Compression Ratio Calculation

[Splitpi]

Using a stock 2G 4G63T as an example.

My known values are:
CR = 8.5:1
Bore = 85mm
Stroke = 88 mm

I can compute the volume of the combustion chamber using the following equation and solving for Vc:
see Equation 1

I get 58.86 cc's of volume in the combustion chamber area, and this is including the volume of the stock head gasket.

Now say I keep the stock head gasket and I don't change the deck height or the volume of the combustion chamber in any way. All I change is the piston size by going 0.020 over. Mind you this is in inches, so we convert to millimeters and 0.020 inches = 0.508 mm.

See Equation 2

This yeilds a new compression ratio of 9.58:1

Is my math correct?

 

[DJYoshaBYD]

no.. that doesnt sound right.. because you are not going with a vertically bigger piston... its just wider.. you will get more displacement, but as long as those pistons are stock, then they should have the same CR

 

[Splitpi]

no.. that doesnt sound right.. because you are not going with a vertically bigger piston... its just wider.. you will get more displacement, but as long as those pistons are stock, then they should have the same CR

Bore is diameter. But since you increased the diameter, you are increasing the volume moving up with each stroke, thus raising the compression. Because what the compression ratio is measuring is the volume of cylinder area versus the area of the combustion chamber. And with the combustion chamber volume being held the same, the compression ratio increases due to the increased cylinder volume.

EDIT:

cbrue122 pointed out the error in my algebra when I was solving for Vc. Vc on a stock 4G63 is 66.58 cc's and not 58.86. I would like to thank him for that. The new CR with 0.020 over is:

8.59:1

See Figure below

Incidently if anyone cares the new displacement of the engine with 0.020 over is:
Displacement = (Pi/4)*(Bore)^2*(Stroke)*(Cylinders) = 2021.4 cc's. or 2.02 Liters, an increase of 0.05 Liters from stock.

 

[DJYoshaBYD]

it will increase the displacement.. think about it.. your compression ratio is basically how much the piston comes to the top of its stroke... you are taking a cylinder that is this big and making it smaller.. so if your piston still only reaches the same point it was before, then you increase the size of the bore, you should probably be lower in CR..

(______________) - cylinder at BDC without bore
(____) - cylinder at TDC of compression stroke without bore
(_________________) - cylinder at BDC with bore
(______) - cylinder at TDC with bore

what I am saying is, that if you are squeezing at an 8:1 ratio, then you bore and keep the stock ratio, then you are actually dropping cylinder pressure because you have a bigger bore, but still only pushing the piston to an 8:1 ratio...

correct me if I am wrong though...

 

[Splitpi]

it will increase the displacement..

You are right in that you are increasing the displacement. Hence a larger volume. If you pack x amount of air in to a space, it will yeild a certain pressure. Now if you increase the amount of air say X+2 into the exact same space you will have an increased pressure due to pushing more air atoms into the space.

This is the same principle or concept as the compression ratio. You are taking the volume of air in the cylinder and pressing up towards head (the stroke). Now the head has a certain amount of volume in it due to its domed design and due to the thickness of the headgasket. However, this volume of the head and the head gasket doesn't change with the increased displacement due to overboaring the engine.

Thus with the over-boared engine you are pushing more displacement into the same area (as the stroke did not change, just the diameter of the piston). This increases the pressure and it increases the compression ratio. Compression ratio is the measure of the cylinder volume and the head volume compared to just the head volume. Since the head volume didn't change and cylinder volume did and for the reasons stated above, the compression ratio increased.

Though my math was off on the first equations. Does that make sense?

 

[Defiant]

You're not going to gain a point-and-a-half compression by doing a cleanup bore. Twenty thousandths will net a loss in compression, as you've enlarged the area. But you won't be able to measure it.

 

[Splitpi]

You're not going to gain a point-and-a-half compression by doing a cleanup bore. Twenty thousandths will net a loss in compression, as you've enlarged the area. But you won't be able to measure it.

You are right, my math was wrong with the first equation and corrected with the second. The compression gain though is 0.08:1

Compression ratio by definition is the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. Thusly if you increase the volume of the cylinder while maintaining the exact same volume of engine when it is at the top of the stroke (i.e. head gasket, head volume) then you are raising the compression ratio (especially if stroke length did not change). Albiet, very small and quite inconsequential to the overall compression ratio. Though the increase will be a square of the bore diameter increase. Meaning if you increased it by 2mm you are adding 4mm^2 of area, which when times by the strike would yield the volume.

My main reason for asking the math check is because I'm just trying to figure out the math involved in calculating engine specs. Sort of a nerd, I know.

 

[Big Woo]

Compression ratio is just a comparison of the total volume of a cylinder with the piston at BDC vs. TDC.

I would also suggest actually measuring the combustion chamber volume with a burette and alcohol. That way you can account for the changes in valve head height due to valve jobs, and various other changes or differences. You should also consider using the burette method to measure the volume on the block side of things with the piston at TDC, and a crushed head gasket in place.

One area that is often missed the the area below the top surface of the piston crown around the outside diameter of the piston and above the top piston ring.

 

[Splitpi]

Compression ratio is just a comparison of the total volume of a cylinder with the piston at BDC vs. TDC.

I would also suggest actually measuring the combustion chamber volume with a burette and alcohol. That way you can account for the changes in valve head height due to valve jobs, and various other changes or differences. You should also consider using the burette method to measure the volume on the block side of things with the piston at TDC, and a crushed head gasket in place.

One area that is often missed the the area below the top surface of the piston crown around the outside diameter of the piston and above the top piston ring.

Good point and I agree. As that is the best way of figuring out the volume of the head after any work has been done.

I do want to point out because I don't think I made myself clear the 0.08 increase in compression with just changing the piston diameter due an overbore, is pretty inconsequential (1% increase actually) and is well within a standard of error from the engine manufacturing process so in most cases it is negligable.

But as Big Woo said, any other work to the engine and you need to do some measuring to find the TDC volume (or the amount of volume that the cylinder volume is compressing into from BDC, and little changes hear and there adds up...

 

[ImportModder]

I admire that you want to do the CR calculations involved in boring out the cylinder. You are correct in that CR does increase when you bore out a cylinder. People forget that the head combustion chamber never changes so you end up compressing more volume than than a stock bore engine. It is very small, so without actually checking your math, it sounds like you did it correctly considering what your resulting change in CR turned out to be.

 

[Big Woo]

(Importmodder)Please be careful here.

The combustion chamber can and often does change in terms of volume.
For example

1: If one has the head resurfaced, that will reduce the amount of volume in a given chamber.

2: When a valve job is preformed the valve seats are cut deeper thereby sinking the valves deeper into the chamber. This increase the displaced volume of a chamber. Additionaly grinding the valves themselves also makes them sit deeper, thereby increasing volume.

 

[Bohrn]

Just did a bunch of this in school(auto mechanic)

the equation is stroke volume + clearance volume devided by clearance volume

now of course stroke volume is pi x radius squared x height. You have these values.

But you DON'T have your clearance volume. this needs to be accurate. Its usually given by the manufacturer, but as stated above, if seats are cut, different valves used, different head gasket, decked block, milled head, combustion chamber work(deshrouding, cleaning etc) that will change.

So really, unless you can accurately find out your actual clearance volume, with head gasket on and spark plug in, then the calculations are not accurate.

 

[motomattxxx]

Just did a bunch of this in school(auto mechanic)

the equation is stroke volume + clearance volume devided by clearance volume

now of course stroke volume is pi x radius squared x height. You have these values.

But you DON'T have your clearance volume. this needs to be accurate. Its usually given by the manufacturer, but as stated above, if seats are cut, different valves used, different head gasket, decked block, milled head, combustion chamber work(deshrouding, cleaning etc) that will change.

So really, unless you can accurately find out your actual clearance volume, with head gasket on and spark plug in, then the calculations are not accurate.

You are the most correct. CR=cylinder swept volume+chamber volume/chamber volume. Other microscopic differences are so negligible that they wont much matter.

 

[fourreGsixty3]

The equation goes as follows:

V1+V2+V3+V4 = CR
-----------------
V2+V3+V4

V1=Cylinder Volume (or Swept Volume)
V2=Deck Clearance
V3=Head Gasket (when compressed)
V4=Chamber Volume ( -dome of piston, or + dish of piston)

If V1 is increased the CR will go up.
If you reduce V2, V3, or V4 CR will go up.

To find Cylinder Volume: Bore x Bore x Stroke x .7854= CV

Bore: 3.3500"
Stroke: 3.4600"