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#1
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compression depression
hi.I just pulled the heads off the 440 in my 73 charger. with the idea of putting edelbrock aluminium heads on.To my disgust the .030 MP "9:1"pistons are .144 down the bore!!!! I new my compresson wasnt high but 8:1?? So does anyone have suggestions? mill the block? Heads? both? Id rather not go to new pistons..She now has 452 heads and I have a pair of 915 heads with 1.6 exaust valves. ps .030 over for figuring the bore is it 4.350 or 4.380.......my head hurts!!!
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#2
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Ha.... I had a similar problem but went the other way....
I wanted 13.8:1 Somehow I ended up with 15.28:1 I would just mill the heads, and get a new valve job done at the same time, clean anything up in there you can and be done with it. You will need to mill them a ton though..... My 452's look closer to 915s than anything else...lol there has been at least 60 off of them. |
#3
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I wonder why mopar called them 9:1, when real world there not even 8:1. The only way is to mill the block, remember if you mill it to much for thoses pistons they will be the only ones that will work. You could find a set of early closed chambered heads like the 516's, 915's and do some more to them. Mill them .040 and steel shim head gaskets, with your current combo your looking at over 9:1 there. The aluminum heads will only hurt the compression. You can mill them way more than the cast irons also. Get the closed chambered heads and mill them .080 of a inch. There should be matieral there. You will get a good quench then, but who likes to mill brand new heads? Not me. Maybe for a race engine.
__________________
68 Coronet 69 Super Bee......new 500 cid comin soon! 73 Duster witha missing 440/727 |
#4
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Excessive milling of the heads/block are not a good idea on stock parts. The decks will get thin, which can lead to cracking and warping. I personally would go with new pistons, it would be safer all the way around.
My $.02, drag-n |
#5
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I do have 915 heads I guess I should price out the $'s involved and compare that to the 1,000 cdn for KB pistons,rings,etc. Does anyone know how many cc's you remove by cutting these heads?? One of the brand X guys suggested a blower....Does anyone make a under hood set up? I'm not cutting the 71 super bee hood I finally tracked down. thanks for the input.
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#6
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cast pistons are not much but forged are best.You remove ,004 th from the head to reduce 1 cc in the chamber .Now when you start to cut big amounts like you need the relationship will change .Not sure of ccs measure first cut 2nd.Youll need to get t o about .050 down the hole with 88 cc heads at +.030 bore with the .028 Mopar gaskets.= 9.65 cr.Perfect on pump gas and iron heads.
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#7
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8:1 CR, good pistons... What are you waiting for...
BOOST!!!!!!!!!!! force feed that thing, and make it a monster.! |
#8
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I concur with drag-n-cuda.....new pistons are the best solution to this problem, both in terms of cost and effectiveness.
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#9
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I put the 0.144" number in my compression ratio spreadsheet, and it came up with 8.13:1 using a 0.020" thick gasket and 84cc head.
Using the same thin gasket you would have to reduce the head volume to about 70cc just to get 9.0:1 compression. If that 0.004" per 1cc is correct (Is this for the open chamber iron heads?) you would need to mill the heads 0.056" (if the heads started at 84cc?) I don't know how much would need to be cut on Edelbrock heads because they are not open chamber heads? |
#10
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It's too much for a head change.
You'll have to go with pistons. I've milled the block, but it's not much cheaper than boring, by the time you machine the intake, to fit and so on. Plus it becomes a pain in the a$$. |
#11
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Pistons.
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#12
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well I've decided. after ccing 3 sets of heads and my bro sending a page from the MP Cassis Manual . The 915 heads I have are 79.5 cc's so if I cut the heads .060 I will be around 9.3:1 with the steel shim head gaskets. I know they won't flow like the eddy's but I will be better off than I was. Thanks to everyone for the input! ( the manual page is posted in the head milling max thread)
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#13
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*cough* HEMI LENGTH CONNECTING RODS WITH CHEVROLET WRIST PINS *cough*
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#14
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Sorry, that won't work either--I typed before I thought. You could however re-use your pistons, upgrade the rods to something in a Hemi length with a 1.094" wrist pin, and come closer to zero deck. I am not going to go to the compression calculator to figure it, but if I remember correctly that is 10ish:1 with big chambered heads.
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