Rb30/25 Comp Ratio

[...Joshwa]

Hi all,

I am in the process of building a high comp n/a rb30/25 using RB25DE pistons. . I am aiming for a compression ratio of around 11-11.5:1

I was just wondering if anybody would be able to tell me the stock comp ratio (with the 25DE pistons in the 30 block with a non shaved head on a stock RB25 head gasket). .

And also how much the head has to be shaved/how much the combustion chambers cc at to achieve the 11.5:1 comp ratio (once again on the stock RB25DE pistons and head gasket)

Thanks a lot,

Josh

[SKiT_R31]

Hi all,

I am in the process of building a high comp n/a rb30/25 using RB25DE pistons. . I am aiming for a compression ratio of around 11-11.5:1

I was just wondering if anybody would be able to tell me the stock comp ratio (with the 25DE pistons in the 30 block with a non shaved head on a stock RB25 head gasket). .

And also how much the head has to be shaved/how much the combustion chambers cc at to achieve the 11.5:1 comp ratio (once again on the stock RB25DE pistons and head gasket)

Thanks a lot,

Josh

About 10.5:1 with the stock gasket. what your asking isn't a good idea, shaving the head can ruin squash, which combined with the fact that rb25det pistons sit recessed the the rb30 block means you will get hot spots, which will make the engine aweful at that high compression ratio (will overheat, need retarded ignition timing and extra fuel). You need to deck the block all the way which will bring the compression ratio to about 12:1, which is manageable with 98+ ron pump fuel and good management/tune. You might be able to get a reputable machinist to open up the chamber to reduce compression (whilst p&p'ing) but probably not the way to go. An alternative is using rb25det pistons, decking the block, shaving the head and running a thinner head gasket, which might get you up around where your asking (in the 11's) but i couldn't know for sure as no one has done it. A benefit of this is you could then purchase oversize forgies, increase capacity by ~ 50cc (for max oversize) and have lighter and stronger pistons (lightweight helps revs slightly with stock unbalanced motor and rods, combines with forged rods and a balance and it should be quite a weapon.)

not sure if im babbling, but the point is, the compression you will get from the 25det pistons is the most you will be able to get with them recessed the way they are. I'm assuming you want the 25de pistons for a cheaper build, so I'd recommend leaving the head as is, depends on what power your after - good flow in and out plus cams will see you 150. Though it would be smart to do the deck shaving while the block is out and bare, since its not overly costly compared to what it would cost in the future when you want more power of hit problems. I would prefer to do that and then decomp it with a thick copper gasket, than go with the recessed pistons with any higher than stock compression, as the hangover will be more costly.

Edited January 28, 2009 by SKiT_R31