XB9 Piston -> XB12 Motor write-up thoughts (long and not 100% complete yet)

[upthemaiden]

When the XB9 piston is installed into the XB12 motor, the added volume of the XB9 piston will decrease the combustion chamber volume of the motor.

That's the part I was referring to. I don't know much about the stuff, but saying that the displacement would stay the same doesn't sound right to me. I would think 'combustion chamber volume' would be the same thing as displacement. Even if the piston only moves the same distance, moving the position of the piston higher or lower would make chamber volume bigger or smaller when the intake valve is open, letting more or less gas/air into the chamber, increasing/reducing airflow that travels through, increasing/reducing the displacement. Right? Who knows.

[Sirius815]

Nope - displacement is how much volume is "displaced" when the piston moves up and down. The combustion chamber volume is the volume when the piston is at top dead center. The RATIO between top dead center and bottom dead center is compression ratio. The VOLUME between top dead center and bottom dead center is the displacement.

Yes, there is less total volume at bottom dead center (12.1cc less due to the dome on the xb9 piston), however the same decrease in volume is present at top dead center. This is why the displacement does not change, but the compression ratio does.

Trust me, I'm just a random guy on the interwebs ;)



In all seriousness, do a search on google and you'll find the difference explained with pictures - probably in either a wiki or a how-stuff-works article.

[anrkizm95]

^ schooling

[upthemaiden]

Very good, makes a lot more sense explained that way!

[pnuts]

Sorry for bringing this old topic back to the day light.

But how many of you guys have done XB9 in XB12 piston conversion? I need to take my heads of due to leaking valve seals. I want to check the condition of my valves and heads overall and I was thinking while the heads are off I could do some port and polish and maybe upgrade my pistons.

What do you guys prefer, XB9 pistons or Wiseco 10.5:1 pistons? Or something else?

Thanks

[Sirius815]

EDIT..I'll reply later.

[theMelvster6]

If you plan to run it reliably on the street...I wouldn't run it over 10.5:1 on 91octane pump gas. Anything really above that and you've gotta start running race fuels to eliminate detonation, pre-ignition.

[pnuts]

I run 98 octane here which is similar to your 93 I guess. If that makes any difference to piston upgrades...

[theMelvster6]

More compression is tough on the rods and crank. 10:1 compression stock xb9 pistons yield 12:1 compression in an xb12....10.5:1 compression forged xb9 pistons yield 12.5:1 compression in an xb12...anything above 11:1 compression and you are looking at a strictly race fuel 100+ octane engine. Also...it's not wise to run that much compression on stock crank and rods unless it's built to handle it. 10.5:1 compression is a safe limit to stay pretty reliable. Granted...depending on your cams...if there is a substantial amount of valve overlap, your engine "leaks" compression and that's how some guys get away with running more compression without issues.

[pnuts]

Are there any notable benefits in using angled domed pistons? I was thinking which one would be better, 10.5:1 flat top Wiseco or CP pistons or 10.5:1 Wiseco pistons for Thunderstorm heads with 15degree squish area? That of course would require some machine work to be done to combustion chambers but my heads are getting ported anyways so that ain't a problem