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#2291584 - 04/21/17 12:39 PM help calculate my comp using 1/2"downfill method
KOS Offline
pro stock

Registered: 08/17/03
Posts: 1270
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

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#2291608 - 04/21/17 01:19 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
Cab_Burge Offline
Too Many Posts

Registered: 08/23/03
Posts: 28376
Loc: Bend,OR USA
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)

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#2291611 - 04/21/17 01:22 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
KOS Offline
pro stock

Registered: 08/17/03
Posts: 1270
valve reliefs are calculated in the down volume.....thx cab ill ck it out.

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#2291617 - 04/21/17 01:31 PM Re: help calculate my comp using 1/2"downfill method [Re: Cab_Burge]
KOS Offline
pro stock

Registered: 08/17/03
Posts: 1270
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

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#2291618 - 04/21/17 01:34 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
madscientist Offline


Registered: 09/17/14
Posts: 1357
Loc: Washington
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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#2291640 - 04/21/17 02:14 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
KOS Offline
pro stock

Registered: 08/17/03
Posts: 1270
well had a buddy run it through his program and came up with 14.6

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#2291646 - 04/21/17 02:24 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
LowDeck451 Offline
member

Registered: 03/01/17
Posts: 27
Loc: California USA
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.

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#2291647 - 04/21/17 02:29 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
LowDeck451 Offline
member

Registered: 03/01/17
Posts: 27
Loc: California USA
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.

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#2291660 - 04/21/17 03:09 PM Re: help calculate my comp using 1/2"downfill method [Re: LowDeck451]
KOS Offline
pro stock

Registered: 08/17/03
Posts: 1270
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

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#2291671 - 04/21/17 03:22 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
Cab_Burge Offline
Too Many Posts

Registered: 08/23/03
Posts: 28376
Loc: Bend,OR USA
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)

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#2292026 - 04/22/17 09:57 AM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
polyspheric Offline
top fuel

Registered: 10/10/07
Posts: 2062
Loc: New York
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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Boffin Emeritus

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#2292043 - 04/22/17 10:39 AM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
fast68plymouth Offline
master

Registered: 01/20/03
Posts: 9407
Loc: So. Burlington, Vt.
I get 14.69
_________________________
68 Plymouth Satellite, 383, stock 906's, 3550lbs, 11.18 @ 123, 1.51 60' PRH is a Comp Cams W/D......competitive pricing on entire line. Custom cams available. ** dealer for Indy Cylinder Heads **

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#2292045 - 04/22/17 10:40 AM Re: help calculate my comp using 1/2"downfill method [Re: polyspheric]
B1MAXX Offline
mopar

Registered: 05/07/05
Posts: 633
Loc: Apollo, PA.
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

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#2292057 - 04/22/17 10:58 AM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
fast68plymouth Offline
master

Registered: 01/20/03
Posts: 9407
Loc: So. Burlington, Vt.
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 Plymouth Satellite, 383, stock 906's, 3550lbs, 11.18 @ 123, 1.51 60' PRH is a Comp Cams W/D......competitive pricing on entire line. Custom cams available. ** dealer for Indy Cylinder Heads **

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#2292067 - 04/22/17 11:11 AM Re: help calculate my comp using 1/2"downfill method [Re: B1MAXX]
B1MAXX Offline
mopar

Registered: 05/07/05
Posts: 633
Loc: Apollo, PA.
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

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#2292071 - 04/22/17 11:17 AM Re: help calculate my comp using 1/2"downfill method [Re: B1MAXX]
B1MAXX Offline
mopar

Registered: 05/07/05
Posts: 633
Loc: Apollo, PA.
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.

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#2292097 - 04/22/17 12:06 PM Re: help calculate my comp using 1/2"downfill method [Re: B1MAXX]
LowDeck451 Offline
member

Registered: 03/01/17
Posts: 27
Loc: California USA
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.????

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#2292099 - 04/22/17 12:11 PM Re: help calculate my comp using 1/2"downfill method [Re: fast68plymouth]
LowDeck451 Offline
member

Registered: 03/01/17
Posts: 27
Loc: California USA
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.

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#2292151 - 04/22/17 01:49 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
KOS Offline
pro stock

Registered: 08/17/03
Posts: 1270
had two other guys run it for me and they where 14.5ish......

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#2292167 - 04/22/17 02:17 PM Re: help calculate my comp using 1/2"downfill method [Re: KOS]
B1MAXX Offline
mopar

Registered: 05/07/05
Posts: 633
Loc: Apollo, PA.
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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