Originally Posted by
Z'Stein
They are breaking at the pin. I just machined one so that it lines up with the sleeve at BDC and took the car out and ran it a little. Revs quicker and accelerates quicker. Don't know the long term effects yet, but so far seems to be a go. I'm not sure it's piston-rock. That is just something I'm wondering. Thanks for the response.
As a fellow machinist and engine builder (Subaru in my avatar is 589 to the wheels on E85!!) I wonder the same things! these engine mfg have been doing this for a long time with that being said I am sure there is a good reason for the skirt being so much longer. Personally i havent figured it out yet and will eventually get to it, right now I am concentrating on porting and port timing. In my 1:1 experience its not typically the high RPM's that kill an engine(unless you have oiling issues) its midrange torque that will send a rod through the block. Detonating while your in that range will break parts. once you get passed peak torque, momentum becomes your friend.
I have not actually looked at the relationship to the long skirt to the exhaust port but it seems to me if you were to bring the piston to TDC and then scribe the piston skirt from the exhaust port opening that would be the shortest that the skirt could possibly be. If that port opens up to the crankcase you would end up with exhaust waste in the crankcase which would in turn end up back in the CC. This would eventually turn everything inside the engine into a black nasty mess! You would also loose crankcase pressure. Keep in mind that the exhaust system sends pulses backwards.
Roelof , The crank is not going to flex it is Hardened steel. It is supported by one huge bearing and a smaller one meaning that the area supported by those bearings will not move unless the bearings move. So you are left with a very short amount of the crank sticking out past that point and connecting to the rod. Hardened steel will break before it flex's thats why they harden them! Only way the piston will hit the head is if the rod is either stretched or the bushing is worn out. Not sure which engine is being used but a billet 6061 or 7075 piston more than likely will not break but can deform slightly under stress but a hypereutectic piston will break and snap off as they have more silicon in them giving lighter weight and better friction coefficiency . more food for thought!
How about a pic?