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Wössner pistons for SC


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Has anyone here first-hand knowledge about using Wössner 10.5 CR pistons in 911SC with 930/10 204hp engine?

Otherwise stock, using K-Jet and stock cams. They are only providing 10.5 CR and 11.5 CR set, there is no OEM 9.8 CR option. Mahle sells OEM CR set, but they only sell both cylinders pistons, no option to buy just pistons.

If someone has experience of Wössner 10.5 set with stock injection, please share pros/cons, how it feels, what are the differences to stock. Or is it a totally bad idea?

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Got my set of Wössner 95mm 10.5 pistons from Design911 today. Beautiful pieces of metal ❤️ So sad these will be buried into the engine..

My take is you are overthinking this a bit.  Let me explain. In a 2.0 911, an S cam is a bit peaky.  A 906 cam even more so.  But an S is still very usable on the road and very fun, even if you n

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I’ve got 98mm Wossners to put in my ’79SC (will be 3.2SS) on CIS.  They came recommended by someone I trust, although I’m just at the cleaning the crankcase stage, so no real world experience of using them yet.

Edited by PeterK
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I have 10.5:1  Wossners in my 81 SC, but I am running Webers and DC20 cams. But they fit fine are 117 grams lighter than the standard and all 6 piston and pins were exactly the same weight.

With their Slipper design, I found it hard to put the pin snap rings in and got the Stromski tool which did the job perfectly

Still running single  spark and it goes well

 

Cheers

Mike

Edited by MikeB
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This small compression bump is very common when rebuilding.  The idea that it is problematic is 99.9% coming from the US and most often associated with the wider 98mm pistons IN ADDITION TO the wedge shape crown making the burn/flame front/propagation with the off centre spark an issue.   But, but, but.  It really is a US issue - most likely because poor quality fuel, maybe high summer temps as well.  I don't think in 20 years I have ever heard of it being an issue in the UK or even Europe. I think if you run 98+ octane UK pump gas, you should be fine.

If you are in this deep, I would strongly recommend a cam change as well (if not already on the shopping list).  A well running SC with cams and compression making 230+hp is nice package that the true enthusiasts who understand this stuff will always appreciate.

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4 hours ago, Richard Bernau said:

This small compression bump is very common when rebuilding.  The idea that it is problematic is 99.9% coming from the US and most often associated with the wider 98mm pistons IN ADDITION TO the wedge shape crown making the burn/flame front/propagation with the off centre spark an issue.   But, but, but.  It really is a US issue - most likely because poor quality fuel, maybe high summer temps as well.  I don't think in 20 years I have ever heard of it being an issue in the UK or even Europe. I think if you run 98+ octane UK pump gas, you should be fine.

If you are in this deep, I would strongly recommend a cam change as well (if not already on the shopping list).  A well running SC with cams and compression making 230+hp is nice package that the true enthusiasts who understand this stuff will always appreciate.

Yes, I already always run it on 98+ octane fuel, nothing else. I know that going all-in would be the way to go for maximum power and response, but It would be quite expensive to go for ITB+EFI and cams upgrade now. Not to mention 3.2SS cylinders and pistons. I might be interested to take that way later (ITB+EFI + cams), but now I would just like to get my engine bottom and top end to be properly serviced and future proof, possible for further upgrades.

That is why I am interested in this higher compression piston set now, and especially with the otherwise stock engine (well, I already have Dansk pre-74 exhaust). I am not really seeking major upgrade now, but to keep the perf AT LEAST at the same level it was before, preferably with some extra.

 

 

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Someone on Rennlist said that symmetrical shape of Wössner pistons would require double spark. Mahle pistons have more asymmetrical wedge shape.

Is there any truth behind this? There clearly is noticeable difference in the shape though..

Wössner:

product_3267_a177a55654a231b73065eb46f4bproduct_3268_82059db0c0d05d0017ff0cc54ac

Mahle:

407318_359383924073744_1977940941_n_05a0

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10 hours ago, suckho said:

Someone on Rennlist said that symmetrical shape of Wössner pistons would require double spark. Mahle pistons have more asymmetrical wedge shape.

Is there any truth behind this? There clearly is noticeable difference in the shape though..

Wössner:

product_3267_a177a55654a231b73065eb46f4bproduct_3268_82059db0c0d05d0017ff0cc54ac

Mahle:

407318_359383924073744_1977940941_n_05a0

I'm 100% certain you don't need twin-plug ignition for an engine with a 10.5:1 static compression ratio, CIS induction and 98 RON petrol.  Twin-plug ignition will allow you to run less advance and give a cleaner pick-up but the engine won't self destruct.  Twin plugging the head is actually pretty cheap, it's the ignition system that's expensive if you want a traditional look.  Go for a crankfire or coil pack approach and it's not that unreasonable.  Nothing to stop you twin-plugging the heads and retro-fitting the ignition system at a later date.  This will slightly lower the CR anyway.   

That said, the Mahle pistons are more closely designed for the 911 head, the existing plug location and the CIS induction. So they probably do perform better in our engines. 

+1 for the Carrillo pistons. I've got them in my SC - trying to find a picture somewhere before they were fitted. 

1 minute ago, jehatwhitland said:

I'm 100% certain you don't need twin-plug ignition for an engine with a 10.5:1 static compression ratio, CIS induction and 98 RON petrol.  Twin-plug ignition will allow you to run less advance and give a cleaner pick-up but the engine won't self destruct.  Twin plugging the head is actually pretty cheap, it's the ignition system that's expensive if you want a traditional look.  Go for a crankfire or coil pack approach and it's not that unreasonable.  Nothing to stop you twin-plugging the heads and retro-fitting the ignition system at a later date.  This will slightly lower the CR anyway.   

That said, the Mahle pistons are more closely designed for the 911 head, the existing plug location and the CIS induction. So they probably do perform better in our engines. 

+1 for the Carrillo pistons. I've got them in my SC - trying to find a picture somewhere before they were fitted. 

PS Just took a look at the link to see the price and my Carrillo pistons don't look anything like the ones on the Design911 site - mine are very similar to the Wossners.

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23 minutes ago, suckho said:

Those are 10.5 as well.

I know this, but the design is very different (if that’s not a generic photo) to Wossner, JE etc

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22 hours ago, suckho said:

Yes, I already always run it on 98+ octane fuel, nothing else. I know that going all-in would be the way to go for maximum power and response, but It would be quite expensive to go for ITB+EFI and cams upgrade now. Not to mention 3.2SS cylinders and pistons. I might be interested to take that way later (ITB+EFI + cams), but now I would just like to get my engine bottom and top end to be properly serviced and future proof, possible for further upgrades.

That is why I am interested in this higher compression piston set now, and especially with the otherwise stock engine (well, I already have Dansk pre-74 exhaust). I am not really seeking major upgrade now, but to keep the perf AT LEAST at the same level it was before, preferably with some extra.

Are you thinking you need ITBs +EFI if you upgrade cams?   If so, that is not correct, I was only suggesting cams that work with CIS.

13 hours ago, suckho said:

Someone on Rennlist said that symmetrical shape of Wössner pistons would require double spark. Mahle pistons have more asymmetrical wedge shape.

Is there any truth behind this? There clearly is noticeable difference in the shape though.

 

Yes and no.  The Wossner shape does divide the combustion chamber and the spark is not central, so in theory it is a problem.  The Mahle "Max Moritz" shape piston is the only one designed to run single plug with the wider 98+mm pistons that I am aware of.  But that isn't the end of the story.  Sure, twin plug is a better solution but it isn't necessary.  As I noted before, the US guys either believe its necessary or have specific conditions that actually make it necessary.  Such as gas quality, 98mm width, high temps, use on race tracks etc.  All/any of those things might make you conclude twin plug is desirable (maybe even necessary) and so that became the received wisdom in the US.  Think about it - if you were building motors for a living, you would give the advice to go twin plug just out of an abundance of caution.

As I said before, I have not in 20 years of watching this kind of thing seen any example of a 3.0 engine having a problem with the higher compression, wedge shaped pistons.  I can't even recall any example of a problem in a 3.2 motor with JEs or similar and there are a few of them floating around.

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Posted (edited)

Set of Wössner 95mm 10.5 CR ordered from Design911. Let's hope they are a good match with original Mahle cylinders, stock cams, and K-Jet.

Edited by suckho
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I have a boxed set of standard euro SC Mahle Ps&Cs that were advertised before the subs were introduced but you can't see them now as I haven't gotten around to subscribing yet.  However, you can actually buy them cheaper from Design911 than I originally paid for them!  So, I've decide to keep them anyway, along with the original cylinder heads, in case I ever take the engine back to stock.

I used Carrillo CP-X series 10.5:1 pistons - https://cp-carrillo-catalog.cld.bz/CP-Carrillo-Catalog-Collection/CP-Carrillo-Product-Catalog/110/  These look very similar to the Wossner pistons in terms of the crown shape - https://www.forged-pistons.co.uk/webshop/wossner-automotive-forged-pistons/porsche-forged-pistons/ - and are available in the same 95mm and 98 mm sizes. As far as I know, they are both good pistons - as are the JEs.

 

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Ok, I see.. Not much (or any) first hand experiences of 10.5 pistons in stock SC, but we'll see.. I'll let you know how it runs when I get my car in one piece again.

Edited by suckho
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On 07/10/2020 at 09:47, SP72 said:

I know this, but the design is very different (if that’s not a generic photo) to Wossner, JE etc

That's a generic picture. Maybe of a Porsche piston, but in that case for a modern one with 4 valves per cylinder. 

As a combustion system responsible for an engine manufacturer, I don't like the design of the 10.5 CR JE, Wössner and Carillo pistons very much. Not that they designed it wrong, but with the combination of combustion chamber volume, generous valve pockets and increased compression ratio, it is what it is. 

The Max Moritz/Mahle Wedge design looks better, due to the shallower valve pockets requiring less compensation elsewhere. The disadvantage with those is of course that it limits camshaft choice. 

That being said, even the JE/Wössner/Carillo piston probably work fine and that's what counts in the end. 

 

 

 

 

 

 

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Got my set of Wössner 95mm 10.5 pistons from Design911 today. Beautiful pieces of metal ❤️ So sad these will be buried into the engine..

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Edited by suckho
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  • 2 weeks later...

Max Moritz P's & C's out-of-stock back in the day when I had my SC engine rebuilt - sob :(

Regret not waiting to this very day. 

 

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On 11/10/2020 at 10:30, suckho said:

Ok, I see.. Not much (or any) first hand experiences of 10.5 pistons in stock SC, but we'll see.. I'll let you know how it runs when I get my car in one piece again.

You won't have any trouble with that CR in a stock SC . Truth be told the static CR  of these pistons(I've had them all)  in my experience is difficult to achieve as the bore size increases. You can measure this, dry build your engine. Do your deck heights and head clearance and fit appropriate base gaskets to suit. Cam choice will be a more important factor .  Twin plugging became necessary to provide a more even burn and a faster flame front on the 100mm 964 pncs running 11.3:1(from memory 🤔

In the race car applications it was for redundancy and performance in the 2.8 RSRs . It really proves itself when the the bore sizes increase . You can comfortably run a 964 on either circuit as the engine runs knock sensors. 

Max Moritz pistons are a better design for a single plug application but there's nothing wrong with the symmetrical design of JEs CP and wossner.  They run just fine on a single plug application. Even on MFI. 

I'm not a huge fan of their cold clearances(due to their type of material construction)  but if you can live with the oil burning at cold  start up and potential piston slap, they're fine.

Mahle are always the better option but you pay for what you get. 

With the high octane fuel available at the pumps it won't be a problem. 

D

 

 

 

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On 12/10/2020 at 14:05, Helmantel said:

The Max Moritz/Mahle Wedge design looks better, due to the shallower valve pockets requiring less compensation elsewhere. The disadvantage with those is of course that it limits camshaft choice. 

What’s the limitation on the camshafts for the Max Moritz/Mahle pistons?

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On 07/10/2020 at 13:59, Richard Bernau said:

Sure, twin plug is a better solution but it isn't necessary.  As I noted before, the US guys either believe its necessary or have specific conditions that actually make it necessary.  Such as gas quality

Problem is the quality of petrol in US, think they get 91 octane in some states so IIRC they twin plug anything close to 10 CR & above. Apparently due to the higher octane levels here its only necessary when you get close to CR of 11.

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15 hours ago, WP0ZZZ said:

What’s the limitation on the camshafts for the Max Moritz/Mahle pistons?

I would take the camshaft supplier's recommendation on that so you have some comeback if it doesn't work out :) 

We know from experience something like a GT2Evo or 993Supercup cam is about as far as you can go and works with even the dome pistons as they are relatively high lift but relatively low overlap and I doubt that an MM piston has much more clearance (but could be wrong).  So that may be the practical limit.  Those are also often the go to kind of cams as they work with common plenum injection, but once you are ITB'd, you have a whole load more options.  Something similar to an S cam in a big capacity engine is a very nice engine.  I have said it plenty of times before, but 30 years ago my mechanic in NZ was running a 3.5 with S cams, bored MFI stacks that were actually EFI on Motec making 350+hp - helluva nice motor.

Most everything in Porsche world has been done before - so find something that works well and copy it.  No need to reinvent the wheel.

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16 hours ago, WP0ZZZ said:

What’s the limitation on the camshafts for the Max Moritz/Mahle pistons?

If you still have CIS then that will limit your camshaft choice more than the MM pistons due to the reversion issues. 

As Wayne says.... 

Yes, 3.2 Motronic manifold intake systems with hot cams have lots of problems at lower RPM. He has direct experience with these - they are fine at RPMs above 3000, but you wouldn't want to drive them. Part of the problem is the charge momentum. When the valve closes, the air hits the back of the valve and bounces back up, like a rubber ball. This charge reversal then intereferes with the proper mixing of fuel (particularly on the CIS systems where the fuel insertion is not sequential - meaning in time with the opening and closing of the valves). This fuel reversion effect is mitigated by taller manifolds and high butterflys on MFI cars.

The bottomline is that this is not a myth. Yes, you can run hot cams on a CIS car and / or an 3.2 manifold car, but you will have problems at lower RPMs...

-Wayne

Edited by GaryH
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