DIY Compression Calculator


本田 Gearing Authority
Mar 26, 2008
Many of us are tempted by the available plug and play compression calculators out there - even myself, as math is definitely not one of my fortes. 1 + 1 = 11 apparently when I'm doing calculations... :angry2:

Luckily there are those who are much smarter than I am and have the patience to share their knowledge with the rest of us.

So for those new to the game, or who've been in the game for a while but never truly knew what was going on - this following information will definitely help you safely extract the potential in any build, regardless of mild vs. wild.

Taken from this thread:
DIY Compression Ratio Calculation - Honda-Tech

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 .0007854 x 89 = 458.61 (cc)

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

(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 .0007854 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 .0007854 x .762 = 3.92 (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 refference 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-

458.61(cc) + 42.7(cc) + 3.40(cc) + 3.92(cc) - 6.93(cc) Dome
------------------------------------------------------------------------------ = 11.64 : 1
42.7(cc) + 3.40(cc) + 3.92(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 preffer calculating volumes in (cubic inches) you can use (Pi*.25) or (.7854) 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

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 (P72AO) = -0.60 cc (dish)
*JDM GSR(P72OO) = +2.52 cc (dome)
*B18C5 (P73AO) = +3.64 cc (dome)
*JDM ITR (P73OO) = +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)


Active Member
Sep 27, 2007
Great info RA, I used the zeal calculator when i done my build. I find thing like that really hard to get my head around zeal is so so so easy.

EG-nath EG owner
Sep 17, 2010
theres a program you can get for your PC made by 'performance trends inc' that does teh actual maths for you, you just enter the values

i got bored of doing the calculations manually for my motorsport engineering degree