924Board.org

[hitbyastick]

I rebuild the engine with mahle flattops as my original pistons had broken ringlands. I had the block bored to 87mm and the connecting rod bushings put in to run the 22mm wrist pin. I didn't really do any other major changes to anything. After doing the calculations online I came up with a roughly 8.3:1 compression ratio. After actually putting the engine back in the car I did a compression test after having some issues with pinging. The pressure was somewhere around 185 which is way too high.
Can I space the head? Any other thoughts on how to solve this blowing up problem? I read something about rotating the pistons 180 degrees which alleviates the pressure problem but shortens the life of the engine significantly.


If not there might be a lot of 924T parts for sale soon.
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1982 Porsche 924 Turbo

[fiat22turbo]

You need to do one or more of the following: Run higher octane fuel, add an intercooler, inject alcohol, inject water, reduce the boost level, richen the fuel mixture or back off the timing.

Moving to a more programmable fuel and ignition system could help alleviate the problems, once you get it tuned properly.

Either way, a wide band oxygen sensor can help you fine tune your setup.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[ideola]

In addition to Stefan's list, you can run a thicker MLS gasket to reduce CR.
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erstwhile owner of just about every 924 variant ever made

[gegge]

The static compression is fine. Are the piston rings sealing too good? I think that will clear after 1000 miles.

The arrow does only point the direction of the wrist pin offset, and the offset on the original pistons are on the major thrust side/exhaust side. They should point backwards, NOT to the front. That could be the reason for the pinging.

I would check the cam timing one more time. Sounds like the intake is closing way too early and raising the dynamic compression. Use this calculator: http://www.rbracing-rsr.com/comprAdvHD.htm

Have you done something with the turbo or exhaust? More backpressure will raise the dynamic compression.
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[hitbyastick]

I'm running megajolt ignition which is like megasquirt, except it only does the ignition part. Even running the timing to near 0 advance under load, it still pings. The fuel system was tested and works as it should. I tried a colder plug and a number of other things. The rods are knocking now, so I gotta replace the bearings before I go any further.

So I need to make the arrow on the pistons point right at the flywheel? I'll try that. That seems like a trivial thing as long as it'll be reliable afterward.

The cometic MLS headgasket looks awesome too!


Christoph
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1982 Porsche 924 Turbo

[gegge]

This is bothering me since I am running Mahles as well in my '81 turbo. Well - it is not actually running yet...

I just can´t understand why the numbers are so high. 175 psi should be impossible with low 8.3:1 compression. The bore, stroke and rods are fixed. Then there must be something wrong with the varibles like static compression or the opening of the valves. Have you adjusted the valves? Large play would increase the pressure to some extent.

Standard head gasket? Standard 931 head within the limit? Some 5-6mm between block deck and piston at TDC?

Sounds like you got close 9.5:1 or the cam is 40° off
The gauge is reading correct?
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[hitbyastick]

Gegge,
trust me I was really suprised. Here's the measurements:

Bore 87
Stroke 84.4
Head Gasket Bore 87.8
Deck clearance 5.4
Head Gasket thickness 1.7
Head Chamber Volume 22.5ccs

I checked the valves, but I hadn't really adjusted them. Cam timing seemed alright, as the marks always lined up. The car actually ran well until any sort of boost above 2psi then the pinging occured. I'm running megajolt so I pulled a lot of timing advance out of it, still pinging. Compression pressure showed near 180 on the gauge (well two gauges actually). Dropped the spark plugs to a colder range, still pinging. Adjusted fuel mixture to the nth degree, still pinging. I will be perfectly honest however. I opted for the cheap-o AAP pistons AND I installed them with the arrow pointing towards the front of the car, which I have now learned is all wrong. I'm getting Mahles on monday and hope to have the engine back in the car sometime around wednesday. The pinging I heard may have been the piston rocking around in the bore, as they are pretty badly scored on the piston skirt already.

I will let you know what I find.

Re-measured and calculated now I come up with 8.7:1 Still a bit hot I think?
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1982 Porsche 924 Turbo

[TurboJak]

Did the same job on my 931.
Renovated the engine and used Mahle flattops, though I also removed 1mm of material from top of the flattops.
I used standard gasket and the car ran fine for 2 weeks until the head gasket blew between nr 2 and 3... Didn't here any pinging or other warnings, car just suddenly lost power
Recalculated CR to around 8.9:1 (my 931 head is missing alot of material).

I removed top with engine in car and installed a 5-layer Cometic gasket giving a CR of around 8.5:1. Engine is running fine now for some month, though of cause lost some power with a lower CR... But its a PITA to change head gasket when you just installed the newly renovated engine 2 weeks back
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'76 924 + '82 924T engine + GT look

[gegge]

But i stilll find the 175psi figure higher that expected for a 8,7:1 ratio. Please note that if the deck or head has been milled the inlet of the cam will close earlier than stock and increase dynamic compression if the cam gear is fixed and not adjustable - not much but still.

TurboJak: Could be an old/weak headgasket? Did you retorque the headbolts according to the manual? Wonder if the flattop design is a lot worse than dished..? There are several turbocharged N/A running high 9,3:1 without any problems and they even got valve pockets in the slightly dished pistons and a flat roof in the combustion chamber.

I am using a hotter cam (268°) and shorter rods (142,5mm) together with a 5-layer MLS in order to keep the static comp down to approx 8:1 and dynamic even lower. And yes, my head has been skimmed a couple of times as well.
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[Mike9311]

I second the "check the cam timing" suggestion

How are the rings located on the Mahle piston? I would think it isn't much difference but most people ignore the cavity around the piston to the top ring that is created once in the bore. Is the Mahle ring higher which would be one thing that would add to compression?

I also think it was mentioned that the Mahle's have thicker rings which should improve sealing and keep effective compression at higher levels?

I am not familiar with these pistons so I don't know how they compare to stock.
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

[hitbyastick]

Well I put most of the engine back together. Flattops and all. Then I simply hooked up the starter and sort of bench tested the compression. Even with the rings not entirely seated yet, I already got a reading of 168-ish psi, and that's on two different gauges.

I think what you're saying is a good point, as to the location of the piston rings. I have another set to compare them to, so maybe I'll do that tomorrow. I guess decking the pistons is a good way to lose compression but I didn't want to take any strength out of the crown.

I think a cometic MLS gasket is a better approach, but to each their own. I know a lot of VW guys stack two headgaskets on the ABA 4 cylinder motors with surprisingly good results.

I also measured my head height and it looks like it has been decked, but very little.
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1982 Porsche 924 Turbo

[Rasta Monsta]

Mine's been decked a crapload, reading this is making me ill.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[fiat22turbo]

Uh yeah, but you've got an MLS gasket, an intercooler and custom pistons. Plus you haven't been hit by a stick recently....

With the UTCIS and eventually the EDIS ignition, I think you'll be fine.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Rasta Monsta]

Maybe you should bring that confidence over to the firing party this weekend.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[gegge]

Hmm. 168psi is a whole lot better than 185psi - but still too high

What did you do to lower the pressure? Was it only the 180° piston fix?

De-shroudding the valves in the head, thicker gasket, hotter cam, adjustable cam gear are other alternatives in order to the the compression down.
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs