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DIY Guru
96 Ranger-stock
637 Posts
Discussion Starter #1
How to figure Compression Ratio

I have noticed very few know what their real compression ratio (CR) is. Most think since they have 9.0:1 pistons, that they have a 9.0:1 CR engine. The pistons in the engine are only one part of the formula. Several other things can affect CR, such as having the head and/or block milled, over sized valves, even a basic valve job can and will change CR.

The basic formula to figure compression ratio is fairly simple, but getting the volumes needed for the formula will take a few pieces of specialized equipment.

The formula is V1+V2+V3+V4+V5/V1+V2+V3+V4


V1 = Combustion chamber volume
V2= Head gasket compressed volume
V3= Piston volume, either a + or – depending on design
V4= Piston to deck clearance volume
V5= Swept volume of the cylinder

Now it is the time to start filling in the volumes to work the formula.

I am going to start with V5 or swept volume of one cylinder.
To find this there is another formula that needs to be worked.
The formula is for finding Cubic inch displacement or CID
Since most have a 2.0l 4G63T engine, that is the base engine I will work with.

The formula for CID is
Pi x R sq x S x C

Pi = 3.141
R = Radius of the cylinder bore or half of the diameter squared
S= Stroke of the crank
C= the number of cylinders of the engine

The bore of the 4G63 is 85mm and the stroke is 88mm

To convert from mm to inches multiply by .03937

85mm bore x .03937 = 3.346 inches
88mm stroke x .03937 =3.465 inches

Now to figure R squared

3.346/2= 1.673

1.673 x 1.673 = 2.799

Now to figure CID

3.141 x 2.799 x 3.465 x 1 = 30.463 CI

Now to convert back into metric measurement

30.463 ci x 16.387cc = 499.197cc

V5 = 499.197cc
The same formula is used to figure Head Gasket compressed volume.
I am going to us a Fel-Pro Composite 9627PT
The Fel-Pro has a bore dia, of 3.445 and a compressed thickness of .039

3.445/2= 1.7225
1.7225 x 1.7225 =2.967

3.141 x 2.967 x .039 = .363

.363ci x 16.387 = 5.948cc

V2= 5.948cc

Sometimes the manufacture will publish the compressed HG volume.

Now the two most difficult volumes have been figured, the rest will go rather quickly.

Piston to deck clearance is fairly simple, the 4G63 is called a “Zero” deck engine, meaning the top of the pistons is flush with the deck of the block.

If the block has been surfaced, the pistons may protrude above the block deck by a few thousands.

This still need to be measured.

Now on to the pistons, V3

Wiseco pistons are a common choice in builds.

page 64 and 65

K559M85 6bolt 17cc dish (8.3CR)
K560M85 6Bolt 10cc dish (9.0CR)
K625M855 6Bolt 14cc dish (8.5CR)

Now on to the head, It is commonly advertised that the 4G63 cylinder head is 47cc.

47cc may be what the engineer who designed the head wanted, but in real life it may be larger or smaller than that. Starting with the foundry that cast the head blank.

Now let’s look how all this works together.

I have four short blocks in the shop I measured.

(SB1)The first one is a 6 bolt, stock rod, STD bore , 63DT pistons.
The block has been milled, the piston is .001 below the deck and has a 21cc volume, this includes the piston dish.

(SB2)The second block is a 6 bolt, Manley Turbo tuff rods, Manley pistons, Part number 6625M885 (.020 over), the block has been decked and the pistons protrude above the deck .003 Dish volume of the pistons is 16 cc

(SB3)The third is a 7 bolt, stock rod, STD bore with 63DTF1 pistons, The block has been milled, the pistons are .002 below the deck, The volume of this combo is 14cc

(SB4)The fourth is a 7 bolt with EVO pistons, E9K1, rods are unknown, the block has been belt surfaced, the pistons sit .060 below the deck, the volume on this combo is 15cc ( This engine was done at another shop)

Now so you do not have to do the math longhand, I would suggest downloading an engine calculator.

This is the one I use.
Download Virtual Engine Calculator by Challenger Engine Software

Now we need some head info that contains the combustion chamber volume.

Now lets figure some real world compression.

The bore and stroke is simple enough, along with the gasket bore.

Stroke is 88 mm = 3.465
Bore is 85mm = 3.346
Gasket bore is 3.445 (felpro 9627PT)

Since I have cc’ed the short block,SB1, we will use that number in the “Dome cc’s” slot, so that is 21cc

Now in the “Gsk + Deck” slot, Just use the HG thickness, since the volume above the piston to the deck was figured when the short block was cc’ed
HG Thickness= .039

Now we need the head spec.

This head has a thickness of 5.191 and is 46cc

Input it all in and you end up with a CR of 7.55:1

So this shows that the engine has even less compression than what is stated. Even after machine work, like block and head surfacing.

A Stock 6 bolt is stated to be 7.8:1 compression ratio

Let’s look at another at another combo.

This time we will use SB2 and head with 44.25cc
Same Felpro composite HG.

Now since the pistons are .003 above the deck, we will subtract that from the HG thickness, making the gasket thickness .036

Also this is a .020 or .50mm overbore block
So the bore has to change from 3.346 to a 3.366 bore

The wiseco piston PN 6625M855 is listed with a -14 dish, and is listed as 8.5:1 CR piston

In the block I measured a 16cc volume, the 2 extra cc’s is from the gap above the top ring and between the side of the piston and the cylinder bore.
When the math is run this combo works out to be 8.44:1

2,638 Posts
Do you have a way of calculating the dynamic compression ratio?

DIY Guru
96 Ranger-stock
637 Posts
Discussion Starter #3
That would take a computer program and a plot map to do.

Also would need flow rate of the head and a lobe map of the cam along with lb/min of turbo flow at every rpm of both engine and turbo.

I am not that smart....

1,055 Posts
Lol, I see where you understood dynamic, but N/A has a dynamic compression also... but exactly, the cam profile is one of the huge factors.

EDIT: Quickie Google search:

Needs more digging for more understanding... I was reading about this a few years ago..

DIY Guru
96 Ranger-stock
637 Posts
Discussion Starter #5
Every engine has a dynamic compression, I am just used to dealing with more turbo engines than NA lol
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