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Change Pistons -- Must rebalance? |
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amx39068
Supporter of TheAMCForum Joined: Feb/21/2008 Location: Arizona Status: Offline Points: 11576 |
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I build all my engine using more of a flat top design on the piston and as close to zero deck as possible except on the rare occasion that the original pistons are still usable. I almost always also get the rotating assembly balanced again except on those rare occasions when I can reuse all the original parts. The gentleman stated he is going to just add a cam and different heads so no need to balance anything.
There is an alternative that may be easier still and that is to use a set of Harland Sharp bridged roller rockers if what you described is all you are going to do to the engine. They will handle anything you want to throw at them with your combination and are adjustable to accomodate the new cam base circle difference. The big benefit is you won't have to remove the heads rather you can just bolt them on, and get new pushrods that are the correct length for the new cam and rocker combination and save youselft from having to remove the heads altogether.
The rockers are the ones called pedestal about halfway down the page at http://www.harlandsharp.com/products.htm and are priced at $488 from their online store
Edited by amx39068 - Apr/15/2012 at 10:11pm |
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19611 |
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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.
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Frank Swygert
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SKeown
AMC Addicted Joined: Jul/30/2009 Location: Texas Status: Offline Points: 3085 |
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The benefit of a quench pad on an otherwise dished piston is the cloose proximity of the pad to the head forces air across the chamber. This serves to cool the combustion chamber, and the clooser the piston comes to the head the better. Yes, some have attempted to eliminate detonation "ping" by using thicker or double head gaskets only to make it worse.
Engine ballance: People get rediculous about balancing engines. Would I use a flywheel or damper that was way off, no. Would I rebalance a rotating assembly due to replacement pistion's weight differing from those being replaced, no. If the new pistons are considerably heavier I might look for a way to reduce that some. Otherwise I would just not worry about it. Some people overbalance, some underbalance, besides no additional power is gained no matter what.
SKeown
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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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 ->
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. ....
Again, mixing issues......... |
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amx39068
Supporter of TheAMCForum Joined: Feb/21/2008 Location: Arizona Status: Offline Points: 11576 |
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Steve, The only caveat would be that an out of balance engine will not rev as high as a balanced engine so on some setups, you may not be able to get to the max RPM required to reach max HP but otherwise a mostly stocker engine I would tend to agree. The place where it gets tricky is if the new pistons are lighter than the old. Its not all that hard to take weight out of them but quite a bit harder to add weight to them! |
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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SKeown
AMC Addicted Joined: Jul/30/2009 Location: Texas Status: Offline Points: 3085 |
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Lighter would be no problem at all, as a matter of fact, that would be a blessing. Were talking recipricating not rotating, besides how does one factor in friction. Like I've said, people get crazy about this subject. I saw a Pontiac engine that was as smooth as silk, but had one wristpin that was 21 grams heavier than the others. An engine's ability to rev is not attributed to precise balancing one way or the other. Yes, it's good to get the bob weights equal and balance the crank/ flywheel & damper to that, but it's only going to be exact at a specific RPM range anyway. Lighter reciprocating weight is conducive to quicker reving and sustained engine life.
SKeown
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