Rb25 And Rb30 Cranks

[Robbo]

I've got a 2 mates who are both building RB30DET hybrids.

I've been wondering, if its possible to put an RB30 crank into an RB25DET block, with the appropriate rods.

RB30E - 86mm bore and 85mm stroke.

RB25DET - 86.0mm bore and 71.7mm stroke.

The reasons I'd like to use the crank rather than the whole RB30 block are that;

- I'd like to retain the oil squirters from the 25 block.

- Also because the engine is destined for an R33, if we use the RB25 block we won't have to worry about slotting engine mounts, or cutting bonnets.

- No dramas about VCT oil galleries

- And finally it will be easier to organize the timing belt.

So why don't people do this method more often? Is it because finding rods to suit will be hard and/or expensive?

Thanks

Robbo.

[SLIMVL]

you would be waistin your time, it would not increase the capacity,cos you are limated by deck hight, i think that a rb25 is about 30 mm shorter or something. it would be posible but pointles

[B0oStEr]

I doubt it is possible as I doubt the crank and rods would clear the block.

If they did the rod angle and ratio would be far from optimal.

Poor torque and high piston speed.

[R31Nismoid]

if you must, use a 26 crank/rods

although you aint really gonna get much more out of it.

100cc tops.

[Sydneykid]

As you correctly pointed out the RB30 crank has a 13.3 mm longer stroke then the RB25. From memory an RB25 conrod is ~114 mm centre to centre. That means you would need ~100 mm rod to fit the RB30 crank to the RB25 block and still have the pistons not protrude from the block at TDC. You would need to have conrods specially made for this application (ie; not cheap).

That's a rod/stroke ratio of ~1.17 to 1, and that's the problem. Many people consider the RB25/26 rod/stroke ratio to be less than optimal at 1.6 to 1. With the RB30 at 1.7 to 1 being much more desirable. A rod stroke ratio of 1.17 to 1 is unthinkable. It would place far too much side load on the conrods as they have to lean over on rotation. They would fatigue and fail very quickly.

That's why OS Giken 3 litre kits for RB26's come with ~18mm block spacer (sits between the block and the cylinder head), that way they can accomodate a decent length rod.

If it was easy, many many people would have done it. But it is neither easier or cheap and will not give a workable result anyway.

cheers

[Robbo]

SlimVL: Increasing a stroke increases displacement, regardless of deck height.

B0oStEr: "Poor torque and high piston speed"

If an RB30DET is proclaimed as a torque monster, and its torque is due to it's stroke, then why would you consider the exact same bore and stroke to have poor torque?

And do shorter rods really give a greater piston speed?

Sydneykid thanks very much.. I knew there was a reason why people didnt do it.. just couldn't think of what it would be.

I had the idea in my head and I could only see positives! All the benefits of the 25, all the power and torque of a 30, only even more revs due to lighter/smaller rods. Ohwell, scratch that idea... 30 block it is then.

Robbo.