Re: help calculate my comp using 1/2"downfill method
[Re: KOS]
#2291618
04/21/17 03:34 PM
04/21/17 03:34 PM
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Joined: Sep 2014
Posts: 4,457 Washington
madscientist
master
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master
Joined: Sep 2014
Posts: 4,457
Washington
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The method in th Chrysler book is the easiest way to do downfill compression ratio calculation.
It gets a bit tricky when the deck of the piston is positive. If you are using a zero or negative deck, it's pretty straight forward.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: help calculate my comp using 1/2"downfill method
[Re: KOS]
#2291647
04/21/17 04:29 PM
04/21/17 04:29 PM
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Joined: Mar 2017
Posts: 43 California USA
LowDeck451
member
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member
Joined: Mar 2017
Posts: 43
California USA
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well had a buddy run it through his program and came up with 14.6 Hmm...Well, I could be wrong, but I don't see where. Interesting that we are 1 full point apart.
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Re: help calculate my comp using 1/2"downfill method
[Re: LowDeck451]
#2291660
04/21/17 05:09 PM
04/21/17 05:09 PM
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Joined: Aug 2003
Posts: 1,573
KOS
OP
pro stock
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OP
pro stock
Joined: Aug 2003
Posts: 1,573
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Do you know your gasket bore dia.? Also, it looks like your 129cc downfill puts your deck height at 1cc 'positive'. A 4.5" cyl., .5" deep calcs out to 130cc. I subtracted 1cc from chamber volume, took a guess at your gasket bore dia. at 4.6", and the compression calculator said 13.66 to 1. I'm pretty sure this is correct, but it does get confusing at times! Hope this helps. gasket bore we used was 4.560 we tried different bore sizes not much diff.....wow 13.66 thought it woulda been more.thx for the input
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Re: help calculate my comp using 1/2"downfill method
[Re: KOS]
#2292026
04/22/17 11:57 AM
04/22/17 11:57 AM
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Joined: Oct 2007
Posts: 4,205 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,205
New York
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The .500 method isn't magic, as Cab_Burge says whatever depth is needed to cover the dome. .500" is just to make the math easy, you could use .391" down.
Boffin Emeritus
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Re: help calculate my comp using 1/2"downfill method
[Re: KOS]
#2292057
04/22/17 12:58 PM
04/22/17 12:58 PM
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Joined: Jan 2003
Posts: 14,491 So. Burlington, Vt.
fast68plymouth
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 14,491
So. Burlington, Vt.
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1205 swept 76 chamber 19.8 gasket -1.3 dome vol -6.5 deck ---------- 1293 total 88 total minus swept ---------- 14.69 (1293/88)
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: help calculate my comp using 1/2"downfill method
[Re: B1MAXX]
#2292097
04/22/17 02:06 PM
04/22/17 02:06 PM
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Joined: Mar 2017
Posts: 43 California USA
LowDeck451
member
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member
Joined: Mar 2017
Posts: 43
California USA
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guys I need to understand this when I saw fast's reply I became confused....believe me all this was meant in a positive light and meant to learn from.
Here is the way I have done this...please point the error of my ways. 1/2 bore times itself 2.25x2.25=5.06 times pie 5.06x3.14=15.89 times stroke 15.89x4.625=73.52 time ci to cc factor 73.52x16.387=1204.77 I rounded this to 1205cc
add head cc 1205+76=1281 add gasket cc... use same formula as above but used4.56 for bore and .074 for thickness times ci to cc factor 1.20x16.387=19.66 I rounded this to 20cc 1281+20=1301 add or subtract deck vol which in this case is -1 based on a -.500 perfect deck vol. of 130cc used same as above but used 4.5 for bore and used .500 for thickness
1301-1+1300 total vol at bdc 1300
ok vol at tdc would be 76cc+20cc-1cc=95cc
1300/95=13.68
Me too.????
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Re: help calculate my comp using 1/2"downfill method
[Re: fast68plymouth]
#2292099
04/22/17 02:11 PM
04/22/17 02:11 PM
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Joined: Mar 2017
Posts: 43 California USA
LowDeck451
member
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member
Joined: Mar 2017
Posts: 43
California USA
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1205 swept 76 chamber 19.8 gasket -1.3 dome vol -6.5 deck ---------- 1293 total 88 total minus swept ---------- 14.69 (1293/88) I guess I'm not understanding the '-6.5 deck' in your calc. Thanks.
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