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Old 10-27-2002, 10:32 AM
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Post "Sucker" Pistons

Just a post to my fellow gear heads to beware of what I call "sucker" pistons, Was years ago I ordered a rebuild kit, was just a basic rebuild with a few upgrades for a 350 chevy, no big deal. It came with Silvolite cast pistons, when I assembled the engine I didn`t pay attention to where the piston was on TDC, but then noticed when I had to pull the heads for R&R, the compression height of these pistons was 1.555 instead of the stock height of 1.560, and it didn`t state this anywhere in the kit or the order, all it said was "flat top cast pistons" yes, there flat tops, but with 5 thousanths cut off them, they may as well be dish top. this hurts performance in many ways, this opens the Quench way up, which hurts combustion effientcy, and at least a half a point or more loss in compression ratio. so giving a tip, before you order, know what your getting. don`t get took by "sucker" pistons like I did.
DV

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Old 10-27-2002, 11:24 AM
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You sure that's not because they had a different rate of expansion?
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Old 10-27-2002, 12:47 PM
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You know an awful lot about quench, etc to not know why these pistons are like this.
These are for blocks that have been rebuilt a few times or have bad deck surfaces that need to be milled down so far a standard piston would stick out of the bore.
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Old 10-27-2002, 02:25 PM
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The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .040" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness of the water- cooled head.
Dish (reverse combustion chamber) pistons are designed for maximum quench area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel.
Opening up the Quench area gives a better flame travel not worse. You can get a 12.5:1 compression with dished pistons.The flame don't have to travel over the top of a piston so you get a faster burn/flash during combustion,and less pre-ingintion.
George <img src="graemlins/sweat.gif" border="0" alt="[sweat]" />

[ October 27, 2002: Message edited by: 1BAD80 ]</p>
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Old 10-27-2002, 05:34 PM
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hey you guys if you read the fine print from the rebuild piston makers ALL OF THEM they state that if this piston is a replacement piston it will be up to .020 shorter compression height than the factory piston. this is to allow for excessive block decking and head milling that the piston manufacturer can't controll.!!! read the book. most offer flat tops in both compression heights but you have to ask for them. find a good parts man.
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Old 10-27-2002, 06:10 PM
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I was stateing I was not told this, or given reason why, and there is no reason why on there web site, so in the end i`ll say this, since you claim that I know so much about Quench, you tell me why Tim Moore builds all his engines with the closest possible Quench he can? when you look at the power his engines make then you can tell him it`s wrong.
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Old 10-27-2002, 06:54 PM
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hey you guys the oem's and the aftermkt piston manufacturers have spent uncountable hours and probably millions of dollars over the last forty or so years to come up with .040 as optimal quench on a normally asperated gasoline engine. makes good horsepower isn't prone to carbon problems and tollerant to bad gas.
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Old 10-27-2002, 11:14 PM
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How could you possibly tell 0.005" shorter piston heights was costing you hundreds of horsepower. I doubt you would even be able to tell. Heck the production tolerances are probably +-0.003" on the height?
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Old 10-28-2002, 03:16 AM
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hey 4-jaw i've seen tolerance variations of .050 on small block fords and chevys both, i had one 400 small block that had .022 from the center block between the cylinder to the rear edge at an angle, needless to say the owner always had head gasket problems before we decked it.
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Old 10-28-2002, 08:29 AM
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"It is hard to recommend a desired Quench clearence that will be correct for all engines. But I think 0.038 inch of clearence between the quench surface is a absolute minimum in an engine with a 4 inch bore and about 0.007 inch of piston to wall cleanrence, or if the piston to wall clearence is greater, say 0.009 to 0.010 inch, this may not be enough, a safe figure for all around performance is 0.040 inch. anything up to about 0.045 is prabably okay, and i wouldn`t worry about a little extra as long as i was close to these recommendations. But remember, If you let the Quench open up too much, to 0.060 to 0.065 inch or more, your going to loose power. this much opening at the quench will allow excessive amounts of the intake charge to remain in the Quench area as the piston reaches TDC, and especially if the piston has a high dome to obstruct the spread of the flame front, these gases may not be properly combusted during the ignition phase, this Significantly reduces Combustion efficientcy." Taken from "Power Secrets" by Smokey Yunick
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Old 10-28-2002, 02:32 PM
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This is pretty typical of most off the shelf smallblock stuff.I have yet to see a 350 that hasnt been decked go together with less then .020" piston t deck clearance.Add that to a .038" head gasket and you will see that quench is really closer to about .060" on most engines that havent had the block decked.Now here is a scary one.I pulled down a 454 crate motor that had deck height variation of .070" from one end of the deck to the other on one bank.the entire deck was tilted both toward the front and the outside of the block,so the front outboard corner was .070" lower then the rear innner corner.This thing had a light knock that nobody could pinpoint,but when the small bit of the piston that is flat on the chamber side of the piston was shiny from whacking the head,I knew what was wrong.So now you know why piston manufacturers arent in any hurry to make pistons any taller then they have to.
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Old 10-28-2002, 02:38 PM
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AMEN SUPER STREETER; good ol mass production.
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Old 10-28-2002, 03:40 PM
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[quote]Originally posted by DoubleVision:
<strong>I was stateing I was not told this, or given reason why, and there is no reason why on there web site, so in the end i`ll say this, since you claim that I know so much about Quench, you tell me why Tim Moore builds all his engines with the closest possible Quench he can? when you look at the power his engines make then you can tell him it`s wrong.</strong><hr></blockquote>

I don't know if you're refering to my reply but I meant that as a compliment, not many people know the importance of proper quench distance. It seems you know what you are doing and figured you should know about pistons with a lower compression height.
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Old 10-28-2002, 04:04 PM
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In my other reply I just wanted to tell about the quench area because you mentioned about it.It could have been the cast piston had a core shift,who knows, but KB pistons are usually good.
At least this topic recevied alot of
attention.
Hope a few newbes learned about quench ,
It started me thinking again.
Have a good day. Thanks for the warning about kits
George

[ October 28, 2002: Message edited by: 1BAD80 ]</p>
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Old 10-28-2002, 07:26 PM
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Yes Jason, i was referring to you, My apologies for taking what you wrote wrong, I guess I should try and open my mind more to what things might mean. Upon doing a compession check, and finding i had 150 psi compression with 64 cc chamber heads, i was much confused on why it was this low, when i removed the heads i found out. my block has never been decked, had i knew the pistons was gonna be this way, i would have had it decked. again, My apologies Jason, thanks to you, i`ll be viewing the post with a different point of view. Thanks again.
DV
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