Rb30det With A Cr Of 9.0:1 ?

[Cubes]

You are going to have to look at custom pistons to get the 9:1 comp ratio.

It costs stuff all more for a custom set but at least you get the squish and comp ratio you are after.

[Scribble]

Any info on the 8.5's? I know stockymcstock said standard SIZE rb30 n/a pistons would get ya 8.3

Customs again?

Edited November 11, 2005 by Scribble

[Roy]

Gibson also rebuilt their engines every race meeting, oh and it was a race car running race fuel, we are talking street cars here that have limited fuel choices.

<{POST_SNAPBACK}>

I got no idea what compression ratio is ideal, but the Group A cars were meant to be running on control Shell fuels. I seem to recall the fuel was not too exotic, actually i dont recall there being any mention of a fuel change when they changed from Grp As to V8s and these days they do run Shell Optimax. What the equivelant back in 90-93 was im not sure, but it wasnt too exotic

[PUS16]

why do you want to run such a high C/R anyway? sacrificing reliabilty, my motor is 30/25 i have a C/R of 7:1 as it allows me to run higher boost and then bring my to4e66 turbo into range(round 25psi) allowing it to make good power. 480bhp with standard manifold, standard cams and stock camgears!

The bathurst sierra cosworths ran 5:1 C/R allowing it to run up to 45psi boost to bring the turbos into range!

[R31Nismoid]

7.1 holy god, that would be a pig of a car off boost.

I aint heard of that since the VL boys out West and thier "fully sick decomp plate" for 11ty billion psi

You dont need it to be that low at all for 25psi

[GTR_Junkie]

The reason i am wanting the CR of 9.0:1 is to allow me to make my target power with as low boost as possible.

Cheers

[Cubes]

Stick with the 9:1 it will work well, especially with a set of lumpy sticks.

Problem is there are no on the shelf pistons that get anywhere near that.. 8.6:1 I think is the closest you will get.

Unless CP's run some wierd pinheight, dome config.

[GTR_Junkie]

well my engine builder knows what he's doing so if he says he can get it i will go with his word

[PUS16]

it may not need to be that low but as it is my daily driving car on pump fuel with no additives it suits my purpose and i still have full boost by 3000rpm! and i have a mate of mine with a vl that had 9.5:1 c/r making 400bhp on 22psi and he melted 2 pistons as apposed to any mate of mine that has the exact same setup except he was running 7.8:1 c/r making 412bhp on 26psi and hasn't had a drama so imo i'd rather be on the safer side rather than pushing it to the brink! But that's just me

[R31Nismoid]

motor's dying are generally due to

1) owner

2) tuner

very rarely the motor itself. If it melted a piston, it was on the edge.

Anything thats on the edge will die

[Cubes]

Munro's GTR was setup to run a 9.6:1 comp ratio if I remember correctly.

Never did hear what happened to that.. I know it spun a bearing on its first couple of outings..

[lukits01]

Can anyone please point me in the right direction to find RB30 off the shelve forged pistons that would give 8.5:1 or higher compression ratio?

A store that sells them for reasonable price?

[discopotato03]

One solution is JE forged VG30DETT pistons at 87.5mm . The gudgeon pins are larger so rods need work .

If boost pressure is a measure of resistance to airflow then high boost is a lame excuse to get more air in . If you reduce the resistance to flow then same volume at lower pressure will make everything else easier . High boost pressure needs high fuel rail pressure/expensive pumps/expensive fuels/etc .

The RS500 YBD engine had a dog of a turbo making an unreliable pig of an engine for road or Grp A . You need to look at better examples than this .

Cheers A .

[MattSR]

I agree with disco, the YBD had some f**ked mods done to it to make the shitbox turbos work on them. It was a very sad turbo system indeed...

[lukits01]

I was talking to Mick from Rocket Industries and he emailed me this:

The RB25 head has a a larger combustion chamber than the the RB26.

The dome on the AP332104 is .110" high. The dome on the AP332105 is .155 high. This will give more comp.

I spoke with our RB expert here and he reckons that the AP332105 might be a better option.

I believe the AP332104 are ARIAS pistons that give 8.5:1 CR and AP332105 are pistons that are used for RB26/30 convertion. I was advised to use the AP332105 pistons for RB25/30 convertion. Two questions though:

1. What kind of CR would this give

2. Would the higher piston sit proud of the block requiring thicker metal head gasket

[Cubes]

Still... I wouldn't be going by what some one says..

Its much safer to simply measure it all up, you then know what + or - dome required. My Rb25 head cc'd up at 62.2cc's.

The Rb25 and 26 pistons also vary in their Pin height. So this is something also to look at if you want to optimize squish, which why wouldn't you for a little bit of research.

Edited November 25, 2005 by Cubes