Can someone double check this information? The compression I'm getting from the c-speed calculator is much lower than I'm expecting, and with reading the ctr pistons stickied article, I'm not sure what to trust right now. I'm trying to get somewhere around 11.7:1 compression.

block: b18c
head: b18c5
pistons: CP Pistons (SC7135) 11.5cr, 4.1cc Piston Dome displacement
3 layer OEM headgasket
compression from c-speed = 10.7:1

Also, I tried the CP Piston (SC7115) which is 12.5cr in a b18c1 (I'm running a b18c5 head), 6.9cc Piston Dome Displacement. This gave me 11.3:1 compression.

What am I doing wrong here?

 

81mm bore.

 

up

 

Here is the mathematical formula for det. compression ratio. This will help you determine it yourself.

Displacement = (Bore ÷ 2)2 x 3.14 x Stroke x 16.387
Head Gasket space = (Bore ÷ 2)2 x 3.14 x Gasket thickness x 16.387064
Deck Height space = (Bore ÷ 2)2 x 3.14 x Deck Height x 16.387064
Compressed Volume = Head Gasket space + Deck Height space +
Piston Top volume + Combustion chamber volume
Uncompressed Volume = Compressed Volume + Displacement
Compression Ratio = Uncompressed volume ÷ Compressed volume

 

you'll need to know the compression height of those pistons to get the right #'s.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 743power &raquo;</TD></TR><TR><TD CLASS="quote">you'll need to know the compression height of those pistons to get the right #'s.</TD></TR></TABLE>

Where can I find compression height information? I have piston dome displacement information... is this the same thing?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Takumi Fujiwara &raquo;</TD></TR><TR><TD CLASS="quote">

Where can I find compression height information? I have piston dome displacement information... is this the same thing?</TD></TR></TABLE>

no, compression height is the distance from the center of the pin bore to the flat of the piston. It is used to determine the "deck height space" in that formula asubenett gave.

 

I forgot to mention to use inches instead of mm for the formula above.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by asubennett &raquo;</TD></TR><TR><TD CLASS="quote">I forgot to mention to use inches instead of mm for the formula above.</TD></TR></TABLE>

so everything in the above formula should be in inches?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Takumi Fujiwara &raquo;</TD></TR><TR><TD CLASS="quote">

so everything in the above formula should be in inches?</TD></TR></TABLE>

yea, unless you leave out that 16.XX multiplier at the end.

 

I'd just plop the piston and conrod onto the crank and measure the piston-to-deck height to calculate that volume.

Also, the piston displacement volume should be subtracted from the total combustion chamber volume, not added, since it's decreasing the volume of the combustion chamber and not appending to it.

 

Why would you do that when calculating compressed volume. Maybe I am missing something.

 

Do what? Measure the piston-to-deck height or subtract the dome displacement?

 

do this

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by IN VTEC &raquo;</TD></TR><TR><TD CLASS="quote">Also, the piston displacement volume should be subtracted from the total combustion chamber volume, not added, since it's decreasing the volume of the combustion chamber and not appending to it.</TD></TR></TABLE>

 

also, if the piston is "out of the hole, the volume will be negative and vice-versa (when it is below deck, positive)

 

If the piston has a dome, then it's taking up volume and making the combustion chamber smaller.

Example (assuming a piston of 0.0cc displacement):
Cylinder head combustion chamber volume = 41.5cc
Head gasket volume (.026" thickness, 81mm bore) = 3.4cc
Piston-to-deck height (assume .000" for simplicity) = 0.0cc

Total = 44.9cc of total combustion chamber volume if you had a 0.0cc piston.

If you increased the piston dome displacement (by adding dome) to +5.0cc, then you need to subtract that 5.0cc from 44.9cc to get 39.9cc. Result: decreases your compressed volume. That's where you get your compression ratio increase from by running more dome.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">also, if the piston is "out of the hole, the volume will be negative and vice-versa (when it is below deck, positive)</TD></TR></TABLE>
And that too, of course. Forgot to mention that.


If you think about it all logically for a second, it makes perfect sense.

 

Your right makes perfect sense. Thanks for the clear explanation.

 

Finally decided to run the numbers from scratch, and as expected, my numbers don't match up to anything. Here is my calculations:
Static Compression Ratio = Vol.@BDC / Vol.@TDC
Vol@BDC=swept vol. + cyl head vol. + headgasket vol. incl. piston to deck height - piston dome vol.

Vol@TDC=cyl. head vol. + headgasekt vol. incl. piston to deck height - piston dome vol.

Swept vol = (pi x [81mm]^2 x stroke)/4000

Head gasket thickness incl. piston to deck height = [(81mm)^2 x PI x .864]/1000
note: (0.029 in. + 0.005 in. ) x 25.4 mm per inch = 0.864 mm)

Cylinder Head volume: 42.7cc (R head)
Head Gasket thickness (stock): .029inches
Piston Dome volume for cp 11.5:1 pistons: 4.1
Piston to deck height: .030inches
B18c1/5 stroke: 87.2

I'm getting over 12:1... something has gotta be wrong.

 

Here is the main formula I'm using:

static compression ratio = vol.@BDC/vol.@TDC

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Takumi Fujiwara &raquo;</TD></TR><TR><TD CLASS="quote">Here is the main formula I'm using:

static compression ratio = vol.@BDC/vol.@TDC</TD></TR></TABLE>

you're assuming the stock PTDH, CP gives the compression height on their spec sheets, I am pretty certain.

Also, are you converting everything to same unit of measure? You have PTDH and HG volume listed in inches and everything else is metric.

 

what is PTDH?

 

piston to deck height

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 743power &raquo;</TD></TR><TR><TD CLASS="quote">
Also, are you converting everything to same unit of measure? You have PTDH and HG volume listed in inches and everything else is metric.</TD></TR></TABLE>

I'm using 0.864 for piston to deck height.

- head gasket thickness of 0.029 in. and piston to deck height from the gasket as 0.005 in. converted to mm.

 

Where did you get the .005" deck height clearance from?

I use a deck bridge with a .001" precision dial indicator and measure both sides of the crown and take the average of the two.
https://honda-tech.com/zerothread?id=579672

 

I used this walk-through found here:
http://www.team-integra.net/se...D=233

I need to figure out the compression before I order the pistons... these formulas are so frustrating. Online calculators that people don't trust, differing compression formulas with different units of measure... where is the "final answer" formula? I used 3 sheets of paper to find out mine is different than the 2 other compressions I got from 2 different calculators.