Change Pistons -- Must rebalance?

 

farna wrote: I believe the reason is the edges of the dish in the piston and the outer edges aren't exposed to initial combustion and don't heat up (and stayed hot) as much as with the piston further down in the cylinder. Once those edges get hot enough to ignite the mixture as it comes in you get pinging (detonation). Hot edges would explain why the top edges of the piston disintegrate with detonation too -- they are hotter and therefore softer, and the detonation literally knocks pieces of the weakened (softened) metal off.

Not even close.......... it's not hot or cool edges.

Search for say 3 years ago some things I posted here on quench.


>> Dish the pistons to bring static compression back to stock and it won't ping on regular any more -- or install a cam that bleeds a little cylinder pressure. At any rate -- lower dynamic compression and ping goes away. <<
You are mixing things here a bit - you changed the SHAPE of the chamber by dishing.
I fully understand the issues..... I took classes in engine design.  I've posted at least a dozen - maybe more times - the FACTS on detonation, quench, chamber design, etc.
Sorry, "strokers" aren't engineers or designers, they put things together until it works. I don't take my information from them. No offense to Jeep folks running a stroked engine.............

the total miss on how and why of quench and ping shows that you really shouldn't be advising on quench distances or design.

Not right, not even close ->

Hot edges would explain why the top edges of the piston disintegrate with detonation too -- they are hotter and therefore softer,

 sorry - that's not how it works. Detonation blows away the barrier layer of unburned gasses, that layer keeps the piston head cool. Otherwise actual combustion temperatures under normal conditions exceed the melting point of aluminum.

I also know all too well the effects of static vs. dynamic compression.... ...my Javelin suffers lowering static and yet raising dynamic with cam.

Remember - I just built a 4.0 - and had .020 shaved off the head.  I run "cheap gas" in it - and AVERAGE just a bit under 22 mpg commuting to and from work through town.

....

......8.7:1 compression, no ping on regular gas. Increase compression and it will ping on regular and even mid grade under a load with the stock 4.0L cam.

Again, mixing issues.........

Bill, look at a stock 4.0L. Piston is 0.060-0.080" down in the hole with the head gasket. 8.7:1 compression, no ping on regular gas. Increase compression and it will ping on regular and even mid grade under a load with the stock 4.0L cam. The stroker guys have been over this a lot! It's not static compression that's the concern, it's dynamic (or "running") compression. A cam with a little more overlap than stock bleeds a little cylinder pressure and will work fine with a bit more compression. IIRC if you put a 258 crank and rods (or use the 4.0L rods with special pistons) static compression is boosted to about 9.5:1. The stock cam will ping at that compression. Dish the pistons to bring static compression back to stock and it won't ping on regular any more -- or install a cam that bleeds a little cylinder pressure. At any rate -- lower dynamic compression and ping goes away. With quench at 0.040" or under (might be able to go closer to 0.050") you can run higher compression and not ping. I believe the reason is the edges of the dish in the piston and the outer edges aren't exposed to initial combustion and don't heat up (and stayed hot) as much as with the piston further down in the cylinder. Once those edges get hot enough to ignite the mixture as it comes in you get pinging (detonation). Hot edges would explain why the top edges of the piston disintegrate with detonation too -- they are hotter and therefore softer, and the detonation literally knocks pieces of the weakened (softened) metal off.

farna wrote: You don't have to worry so much about the quench if compression is down. 9:1 or more you probably need to get the quench at 0.040" or less -- zero deck height and stock head gaskets(most stock type head gaskets compress to the 0.040"-0.042" range). You're right on the verge of having to have quench correct at 9.5:1, but you CAN get by with running up to 0.050" or so down from the deck, just have to retard timing a little. That means you're giving up a little power, but shouldn't be that noticeable. With the quench right on you can run a bit more compression without retarding timing. The important thing is to not have detonation, whether you do it by having a good quench dimension or have the piston further down in the hole and reduce compression to under 9:1 is really beside the point.

What wipes out the lobes is valve spring pressure, not the cam itself. Stock valve springs shouldn't wipe lobes after cam break-in (additive needed for break-in, and I use it for the next oil change just to make sure, but not after that). You can go a little over stock pressure, but not much, without having to use an additive. A high rpm engine that needs high pressure springs must use an off-road/racing oil or an additive.