help calculate my comp using 1/2"downfill method

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

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.
Do you know the valve reliefs CC? If so deduct that from both measurements to keep it even

valve reliefs are calculated in the down volume.....thx cab ill ck it out.

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

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.

well had a buddy run it through his program and came up with 14.6

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.

Hmm...Well, I could be wrong, but I don't see where. Interesting that we are 1 full point apart.

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

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
The first calculation should be accurate
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
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

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.

I get 14.69

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

1205 swept
76 chamber
19.8 gasket
-1.3 dome vol
-6.5 deck
----------
1293 total
88 total minus swept
----------
14.69 (1293/88)

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

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.

Me too.????

I guess I'm not understanding the '-6.5 deck' in your calc. Thanks.

had two other guys run it for me and they where 14.5ish......

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)

The deck isn't figured into the fill volume, but certainly has to be figured into the compression ratio calculations.

The pistons stick out of the bore .025(6.5cc) and you guys aren't accounting for that.

The cr is 14.69 based on the info in the original post.

It's not really debatable.......... It's just simple math.

If it was zero deck, you'd get rid of the -6.5 for the deck height and you'd have 1299.5/94.5 = 13.75

OK, not meaning to debate really, just trying to understand.
Thanks!

believe me its in there. a number less than 102.9cc when actually measured would be would be above deck.

To my way of thinking, the fill down is used to determine the dome/dish volume...... Period.

I don't move the piston a given distance from tdc to determine the fill down distance(well, at least I've never done it this way), it's the distance from the top of the block.
A .500 fill down is done at .500 from the top of the block.

I suppose it could be done by moving the piston a specific distance from tdc, but which method of doing it would need to be clarified.

.500 from tdc, or .500 from the top of the block.
One way takes into account the deck clearance, one way doesn't.

I guess we'd need the OP to clarify which way it was done to know if it's 13.7 or 14.7.

My calculations are based on the fill volume being measured with the piston .500 from the top of the block.

The way I understand it is, .500 down from TDC takes the dome (above deck), valve notches (below deck) into account, and that the amount more than, or less than the perfect .500 volume (130cc in this case) indicates 'more', or 'less' chamber volume.

The "problem" with this example is...... The pistons stick out .025".

If you move the piston .500" from that position it's only down .475" from the top of the block.

So, you're going to seal up the piston to the bore, put your piece of plexiglass on top of the block and fill it up.
The OP said they observed 129cc of volume to fill it up.
The piston isn't .500" from the top of the block, it's only .475" from the top of the block(if it's .500" from tdc).
The area for the 4.5" x .475" bore is 123.8...... Not 130.3.
So if you observe 129cc fill volume, you have 5.2cc(+)worth of valve pocket, not 1.3cc(-)worth of dome volume.

You've poured 129cc of liquid into what should only be a 123.8cc hole.

That would give you the 1299.5/94.5 = 13.75 result.

1205 - swept
76 - gasket
19.8 - gasket
5.2 - dish vol
-6.5 - deck
------------
1299.5
94.5 total minus swept
-----------
13.75(1299.5/94.5)


In this case, whether the .500" dimension is from tdc or from the top of the block changes the results by almost 1 point.

It looks like this is a case where a slight difference in how some terminology is interpreted makes a difference in the bottom line.

I use the fill down volume to determine the piston dish/dome volume by itself.

The OP would have to clarify how they did it to know what the cr actually is.

The 5.2cc valve notches increase chamber volume, the +.025 deck height= 6.5cc- decreases chamber volume, the difference-1.3cc is the effective 'dome' above deck, and is subtracted from chamber volume. Also, if you moved a domed piston down from the top of the block, where would you measure from, the top of the dome, the edge of the piston? Still wouldn't tell you how much volume is above deck. I believe the only way to account for an odd shaped domed piston top (dome above deck, notches below deck) is to move the piston down from TDC, pour and compare to an ideal cylinder volume. I agree, the OP should describe how he got his 129cc.

The bottom line is, the difference in this case is 6.5cc, depending on how you measured it, which is the .025 of deck.

To your point about the irregular piston shape affecting the value.....
The 4.50 x .025 area is 6.5cc.

If the fill down was done at .500 below tdc, the piston dish is 5.2cc.
If the fill down was done at .500 below the top of the block, there is a dome volume of -1.3cc.

The difference between the +5.2 and the -1.3 is still 6.5cc.

Think of it this way.......
Assuming this irregular shaped piston in a 4.50 bore is completely below the deck at tdc by .100(an arbitrary number).
Now slice .025 off the top of the block so the piston is .075 down the hole.
How much did you reduce the volume?
How much did the irregular shaped piston affect it?

yes Dwayne correct.....i put the piston few inches in the hole greased it up and moved it up the bore till it was .500 down not taking into account its .025 out of the hole thats why i specified its .025 out of the hole.your math is correct its 14.6.now with .010 or so block growth(alum)14.5ish true.

6.5cc
0cc
Hmmmm........
Now just gotta visualize it in my head. Thanks for your time Dwayne!

.500 down is from TDC not deck.... we are figuring bdc and tdc volumes. why would you use the deck to measure from?

The first thing I do is bring the piston up ...0 the indicator...and drop it .500.... the piston could be above or below deck. the difference in this number from the deck(the perfect number vol.) is exactly what your measuring.

[quote=B1MAXX].500 down is from TDC not deck.... we are figuring bdc and tdc volumes. why would you use the deck to measure from? [/quote

i guess i coulda measured it .475 down but to me it was easier to account for the postive deck after.ive never had to use this method before i usually have piston specs so im alittle green on the topic but i get what your saying.i guess i get where all the confusion was coming from ......

the down fill is for when a piston has any part of itself above deck. dome or the whole thing or both, so that we can measure it. To add the deck height back in to the equation would be double dippin.

10/4

Just to be clear, not everyone does it this way.

I, for one, don't.

I use the fill down method for determining dome volume only.
This is what the OP did as well.

Once I have that value, I can plug it in to the calculation and see where I'm at, and then easily make adjustments to chamber volume, deck clearance, etc if I need to change the cr to meet a target.

Yeah. I keep coming back to the 'From TDC' method.

Just to be clear, not everyone does it this way.

I, for one, don't.

I use the fill down method for determining dome volume only.
This is what the OP did as well.

Once I have that value, I can plug it in to the calculation and see where I'm at, and then easily make adjustments to chamber volume, deck clearance, etc if I need to change the cr to meet a target. [/quot

good to know im not the only one thinking the same way.....thx Dwayne for clarifying that.

The "tdc method" does not solve for dish/dome volume. It combines the dish/dome volume with the deck clearance.


Using previously mentioned perfectly flat 4.00 piston.......

The fill down for 4.00 x .500 is 103cc.

If you measured the .500 from tdc, and observed a fill volume of 98.9cc, what would you call the dish/dome volume?

The piston itself has zero volume.......the fact that it's sitting above the deck .020 is where the reduction of 4.1cc's comes from.

Which way you do the math won't change what the actual cr is, it just makes more sense to me to use the actual dome volumes and deck volumes instead of combining them.

Either way will work, just make sure if you're comparing notes with someone else that everyone is on the same page.
As this thread has shown, when you're not.........the results can be way off.

Dwayne thanks for your patience and willingness to help/teach us.
Don