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Old 07-18-2003, 11:13 AM
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vortec heads, quench height revisited

hey all,

i just had a rather frustrating conversation with the guys at JE pistons, who basically wouldn't tell me anything i didn't already know except that i'm apparently an idiot.

I have a new set of L31 heads sitting in my garage. I want to install them on my 350 Small Block.

I tore off the old heads (cast iron 882's). I have a set of speed pro 345 pistons. I'm measuring a piston to deck height of .048".
My composition head gasket is .028".
That gives me a quench height of .076"
Now everyone keeps saying "don't increase your quench height, or you'll face detonation problems."
well detonation isn't the product of only this one measurement. other factors like air/fuel mixture and ignition timing have to be considered.

So my question is this: even with my quench height of .076", if i mess with the timing and fuel mixture enough, is there any way i can get this engine to simply run, aside from whether or not i'm getting the most possible performance out of it, without destroying my motor? i'm willing to tune it down to avoid detonation if it means i can avoid stripping my block down to the casting and decking it, getting new pistons, etc.

more info: I have a cam with adv. duration of 292/292 and .480" gross lift. This should bleed off some compression.

I just want this engine to run. I'm not planning on racing it, just driving it. please. give me some words of encouragement, i could use it.

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Old 07-18-2003, 12:19 PM
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I don`t know who told you this, but Quench helps combustion, it helps a engine to not detonate, and .076 Quench is too much, it needs to be closer, Smokey Yunick said "When it comes to Quench, .040 is about the safe limit, but anything over .060 and you`ll loose power" the SP345 pistons are "sucker" pistons as I call them as they have .020 cut off from them with a compression height of 1.540, there the reason why your pistons are so far down in the bore, I have these same pistons in a 350 with 64cc heads and my compression is 150 PSI with a small 210 duration cam. here is a link to lead you to some good pistons that should suit your needs, they have a D shape dish so the Quench area is still active, plus they are the correct compression height of 1.560. and they`ll help keep compression in control, you can get them here: http://www.northernautoparts.com/Pro...ctModelId=4621
Remember when you go with a tight Quench area, to check the pistons side wall clearance, as too tight a Quench area and the piston might kiss the head. good luck.
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Old 07-19-2003, 10:04 AM
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That's the sorry pistons I've mentioned in the past. Another source of good pistons & a chart of dish & head sizes vs. compression ratio is here: http://www.kb-silvolite.com/page25.htm Their catalog is free and has a lot of good information about engine building. I recommend it highly. As far as running what you've got, you might try the Felpro shim head gasket, which is only 0.015" thick. Be sure your head & block surfaces are almost perfectly flat, though. Shim gaskets are not too forgiving of uneven surfaces. I've heard of even new heads being uneven. This might explain the stories of Vortec heads cracking. With that big of a cam, you might be ok. Crane recommends 10:1 or over with their 286* SNS cam. You're probably in the low to mid 9's now, so you probably won't destroy anything, but your bottom end might be a little soggy & your fuel economy won't be what it could be.

Last edited by jimfulco; 07-19-2003 at 10:42 AM.
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Old 07-19-2003, 02:07 PM
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you recommended the H126 HP Sealed Power pistons... so let me try to clear up some confusion on my part, because there isn't much info about the piston on that website.

This is a flat top piston, with the dish on the valve side, correct? giving me the flat surface I need on the quench face. what's the volume of the relief? I have some concerns about compression. I couldnt find any info about these pistons online.

So they have the proper 1.56" compression height... they're hypereutectic... this is sounding good.

I was looking at a set of Keith Black KB106's or KB120's which have a +7cc one sided valve relief and should produce 10.2:1 compression with 64cc heads (and some of that will bleed off due to the large duration of my cam)
These retail for only $173 a set, as opposed to the $230 for those Federal Moguls you mentioned.

opinions?
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Old 07-19-2003, 04:09 PM
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I am unsure of the cc size of the cup in the piston, I haven` found any info on it either, it seems no one carry`s these pistons. I`m sure the KB`s will suit you fine as long as you take the time and pay careful attention to the ring gaps. A flat top piston would likely serve you well with the cam you`ve chose, when I did your post I was in a hurry and didn`t pay a lot of attention to your cam size.
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Old 07-20-2003, 12:22 AM
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Re: vortec heads, quench height revisited

Quote:
Originally posted by blndweasel
hey all,

i just had a rather frustrating conversation with the guys at JE pistons, who basically wouldn't tell me anything i didn't already know except that i'm apparently an idiot.

I have a new set of L31 heads sitting in my garage. I want to install them on my 350 Small Block.

I tore off the old heads (cast iron 882's). I have a set of speed pro 345 pistons. I'm measuring a piston to deck height of .048".
My composition head gasket is .028".
That gives me a quench height of .076"
Now everyone keeps saying "don't increase your quench height, or you'll face detonation problems."
well detonation isn't the product of only this one measurement. other factors like air/fuel mixture and ignition timing have to be considered.

So my question is this: even with my quench height of .076", if i mess with the timing and fuel mixture enough, is there any way i can get this engine to simply run, aside from whether or not i'm getting the most possible performance out of it, without destroying my motor? i'm willing to tune it down to avoid detonation if it means i can avoid stripping my block down to the casting and decking it, getting new pistons, etc.

more info: I have a cam with adv. duration of 292/292 and .480" gross lift. This should bleed off some compression.

I just want this engine to run. I'm not planning on racing it, just driving it. please. give me some words of encouragement, i could use it.
You could always mill a little off the heads, (Up to .20). You can mill .30 or more if you want, but you will have to mill the intake to match, and you will need to check your pushrod length and rocker geometry.
I'm running 10.5:1 on 91 octane gas with a 230 duration @50 cam and having no problems. I think my quench height is about the same as yours.
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Old 07-20-2003, 06:45 AM
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Maybe I am confused, but I dont believe milling the heads will change quench aside from adding a smidge more because of reduction in combustion chamber width(because chambers taper). It would not be enough to make any difference. The only real factor would be slight increase in compression.

Chris
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