Tag Archives: compression ratio

How to Compute Compression Ratio, Head Volume, Dish, etc.

I am planning a motor build for Project Domino. There is nothing worse than spending money on parts to only figure out you ordered the wrong thing. In this particular case, I wanted to make sure I ordered the right pistons the first time to achieve a reasonable compression ratio for pump gas.

Compression Ratio

I want to get to the basic math without going too deeply into how engines work, so I will provide a brief term overview. I will use Domino’s EJ207 for an example.

Computing Displacement Volume

Displacement/Swept Volume refers the volume displaced from the piston moving up (top dead center) and down (bottom dead center).

It is computed by the surface area of the piston times the stroke of the piston, times the number of cylinders.

displacement per cylinder = π * (bore / 2)² * stroke

total displacement = π * (bore / 2)² * stroke * num_pistons

eg. EJ207’s displacement = π * (92mm / 2 )² * 75mm * 4 = 1994cc

Computing Compressed Volume

Compressed/cleared volume refers to the volume between the surface of the top of the piston when at top dead center and the top of the cylinder head.

compressed_volume = piston_dish + ((bore / 2)² * (piston_deck + gasket_thickness)) * head_volume

eg. EJ207 compressed volume = 6cc + ((92mm / 2 )² * (1.42mm + 0.6mm)) + 50cc = 69.5cc

Computing Compression Ratio

Engine compression is essentially taking the displaced volume and squishing it into the cleared or compressed volume as the piston travels from bottom dead center to top dead center. Compression ratio is therefore:

compression_ratio = (cylinder_displacement + compressed_volume) / compressed_volume

eg. EJ207 compression ratio = (498.5cc + 69.5cc) / 69.5cc = 8.2

Now What?

Now that you have the basic idea of how the compression works, how is it useful?

Given some fixed variables, you can figure out how to adjust the others to achieve a reasonable compression ratio.

Note: if you are not increasing the head volume, then the only way to change the compressed volume is to modify either the piston dish, piston deck, or gasket thickness. Ideally, you modify it with the piston dish for reasons I will not go into. Rods length can change the piston deck, so consider it a factor when selecting the pistons.

The main way to change displacement is modifying the bore and stroke. For EJs, 2.1 stroker is *roughly* a 92mm piston with a 79mm crank, a 2.34l destroker is a 75mm crank with 99mm pistons, a 2.6 stroker is a 83mm stroke with a 99mm pistons…etc.

Applied Math

I wanted to build a 2.36l destroker with +2mm long rods. I won’t go into all of the reasons, but the thrust of it is that I wanted more displacement while keeping somewhat in the high-revving spirit of the EJ207.

Computing displacement is easy, we already have all of the variables:

displacement = π * (100mm / 2 )² * 75mm * 4 = 2356cc

I know I want to hit a target compression ratio of somewhere around 8.5-9.0, we don’t have the compressed volume yet.

We need: piston dish, compute piston deck, and factor in gasket thickness. We will keep the head volume at 50cc. I also don’t want to order custom internals, so we will stick with Manley’s catalog.

The long rods are +2mm longer or ~132.5mm center to center. Destroker pistons made for standard rods have a compression distance of 32.6mm. To keep piston deck constant, and not have the piston sticking out of the block, we need that to be 2mm less. Pistons for 79mm cranks are 30.7, and 83mm cranks are 28.7mm. So, the 79mm crank pistons are good, or ~1.9mm less, which makes sense. The stroke difference is 4mm / 2 = 2mm. So, the new piston deck is 1.53mm to account for the 0.1mm difference.

Manley’s piston has a -17cc dish.

compressed volume = 17cc + ((100mm / 2 )² * (1.53mm + 0.6mm)) + 50cc = 72.3cc

Compression Ratio = ((2356cc / 4) + 72.3cc) / 72.3cc = 9.15

That is a little on the high side, so how could we hit a lower target? Basically by increasing the compressed volume. If we wanted to hit say 8.5, what would it have to be?

((2356cc / 4) + X) / X = 8.5
=>(8.5 * X) – X = 589
=> X = 589 / 7.5 = 78.5cc

Or, we’d need 6.2cc more compression volume to achieve a compression ratio of 8.5. This can be done with the head gasket, or finding another piston with more dish, or increasing the head volume.

Appendix:

I found this helpful table on NASIOC for EJ motors. Hit the source link if you want to see it on that site.

Source: Titter
Source:Titter