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help calculate my comp using 1/2"downfill method #2291584
04/21/17 02:39 PM
04/21/17 02:39 PM
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KOS Offline OP
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well i have all the measurements just need to figure out the math ive never did it this way before so i need a bit of help.
4.5x4.625 bore+stroke
129cc 1/2"down volume
76cc head
.074 head gasket
piston .025 out of hole.

my ruff guess is 14.25to1 ish

Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2291608
04/21/17 03:19 PM
04/21/17 03:19 PM
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Go to KB, United Machine, web site and use their on line compression ratio calculator and plug in your specs. with both + .025 deck height and then .500 down specs. up scope
Do you know the valve reliefs CC? If so deduct that from both measurements to keep it even work confused


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2291611
04/21/17 03:22 PM
04/21/17 03:22 PM
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valve reliefs are calculated in the down volume.....thx cab ill ck it out.

Re: help calculate my comp using 1/2"downfill method [Re: Cab_Burge] #2291617
04/21/17 03:31 PM
04/21/17 03:31 PM
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Originally Posted By Cab_Burge
Go to KB, United Machine, web site and use their on line compression ratio calculator and plug in your specs. with both + .025 deck height and then .500 down specs. up scope
Do you know the valve reliefs CC? If so deduct that from both measurements to keep it even work confused


no calc on that website using .500 downfill .....thx anyhow

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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madscientist Offline
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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
Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2291640
04/21/17 04:14 PM
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well had a buddy run it through his program and came up with 14.6

Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2291646
04/21/17 04:24 PM
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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.

Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2291647
04/21/17 04:29 PM
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Originally Posted By KOS
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.

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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Originally Posted By LowDeck451
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

Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2291671
04/21/17 05:22 PM
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What I would do is to use the positive deck height spec. with the valve relief specs and then do the same calculation with the piston down .500 with all the other specs remaining the same if you want to know the differences shruggy scope
The first calculation should be accurate shruggy
On your deal using the .500 down with no valve reliefs and then using your actual fill volume will tell you the actual dome volume with the valve reliefs shruggy
I use the .500 down method or what ever depth is needed for hemi pistons to get them below the deck to figure out their true dome volume is up


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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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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.


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Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2292043
04/22/17 12:39 PM
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I get 14.69


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
Re: help calculate my comp using 1/2"downfill method [Re: polyspheric] #2292045
04/22/17 12:40 PM
04/22/17 12:40 PM
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VOL. AT BDC 1300CC
VOL. AT TDC 95CC
1300/95 13.68
this was with 20cc for gasket....4.56x.074

you need 69cc head vol. for 14.7

Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2292057
04/22/17 12:58 PM
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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
Re: help calculate my comp using 1/2"downfill method [Re: B1MAXX] #2292067
04/22/17 01:11 PM
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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

Re: help calculate my comp using 1/2"downfill method [Re: B1MAXX] #2292071
04/22/17 01:17 PM
04/22/17 01:17 PM
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ok I am beginning to see the fork in the road. The out of deck is in the down fill of -1cc. there is no reason to factor this. With the piston .500 down and 0 deck it should be 130cc with the positive deck vol , pocket relief, and or dome dish. we have 129 which is -1cc factoring all of that.

Re: help calculate my comp using 1/2"downfill method [Re: B1MAXX] #2292097
04/22/17 02:06 PM
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Originally Posted By B1MAXX
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.????

Re: help calculate my comp using 1/2"downfill method [Re: fast68plymouth] #2292099
04/22/17 02:11 PM
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Originally Posted By fast68plymouth
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.

Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2292151
04/22/17 03:49 PM
04/22/17 03:49 PM
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had two other guys run it for me and they where 14.5ish......

Re: help calculate my comp using 1/2"downfill method [Re: KOS] #2292167
04/22/17 04:17 PM
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When measuring actual down fill you cannot factor the deck height into the equation...

assume you have 4.00 bore, no valve pocket, no dish or dome for simplicity sake.

2x2x3.14=12.56x.500=6.28x16.387=102.9cc down fill.

we measure at .500 and get 110cc

110-102.9=7.1 cc extra vol. this mean/shows the piston is below deck .035 (2x2x3.14x.035=7.2cc)

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