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  #1  
Old 03-18-2005, 01:17 AM
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Default 318/5.2 Magnum deck clearance (quench height calculations)

I've now read a couple stating theie pistons were .050" in the hole. With a .047" gasket (supposed OEM size), that's a .097" quench height (which would explain why these engines have terrible economy and knock so blasted much). But, I have some contradicting info...

With block deck height: 9.058-9.060, stroke: 3.31, rod length: 6.123, and piston compression height: 1.81, I come up with -.006-.008 deck clearance. The piston tops would be .006-.008" out of the hole @ TDC. With a .047 gasket, that would leave .039-.041" deck clearance, which is about where it should be. What am I missing here? We know the block deck height, rod length, and stroke are all correct, but I'm not 100% on OEM piston compression height. I'd very much appreciate some light on the subject.

Last edited by Username: : 03-18-2005 at 01:19 AM.
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  #2  
Old 03-18-2005, 02:23 AM
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hello, the mopar race book lists 273-360 blocks with deck hgt of 9.60.
the 318 piston C.D. is 1.74. 1/2 of stroke +6.123 +1.74= 9.5195. this gives you a -.080 down in the hole. just food for thought.
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  #3  
Old 03-18-2005, 03:16 AM
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it is my take on this that IF everything was as spec'ed, piston height would be at (or very close to) deck height when the piston was at TDC... BUT... while the holes are pretty close to where they should be, and the pistons, rods, and crank are where THEY should be, the block deck is not.

all blocks are cast oversize, so to speak, so there is plenty of material for machining. when the actual machining was done to the blocks, there was just enough material removed to get a good, reasonable true sealing surface, except where tolerances were critical. this would mean that when the deck was machined, enough material was removed to get a good surface in something like 99.7% of the blocks... and no more was removed. my figures may be off... the actual numbers may be higher or lower... but not by much.

this would leave a lot of metal to yet be removed (in a "perfect world"), but we dont live there. in actual life, the cost of labor and tooling is pretty high, and time is literally "of the essence", so when they figured out the bare minimum that HAD to be removed from the MAXIMUM number of blocks to meet the "quota", they stopped machining. in reality, the 'block decker' (if that is what you want to call the machinist that milled the deck) could probably finish 3 blocks to the 'minimum specs' numbers, and do it in one or 2 cuts in the same amount of time it would take to mill ONE block to 9.058-9.060.

and things get/got worse... the late model 440's had the pistons as much as .200" down in the holes! i have paid a little more than "passing attention" to this, because i have one of those "low compression 440's", and i was thinking about doing something about that. i even went so far as to inquire at local machine shops about milling costs. partly because of funding, but mostly because of indecision, i have put this idea on hold for a while.

just because the compression is low isnt a bad thing. it could be a very GOOD thing... if forced induction is in the engine's future. you DONT want a high compression engine for turbo or supercharging. a 7.5:1 CR is an IDEAL engine to just 'bolt on' 6-8 psi forced induction system, or god forbid... nitrous... with little or no modifications. at that boost level, an intercooler is not even needed.

if increased compression is what you want, though, you can avoid a LOT of machining (and related costs) by carefully shopping around for pistons. if you know what the minimum distance is from the deck to the top of the piston, you can often find the correct compression height pistons from any number of manufacturers, and select a set that will give you nearly 'zero deck height' or even more. and the .039" thick felpro gaskets arent your only choice, either. both mopar and mr gasket make steel shim gaskets that compress to .019-.020". the pistons should be able to be had for around $400 or so, but you may have to do some looking to find them. or custom pistons can be had for around $150 a set more.

the drawback to milling heads to improve compression is that if for any reason a head should need to be changed, then you have to machine the replacement to the same specs as the head being replaced, and that isnt always easy. then if you upgrade to edelbrock or other aftermarket heads, you have to do it all over again. if you cut the block, you still have to machine the intake surfaces of the heads (or the intake manifold) so the intake will fit. milling the intake isnt much of an option, xince it will then be "engine specific". and again, if a head (or heads) are replaced, the intake will no longer fit ANYTHING, and becomes a very poor boat anchor.

and fwiw, quench and squish are pretty much usless with piston-to-head clearances of more than about .050"... there just isnt any effect. i guess that it is there... technically... but it does absolutely no good. kinda like mammary glands on a boar hog...
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  #4  
Old 03-18-2005, 11:47 AM
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Quote:
Originally Posted by perfmachst
hello, the mopar race book lists 273-360 blocks with deck hgt of 9.60.
the 318 piston C.D. is 1.74. 1/2 of stroke +6.123 +1.74= 9.5195. this gives you a -.080 down in the hole. just food for thought.
So we were way off with compression height. I had a feeling that's what it was. I assume the stuff from the Mopar race book is LA engine material. Would you happen to know if the Mangum pistons are any different?

.0605-.0805 is almost hard to believe. I wonder what the heck the engineers were thinking. That leaves a ridiculous QH with an OEM gasket (.1075-.1275 ). I guess we'll be ordering some .005" tin foil lol...

Last edited by Username: : 03-18-2005 at 12:28 PM.
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  #5  
Old 03-18-2005, 12:20 PM
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creative1:

We're no first timers with engines, just Mopar engines. Mopar keeps everything so damn secrative. We're having to DIG for certain engine specs. I coun't find a thing on compression height of factory pistons (specificly Magnum, if different), or OEM gaskets. I only know of one 'independant' shop that's actually taken measurements of the factory stuff before throwing in after-market. Even there, I have nothing to compare them to, so who knows how accurate they are (such as the 1.81" CH supposedly measured).

I'm one of those GM F'ers. My recent LS1 build showed an excellent performance gain with bringing the factory QH of .045 down to .024. The additional compression, from a detonation standpoint, seemed almost neglegable because of the QH. Resistance to knock went way up. We ended up milling the heads .030 as well to bring the STR to 11.2:1 with the factory cam and 28* total timing, with ZERO K/R at even a 192* coolant temp. Though I wouldn't go any tighter than .026 with an iron block running to 6k, aluminum blocks grow, so we easily got away with the .024 (@ room temp) QH.


EDIT: I forgot to add...This isn't a 'build-up'. I just discovered a blown head gasket (though a very small leak), and have to order new gaskets. I need gaskets here before pulling the heads. Being a dd hauler, there's no time for pulling the engine to have it measured and decked. I need a close 'guestimation' of the deck height before ordering the gaskets. Cometic can cut .005, .010, .016, .020, .027, and so on, on a 3.940 bore. If I'm working with .0605-.0805" DH, a .005" coated 'shim' will be used.

Last edited by Username: : 03-18-2005 at 12:43 PM.
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  #6  
Old 03-19-2005, 12:17 AM
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I acquired some more independant measurements today. Dan 'The Fastman' said his couple have been .060-.062 in the hole, and the engine builder at KRCPerformance said theirs have averaged low-mid 70's. I guess Chrysler just didn't care much for keeping tolerances. It's shocking they very THAT much. At over .060 QH, I imagine it probably wouldn't make much difference what it was. You'd have no 'squish' with such a gap.

I guess I'll be ordering some .005" tin foil and hope for the best lol...

Last edited by Username: : 03-19-2005 at 12:18 AM.
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  #7  
Old 03-19-2005, 01:48 AM
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Quote:
Originally Posted by Username:
I guess Chrysler just didn't care much for keeping tolerances.
Now listen here chevyboy! i find it hard to believ that you can say that being a cheap ass chevy guy! for one, the reason why they are so far down is for long term use, ie, machining!
one of the things you will notice on our junk dodges,
look at the crank, not only is it better than your chev, but its more up in the block, for strenght
look at the cam oiling they dont go flat all th etime like you chevs cuz dodge "thuoght" when the built their engines.
look at the rods, they are long, guess what? not only does it mean the engine will last longer, but it will beat you to the end of the strip, out pull you up a hill, do you know why chevs stretch rods?
look at the stupid valve spacing of the heads!?!?!?!? not only are they too close (ineficient), but chev puts the spark plugs in a hole! guess thats for oil burners :flip:
well, lets see, lets put the same ports on all small blocks! yeah, thats cheap! an dit makes the small motors sooo torkey and efficient!
not to mention i have seen so many of your quality chevs with the crank, or the cam not in the center of the block! jeez! new timing chain? wont be the same on many of em, some too tight, some too loose, ever wonder why?
i dont think they ever made a block that shifted more than a chev!
no cam plate
oh, the adjustable pushrods? well, the only reason they are there is because they could never live without them, cause chevs "quality" was too poor to put the valve gear in the same spot. true!
oh, yeah, dodges always have the rockers coming off, right?
the reason why people build chevs? cheap
the way they are built? cheap
parts? cheap
cheap, cheap, cheap!
wanna talk trucks? how bout that steering box that the frame breaks around? how bout that.............................................. ...........
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  #8  
Old 03-19-2005, 02:11 AM
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Quote:
Originally Posted by dodgetkboy78
Now listen here chevyboy! i find it hard to believ that you can say that being a cheap ass chevy guy! for one, the reason why they are so far down is for long term use, ie, machining!
one of the things you will notice on our junk dodges,
look at the crank, not only is it better than your chev, but its more up in the block, for strenght
look at the cam oiling they dont go flat all th etime like you chevs cuz dodge "thuoght" when the built their engines.
look at the rods, they are long, guess what? not only does it mean the engine will last longer, but it will beat you to the end of the strip, out pull you up a hill, do you know why chevs stretch rods?
look at the stupid valve spacing of the heads!?!?!?!? not only are they too close (ineficient), but chev puts the spark plugs in a hole! guess thats for oil burners :flip:
well, lets see, lets put the same ports on all small blocks! yeah, thats cheap! an dit makes the small motors sooo torkey and efficient!
not to mention i have seen so many of your quality chevs with the crank, or the cam not in the center of the block! jeez! new timing chain? wont be the same on many of em, some too tight, some too loose, ever wonder why?
i dont think they ever made a block that shifted more than a chev!
no cam plate
oh, the adjustable pushrods? well, the only reason they are there is because they could never live without them, cause chevs "quality" was too poor to put the valve gear in the same spot. true!
oh, yeah, dodges always have the rockers coming off, right?
the reason why people build chevs? cheap
the way they are built? cheap
parts? cheap
cheap, cheap, cheap!
wanna talk trucks? how bout that steering box that the frame breaks around? how bout that.............................................. ...........
And you're 26yo? That has got to be the most childish post as well as the most ridiculous argument I've ever seen in a forum.
Where the hell do you come off insulting someone over something so ridiculous?
Congrats on the time you've wasted to contribute such immaturity and nonsense to the thousands that view this site. Bravo...

Last edited by Username: : 03-19-2005 at 02:12 AM.
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  #9  
Old 03-19-2005, 03:14 AM
TK TK is offline
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I dont see what age has to do with it, but ok, and, i wasnt insulting you. see, heres the funny part, i like chevro's, yes, i do, now, my point was this. all day today, i had this chev guy picking on my piece of shit race truck, dodge this, dodge that, then, i get on here, and a chevy guy goes from asking advice, to picking on my brand, its frustrating. yes, we all know, i have said on here bout those dumb magnum intake ports. and i hate those annoying starters, but please dont pick on the things hat chrysler has going for it. keep a open mind, and you might be turned into a dodge guy.
sorry it sounded like i was aiming it at you, it was not intentionally meant to, just chevy venting, to a chevy guy. now, are those arguments really nonsense?
the good news? i can come up with a list twice as long for chrysler, he he!
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Old 03-19-2005, 04:41 AM
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I don't ever recall stating 'I hate Mopar.' You just assumed so after I mentioned the 'G' word (GM) and attacked. This is a Chrysler/Mopar forum, setup to discuss Chrysler/Mopar designs, info., and flaws, which is what I'm here to do. If I disliked Chrysler/Mopar for whatever reason, I wouldn't own a Chrysler/Mopar vehicle, and wouldn't be on this forum asking technical Chrysler/Mopar questions. I'd be digging up specs on a Porsche or LS1 forum.

I just made a light, well backed-up comment in regards to the subject of this thread. I have no problem with the 'brand' Chrysler/Mopar/Dodge. I'm a mechanical engineer, and a perfectionist on top of it, so maybe such a LARGE variation in such an important part of the engine gets to me more than the average Joe.

Food for thought, .030 difference between banks of a 5.2 is a .572:1 static compression difference. Factory injector flow varies up to 10% (+/- 5% out of spec), who's to say you don't have one of those lesser flowing injectors feeding a cylinder in the higher compression bank? PING The belly pan gasket could be one reason, PING bad gas could do it as well. PING But lets be honest here...

Last edited by Username: : 03-19-2005 at 04:45 AM.
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  #11  
Old 03-19-2005, 01:33 PM
TK TK is offline
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yeah, well, i have to admit, you just made sense on the pinging issue. so, what can you do? no ones ever brought up this in here that ive seen. my buds doing a magnum 360, it has a .050, he's gonna mill it .030. but, they are all the same, (within.010) theres not a difference of .030 that sounds crazy, maybe not, but is there a possibility that this engines your workin ons been abused? stretched rods? who knows
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Old 03-19-2005, 08:05 PM
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The block would have to be decked. Not just one bank to even out the deck clearance of the two banks, but both, to lower the deck clearance and quench height. Lets use the 5.2 Magnum as an example because I now have all its specs...

If the 5.2 Magnum was at its 'blueprint' spec, we'd have .047 deck clearance. If you add a .047 thick OEM gasket, that leaves you with a .094" QH (hot days, bad gas, lean A/F mix, vacuum leak (belly pan)...PING). To get the 'squish' effect of a tighter QH, you'd have to mill the 9.58" deck hight of the block down at least to 9.533", which would leave .000" deck clearance, and .047" QH with an OEM gasket. Using a .026" thick MP Performance gasket would mean keeping the RPMs under, say, 6K for rod stretch reasons, but that's an excellent quench height. Because of the additional fuel atomization, the 'swirl' in the chambers, having the majority of the charge in the actual chambers, and the additional cooling of the exhaust valves, plug tips, gaskets (if copper), and any other hot spots in the chambers and piston crowns, performance, fuel economy, and resistance to knock would be MUCH improved.

The only problem in doing so would be a much higher compression. Though cutting valve reliefs into the pistons, and deburing and polishing the head chambers (usually the case with a performance build anyway) would bring it back down to a pump gas freindly level.

Now, there's another way of dropping the deck clearance and quench height...Taller after-market pistons. The drawback to doing so though is a heavier piston, with the additional weight being at the top of it (the very end of the rotating assembly), which is the last place you want to add weight. The positive to this method would be compression choices with the many crown designs available. The block would still need to be decked to ensure both banks are of the same height, and 'square', so cylinders 1 and 2 have the same compression and QH of their apposing ends/cylinders, 7 and 8.

If you're EXTRA interested in combustion chamber efficiency, grab some popcorn and a coke, and have a gander here:

http://www.theoldone.com/articles/The_Soft_Head_1999/

It's probably the most detailed write-up I've seen on combustion chamber and piston crown design.

Last edited by Username: : 03-19-2005 at 08:11 PM.
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