 Posts: 2669 Location: Vancouver,B.C.

I gotta echo what Zuffen says.Isn't 8.7/1 more desireable or are you worried about having timing problems or ? Also I gotta ask is the head gasket stock like what would come in a standard gasket set?
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1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
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-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

CR on a turbo is a big can of worms! Since I only pretend to be an engineer, I'm going to shoot low on compression and just bring the power back up with boost. I think there is more flexibility going this route than starting with high compression and using less boost. The drawback using lower compression, of course, will be less response off boost and a bit more lag. So I loose some bottom end, back gain it back (plus possibly more) in the top.

I think it's somewhat a personal opinion too. I can live with the drawbacks. I don't need tons of power in traffic and I don't mind waiting a second for the boost to kick in if I do. I like the "kick in your back" feel of sudden turbo power. I just think it's more fun regardless of whether it's faster or not

There are lots of fast cars out there. Heck, my roommate just bought a camry that uses premium fuel and puts out 200hp. I like the 931 because it has character.

On top of all this, my head is a mess from some copious detonation. It will probably need a couple cc's taken out just to smooth the combustion chambers back down anyway.

ps- The numbers above are for a stock head gasket. I both measured a used gasket (already crushed down) and solved for the gasket height and the numbers agreed amazingly well. So I think that mystery is finally solved.

nick

Paul · Posted: Thu Jan 13, 2005 11:48 am Post subject:

Here's the specs for the wife's Audi TT:

A Type K04 turbocharger forces air into the combustion chamber via two charge-air intercoolers. Compared to the less powerful 132-kW unit, a modified airflow path, together with a larger air cleaner, ensures that the air reaching the engine has a temperature only about 30¡C higher than ambient. This 80% charge-air intercooler efficiency keeps the engine supplied with an optimum flow of oxygen-rich air. Maximum boost pressure at the intake manifold can reach 200 kPa (29 lb/in2).

The higher-output 165-kW (225-hp) engine uses a high compression ratio (8.9:1) for a turbocharged unit. The 1.8-L engine was extensively modified to attain the higher output reliably. The pistons, connecting rods, and big-end bearings were uprated to accommodate the higher combustion pressures. The double-mass flywheel and clutch were modified to match the greater performance. The intake manifold, turbocharger, and exhaust manifold are new.

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

Anyone out there know what the bowl volume of a standard 924 piston is? Euro or US, doesn't matter. I'm looking for cheap production alternatives.
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1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

924RACR · Posted: Thu Jan 13, 2005 11:46 pm Post subject:

2 bar boost in a production engine??? I'm gonna take a huge leap here and expect that they're talking about absolute pressure, not gauge, meaning they're only running 1 bar boost on the gauge (=14.5psig). Can't believe that a production car would go out with more than 20psig, and even that would be a stretch.

Geeze, freaked me out for a second there!
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Webmeister
'79 924 #77 SCCA H Prod racecar
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Alex Roy · Posted: Fri Jan 14, 2005 5:02 am Post subject:

It read to me like that was the maximum boost of the intake, IE: un-wastegated

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

I think the EVO's run something like 8.8:1 and 19psi. This is with a carefully engineered combustion chamber. I'm just slapping parts together and hope I can reach 250hp reliably. Anything else is gravy

nick