I have been reading some of the threads about modded motors but have not heard anything really about how compatible 968 intake valves would be with factory pistons.

Naturally on heads that are pre-GTS machine work is needed to get these bigger valves to seat properly. However what about with the pistons? Will the reliefs need to be cut deeper, especially with GT cams to support thw bigger valves.

I think I will go get some breakfast now and stop thinking.

 

All stock heads that came with 37mm intake valves are going to require work to accept 39 mm 968 intake valves, including GTS. I would think that you would need to increase the reliefs in the stock pistons but don't know for sure. But by the time you've done this and bought GT cams, you're at supercharger money.

 

so, look no farther than the two pistons that are fitted for the 928S4 and '85-86 S3.

there are barely even any reliefs on the 85-86 pistons, yet it has the cam with the greatest lift.
the S4 pistons have HUGE valve reliefs. mosty because Porsche solved the offset problem and was able to run the proper offset piston on each cylinder bank. (pistons had to be reversed so, symetrical valve reliefs were needed) not an indication of the requirement, as the compression ratios were controlled by deep cuts and the piston dome.

since the piston never gets close to the valve (intake) on the way down, and on the way up, its the closer of the two, but look at the 85 again. barely any cut out for exhaust, so i wouldnt worry about it.
worst case, nothing that a dremmel coudnt fix in short order.

 

So basically Mark you are saying the S4 pistons will accomodate the higher lift cam in addition to 39mm valves? If so, that is great! I will these options on my to do list for the eventual overhaul

 

Seems to me that the clay displacement in the third photo indicates that some kind of valve is extremely close to the cutout in the piston top. What am I missing?

 

Unless you are building a stroker, I would forget about the bigger valves.

 

I wouldn't rule them out just because the motor would remain at 5L. I am still waiting to see if Dennis will post about his valve upgrade on his Screamer build.

 

major correction guys! SORRY! Just to be clear!

So, the S4 has a much shallower head combustion chamber. thats why it needs the deeper valve cuts. (I would guess). the flat top piston, has a much deeper combustion chamber, and thats why the pistons dont need much valve relief.

so, if the s4 pistons and GT pistons are the same, well, you get extra depth and protection knowing that the exhaust valves are much deeper and wider than they need to be . (exhaust pockets are same size as intake pockets to make the pistons universal). the S4 heads are said to be the same as GT, so that means GT cams will work (only 1mm more lift anyway). BUT, the radius of the valve pocket is for the smaller 928 valve vs the 968 intake valve, so the question is, is it big enough. I thought the exhaust valves on the 968 are not any different, but I could be wrong there.
Bottomline, i probably would dremel out 1-2mm radius on the intake valve pocket if fitting the heads with 968 intake valves using S4 pistons. easy mod, and the exhaust pockets are way over cut anyway so they are not an issue, unless you are using S4 intake valves for your exhaust valves.

 

you got it!! Yes, this was the result that led us to know that the piston cuts were a little shy. 1mm shy, so we hand cut them afterwards. this has happened every time ive had them cut. even the third time where I had the machinist move the pattern down 1mm. next time, ill have him move it 2mm more!
anyway, we dremmeled or air tooled the 2mm of material to make sure the valves didnt hit.

 

So, from my last post, the cams will work, but the intake valves are suspect. probably need to grind off 1mm of material to be sure.

 

DREMEL in balanced reciprocating machinery oh my god help us !

You will need to widen the standard intake slot in the s4 piston to accept the 968 (928 PN) valves, as with any custom build you measure the clearance anyway.

However as Lizard has indicated unless you are building a monster you are wasting your money, the stock heads are well and trully adequate for any cam upgrade stroker , Greg gray has done lots of research on the flow characteristics of the heads and will confirm the above .

If you fly cut the piston (widen ) please have them machined so they are identical and then balance it as an assembly not the barbarians way .

 

'87-95 valve cuts have 40mm diameter. As valves are 37mm Porsche saw need for 1.5mm clearance for them. Based on this I think 39mm valves needs 42mm cuts and thus 1mm needs to be taken off from piston.

How deep cut is do not have anything to do with it. People have been using at least 1mm higher lift cams than GT setup so 11mm intake and 10mm exhaust lobes should be ok with stock cut depth.

 

oh brother. yeah, im a barbarian. what are you racing, an F1 engine spinning 18000rpm??

cutting 1mm of material off the valve relief isnt more than a couple drops of oil weight on one piston vs another or the crank. it is pobably less than a gram each and certainy if you do the same for all pistons, it wont be a huge issue.
plus, we are talking 6500rpm engines here. not a big deal. Hey, no problem being a little ****.

so yes, if you had the choice, when buildig the engine make the cuts and then balance the entire rotating assembly. but, if you are doing a valve mod, there is no real need to pull the block to cut the pistons for this amout of material removal. I dont think even GB would have a problem with widening the smilie faces on each intake valve relief by 1mm. Greg??

 

What about depth of the relief? Will the factory depth be sufficient?

Seriously, I am trying to absorb more knowledge about the modifications that can be done to these motors (in addition to using the search button).

 

the factory relief works for the S4 and the GT cams, as well as the 1mm more lift for modded cams. (and Im sure more as well). the point is, the engine at 45degrees BTDC, allows the cams to be rotated with no valve touching the piston. this is the main reason for the reliefs. we all know that the engines are 'interference" engines. But, can you skip 1 or 4 teeth on the timing belt before something hits. .