The Porsche 924 Owners Club

924 turbo series 1 forged pistons
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Author:  Colin [ Sun May 24, 2020 9:30 pm ]
Post subject:  Re: 924 turbo series 1 forged pistons

Just finished putting the block/crank/pistons together. I used the Mittelmotor 87mm JE pistons and, as I said before, had to deck the block to get to my target CR ( 8.2:1 ) because the MM pistons have a compression height 4mm lower than the Porsche/standard design. Took 1.15mm off the block and that leaves the pistons inset 2.85mm below to deck height:-

Colin MM piston height.jpeg
Colin MM piston height.jpeg [ 101.42 KiB | Viewed 1148 times ]

This gives 16.9cc in the unswept bore portion that is added to the piston bowl volume, head chamber volume and gasket thickness volume when calculating your CR. MM say it's 10cc, which implies the gasket compresses to 1.68mm - I've found 3 old head gaskets ( Goetze and Reinz ) from prior years and they measure about 1.75mm "used", so 10cc is about right. To re-iterate, the MM JE pistons have a 23.1cc bowl and my skimmed head is 19.7cc

Rawiz, are those 924GTS 87.75mm pistons the ones Cambridge Motorsport are advertising? I was a bit confused because the web page title says 924 GTS and a side note says 944 GTS. Either way, they look like wide bowl low CR 924 turbo pistons to me. They also look a lot heftier than these MM JE pistons I've got...maybe a better bet for every day driving use. If you want to measure your head combustion chamber volume, I can send you details on how to do it - it's a straight forward process.

I've found two wrinkles with these JE pistons. One is that the ring pack is generally "oversized" so you can file the ring gaps to exactly match your intended application and the specific value of each individual cylinder. Seems this is seen as a big plus in USA engine tuning circles but it left me having to file up to 0.1mm off most of the ring gaps. Having manually filed a lot of rings in my youth - it's a pain - I found Hutchinson Motorcycles import the standard US manual ring filer at a great price - bliss!

Colin Ring Gap file.jpeg
Colin Ring Gap file.jpeg [ 127.56 KiB | Viewed 1148 times ]

The other wrinkle is that these pistons use the JE wire C clips to retain the gudgeon pins, like we have on the standard pistons. Unfortunately, they are much stronger than the standard ones and fiends to put in! They're supposed to be better for vey high rpm engines but that's not really what we do with 924 Turbos.

Hope that all helps!

Author:  Rawiz [ Tue May 26, 2020 10:06 am ]
Post subject:  Re: 924 turbo series 1 forged pistons

Thanks Colin. Yes, Cambridge Motorsports. Trying to check what they are and how many they have. Also not easy to get hold of them on phone or email... Sounds like they could work then.

Author:  Rawiz [ Tue May 26, 2020 6:14 pm ]
Post subject:  Re: 924 turbo series 1 forged pistons

Got Cambridge motor sport on the phone. No stock but have had them made before for eg 924 GTS. Listed price apparently about right, <£1,000 for a set of 4, lead time about 4-5 weeks, from JE pistons.

So they could make the same ones they have done before. Or... Is there an exact full spec for ‘ideal’ 924 turbo S1 first over bore forged pistons somewhere...?


Author:  Rawiz [ Wed May 27, 2020 8:45 am ]
Post subject:  Re: 924 turbo series 1 forged pistons

It’s quite a form JE Pistons fill in for pistons:

A lot more specs than are listed on the 924 turbo repair manual page for pistons:

Presume people have spotted this french company claiming Mahle piston stock too? ... -mm-121040

Author:  Carrera RSR [ Wed May 27, 2020 8:59 am ]
Post subject:  Re: 924 turbo series 1 forged pistons

Thats on key reason why I didn't order JE...... despite making them for vendors and users they want the inside leg measurement of the engine builder and his mothers maiden name to get them made. You will need to know your onions to place an order or get your engine builder to fully measure your block and head to calculate the exact measurements. I guess JE do not want any come back that they made a manufacturing error.

As I was going through this process with JE and trying to find expertise in the measurements required the Omegas came available, so I jumped on those.

Author:  Colin [ Wed May 27, 2020 9:43 am ]
Post subject:  Re: 924 turbo series 1 forged pistons

JE Pistons have an order code laser etched on the underside of the Pin boss. The MM JE race pistons I've got are 1022542 ( it's incredibly faint so I missed it first look ). Maybe you could work through Cambridge using that as a guide and see if they can replicate them with JE?

Obviously, being race pistons, they've thrown the features at them - much lighter than standard, thin pins, ceramic crowns, coated skirts, 2618 forging - so they're maybe much more expensive than the apparently simpler design Cambridge show on their webpage. Also, they use slightly higher clearances than the more standard cast/forged pistons so start up piston slap noise and oil consumption is still an open question for me!

If you're happy with a "normal" spec ( you said you wanted reliable road driving ), the Cambridge web page JE pictures look like a standard piston of maybe 7.5:1 that could be used in an S1 turbo. Are Cambridge willing to release more detail on the design they've previously made which you could only minor modify to a smaller overbore like 87mm? The key issues are bowl volume, radii and depth; compression height ( Pin centreline to piston top face ); pin diameter; space between pin bosses; clearance between pin axis and piston undercrown ( con rod clearance ).

The Mahle's also make sense in that you don't intend to up the power much so cast pistons should do fine. I think my experience of using these ( at 8.5:1 CR version for S2's ) in a race GT at 0.9b shows you need forged pistons if you use the car hard! I bought some bits from Rose Passion during this rebuild with a good experience - they had bits on the shelf for immediate delivery while Porsche UK has been closed down. The web price is more than the MM JE pistons...

Which makes Steve's rationale for settling on Omega's very sensible - can you wait for a group buy? I would have gone with the Omega route except my time available for this rebuild is severely constrained - yet another grandchild on the way!!

Author:  Rawiz [ Thu May 28, 2020 11:11 am ]
Post subject:  Re: 924 turbo series 1 forged pistons

Omega don’t seem that keen, they pointed me to ordering via I rang them, they also mentioned the 20+ minimum order from Omega. It was mentioned that there was also a very good racing piston manufacturer in Europe (that would do just a set of 4, sounded like <€1500) but would again need the full spec or a piston to copy.

Author:  Carrera RSR [ Sat May 30, 2020 9:22 am ]
Post subject:  Re: 924 turbo series 1 forged pistons

Couple of sets of ARP rod bolts for sale. Imperative to replace in a turbo rebuild according to FSM.

Or buy stock bolts now available from Porsche for similar money but not made to the higher specs offered by ARP

Author:  Rawiz [ Thu Jun 11, 2020 9:56 am ]
Post subject:  Re: 924 turbo series 1 forged pistons

FYI I’ve sent a 924 turbo piston to Neil Bainbridge to get a quote for making improved forged metal versions.

Author:  Carrera RSR [ Thu Jun 11, 2020 12:15 pm ]
Post subject:  Re: 924 turbo series 1 forged pistons

Rawiz wrote:
FYI I’ve sent a 924 turbo piston to Neil Bainbridge to get a quote for making improved forged metal versions.

Just some notes from my side to help. I would aim for S2 central bowl design versus S1 offset bowl. I would aim for a durable crown to be more detonation tolerant, the S1 piston is not a durable design and depth. I would ensure the ring lands are improved to to be more robust and set tolerant than the S1 or indeed S2 design. I'd at least add a skirt coating to help avoiding scuffing. A top ceramic coating would be nice to have, bit didn't have this on the Omegas when we bought them

I think I posted Wes' image of the Omega v S2 OEM piston to see the differences of crown and ring design. The Omega piston is quite a beast. Also similar to the JE design that Colin posted but that has a squarer bowl line, the Omega is curved.

I wonder how mine look after 6yrs of spanking and det events?

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