bad_monkey

I've got a cunning plan, but before I go ahead with it there's one area of uncertainty I'd like to clear up.

Will the S2 mahle or KS piston fit on to a 2.7 rod? i.e. are the wrist pins / gudgeon pins the same?

Is the distance between the piston face and the wrist pin the same? In other words, will the piston change the overall "height" of the piston?

In case you can't work out the cunning plan / haven't heard me go on about it before, I'm looking to build a 2.7 16v - essentially a destroked s2/968.

Before I buy stock 16v pistons I'd like to check the measurements.

(PS - the usual plea - if you want a complete 2.7 head in exchange for a complete 968 or S2 head for you 3.0 or 3.2 turbo - PM me.)

bad_monkey

OK, no-one. I'm going to bring in a set of S2 pistons in a couple of weeks, then the search is really on for an S2 or 968 head in great condition.

J1NX3D

i think they're all asleep Charlie :P its nearly midnight in Hawaii so early hours further east.

TurboAngel

Hi Charlie, I have a set of 2.7 rods/pistons and also a set of S2 ones on the shelf so I will look into it for you today. I was thinking along the same lines, but perhaps the compression ratio will be very low?

Jfrahm

All the 944 series cars use interchangeable rods. To adjust for stroke the piston pin is relocated. Thus, the pistons should fit on the rods. You might need 2.7 pistons cut for 16v valve reliefs or something. You would want to see if you are using the suspect S2 and early 968 "1R" rods and not use them for any performance application. Offhand I do not know what rods are used in the 2.7 but I would guess the 1R was used once the supply of the earlier cast 944NA rod was exhausted.

bad_monkey

Thanks guys! TurboAngel - that would be greatly appreciated. As Jfrahm points out the piston pin location is the critical thing. I've done some numbers below, but as it's all just math with some assumptions, measurements would be great.

Everything I've read here, and over on the 928 forum indicates that the 2.7 retained the NA rod specs and piston pin placement, only gaining displacement through the 104mm bore.

Theory part: assuming we're keeping piston height the same, the difference in the wrist pin placement is then going to be the difference in the crank offset from centreline, which is half the total stroke change. So 88-78.9 = 9.1mm / 2 = 4.55 mm - which according to very rough calculations drops the static compression to about 7.6:1. So, on paper it looks like 3L pistons won't work off the shelf.

Which brings me on to Jfrahms comment on rods. A 4.55mm longer rod would compensate for the wrist pin placement and leave the displacement and stroke the same - so custom rods would probably work. Last time I looked that's at least about $1k USD. Probably more than machining the pistons to add valve pockets, although you have the benefit of lighter rotating assembly.

Then there's custom pistons - an interesting idea. The JE pistons used in Henks build were 425g - at least 150g lighter per piston. So, a piston set could be bought with the corrected wrist pin placement, but again more $$ than getting the 2.7 pistons machined and coated.

Another idea would be to offset grind the crank to make up the offset from centreline - if rods could easily be found with the new big end diameter, correct length and pin setup. Grinding probably couldn't make up all of the change - you'd need to reduce 8mm in diameter to make a 4mm change in centre and that seems like a lot.

So, that's the thought process for the record. Pending actual measurements from TurboAngel I'm now inclined toward machining/reconditioning the 2.7 pistons to achieved the 16v swap.

The next challenge will be to find someone who can take the pattern from the s2 / 968 piston and machine it into the 2.7 pistons. The good news is the high-compression "smiling ninja" 2.7 pistons seem to have plenty of material to start with.

MAGK944

If your 2.7 "smiling ninjas" have the same material as my 2.5 "smiling ninjas" there is a lot of metal to work with. I had the valve pockets fly cut to avoid piston to valve contact and recovered compression by skimming the head. Might consider doing this when you get your extra valve pockets machined. You will have to have the coating restored on the tops but there is nothing quite like the peace of mind a non-interference engine.

bad_monkey

That's the type of pattern. I'd say while machining pistons it would be a good time to have the top ceramic coated, and possibly lighten/match the pistons.

It seems likely that skimming the head would be a good idea too.

Machining pistons doesn't seem to be your regular type of activity - MAGK944 where did you get yours done?

bad_monkey

Here's an interesting pic or two of 928 S4 vs 968 vs CP custom pistons -



from this thread : http://speedtalk.com/forum/viewtopic...=18847&start=0

Looks like just copying the 968 design should be possible - though the CP piston gives us some hints too.

MAGK944

SwainTech do good piston coatings, tops and sides that are compatible with alusil bores, quite reasonable prices too.

Fly cutting pistons is very common, most domestic engine shops should be able to do it.

bad_monkey

Thanks - I'll look into it locally and see who's who, although it wouldn't be prohibitive to ship the pistons.

Found this video which seems relevant;

944CS

you can't use the S2 pistons, the crank stroke is different but the rod length is the same. thus you will lose all of your compression.

bad_monkey

Thanks Chris.

This confirms what I worked through, assuming that the rod length was the same, above.

I'm going to plan on having 2.7 pistons flycut with 16v valve reliefs.

I'd love to have the cash to drop on custom internals - lighter weight for more RPM with an oversquare setup - but at the moment, my approach has to be spending the least first for the most improvement second.

If I can get the 16v breathing going, then that's a good start.

bad_monkey

Having thought about it a while, I realised that if a 155mm rod could be found, then compression height would be only 0.45mm higher than stock using the S2 pistons on the 2.5 crank to achieve a 2.7 16v.

So, where do you get 155mm rods? Turns out Ford Duratec / Mazda L Series 2.3 rods are;

155mm centre to centre
53mm crank bore vs 55mm Porsche
21mm pin bore vs 24mm Porsche

So, I wonder how hairy it would be to bore out some Duratec rods to the stock size? 2mm on the big end diameter doesn't sound so bad (1mm each side). 3mm on the piston pin bore diameter might be a bit much? The idea would be to offset the pin bore by the 0.45mm difference in compression height to bring things back to spec.



In any case, the duratec/mazda rods are cheap and plentiful, and there are higher performance options floating around too - Eagle h-beam rods are about $345. Within a hobby budget for sure.

There seems to be an Audi rod that's 155mm long too - 20mm pin bore, 50.6 big end. Anyone know of any other options?

I'm pretty sure fly cutting the pistons would work too - just this way there would be the choice to move up to lighter off-the-shelf 16v pistons later.

I'm also going to call a couple of local machinists and see how indicative costs stack up - fly cutting pistons versus boring out rods - just to see what cost there is in the work.

bad_monkey

Need to be sure before I start selling off the top end of my engine