Engine Refresh

[TTM]

Hi Graham,

Sorry if this sounds a bit rude but any (serious) machine shop would not rebore a block without having the pistons in hand to make sure they are doing it within the correct tolerances.
Also, the 0.005 mm clearance works well with standard engines back when they were new and running low boost, and I don't believe you will want to run it in stock configuration.
We all got away with tuning them up as when we bought them they were already quite a bit worn and could handle more boost and abuse without scratching the bores (too much). I would not use the standard tolerances on a modified engine. A bit looser is better and safer.
At least that's my experience and that seems to be a general consensus between all the folks and machine shops I have talked with.

 

[blade7]

Need to source pistons now and probably go to forged rods, but still waiting for some ARP head studs to arrive and be fitted before I can collect everything and get it all back to the OPC.

I think Porsche pistons will be expensive so may go with forged Wossner, but I'm still waiting for prices. Any thoughts?

I think Wossner will make custom pistons in any size, but they want a drawing with all dimensions, and take weeks. I'd get a set of standard 1st oversize Wossners that are available off the shelf for around £600 and have the bores correctly sized to match.

 

[GPF]

Hi Graham,

Sorry if this sounds a bit rude but any (serious) machine shop would not rebore a block without having the pistons in hand to make sure they are doing it within the correct tolerances.
Also, the 0.005 mm clearance works well with standard engines back when they were new and running low boost, and I don't believe you will want to run it in stock configuration.
We all got away with tuning them up as when we bought them they were already quite a bit worn and could handle more boost and abuse without scratching the bores (too much). I would not use the standard tolerances on a modified engine. A bit looser is better and safer.
At least that's my experience and that seems to be a general consensus between all the folks and machine shops I have talked with.

Hi Thom, no worries mate - thanks for chipping in

However, I'm not too sure I entirely agree with the first statement. I can, of course, see where you are coming from, but having chatted on the phone Serdi seemed quite happy to do it.....so did the shop that did the engine in my Noble - and we took that from 349 bhp straight up to 612! Still going strong 10,000 miles later!

The 0.005mm isn't a proposed clearance btw, it's just the machining tolerance.

This is lifted from an email following inspection :

BORES ARE AT PRESENT ø100,025MM SO WE HAVE TO HONE TO FIRST OVERSIZE

Then from an email listing work done after machining completed :

1 - ROUGH HONE TO PRE POLISH DIAMETER
2 - POLISH AND FINAL LAP TO 1ST OVERSIZE TOLERANCE GROUP 0 (ø100,50MM ± 0,005MM) THIS WILL ALLOW PISTON DIAMETER ø100,48MM TO BE USED


(Sorry for the caps - they are theirs not mine)

You are right, I don't intend to run it at standard boost etc, but I don't feel the need to push it any further either - I'm not interested in making it a monster - so I think that standard oversize pistons will be fine. Blade, I've not seen any Wossners for £600 anywhere though?

 

[PSH]

hmm.. I'm a little with Thom on this one and surprised that Serdi did the bores without knowing the spec of the planned pistons? In general, when reboring, you would work to the tolerances of the pistons being fitted as different alloys have different expansion rates and thus each piston manufacturer will carefully give details of the bore tolerance required, along with ring gap for normal street use. You may also find that the tolerance required varies between cast and forged pistons. Ring gap may need to be opened depending on planned boost levels and thus heat generated, there are formula's out there to work this out ( a google search should find one) which IIRC will tell you what gap is needed for road, track, race and drag setups. I worked out my own gaps and then phoned the piston manufacturers tech department in Argentina to see if they agreed with my maths, they did and the car has now been running for more than 2 years with no smoke even when cold. Oil consumption is zero off boost (normal driving) and within Porsche spec for a new car when driven hard, so I think and hope that all is well in the engine department.

I'll go a touch a big lump of wood now just to play safe..

Pete

 

[blade7]

Blade, I've not seen any Wossners for £600 anywhere though?

I did a quick search on ebay and the main US importer does them for $640, an Italian supplier has them for £720. I'd try the UK importer and see what they can do. As has been said the piston to bore clearance is critical with Alusil. The original pistons have an expansion rate very similar to the bore hence run very, very tight clearances. Aftermarket forged pistons usually run a lot more, typically at least 0.004"

 

[GPF]

Hi Pete,

I'm not saying that Serdi did the honing. Just that from the chat I had with them when I contacted them to see if they could do it, it seemed that would be what they would do - check the bores, and my original pistons, and then if they found that the block needed honing they would do it to a standard oversize and I would need to source some appropriate new pistons. Maybe I misinterpreted them, but I don't think so. They certainly didn't say that they would need the new pistons on hand.

I get that different pistons will need different tolerances, but if presented with a pretty perfect, "standard" size of bore, that the pistons are intended to fit....you know?

I was swaying to the Wossners because they are very similar to the originals in the material used, they don't expand the way J&E's do for example. Maybe I would be better off just biting the bullet (again!) and sticking to Porsche pistons. I really don't see how that could go wrong? I'm sure Porsche don't produce blocks for specific sets of individual pistons??

Blade, thanks. I'd already contacted the Italian company.

 

[GPF]

Morning Chaps,

Just checking what I wrote when arguably a tad under the influence having just got in at 1.30.....lol

I guess I'm in danger of missing Thom's original point about tolerances for non standard setups.

Hmm, need to look at that. Thanks for the great advice guys - please keep it coming I'm more than happy to take it in board.

Now, where's that bacon

 

[PSH]

Morning Graham....perhaps Serdi were thinking that Porsche pistons would be used if needing a larger size, in that case then they probably could machine the block in confidence working to Porsche specs. Why not consider IASA full race forged pistons which is what Serdi recommended for mine. They have far less friction than standard Porsche pistons and thus help in throttle response time.

Cheers

Pete

 

[blade7]

please keep it coming I'm more than happy to take it in board.

OK, at the risk of telling you what you already know. Cost/time being no object probably the best bore surface is a Nikasil type coating, Porsche have used this on all 911 turbo engines AFAIK. I think only Capricorn do this in the UK on car blocks at present. There's 2 alloys commonly used for forged pistons, 4032 and 2618, they expand at different rates so require different clearances. I think people have slipped up trying to run too little clearance on 2618 JE pistons in Alusil bores. Aftermarket Mahle pistons are available but the anti scuff coating was changed a while back, and there's opinion it isn't as good as it used to be. I wonder if new OE Mahle pistons from Porsche are also affected ? So, too little clearance and the pistons will seize or at least scuff the bores, too much clearance and it will be noisy. I'd always go top limit on bore clearance and ring gap on a modified road engine.

 

[GPF]

Hi Blade,

You're right, I do know most of that but thanks anyway - I also know that my knowledge only scratches the surface and your very good summary pretty much covers everything that I do know!

I just didn't want steel liners and Nicasil coating dried up as an option (I think you're right about Capricorn, and also felt they wanted to build an engine, rather than just sort the block) plus I wanted to keep some basic originality (it is a Turbo S after all) so decided to stick with Alusil.

Don't want 2618 for the reasons you give, hence favouring Wossners because they are 4032 and so closer to the characteristics of the originals. I think Mahle swapped to ferroprint for some enviromental reason to do with the use of the original coating but I can't remember what it was called just now. What I don't know is what coating Wossner use, most of the blurb refers to ptfe but we need a ferrous coating. I presume their K9477's for our blocks have that instead, but I need to find out. I'm set on 4032 ali though. I have 2618 J&E's in the Noble, and although they work great, you do get some audible piston slap on cold start up. It's not a problem and soon quietens down but can sound a bit too much like a bag on spanners if there is an audience, which it tends to attract!

How difficult it may be to get the clearance right is what (or maybe just one of the things!) I have possibly underestimated based on the great advice here!

Pete, full race sounds likely to be a tad pricey and I don't want to be re ringing this anytime soon! Equally, I will quite possibly release him back into the wild in the near future too so something less exotic may be prudent for the next owner. Not sure though - it seems I have lots to look into a bit more seriously next week.

Thanks guys. Anything I'm still missing, or plain wrong about, please feel free to dive in with. As I say, I'm well aware that I don't know it all and always happy to take knowledge and advice on board!

 

[blade7]

Other than JMG/SPS, PSH(Pete) and Turbotim I can't think of anyone in the UK that have had 944 blocks machined in the 8 or 9 years I've been on the forums. I tried to order some custom Wossner 3.0 turbo pistons couple of years ago, only for the US supplier to say Wossner had told them not to supply to Europe as it upsets their distributors there. Although there was a way around that, I just lost interest at the time.

 

[TTM]

Wössner pistons do require much more clearance than the Mahles as the aluminium they are made of expands a lot more. Think it's something like 0.04mm which is more than even the wear out clearance of Mahles. Apparently they still work but I don't get the point. I would always prefer to pay 30% more for Mahles.

 

[PSH]

Hi Graham

I don't recall the pistons being too expensive...iirc the machining and pistons was around £1500....admittedly this was a few years ago and we hadn't destroyed the value of the pound...lol

 

[924Srr27l]

Wössner pistons feature a unique PTFE coating on the skirts and a grade of 4032 Alloy (T6)

which allows the engine builder to run a closer piston to bore tolerance then he might do on another make of piston. This helps remove the slapping noise during cold start, common place among engines running forged pistons due to larger then OE bore clearances. The crowns of the pistons are also polished to better resist detonation on a high performance engine. Polishing allows a more even spread of heat inside the chamber which will help prevent "hot spots" on piston crowns.

I've used them in my 2.7 but N/A not Forced induction, so far so good, good Power and no oil consumption etc..

Stock cast 2.5 N/a Piston v 968 rods & Forged Wossner made to order to suit my block spec / bore

R

 

[blade7]

The only current Mahle pistons I can locate online for the 944 turbo are the Mahle PowerPak Plus. These are 2618 alloy so may be cold knockers...

 

[blade7]

I wonder if the piston question was ever resolved...

 

[Eldavo]

Nope.

I’m still buying Wossners, spoke to a contact yesterday at Autosport that I was given - getting them for me at cost as a favour to a mate of a mate. I’ve done my research and happy that it won’t be a costly saving in the long run.

 

[blade7]

I was wondering about GPF's rebuild, but you both had the same choice to make.