Porsche Enthusiasts Club Forum

Excellent.. One of the parts of engine building I actually really enjoy and also the part few builders bother with.

Do you have a dremel? A compressor? Just a thought....

I am sure Wes won't mind posting his pics??!!

Here's some Omega forged pistons he's just bought off the shelf, said to have been spec'd by Prodrive for a 924 turbo S2 engine with a 8.5:1 CR. You can see the ring lands are massive, the skirt is a touch shorter and the dish measures 35cc's versus an S1 turbo piston of 45cc (7.5:1 CR) price was in my view a very reasonable £550 all inclusive for a piston which looks to be built for strength rather than lightness. These are the only 87.5mm's on the shelf and I have committed to the 87mm's. There is a set of 86.5mm's available but will need a virgin/in tolerance block. They will forge more pistons but require a 12 piston/3 set order.

As a reference, here's JE's stock 931 piston option

Those Omega pistons are probably the most interesting pistons I have seen in years.

A very deep bowl, I mean really deep, and a wide squish.

I would suspect these pistons will produce lots of torque off boost at low rpm, which may be exactly what a 924 turbo could do with.

Steve, machine your block out another 0.5mm and buy a set of these, how many more do they have in this size??

The price is good as well, and with a skirt coating already. Awesome.

I think the JE will make more peak power, but that Omega piston crown design is really interesting for a turbo piston.. certainly will have trouble detonating that top ring land and crown side away!!

It may not make quite as much "Peak" power as the JE, but to be honest, I think the extra torque is a much more worthy target, and that chunky crown may allow you to run much more boost while retaining a nice deep cool flame trap at the bottom of the bowl.

Very interesting..

Steve, I also replied to your post on the 944 section in PCGB with 924 turbo badge pannels with 944 turbo intercoolers.

This is something I have done a lot of research on, if you need some help implimenting it, let me know.

I am not planning on being a presence on PCGB forums, just keeping an eye on it for when my or my company name is mentioned, to keep records straight and minimise confusing or anyone missquoting me on there, sometimes people take a little info and get the wrong end of the stick, so I need to Police my name being included in that misinfo.

Jon, My understanding is they had three sets left on the shelf. Wes bagged the 87.5mm, I have bagged the 87mm and there is a set of 86.5mm left which will need a virgin block.

They will make more with any spec you wish but with a minimum order of 12 pistons/3 sets per spec. There may well be more people to buy into these on 924OC. Once my current financial haemorrhaging has stopped I'll be up for another set, but that will be another 12 months away. I will build another engine with all things I don't nail on this build.

Once my engine is back in the car and run in you will still have the fun of playing with tuning it later this year or early next. Thanks for the comments regarding the FMIC ducting. You did make some valid points and I may leave it alone until I am back on the road and we are both final tuning the car.

Steve, I don't mind you posting the pics, just not got round to it myself

Jon, nice to read your thoughts on the Omegas. I sent Omega an S1 piston weeks ago with a letter detailing possible mods I'd like them to make.
Turns out they had 'extras' left over from when they used to make them for Prodrive and the manager let me have them minus the coating cost because they were old stock and I was basically doing him a favour.

I took a small risk and bought them blind, already aware of Omega/Accralite's reputation.
I was more than a little excited when they turned up, and they have features that I asked for in my letter.
The manager described them jokingly as 'tractor' pistons with regards to the strength and bulk and described the 24mm pin as massive compared to modern equivalents of ~20mm.

They're not light, although I've not had chance to accurately weigh them yet on digital scales. On crap kitchen scales they look to be a little lighter than OEM cast pistons.
Personally I wouldn't trade strength for lightness but I am a little too much 'old-school' sometimes.

I knew you'd approve of the wide squish area.
Steve needs to lower his CR a bit and personally I think his best option is to have some cc's taken out of the head chambers, probably to ~28cc, 21.5cc being standard.
I think if he goes the Cometic/thicker headgasket route and increases the squish/quench to greater than 0.050" he'll lose all the benefit and possibly become more detonation prone. From what I understand it's better to have a higher CR and proper quench than have lower CR and no quench??
What are your thoughts on this?

Another few pics poached from Wes!

Those Omega pistons certainly do have a mightly fine squish

I think squish and quench are extremely important.

A couple of good examples, the 924 Non turbo lacks good squish, tackle this and the low to midrange performance comes on leaps and bounds, but because it does not massively hike the peak BHP figure that everyone likes to judge, improvements hear rarely hit the headlines. The same is true of the NA head, which can be modified to make a real winner.

The other example were the pistons that Steve linked to in another post on 924.org, truely aweful pistons in my opinion, especially the seller trying to palm off all their drawbacks, including the poor squish/quench design as not being important.

The only thing I am not sure of with the Omega pistons is their very deep bowl, and at higher rpm if the flame front will completely burn the charge at the bottom, or if the charge down their may extinguish the flame front at the end of its "push" as it reaches the bottom of the bowl.. However, on the other hand, it may even help the engine scavenge the last drop of power from the charge at lower rpm when it has longer for the flame front to burn into the bowl with a reducing surface area..

All very interesting.. I would love to see a picture of the tops of the crowns on these pistons after 1000 miles or so, that will tell a really interesting story of how the charge swirls into the crown, where the charge boundary remains and how complete the burn is.

I think it will be ok.

I believe that on a turbo charged engine it is important to have a good squish/quench versus ideal CR, ideally you would achieve all things, but with a higher CR than ideal you can adjust the boost to the engines efficiency band, but you cant adjust anything to make up for a lack of squish/quench.. not to mention that an engine with high CR and good quench you can run higher boost in some or most cases, than you can on an engine with low CR and no quench..

Like everything in engine building and tuning, its all a balancing act and every decision is a comprimise, you just need to balance and comprimise your choices dependent on what you want to achieve.

So another question Jon. Looking at assumed cc figures and getting CR below 8.5:1 and closer to 8.0:1 it looks like I may need to gain 6-7cc's from the head and/or HG. Clearly we don't want to use too thick a HG to maintain squish, nor do we want to loose to much from the squish areas in the head. Any thoughts of best options here?

Here's a pic of a 931 head




I need to measure my block and cc the head to be sure on figures but looking at the numbers Wes calculated we could be looking at a CR with these pistons of circa 8.65:1 with a stock Victor Reinz HG. (could be more if we used the numbers from the US guys!)

So the options are sticking with a Victor Reinz or Goetze stock HG's subject to the compressed values, thicker being better, or going slightly thicker again with a Cometic, but we have had your thoughts on these. And then we need to think about how much material to take out of the head and from where

We previously decided that 8.0:1 CR was the optimum for my S1 engine/turbo but with this piston design could this be pushed north towards 8.5:1 without too many other compromises/issues? If so to what CR figure could be used as a max target?

As for your comment of seeing what the piston crowns look like after 1000 miles, then you can have a look at mine via an endoscope when its in for tuning. If you want to see them closer/first hand then you can pop the head off if you like, as long as its at your cost!!

Not in the country at the moment, so pretty far from the workshop and my cross sectional drawings of the 931 head.

However...

I would be tempted to take about 3cc's out of the head in the combustion chamber roof, which may require a little reseating of the valves and machining a new valve shroud around the head, which is not a bad idea to do anyway, because in stock form that part of the head is lacking in finesse.

Work out how much material you need to remove from the chamber roof to achieve 3cc's, you can do this as a rule of thumb as a theoretical study by putting the head level and injecting 3cc's of oil into the combustion chamber and looking at the depth of it. Hopefully it is just a mm or less

Another 3 to 4cc's I would talk to Omega and see if they could take 3 to 4cc's out of the piston crowns in the north and south positions which would not contributre to squish. Creating a slightly ovalled dish in the piston.

I don't like the area around the spark plug in the 931/932 head chamber and the rectangular shaped recess in each one. That area definitely needs some finesse methinks

I think 3-4cc should be possible without losing quench area or substantially changing the over-all shape of the chamber.

If only they'd let me lob my head on the miller at work. Would be quite easy to CNC machine each chamber identically.

Machining a 16 valve head from a solid billet for the 2.0L block is a dream of mine - they wouldn't allow me the development time but it actually wouldn't be massively difficult using something like Solidworks and taking designs from other 16 valve heads.
Imagine that sat on the 2.0L bottom-end with custom designed ports/plenum, proper exhaust manifold, modern turbo etc. etc.... ahh well, life wouldn't be the same without having dreams...

I am sure someone a few years ago on 924.org found a 16v head with similar bore size and spacing as a 924 which may possibly be "graftable" onto a 924 block...

I am a bit of a fan of the 8v 4 cylinder turbo configuration though.

There is a lot of scope for tuning the 924 turbo, much more than I have seen in the comunity so far.

Rareity of the 924 turbo is holding them back, but a 924 non turbo could be converted to a high spec tuned 924 turbo with no loss of money, as the bits to be added replace the 924 turbo specific parts anyway.

1.8l 20v Jon? I was told by someone whose is well-regarded, that the 20v engine uses a block whose lineage can be traced back to the Ea425.

Its a shame that the bellhousing fixing pattern was revised...

I cant for the life of me remember which one it was, apart from it was talked about on 924.org

I still think the 8 valve head on both the 924 turbo and 944 turbo are good for much more power than has been squeezed out of them so far.

Years ago I did a study of the comparison of 2 valve per cylinder heads versus 4 valve per cylinder heads and why a 2 valve/cylinder head will produce more power in nearly every application, whereas a 4 valve/cylinder head is better for some limited applications or gains, one of which being emissions and economy based and application wise being extremely high reving engines. Which was part of a report for SAE International.

I should dig out my findings and turn it into another article for the JMG articles bit in our website in the performance section, I seem to have had some good feedback from the couple that are already in there.