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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Fri May 15, 2020 6:59 pm 
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Joined: Wed Dec 24, 2014 1:14 pm
Posts: 25
Model: 924 Carrera GT
Got the block back today and machinist says they are regularly doing 50Ra microinches these days as that's what more modern engines use - I guess ours are 40 years old! The old head gaskets could go up to 100 Ra but they probably had asbestos in them...

Anyway, the deck finish looks great and a smoother than the original OE. Without a fancy profilometer it's difficult to say what the finish is but you get no noticeable catching on "the old finger nail dragged across the surface test" of my youth!

As further info for this thread on pistons, Michael Wittke of MM wrote to me re the JE pistons they suppled:-

" the race pistons are rather designed for higher boost values. Fuel efficiency is not the main target, and that is why we do not design a squish area. Sharp edges like squish areas and “hard-to-reach” areas for the flame front, lead to an earlier effect of knocking.
The material is 2618, but you do not have to take care of the clearance or higher expansion coefficient. The bore diameter must always be the “full” value, so for 87 mm you need to bore 87,000 mm. The clearance is always regarded in the diameter of the piston.
Yes, the gaskets are from Cometic and 50 Ra is the limit, but this is a value all machine shops can do anyway as a normal job. The number 4.01301A1 is correct for the 1,0 mm gasket."

Hope that helps Rawiz.


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Fri May 15, 2020 7:10 pm 
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Joined: Sat Jan 30, 2010 11:13 pm
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Location: Somerset
Model: TheOnly Way Is Turbo
Good info Colin. Thanks

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Steve - Joined: Sat Jan 30, 2010 10:13 pm......think I have lost far too much time in this forum.........
1980 924 Turbo Series 1 - Alpine White & Guards Red
Omega forged pistons, lightened and balanced bottom end, ARP studs + Cometic MLS HG, Piper fast road cam, K27/26 3257 hybrid turbo, 951 ducted FMIC, Custom intake manifold, Mittelmotor dizzy & cam pulley, Adj boost - dyno’d at 239.5 rwhp @ 1.05bar, Zeitronix ZT2 & Phormula KS4 knock sensor, 944 turbo all alloy radiator, Hayward & Scott custom tapered race exhaust, GAZ Gold coilovers, Fuch'ed up, Quaife LSD
2017 VW Passat Variant GTE Advance
2015 Nissan Leaf Tekna


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Thu May 21, 2020 4:14 pm 
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Joined: Sun Jan 21, 2018 11:22 pm
Posts: 203
Model: 924 turbo S1
Now also seeing if MAR Motorsport can get Mahle 924 turbo pistons, forged or otherwise.

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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Thu May 21, 2020 4:32 pm 
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Rawiz wrote:
Now also seeing if MAR Motorsport can get Mahle 924 turbo pistons, forged or otherwise.


Mahle were cast for turbo and they are available as mentioned before from Porsche and I guess any Mahle reseller

Forged were done by Kobelschmitt for CGT and they are NLA.

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Steve - Joined: Sat Jan 30, 2010 10:13 pm......think I have lost far too much time in this forum.........
1980 924 Turbo Series 1 - Alpine White & Guards Red
Omega forged pistons, lightened and balanced bottom end, ARP studs + Cometic MLS HG, Piper fast road cam, K27/26 3257 hybrid turbo, 951 ducted FMIC, Custom intake manifold, Mittelmotor dizzy & cam pulley, Adj boost - dyno’d at 239.5 rwhp @ 1.05bar, Zeitronix ZT2 & Phormula KS4 knock sensor, 944 turbo all alloy radiator, Hayward & Scott custom tapered race exhaust, GAZ Gold coilovers, Fuch'ed up, Quaife LSD
2017 VW Passat Variant GTE Advance
2015 Nissan Leaf Tekna


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Thu May 21, 2020 5:03 pm 
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Joined: Sun Jan 21, 2018 11:22 pm
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Model: 924 turbo S1
Just seeing what they’ve got and what they can do. They ‘do’ forged pistons but indeed maybe not for 924 turbo.

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Porsche 924 Turbo S1 1980 two tone green with tartan interior (project).
Lotus Europa S2 in Bahama yellow (running).


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Fri May 22, 2020 10:45 pm 
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Joined: Sun Dec 23, 2007 2:24 pm
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Model: 1983 n/a
Chap(German) has just popped up on FB looking for a piston.
Have directed him our way

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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Sat May 23, 2020 12:32 pm 
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Model: 924 turbo S1
Presently I’ll have another go with JE and Omega (after I see what Mahle and possibly Porsche themselves have got). Garage has started working on my rusty sills now too, I get to see ‘the horror’ on a lift this week (they took off the covers and steam cleaned).

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Porsche 924 Turbo S1 1980 two tone green with tartan interior (project).
Lotus Europa S2 in Bahama yellow (running).


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Sun May 24, 2020 2:05 pm 
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Model: 924 turbo S1
There *might* be some JE Piston forged stock around with these specs (checking), would they work in an S1 turbo?

924 GTS 87.75mm bore, 1.774” C/D, dish dome -35.3mm.

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Porsche 924 Turbo S1 1980 two tone green with tartan interior (project).
Lotus Europa S2 in Bahama yellow (running).


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Sun May 24, 2020 4:09 pm 
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Anything will work if you want to make it happen.

Is that the spec of the piston or are you quoting GTS spec? Assume you mean -35cc and not 35mm? An stock S2 piston has 35cc bowl and assumimg the top of piston is almost flush to deck of the block it will be circa 8.5:1CR. Minus what’s lost in head skims. GTS would need a larger bowl circa 40cc unless the piston sits lower in the bore. S1 piston has 45cc bowl.

Ultimately you need to cc your head. You need to know how the piston sits in the bore. You need to know what head gasket you are using. After all that does the CR work for your build and expectations.

Ignore S1 v S2 specs exclusively. You can blend the best aspects of both to work for your car. Low compression ratio allows bigger boost. High compression allows better off boost throttle response. As I said before. 8.0 to 8.2:1CR could be the best compromise. Iirc CGT and S2 was 8.5, GTS was 8.2, GTR was 8.0 and S1 was 7.5. Small variations in build spec will change final CR

Sorry for the long answer

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Steve - Joined: Sat Jan 30, 2010 10:13 pm......think I have lost far too much time in this forum.........
1980 924 Turbo Series 1 - Alpine White & Guards Red
Omega forged pistons, lightened and balanced bottom end, ARP studs + Cometic MLS HG, Piper fast road cam, K27/26 3257 hybrid turbo, 951 ducted FMIC, Custom intake manifold, Mittelmotor dizzy & cam pulley, Adj boost - dyno’d at 239.5 rwhp @ 1.05bar, Zeitronix ZT2 & Phormula KS4 knock sensor, 944 turbo all alloy radiator, Hayward & Scott custom tapered race exhaust, GAZ Gold coilovers, Fuch'ed up, Quaife LSD
2017 VW Passat Variant GTE Advance
2015 Nissan Leaf Tekna


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Sun May 24, 2020 7:27 pm 
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Joined: Sun Jan 21, 2018 11:22 pm
Posts: 203
Model: 924 turbo S1
That’s apparently the piston specs (except you’re of course right, it’s dome cc). I’m just wanting to have a good quality drive, with reliability and reasonable performance. Not planning to go racing or anything.

P.s. the spec does mention they are for a 924 GTS.

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Porsche 924 Turbo S1 1980 two tone green with tartan interior (project).
Lotus Europa S2 in Bahama yellow (running).


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