Category Archives: Project Domino

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

Project Domino: A New Turbo and a New Tune

It has been over a year since we’ve done anything with the old girl.

As a refresher, our 2001 Impreza RS had(power wise):

  • a full version 8 JDM WRX STI swap
    •  The the version 8 EJ207 engine is still stock; it didn’t seem right break down a perfectly health motor at the time
    • Version 8 STI 6-speed
    • We did the 5×114 conversion from a 07 STI
  • Custom 3″ catless turbo back
  • Custom FMIC with a ridiculously large IC core
  • Big MAF intake
  • SteamSpeed silicone inlet
  • ID1000 injectors + a Walbro 255 lph pump
  • SteamSpeed STX 71 JB turbo for JDM STI twin scroll

In that iteration, it made ~360 whp at PRE facility on pump gas.   For comparison, the SteamSpeed STX 71 on a 2015 STI made about 420 whp on the same day.  With that turbo it was rewarding to push the car, and if you kept the revs up, you could keep it in boost, but realistically it was a 4.5k+ RPM turbo on a stock EJ207.

[Domnio V3]

We wanted to do something a little more responsive.  Enter the SteamSpeed STX 67R+ BB turbo for JDM STI.

We would manufacture small batches of this turbo, and we would sell them all out before we could install one in Domino, but we able to finally get one installed.  It has the smaller 67mm compressor which should make it more responsive, but the “+” means that it still has a GTX30 spec turbine which is a little better suited for a 2.5l.  I think a GTX29 size wheel would be perfect for the EJ207.  Next update to the turbo we can make the change.  🙂

The STX 67 wheel did get us close to 4k RPM, but it still feels big.  In fact, doing pulls, it feels similar to the 71; however, everywhere else, it is night and day.  The BB CHRA is super responsive.  Going on and off the petal, the turbo is a lot more responsive.  It could also be related to the fact that we went to speed density from a MAF based tune.

What is the final result?   It made about 10 whp less with a 4 psi more boost.  Not a bad trade off.

[Dyno Plot]

[Next Steps]

The turbo still feels a little big.  Andy at FTW thought going to a top mount could give back about 500 RPM of spool.  It would probably be best for our usage of the car, but for vanity reasons we like having the huge FMIC.  We could just go to a 2.5l displacement, but it seems like it goes against the spirit of the ej207 swap.

Where do you think we should take the build next?  Let us know what you think.

Project Domino Small Update

Project Domino has been on the back burner for a little while, but we have been making a little progress.

The wiring is mostly done.  Today we got the DCCD all wired up.  The wires got damaged on the transmission, but we got them all cleaned up.

Power steering lines are hooked up to the rack, and the bumper cover is all set.

STI Twin-scroll bracket

STI Twin-scroll bracket

Power Steering Lines
Power Steering Lines

IMG_20150325_211020937
Power Steering Lines

DCCD Pro Wiring Done
DCCD Pro Wiring Done