DC_Legacy

I haven't posted a thread of my own here in a while, but lately I've noticed alot of Compression Ratio questions so I thought I'd give everyone a quick rundown on how to calculate your Compression Ratio or more specifically your "Static" C/R, if you don't already know how to or prefer not to use the web calculators.

Keep in mind this is just an example of how to calculate your C/R based on "known" measurements such as: Cylinder Bore size, Stroke, Combustion Chamber volume, Gasket Bore size, Gasket Compressed thickness, measured or known Piston-to Deck height.

Please Note* This is not intended to substitute for performing actual measurements of your engine's component volumes by conventional methods such as, "CC'ing" an engine with a Graduated Cylinder or taking other specific measurements/tolerances, but if your figures are right this should allow you to get a close enough approximation.

General spec's for "B-series" motors are at the bottom of the page. If machining of cylinderhead and-or engine block have or will be performed, "CC'ing" the combustion chamber and piston-to-deck volume is advised to determine your specific dimensions/volumes. If decking or plaining of the engine block have been performed and you are using "Dished" or "Domed" pistons, you can obtain a measurement of the piston-to-deck clearance by measuring from the blockdeck to the piston quench-pad area, or "flat" side portion of the piston (while at TDC) using a DeckBridge and Dialgauge micrometer.



(measurement of clearance from piston quench to deck using a Deckbridge & Dialgauge)


(example of CC’ing being perfromed with a Graduated Cylinder to determine P-to-D vol + piston dish vol)

In the example below I've listed the C/R mathematics of a Stock spec LS block, w/ 3 layer Gasket thickness (026") or (.664 mm), Stock spec B16 head/Combustion Chamber, and 81(mm) P30 piston





*Cylinder Volume (cc) = Bore(mm) x Bore(mm) x (Pi*.00025) x Stroke(mm)


Numerical Example-
81 x 81 x .000786475 x 89 = 459.24 (cc)






*Combustion Chamber Size (cc)=(*reffer below for stock combustion chamber volumes)

Example-
(Pr3/B16) stock spec =42.7(cc)







*Gasket Volume (cc) = Gasket Bore(mm) x Gasket Bore(mm) x (Pi*.00025) x Gasket thickness (mm)


Numerical Example-
81 x 81 x .000786475 x .6604 = 3.40 (cc)






*Piston-to-Deck Volume (cc) = Bore(mm) x Bore(mm) x (Pi*.00025)x Piston-to-Deck Spec’s (mm)


Numerical Example-
81 x 81 x .000786475 x .762 = 3.93 (cc)

Please Note* If piston compression height causes the piston quench pad to sit above the deck or "Out of the hole" then you must calculate this negative volume or reduced displacement and subtract this figure from your Vol@TDC and "Swept Volume" .

Example- Negative displacement (cc) = Bore(mm) x Bore(mm) x (Pi*.00025) x Deck-to-Piston "quench pad"(mm)

*Piston Dish Volume /Dome negative Volume = OEM or Piston Manufacturer Specs (cc)
Example- OEM “P30”= +6.93(cc) Dome, or “PR4”= -3.2(cc) Dish

Please Note* Piston “Dish” and “Dome” figures are listed minus for “Dish”, and plus for “Dome”. You must invert a pistons advertised “plus” for the Dome to a “minus”, as well as an advertised “minus” for a pistons dish to a “plus” when adding these values to your other calculated volumes. For reference please refer to example below.



(example of Volume@TDC shown above, the "Swept Volume" is the sum of the Cylinder Volume + Volume@TDC)



Now lets add it all up!


Cylinder Volume (cc) + Combustion Chamber Volume (cc) + Gasket Volume (cc) + Piston-to-Deck Volume (cc) + Piston Dish Volume(cc), or - Dome negative Volume(cc)
------------------------------------------------------------------------------ = C/R
Combustion Chamber Volume (cc) + Gasket Volume (cc) + Piston-to-Deck Volume (cc) + Piston Dish Volume (cc), or - Dome negative Volume (cc)



Numerical Example-

459.24(cc) + 42.7(cc) + 3.40(cc) + 3.93(cc) - 6.93(cc) Dome
---------------------------------------------------------------------------- = 11.65 : 1
42.7(cc) + 3.40(cc) + 3.93(cc) - 6.93(cc) Dome


Swept Volume / Vol@TDC

------------------------------------- = Compression Ratio (Static)






Conversion Factors:

Inches to Millimeters= 1 x 25.4
Millimeters to Inches=1 / 25.4
Cubic Centimeter to Cubic Inches=1 x .061
Cubic Inches to Cubic Centimeter=1 / .061

Please Note* If parts/engine spec's are listed in (Inches) and you prefer calculating volumes in (cubic inches) you can use (Pi*.25) or (.786475) in your Bore x Bore calculation factor.




OEM Honda "B-series" Cylinder Head Combustion Chamber Volumes:

*PR3/B16, B17 & *P73/ITR = 42.7 cc
*P72/GSR = 41.6 cc
*PR4 /LS & B20B/Z = 45 cc


OEM Honda "B-series" Piston-to-Deck Clearances:

*B18A/B, B18C1, B18C5, B20Z/B ~ 0.762mm
*B16A ~ 0.508 mm, (B16B unavailable)

Please Note* Piston-to-Deck or "Deckheight" clearances were quoted from Team Integra.net http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=233

OEM Honda "B-series" average compressed Gasket Thickness:

*3 layer OEM =.660mm ~ .740mm




OEM Honda "B-series" Crank/Stroke Specs:

*B18A,B/B20 =89mm
*B18C 1-5 = 87.2mm
*B16A/B = 77.4mm


OEM Honda "B-series" Piston Specs:

*B18A/B (PR4/P74) = -3.2 cc (dish)
*B18C1 (P72A0) = -0.60 cc (dish)
*JDM GSR(P7200) = +2.52 cc (dome)
*B18C5 (P73A0) = +3.64 cc (dome)
*JDM ITR (P7300) = +5.96 cc (dome)
*B17A (P61) = 0.00 cc (flattop)
*B16A (PR3) = +6.01 cc (dome)
*JDM B16A (P30) = +6.93 cc (dome)
*B16B (PCT) = +8.63 cc (dome)
*B20Z (PHK) = -4.04 cc(dish)
*B20B (P3F) = -9.92 cc (dish)

teggything

im just wanting to know what pistons will clear my deck height on a b18a1 bock and a p72-1 gsr head all stock, and i want to go with OEM pistons

clem kevin

This is awesome. Currently building a motor and sometimes questioned how accurate zeal really is.

DC_Legacy

For the most part the Zeal is still a good calculator, however the results I got from my engine combo were quoted (12.75:1) which seemed alittle high in which I did account for Bore size, milling/decking tolerances, gasket size/bore difference, etc.

I took measurements of my Piston-to-Deck Clearance, plus CC'd the Cylinderhead, then added those figures to the other dimensional calculations that bought me to (12.1:1). Which to me, seems reasonable with 93 octane, good tuning, and no visible sign of detonation from looking over the plugs.

clem kevin

it took me a little extra time to figure it out because most of my calculations were measured in inches, by requirement of my class

DC_Legacy

Yeah I put a conversion factor in the post midway down if you'd rather use standard units.

H - Badge Boondocker

Hey is that Pi (3.14) x .00025 for the cylinder volume calc. ( sorry f'n newb

DC_Legacy

right for calculating Vol with (CC), or if using nearly all of the Pi equation (3.14159265) x .00025 = .00078539 ~ .0007854

H - Badge Boondocker

So then I've calculated C/R volume and have 446.565 CC and i've completed the next calculation equaling combustion camber volume with a value of 42.7 (obviously) do you divide this value into the C/R volume and and up with 10.45 (which i'm thinkin would be the C/R or am I just missing something with my rusty *** math skillz)

DC_Legacy

Just to reiderate you would first add-up these different volumes: Cylinder Volume (cc) + Combustion Chamber Volume (cc) + Gasket Volume (cc) + Piston-to-Deck Volume (cc) + Piston Dish Volume(cc), or - Dome negative Volume(cc) (this is called your Swept Volume)

now write the sum of the above down,

now add-up just these volumes Combustion Chamber Volume (cc) + Gasket Volume (cc) + Piston-to-Deck Volume (cc) + Piston Dish Volume(cc), or - Dome negative Volume(cc) (this is your volume at TDC)

now take this sum and divide it into the first sum you wrote down.

h224thgensedan

Good info

were are you guys finding your graduated cylinders at. Went to some local hardware stores but cant find anything that would work except for a piece of lexan to hold the fluid in.

DC_Legacy

My block machinist lent me his. I used plexy glass and vasoline, isoprophal (i think i spelled that right) for the fluid.

h224thgensedan

I was thinking of using vasoline as well. Would come of the head easy plus seal up good. Was thinking about making one. I found a plastic one with a holder and just drill a hole in the bottome of it for a small ball valve. Not sure if it would be as accurate. What do you think.

DC_Legacy

You want to drain the fluid slowly to be accurate or to prevent over filling. As long as you could control it quickly I'd say try it. If not, try looking for one online with a control valve.

h224thgensedan

I think im just going to get the stuff and try it. Do each combustion chamber like 4 times and see if i get any different numbers. The ball valves i was looking at for it were small (1/8")

DC_Legacy

If your careful you can do it without valve, this site has some Pyrex cylinders

http://wardsci.com/category.asp_Q_c_E_1282_A_Cylinders

H - Badge Boondocker

Great help guys the plexi and vas. is a good idea I was going to use an insuline needle for fluid measurement (got it from a friend with diabetes, what where you thinkin?) so a small hole in the plexi and some sealent would work in theory for comb. chambers but a grad. cylinder is the way to go thanx

h224thgensedan

http://www.circletrack.com/techartic.../photo_06.html This is the one i seen a few months back. How i got the idea of making one

DC_Legacy

Looks like the one I used, If you dont find one like that or make one that works for you. You could fill 90% of the volume of your combustion chamber with a non-valved Graduated cylinder, then carefully fill the rest of the space with a CC syringe like what you would find in a Children Tylenol package.

clem kevin

can this be stickied or added to the faq page? i couldnt find it on there....

DC_Legacy

That would be nice, lol...Make it a subcribed thread that why you dont lose it.

clem kevin

one thing i want to know is, the relation between milling a head and combustion chamber size. If i milled .0025" off my head, does that decrease combustion chamber volume much?

DC_Legacy

Yeah it reduces cubic space. I CC'd my combustion chamber to know for sure but if your combustion chamber is circular like that of the PR3 you could just calculate the Area (example. 3.188"*3.188"*.7854) then multiple that by the amount of milling you had performed (.0025") to determine the estimated Volume to subtract from the Vol@TDC & your Swept Vol.

The machine shop that worked on my head told me that they milled my head by .005"-.010" so after CC'ing the combustion chamber I found the volume left over made the mill more so in the .0075"-.010" neighborhood.

clem kevin

where did you get 3.188?

this work is being done on a p72 head.

So i would assume 3.188 would be the cubic inch size, rather than cubic centimeters.

so if i did 41.6x41.6x.0007854x.0025(.0635mm) that would make it .0863cc subtracted from the chamber? seems like that number is pretty low?

DC_Legacy

3.188976378"~3.189"=81mm

It was just an example of the stock B series bore in standard units.

If determining the combustion chamber Vol on the P72 I'd CC for that, although I'll try to dig out one of my college algebra books to post a formula on how to calculate the area for the P72 combustion chamber shape