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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Sat May 09, 2020 1:25 pm 
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Joined: Tue Mar 21, 2017 10:01 pm
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Location: Sweden
Model: 924 turbo
Interesting with another diesel engine developer in here :)

Squish band will be a trade of between combustion velocity and knock tolerance. Especially on 2 valve engine like ours that have very little air motion in the cylinder. Since i don't aim for the moon with boost for my next bottom end im planning I picked up a set of omega pistons when they came up. Maybe even a bit over the top with the squish band, but they where available. Quicker combustion duration means better efficiency and torque, especially at lower rpms when air motion is slow and lower loads.

The omega has a sturdy (and a bit heavy) design, probably will be fairly hard to detonate to pieces. I've seen the top (the squish band) come of a JE piston on a 924 turbo race car here years ago, probably some detonation involved. I think they are pretty good if you keep them in the correct operating window. I think Joakim.S has JE pistons in his engine, and he have run 400-450 hp for years. Though its calibrated in a proper engine test cell, so top quality on that part which helps reliability massively.


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Sat May 09, 2020 2:02 pm 
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Model: TheOnly Way Is Turbo
the colonel wrote:
Just to throw another hat in the ring you could give Troy a bell at EBS, Reno nevada, as they can get any piston to any cr made, with around a 6 week turn around.


JE pistons?

Then + shipping + duty + VAT + local courier handling fee. Circa 30% up to 40% of the original dollar price.

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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: Sat May 09, 2020 2:32 pm 
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Joined: Tue Mar 21, 2017 10:01 pm
Posts: 457
Location: Sweden
Model: 924 turbo
This is an old JE piston, i think it was one of these who let the piston crown fly away.


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Sat May 09, 2020 3:45 pm 
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CedricN wrote:
This is an old JE piston, i think it was one of these who let the piston crown fly away.


I’m guessing that bowl is at least 45cc? I’m guessing the rim would be more detonation susceptible than the Omegas which have huge amount of meat around the tops.

I’m sure the JE are fine for stock to mild tunes. But I’m glad to have the heft of the Omegas for breakfast abuse I give them even though they are probably not so light. I’d also like to see a minimum of coated skirts. Ideally I’d have liked a ceramic coating on the tops of mine. But I had no choice when they came up for sale.

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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: Sat May 09, 2020 6:35 pm 
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Joined: Wed Dec 24, 2014 1:14 pm
Posts: 24
Model: 924 Carrera GT
Extracting dimensions from the JE piston photo, I make it a 78mm diameter bowl by 9mm deep ie about 43cc so a good estimate at 45cc! That would make it 7.8:1 CR.

Top ring groove looks mighty close to the internal radius at the bottom face of the bowl so maybe that's why the piston came apart.
Concur that beefier is better - you don't want to be replacing pistons every 30 hrs as on a race car!


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Thu May 14, 2020 12:32 pm 
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Joined: Sun Jan 21, 2018 11:22 pm
Posts: 198
Model: 924 turbo S1
Is this a sensible piston option from Mittelmotor for an S1 rebuild or are they too big? I’ve not managed to get a quote from Omega as yet.

————————
The pistons I offered have a 88.50mm bore instead of 86.50.
We use them most of the times because many 924 turbo engines have to be bored to oversize.
They have a thermal barrier to increase the thermal efficiency of turbocharged engines and also offer protection against overheating of the piston for high performance engines.
There is also tread coating to reduce friction and increase mechanical efficiency.
The compression ratio is 8.2:1. This set includes all rings, pins and clips.


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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: Thu May 14, 2020 2:17 pm 
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I was told 88mm bore was the largest, but MM are more an expert than me. I also thought that the HG bore was 88m? I have never checked to the sure about that. Just internet/forum reading.

No reason not to work. MM have built more 924 engines than me. No reason to not go to 88.5mm, but it will be the last rebore that block will be able to have unless fitting in liners? At least going to 87, 87.5mm allows another rebore.

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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 14, 2020 6:21 pm 
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Joined: Wed Dec 24, 2014 1:14 pm
Posts: 24
Model: 924 Carrera GT
They are the one's I've just bought from MM, though mine are 87mm. When I took out the old head gasket ( the standard Reinz ), the gasket bore measured 88.44mm, so you might need a special gasket.

If you recall, I was surprised that the pistons have a compression height 4mm less than the standard Mahle pistons and, no surprise, were inset 4mm into the block when I did a dummy build with them. I talked to Michael at MM re these pistons. Their philosophy is that people rebuilding/upgrading 924 engines always end up skimming the head and block anyway, so they shorten the pistons so you have enough room to skim these components until you get the CR of your choice.

They checked my calculations and, with the exception of the head gasket volume, agreed with my measurements. Having closely examined my engine, I remembered we'd skimmed the head a couple of times and it's really at the recommended minimum so shouldn't be skimmed again. I was thinking of fitting a Cometic MLS head gasket but the head surface finish doesn't look good enough and can't be skimmed. So I'm sticking with a conventional gasket, at least for the time being. I'm aiming for 8.2:1 and that works out I need to take 1.15mm off my block head face. The block is at the machine shop today getting done so I'll update you on what it comes out like. By my calcs, the valve timings are changed by 3 degrees because of the decreased crank to cam centres.

That's a lot to take off the block but I checked it as much as I can and can't see any significant problems eg the Head Bolt threaded holes still have enough thread depth to accommodate the bolts.

As a reference my/MM measurements on combustion volumes by component are - MM pistons 23.1 cc; Head Combustion Chamber incl. spark plug 19.7 cc, MM head gasket 10cc; my old Reinz 11.4cc. You add on the volume of the clearance disc above the inset piston so this is where the decked block tunes the CR. Remember an 88.5mm bore has noticeably more swept volume than standard.


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 Post subject: Re: 924 turbo series 1 forged pistons
PostPosted: Thu May 14, 2020 7:06 pm 
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Joined: Sat Jan 30, 2010 11:13 pm
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Model: TheOnly Way Is Turbo
I checked earlier, the Ideola Garage compression calculator has be pulled again. Great tool for check CR, I'll reach out to Dan to see if he can reinstate again. It seems to come and go now his website is down


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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 14, 2020 7:09 pm 
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Model: TheOnly Way Is Turbo
Colin wrote:
I was thinking of fitting a Cometic MLS head gasket but the head surface finish doesn't look good enough and can't be skimmed. So I'm sticking with a conventional gasket, at least for the time being. I'm aiming for 8.2:1 and that works out I need to take 1.15mm off my block head face.


I never got to the bottom of what a 'standard' skim surface finish was versus a 50RA (roughness average) required by Cometic. How smooth or rough is it? I installed mine blindly and its still working :hope: :sweat:

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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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