Teach me how to calculate CR

[entirelyturbo]

As many of you already know, I'm planning an overbudget, underachieving build with an EJ18 block, EJ22T bottom end, and EJ205 heads. I just picked up an EJ18 longblock this past Sunday.

Most are asking why I'm doing this, and it's really for only two reasons:
1) To FINALLY have some sort of a boost fix
2) To do something different, and to do it myself

All suggestions to do something different, and not waste my time, aside (I've heard all of them already, believe me, please, no more), I need some help.

I need to know how to calculate the CR of the resulting engine that will exist when I mate the EJ205 heads to the EJ18 block.

I know it involves the headgasket thickness, the capacity of the heads, and some other stuff, but I dunno what to do with all of it.

The reason I ask, is because I want to know whether the EJ205 heads will bring the CR down to boost-friendly levels, or whether I'm gonna have to have some custom pistons built, which would be EJ18 bore and most likely have a cupped crown like the EJ22T pistons.

Any help would be greatly appreciated!

[napphappy]

Here is a great calculator.
http://www.rbracing-rsr.com/compstaticcalc.html

[greg donovan]

this is a good page for some measurements of ej series motors and various head and block combinations:

http://wac.addr.com/auto/obs/turbo/ejcalcs.html

[tzedek]

With the EJ18 gasket it comes out to 7.86:1 with the EJ205 head gasket 8.33:1 thats from me looking at ejcalcs and typing in the numbers to the link provided above.

[tzedek]

The reason I ask, is because I want to know whether the EJ205 heads will bring the CR down to boost-friendly levels, or whether I'm gonna have to have some custom pistons built, which would be EJ18 bore and most likely have a cupped crown like the EJ22T pistons.

Any help would be greatly appreciated!

EJ205 heads are going to bring the compression down to great levels for a turbo setup, becuase of the larger bore from the 205 and the head volume of the 205 is much larger than the ej18. cheers

[entirelyturbo]

Good stuff! Thanks guys!

[206er]

head volume+gasket volume(quench volume)+piston volume+displacement=total volume at BDC
devide that by the area at TDC(all previous #s except displacement) and that is your CR.
sounds like an interesting combo, from what i hear the ej18 is pretty strong because of the thick sleeves. a low compression ratio cant hurt. not to mention if you do break it they are near free or free to find. you could always just put an ej22e block in as well.

[THAWA]

I dont see the point of replacing forged crank and rods with some different forged crank and rods. It's not like the balancing/specs are any different.

[entirelyturbo]

Well, I can now officially confirm that the bottom ends are the same...

... in fact, the EJ18E, EJ22 (E and T), and EJ205 all have identical rods. All part numbers for the rods all supercede to the same part number, 12100AA240.

Also, the crank part numbers for the EJ18E and EJ22 (E and T) all supercede to the same part number, 12200AA150. However, the EJ205 crank goes to 12200AA240.

I will probably just strip the EJ205 I get for its bottom end, since the rods are the same, and the crank is probably better.

The EJ18 I have does have 230k miles on it, so I will be tearing it apart regardless. I might as well throw the EJ205 bottom end in it.

So, I probably won't be buying an EJ22T then.

[THAWA]

You'll be going to the machine shop to get that 205 crank to fit the 18E block.

[entirelyturbo]

Umm, yeah, you're right. Phase II crank in Phase I engine might cause some issues. Duh. Brain fart.