Rebuilding the M96 oversize 3.8-3.9 litre

Looking at rebuilding the bottom end fully on my engine and it occurred to me that a small increase in capacity properly thought out could be the way to go.

These forged piston sets look to be incredible value as comes in either 99 or 100mm giving 3.8 or 3.9litre respectively.

Being forged perhaps they are lighter/stronger units than the OEM piston which would reduce stress on the standard conrods perhaps (would be looking to retain mine).

https://www.design911.co.uk/mobile/#part;id=132513

My questions are, do we have a supplier for closed or semi closed top deck liners (I don't believe Hartech offer oversize, apologies if this is wrong info) and that leaves Westwood liners which are ductile iron and I have some concerns with material type miss match if those are fitted in an over bored existing crankcase liner unless they can offer a complete thick wall liner that totally removes the old ones?

Additionally would head work be required or could an ECU remap take care of any fuelling issues this increase in cc may create?

:?:

I know Autofarm offer this on their builds. May be worth a call to see if they will sell you the components.

Definitely give Matt at Autofarm a call... he's the gifted engine builder over there and I had similar conversations with him regarding my own project – he is a wealth of knowledge. Great team to discuss these types of projects with too. Hope that helps. :thumb:

Westwood Liners also do a cast iron liner. If only used for road use this is the way to go :thumb: You can utilize the liner up to 4.0 ltr in the 996 crankcases
but it would be wise to use a stronger con-rod. No problems here if machined correctly. :thumbs: cheers.

Does the WW cast iron liner fit in place of the existing liner or is it a milled interference fit inside the exiting one?

How do the differing expansion rates work here. The head bolts to an ally crankcase with the liners top face being milled to the same level, wouldn't the iron liner expand at a different rate to that of the crankcase

Hmm.... I may play Devil's advocate here.

As an engine builder and tuner I would usually say that the Hartech solution is by far the best. But... I regularly attend British Hillclimb events and have done for quite some time, where I have witnessed Duncan A, the boss of Westwood liners, giving his 3.9 Cayman terminal grief off the start line and up the hills. So far despite multiple shock loading type starts on soft racing slicks over many seasons, the engine has been both very quick and reliable.

I'm only throwing this into the mix because although IMO the nickasil liners would ultimately be a better engineered solution, Duncan's car has been very impressive indeed. He is certainly giving the engine repeated hard starts, sometimes after a thorough heat soaking too if a run gets stopped and he has to turn the car around to get another run, all of which is worse case scenario stuff. He is trying as hard as he can to break it! I have been very surprised it has stood this grief as well as it has.

I would like to reiterate that personally, I feel Hartech's liners are a better solution. However if you fancy a capacity change it would be wise to investigate all avenues.

Hmm that is indeed interesting.

I have had some correspondence with WW and they have told me there's been no failures of their liners, they offer a cast or ductile liner. From what they have said Serdi is the firm to use in Uxbridge for the machining and liner install.

Perhaps thrashing these is a better way to operate them, lots of heat quickly?

I also believe that the nickasil liner is a better option (from my research) but I guess this would need to be a test mule for such an install using a ID bore of 100mm unless it's brought straight to market. Understandably reliability could be an issue for the manufacturer and that this liner probably doesn't exist right now.

With a view to viability is anyone able to verify the following?

Are the 997 oem factory pistons forged (any idea of the weight ea in grams?)
Are the 996 3.6 and 997.1 3.8 con rods exactly the same?
Is the 997.1 3.8 crank the same sintered construction as the 996 crank?

:thumb:

Cracking thread this,

:stay:

ragpicker said:

Cracking thread this,

:stay:

Click to expand...

Agreed.

Harv - can you keep us updated on this and how you go forward...?? I'm no spanners man and only get half of whats said, but i do find it fascinating. Would also be interested in seeing any piccies of what you are doing and a write up.


Ta m8


Osh

opcorn:

I will do.
My engine is in bits so makes sensor to do this now if the figures stack up.

Fortunately I have a ability to assemble this motor but I know little generally about how the factory swapped and changed parts in these engines hence trying to quantify the rod and crank differences between the 3.6 996 unit and the early 3.8 997 one (I suspect they are the same).

Ive had a closer look at Westwood's web site and it seems their ductile 'wet type' cylinders are not inserts but stand alone items.

More input required before I decide to build :thumb:

I didn't really want to formally announce this yet but it seems appropriate to do so now - that we are in the process of developing and testing some oversized engines.

The capacity increase is from a 3.2 Boxster S, 3.4 996 or 3.4 Cayman S to 3.7 and for a 3.6 or 3.8 to a 3.9.

When our test programme has been completed they will be available with our Nikasil wet liners.

We are not yet contractually allowed to reveal where our pistons (or indeed our liners) are made (but will be able to do so soon) but the work that has gone into them is enormous from a very respected F1 and Le-mans winning supplier.

Work on compression ratio corrections etc has all been completed as have some of the prototypes and cars will be running very soon.

There will be no need to alter the cylinders heads - just the mapping. The 3.9 increase can be run with standard injectors but we do not yet know if that will be the case for the 3.7 engines for all variants. Some mods to the cooling system are also included (already fully tested for the purpose).

It takes a lot of work and we do not yet have test cars of each type available (presently a 3.6 and 3.8 are the first although the 3.7 engine parts are almost ready to build).

I will not pretend that we will expect to make the product available to the public in the next few weeks (we prefer to conduct long test programmes first) but there may be a need or benefit in combining a rebuild with the use of a suitable customer's car and engine (under strict test agreements between us first). if anyone needs and immediate rebuild and wants to increase the capacity we cannot help yet - sorry!

The 3.7 pistons are presently in manufacture and we have liners already (and for the 3.9's).

Ferrous liners if properly fitted (which is not as easy as it sounds but should be OK from a source that has proven reliability) are usually OK for racing and competition work because you can increase the bore clearance to suit the lower expansion rate and they only ever run flat out anyway (for which the only downside is when the engine is cold or running on moderate throttles as most road users do) - that is as long as the head sealing has been worked out OK and because overall racing mileages are very low annually so bore wear is not too big a problem.

Our reason for our conversion is to make it more attractive for owners considering a premature rebuild (with an older or higher mileage car before it goes wrong and costs even more to fix) to pay little more than for a standard rebuild and get additional performance - but they are designed at present for road use. If we eventually created a racing version it would have a different piston profile and compression ratio (but easily doable in production).

Although stronger rods are already on the market we do not expect this to be necessary (except for racing).

Our wet liners do close the deck at the top and therefore are rigid and provide better heat transfer than a ferrous liner - less problems with the head sealing and probably longer life but for racing ferrous liners should be OK if properly fitted.

Before anyone asks - our pistons will not be made available without our liners fitted by ourselves (or our agents under licence abroad) and they would not suit ferrous liners as the profiles and clearances are all different.

The results for the 3.7's and 3.9 from a 3.6 will be a lot more torque and punch especially under acceleration from low revs and the 3.9 from the 3.8 will feel more punchy.

We are also have further increases on the drawing board but we will not be making the prototypes until we have both satisfactorily completed the test programmes for the present upgrades and customers have put some mileages on their cars without issues.

Creating too much increase in top end power by going too far (or through say turbo charging) can create a lot more problems with auxiliary components and cost a lot more and braking and tyre sizes, suspension all need attention to do it properly.

More torque through effectively "overboring" is a great way to improve performance with few downsides (and remember acceleration is more proportional to torque than BHP). BHP increases will be less than the torque because the greatest BHP occurs at higher revs when the basic breathing of the engine (with standard auxiliaries) restricts maximum flow rates. Furthermore having more torque at lower revs keeps the grip with standard wheels and tyres better and reduces engine stresses that are at a maximum at peak revs.

Sorry - I guess this may lead to a lot of questions but I am too busy developing and testing these engines to enter into any lengthy discussions about them either on the Internet or privately - but this is just to make you aware of what is coming quite soon in the future for those making longer terms plans.

Baz

Thanks for that input Baz (reading between the lines there's a lot of info in ur post) and good to hear you are working on similar options.

Are the self engine builders among us classed as 'public release'? I wasn't 100% sure if your response was offering an option for my build at 3.9l from the 3.6l but as you said you have this configuration running and testing already.

Harv :thumb:

I am guessing that these pistons are the same as the ones we had manufactured for us by Wossner as std and 0.5mm and 1mm oversized to suit 996 3.4, Cayman S. 996 and 997 3.6 and 3.8 engines almost a decade ago. We chose them because the other manufacturers said their coatings would not last in Lokasil - but Wossner had a good reputation and also stated that their coating would last OK in the Lokasil bores. This was when the published information implied that Alusil (which we knew they worked in well) and Lokasil were effectively the same thing at the cylinder bore surface (so we do not blame them for thinking their coatings would last OK).

The pistons were good quality but the coating they also supplied them with did not stand up to running in the Lokasil cylinder bore material as (that we intended to bore out after fitting support rings to provide a low cost solution). We intended to fit a new liner to the cracked bore and bore out the other 4 or 5 bigger - but we were not satisfied with the test results - they didn't last long enough in some test cars.

After much extensive research and communication with the cylinder block manufacturers we decided that the Lokasil cylinder solution was not as good as Alusil and was less reliable than we wanted for our products. They were Ok when fitted with the original ferrous coated pistons but the apparently imposed restriction on that process (or was it the cost?) rendered Lokasil a potential problem long term and we decided we would instead concentrate only on fitting the best liner solution we could and just had to stand the piston cost!

When the later engines started scoring bores we decided to stick to the existing K.S Cast or Mahle forged pistons (all later production and replacement spares) running in either std original Lokasil bores (after offering to fit support rings) or the Nikasil wet liners (that are supported anyway at the top) and still stock a lot of those Wossner pistons if anyone is interested - make me an offer! They would probably work OK in a ferrous liner - but - as you all know - we prefer the better Nikasil alloy solution for an alloy engine with such large pistons and high performance potential.

It was a great plan that cost us a great deal but ultimately backfired (more probably due to our fussiness over product quality than anything else). We had some engines that lasted and others that did not = frankly - not good enough.

However the reliability and quality of our Nikasil wet liners has become legendary now and they are so good - some time ago - we felt we were ready to embark on another project - hence this change.

Our new pistons are not from Wossner (will become general knowledge one day) but have undergone extensive research and development.

We might be prepared to consider building an engine for a suitable customers project but at this stage would not offer just the parts in case there are some development issues we need to incorporate that otherwise - if fitted by someone else and without that - might not carry out the other changes - might fail and harm our reputation.

It sounds like you are ready to go now and if so we will not be ready for your project in time!

We could also make anything from the Boxster 3.2 into a 3.9 - but feel that the auxiliaries would not be up to the performance that would result and hence have decided to limit the increase to 3.7 for them and 3.9 for the 3.6 engines.

The 3.8 to 3.9 we already ran several years ago without problems and hence once that is proven with the new pistons we may (probably well over a year away) go for something even bigger!

I only mention this somewhat prematurely from an availability point of view because it seemed appropriate when the questions were being asked and suggested we didn't offer this as an alternative and felt it would be wrong to say nothing and then (possibly after people had committed to another source or solution) revealed our future plans - to upset anyone.

Baz

I would be really interested in looking at this Baz, I`ve been thinking about an upgrade to my 996.1 C2 3.4, especially in the lower rev range. She`s on 106K miles with no issues and only used as a toy car. I`m also in no hurry to rebuild the engine immediately as I`m enjoying what is already under my right foot :grin:

If you fancy using my car in your development project then let me know :thumb:

Edit : Mine is cable throttle pre-Egas car if that makes any difference Baz ?