BC

Cool. Cheap motor for racing. Well, I mean, not CHEAP, but simpler.

FRporscheman

Would there be any problems using pistons with the wrong number of valve reliefs (and in the wrong place), assuming the pistons do not hit the valves? I'm just thinking about air flow, heat concentration, etc.

largecar379

not really, unless you're in serious competition racing where you need every little advantage you can get.....



In which case, you would not be thinking of doing this at all--

--Russ

atb

I dunno Russ, I think the majority of active 928 racers out there at one time or another ran 5L hybrid motors running 16v heads. Those run pistons with 6 valve reliefs. Kibort, Veninger, Shaw, Lloyd, Osage..., whether or not you'd consider them in "serious competition", I think its a safe statement to say they are all pretty serious about competing in their respective classes.

largecar379

yes, they are, but not really what I was thinking about.....no "slight" intended.


--Russ

FRporscheman

6 valve reliefs? Is that a typo?

So those people run/ran 2v heads with pistons intended for use with 4v heads?

atb

That's correct Arash. The 5L shortblock is from a 32v engine. When they put the 16v head on, an extra set of Valve furrows need to be fly cut because the valves sit in a different orientation than in the 32v head. (plus they are larger).

FRporscheman

Ah. So that's where the 6 comes from - 2 new cuts on a 4v piston. What happens if the new reliefs are not made? I.e. has anyone used 16v heads on a 32v bottom end with stock unchanged pistons?

I appreciate all the info. I want to put a 2v head on my 968 and I want to know if I can leave the pistons alone. Thx

hacker-pschorr

If we had an affordable 5.0 piston to buy with the "proper" 2-valve relief's that would be different. We do not, so this is the only option unless money is no object. If that were the case we wouldn't be talking about a 5 liter to begin with.

atb

No reliefs is asking for a valve crash.
Here's some pics from Greg Gray of a side by side comparison. Those are some deep reliefs.

largecar379

that's really what I meant to write....just came out wrong. After a 20 hour day, things get mangled up.

it would have more to do with one's wallet, than one's skill on the race course.


no harm intended----

--Russ

FRporscheman

Forgive my ignorance, I don't know anything about the 928 engine-building aftermarket - isn't there some aftermarket piston like JE or Wossner or something with special coating for alusil that would work for a 5.0L 2V build? I don't suppose 944 NA pistons will work? They have the 100mm bore and 2 valve reliefs...

ATB, thanks for that picture. So what I'm understanding is that the sole purpose of the reliefs is to prevent contact. And the fact that 928ers cut pistons for 2v builds on 4v pistons and leave the 4 old reliefs, tells me that the effect of extra/misplaced reliefs does not affect anything to a noticeable degree. Sorry if this is a tangent going OT but it really helps me in my upcoming project.

atb

As of yet, no one has come forward with a successful aftermarket piston or coating that lasts in a stock 928 alusil cylinder with any kind of reliability. Lots of stories out there, but I think for the most part no one with a 928 can step forward and say I ran this coating and everythings fine. A lot of folks are getting their cylinders nikasiled and using domestic pistons, rather than trying to find a coating compatible with the alusil. Mahle motorsport has a piston that's supposed to work, HPC and Swaintech both are supposed to have coatings that work, but I don't know of any success stories.

The extra reliefs haven't posed any kind of problem that I'm aware of. The reason for having to use the 4 relief piston is that there was never a two valve 5L 928 motor, so when racers go for the bigger cube shortblock to run in their older 4.5L or 4.7L 16v cars, they have to notch the pistons as shown. The pic above shows a 4.7 piston next to a notched 5L, they look the same size but thats just because I put them together, the 2v piston is for a smaller bore.

FRporscheman

Yup. I'm surprised 944 or 951 pistons don't work for you guys. That's too bad...

Has there been anyone with a 5.0L 16v blown motor? Any trouble with hot spots at the reliefs? I haven't heard anything about that but it's a concern that just popped into my mind and won't go away. I want to build an 8v turbo motor using 16v (4v) pistons.

Vilhuer

Other concern you should think about is piston top and cylinder head shape. Specifically what compression ratio they will produce.

https://rennlist.com/forums/928-forum/344250-head-and-piston-top-cc-numbers.html