Compression Ratio Theory and compression calculation 101

compression ratio

INTRODUCTION

If you look in the dictionary:

Compression as an adjective means something is squeezed (in this case it's air and fuel)

Ratio as a noun means something is divided by something else. It is a math term meaning a quotient.


There are 2 kinds of Compression Ratios (CR):

1. Static

2. Dynamic


The most common one everyone talks about is Static CR.

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1. STATIC CR



1A. DEFINITION

The Static Compression RATIO is defined as the Volume of the Combustion Chamber when the piston is at the very bottom of it's travel (called "bottom dead center" or BDC) DIVIDED BY the Volume of the Combustion Chamber when the piston is at the very top of it's travel (called "top dead center" or TDC).

Static compression ratio is one factor that affects how completely the air-fuel mixture is burned, once it has been lit by the sparkplug. If you burn all of the air:fuel mixture, you make more hp. If there is some leftover unburnt air:fuel mix after the spark has been lit, you have not gotten all the power you can get out of the mix that was just added into the cylinder. This completeness of burn is called THERMODYNAMIC EFFICIENCY (measured in units of energy called joules, pronounced like "jewels").

Remember, basically, you increase horsepower by increasing 3 different types of efficiencies: thermodynamic (relates to burn), volumetric (relates air flow in and out), and mechanical (relates to weight and friction). Improving thermodynamic efficiency is one of the 3 major power-gaining methods available for engine builders.

The relationship between thermodynamic efficiency and static CR is not a simple, direct, linear, straight-forward 1:1 relationship. In plain english, if you increase the CR by 1.5 times, it does not mean the burn efficiency (or completeness of burn) will also increase by 1.5 times. It's a complex direct exponential and inverse exponential equation that relates static CR to burn efficiency.

Like anything exponential and inverse in math, there is an initial rapid increase in thermodynamic efficiency as static CR increases but at some point of CR, the efficiency levels off and plateaus. In other words, at some point, further increases in static CR no longer improves or increases the burn efficiency.


1 B. THE PROBLEMS WITH INCREASING STATIC CR

The problem is that as you increase CR, you increase cylinder pressure and temperature inside the combustion chamber. When air is squeezed hard inside a closed container like a cylinder, the pressure inside goes up the harder you squeeze. As pressure builds up, so does temperature. These 2 (high pressure and temp.) can cause the air -fuel mix to ignite on it's own without a spark from the plug....this is called detonation. So there is a CR level which will cause detonation. The value of that level varies with each engine and depends on combustion chamber's design which limit detonation risk (eg. compact combustion chambers, low surface-area-to-volume ratio combustion chambers, more quench or squish area, swirl filling of the air-fuel mix into the chamber, getting a stratified air fuel mix once the chamber is completely filled, cooling ability of the engine, etc. ... all of these other chamber design factors reduce detonation risk and speeds up the burn rate, against the effect of higher static CR ).

Secondly, as you increase static CR more and more, the cylinder pressures increase more and more. The piston must work much harder to compress the same amount of air:fuel mixture delivered into the chamber due to this higher pressure. This negative work adds more power-robbing or parasitic-losing friction and slows the piston speed momentum which affects the power you produce.

So you can make more power by improving burn efficiency via increasing the static CR up to a point. For street engines, the maximum static CR on pump gas is around 12.5:1 CR if you know how to tune. If you do not, the maximum is around 11.5:1 CR. For a race engine, the point at which cranking pressure causes negative work or parasitic friction and affects power output is around 14:1 CR. Alcohol-fueled race engines can afford to run 15-17:1 CR, since the alcohol cools the chamber and lowers both the cylinder temperature and detonation risk. Methanol race engines run much richer air-fuel ratios (around 5-6:1) than gasoline engines, as well.


In simple terms: for static CR, the first number is the chamber volume at BDC, second number is chamber volume at TDC...the higher the first number is, the more squeeze you have. More squeeze improves burn up to a certain point. This point varies for each engine design.


When we talk about turbos and superchargers, the point at which an increase in static CR that can cause detonation is much lower than in all motor engines.

The reason is : you are stuffing in more air into a closed container with boost. The cylinder pressures build up to a higher level faster. And so does temp. So the risk of detonation is much higher.


If you change cylinder heads (which have different head volumes) or change pistons (with different dome heights), or crankshafts (with different strokes), you will change both the static CR and the dynamic CR.

Many novices do not understand that if a piston comes from one type of engine (eg. CTR pistons) and provides a certain static CR for that engine (eg. in a CTR, the B16B (PTC) pistons give 10.8:1 static CR), it does NOT necessarily mean that you will have the same static CR when you swap those pistons into another engine.

Because the cylinder head volume, stroke, and piston-to-deck height are different between engines, adding the same 81mm bore CTR pistons (using a stock 3 layer head gasket) to a B18B will give 11.6:1 CR, to a B18C1 will give 12.3:1 CR , and to a B18C5 will give 12.0:1 CR!! Remember, this is the same CTR piston that has 10.8:1 static CR in a B16B (CTR) engine.


1C. STATIC CR CALCULATORS ON THE WEB\

just as an example, imm providing honda specs for now.


Here are some sites which provide you with static CR calculators for different Bseries pistons in different Bseries blocks/heads:

http://www.zealautowerks.com/ (this is the most accurate calculator on the web)

http://www.bojangs.com/calculator (this has errors with the PCT piston because it doesn't account for compression height spec differences)

http://www.c-speedracing.com/howto/compcalc/compcalc.php (this has deck clearance errors)


1D. HOW TO CALCULATE STATIC CR FROM SCRATCH YOURSELF


In not-so-simple terms, for the more math-inclined members, you can also calculate this out yourself (i.e. to understand the definition of CR more clearly and what individual factors determine it):




CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)

CR = Compression Ratio

D = Displacement

PV = Piston Volume

DC = Deck Clearance Volume

G = Gasket Volume

CC = Combustion Chamber Volume




More Detail:

Static Compression Ratio = (Volume at BDC) / (Volume at TDC)

[ Aside: Volume stated in this equation above is the combustion chamber volume. ]


where


Volume at BDC = [ (Swept Volume + Cylinder Head Volume + Headgasket Volume (including piston to deck height) - Piston Dome Volume ]

Volume at TDC = [ Cylinder Head Volume + Headgasket Volume (including piston to deck height) - Piston Dome Volume ]

Swept Volume = [Pi x [Bore] ^2 x Stroke]/4000

Call the piston manufacturer for the aftermarket piston dome displacement spec.

See the specs in the tables below for the OEM pistons' dome volumes and cylinder head volumes.

An Example:

B20 VTEC with 84.5 mm BORE and 89mm STROKE, RW pistons with a 7.45 cc Dome Height Volume, and B16A (PR3) head with 42.7 cc head volume, head gasket thickness of 0.029 in. and piston to deck height from the gasket as 0.005 in.


Swept Volume = [Pi x (84.5 mm )^2 x 89.0mm ] / 4000 = 499.1 cc

Endyn/Wiseco Rollerwave Piston Dome Volume = 7.45 cc

Cylinder Head Volume = 42.7 cc

Headgasket Volume (incl. piston to deck height) =
[ [ (84.5 mm / 2)^2 x pi x 0.864 mm] / 1000 ] = 4.85 cc


[ Aside - For the head gasket and piston-to-deck-height part of the calculation you must convert from inches to mm where 1 inch = 25.4 mm:

(0.029 in. + 0.005 in. ) x 25.4 mm per inch = 0.864 mm

cc stands for cubic centimeters where 1000 cc equals 1L of engine displacement or combustion chamber volume]


therefore,

Static Compression Ratio = (Volume at BDC) / (Volume at TDC)

= [ 499.1 + 42.7 + 4.85 - 7.45 ] / [42.7 + 4.85 -7.45]

= 539.2 cc / 40.1 cc = 13.4:1 static CR

in this particular example.


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Some Useful Specs for Calculating Static CR:


a) Piston to Deck Height

B18A/B, B18C1, B18C5, B20Z/B = 0.762mm ( 0.030 in.)

B16A/B = 0.508 mm (0.020 in.)



b) Piston Dome Volume

B18A/B (PR4/P74) -3.2 cc

B18C1 (P72AO) -0.60 cc
JDM GSR(P72OO) +2.52 cc

B18C5 (P73AO) +3.64 cc
JDM Spec R (P73OO) +5.96 cc

B17A (P61) 0.00 cc

B16A (PR3) +6.01 cc
JDM B16A (P30) +6.93 cc

B16B (PCT) +8.63 cc

B20Z (PHK) -4.04 cc
B20B (P3F) -9.92 cc



c) Head Gasket Thickness

Stock 3 layer: 0.74 mm (0.029 in.)

Modified Stock 2 layer: 0.49 mm (0.0193 in.)

Modified Stock 1 layer: 0.25 mm (0.0097 in.)

Aftermarket Mugen 2 layer: 0.47 mm (0.0185 in.)

Aftermarket Spoon 2 layer: 0.45 mm (0.177 in.)

Subtract the stock thickness from the thinner headgasket's thickness to get what the equivalent to head milling would have been. For example, with a GSR head (smallest head volume of the Integras) using the Mugen headgasket on a stock GSR block , the static CR would be lowered by approx. 0.4:1 CR with the 10.5 thousandths difference in thicknesses (i.e. it's equivalent to milling your head 10 thousandths).




d) Cylinder Head Combustion Chamber Volumes


B18A/B: 45 cc

B16A, B16B, B17A, B18C5: 42.7 cc

B18C1: 41.6 cc





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2. Dynamic CR

The piston is always moving up and down but the intake valve opens and closes during this time as well.

As the piston is beginning to squeeze at BDC, the intake valve is beginning to close. The intake valve is not completely shut until the piston is near TDC. There is a connection between the cylinder combustion chamber and the intake port/intake manifold runner, when the intake valve is still partially open. As the piston is squeezing and approaching TDC, some cylinder pressure can bleed up into the intake port which reduces overall cylinder pressure.

If you use your adjustable intake cam gear to close the intake valve earlier (advancing the cam gear), the amount of cylinder pressure bleeding up the intake port is reduced. The cylinder pressure builds up faster and you get a better burn.

If you let the intake valve close later (retard the intake cam gear or use a longer duration intake cam), more cylinder pressure will bleed out or be reduced and the burn will be less complete.

This is why it is important to increase your static CR when you get extremely longer duration cams.

You want a fast, complete burn of the air fuel mix to make power.



3. FINAL THOUGHTS


Remember when we talk about CR, the first number is the volume at BDC, second number is volume at TDC...the higher the first number, the more squeeze...


Original Post can be viewed at: http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=233



EDITED BY MARTINI: Per original writter of this artical I was asked to include a link to the original post on Team-Integra: http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=233