924Board.org

[morghen]

I'm trying to rebuild my 932.
It has 3 good pistons and a bad one..so i need to source a set.
I've looked and looked for a set of OEM S2 slugs that can fit to my application but honestly i dont think i'll ever find them.
I need 87.XX mm pistons. I dont know yet how much i have to cut into the engine block...but the pistons will have to be at least 87.00mm

The OEM pistons are kind of out of the question.
Diamon or JE pistons can be the next best thing but i want to compare a custom piston to a VW flat top.

I've done some searching and there are pros and cons on the flat top pistons...yea flame propagation is not the best but does that really matter in a road going car?

I'm quite drawn to the flat tops as they seem cheap and it also seems they work well on our engines.
Goal is to restore the 932 to working order...probably no intercooler....maybe water injection.
If you have experience or knowledge about the flat tops..please do share.
I'd like to do a comparisson, pricewise and performancewise.
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Supercharger and EFI kits
https://www.the924.com

[WEASEL149]

I don't have direct experience of using any of those pistons but did investigate flat tops and almost went down that route before finding the Omegas.

If you go VW flat top, try to get the genuine Mahle ones with the graphite coated skirts. Cheaper ones have crap rings.
Also, there are cast and 'forged' versions of the Mahles. The 'forged' ones are proven in high power 931's, so I would go that route.
Raceboy and I believe Gegge have direct experience with them, and Racing ran 2 bars of boost pressure with them so they'll stand abuse as good as the others.

Don't forget that going with JE or VW flat top means a 22mm piston pin so you'll need the corresponding bushings from EBS ($10 each).

I think any of those pistons would be fine. I personally wouldn't push the boost without some form of 'permanent' charge-cooling.

I have nothing against water/alcohol injection but think it's best used in conjunction with charge-cooling/inter-cooling, when pushing the detonation threshold on pump fuel.
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1979 UK 932

[morghen]

Thanks, this engine is going into a factory looks and performance car...so boost will be low. In the future maybe i'll swap the engines between my cars and this flat-top engine might see some high boost like 1bar but will get proper cooling then.
_________________
Supercharger and EFI kits
https://www.the924.com

[leadfoot]

With vw pistons you'll drop compression and be non interference most likely... You might consider this if keeping boost stock... and yes they are fairly cheap but buy new rings.
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Rasta Monsta]

Biggest benefit of modern slugs is their utilization of modern tech that didn't exist when our cars were built. Check out the JE website for details.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[juha_teuvonnen]

At this day and age it's not all that difficult to make a copy of a piston if you provide a sample. All you need is a decent 3-D scanner, some CAD software (to play with diameters and etc) and a CNC machine. Coatings are nor stricly required, but highly recommended. There is a number of vendors in the USA that will gladly make you a set of pistons. Diamond, Wiseco, JE, Ross, Arias just to name a few.

You have to be careful with the alloy used to make the pistons. While 2618 is a perfectly valid choice for building a good ol' small block chevy v8, IMHO it's a poor choice for 931. 2618 has very little silica in it (something to the tune of 2%) which means that it survives detonation easier than a stock hypereutectic cast piston. The downside is that it's soft, ring grooves wear out fast and it expands a lot. Consequently you have engineer larger clearance between the bore and piston skirt and end up with a fair amount of piston slap on cold engine. With a typical v8 hotrod having a lopey idle from an aftermarket camshaft and plenty of noise from aftermarket mufflers or lack thereof, you may not even hear the piston slap at the end of the day. The 2618 alloy is soft, so it may not last past 50K miles, which is a non-issue for a hot rod that's occasionally driven for showing it off.

IMHO 4032 is a better choice of material for a 931 or any other turbocharged car for that matter. It has higher silica content (11%) so it does not suffer from the downsides of 2618. Cosworth has a proprietary alloy that is very good. Their product can survive abuse of rally racing, but is far from inexpensive.

While making a forged copy of the OEM piston is trivial, making the one that works well for your particular application is another story. It sometimes takes a few tries before you get it exactly right. Tinkering is half the fun for some of us

If you prefer to get your car on the road fast, rather than pulling engines in and out, Ideola's garage has custom pistons (made by Diamond) available for $695. Dan Beckett is a reputable vendor that knows these cars like the back of his hand. He also has sound engineering knowledge, which is important. I have bought tons of stuff from him and he has never let me down.

You can probably have custom pistons made for less than what Dan is selling them. You are taking the risk that they won't work out exactly right, and you way need to make another set and rebuild the engine again. Buying a known working set from Dan is a bargain in my book.

P.S. No piston built for spark-ignition engine, no matter what alloy it's made from, will survive detonation in the long run. Your engine needs to be tuned correctly. If you have detonation you will destroy any pistons, regardless of brand and material they are made from.