Well it’s known that our J-Series motors come with tight piston ring gaps from the factory. Usually this will be one of the first limitation you reach once you start edging up towards 600-700whp (along with a couple others, depending what motor you go with). As the motor heats up so does the piston and the piston rings. As that happens the Piston rings goes under thermal expansion which ultimately leads to the two ends butting together (as some call “pinching”), which then has nowhere to go and pushes up into the piston ring land (where the piston rings sit..or “grove”) and ultimately breaks the stock cast piston. There have been some (actually only one instance) we have “heard of” with someone running stock pistons and they “alleged” stock ring gap past 800whp…and they claim to 1400whp!! LMAO..Never heard or seen of anyone else on stock pistons past 700-800whp. The only way we could see this happening is if they ran a Meth. based fuel, which has a super cooling effect when injected, thus the pistons not heating up as much and the rings not pinching. Not saying it can’t be done, but personally I wouldn’t put my faith in stock cast pistons much past 700whp for my expensive forged bottom J-Motor, why cheap out and risk it?.
With some research you will see there is case, after case, after case of stock pistons breaking due to piston ring pinching/butting…..and our J32 experienced the same effects around 600whp on stock piston ring gaps (Safe AFR, Safe timing etc). This even more holds true for Boosted applications, as when you compress air it heats up, only compounding the issue. In our situation to make things worse is we choose to run motors with higher compression, with our Turbo motor running 11.5:1, which only adds to additional cylinder temps with the extra compression of that forced induction air. There’s many ways to set up a motor and we prefer the higher side of compression, in doing so the optimal route would have been to gap those piston rings, but that would negate the ultimate reason why we were running a COMPLETELY STOCK J32a3, to find the limits. And if you don’t already know, J32a3’s already running 11:1 comp so it wasn’t a stretch for someone to boost at our CR. However, that extra CR leading to higher cylinder temps, thus heating the piston rings more then say a 10:1 motor, could have lead our motor to failure faster then a lower CR motor….non the less all J motor completely stock will start finding limitation at 6-700whp.
You’ll see from the sample picture in this thread that the piston did not suffer from any type of detonation and the only damage was strictly located in the ring land area, exactly where you can see the piston rings are bent (from butting) and pushed up into the ring land, thus cracking them and causing piston failure…which is only caused by piston ring gap (not the tune, not a bad motor, but stock piston ring gap at 600whp). This motor was salvaged, a new piston installed, left the block completely stock bottom end and all piston ring gaps were gaped appropriately. This motor currently has 8k HARD MILES on it, with MANY MANY WOT pulls (easily 80+ multiple gear pulls and races) while also being a daily driver. This goes to show the J-Series resiliency and dependability, even around 560-600whp (which we feel is the safe limit for stock internals on these motor AFTER ring gap is fixed. With stock ring gap 500whp is what we would run max).
For that reason, if if swapping in a stock motor, it’s best to pull the pistons and gap them before install. This will insure a safe running boosted/nitrous J-series to around 600whp.
The general rule of thumb is this….
– Stock to 350-400whp = Safe to leave them alone
– 450whp-700whp =
Top Ring = Bore Diameter (Inches) X .007″
Second Ring = Bore Dia. (Inches) X .008
750WHP-1000whp (General Rule, Seek guidance from engine builder)
Top Ring = Bore Diameter X .008-.009
Second Ring = Bore Diameter X .009-.010