Jump to content
SAU Community

Recommended Posts

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.

Link to comment
https://www.sau.com.au/forums/topic/84987-rb25-and-rb30-cranks/
Share on other sites

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.

:D cheers :D

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.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • Ok i will get those 310mm. I found one but on a different site. This is the description on those...is it ok? Technical parameters: - Axle: front. - Disc type: ventilated. - Number of holes: 5. - Disc diameter: 310mm. - Total height with center: 54mm. - Thickness (new/min.): 30/28mm. - Designed for brake calipers manufacturer: Sumitomo.
    • You Gregged a whole racetrack!?
    • Look for broken wire or bad connector at the motor. Might not be it, but is worth starting there, as it is easy.
    • Hi everyone, I’m having an issue with my R32 GT-R. Sometimes, when the car goes over a bump or experiences some vibration, the 4WD warning light comes on the dashboard. When I check the code from the control unit in the trunk, it shows Code 19 – ETS Motor. However, everything seems to be working fine — if I turn off the engine and restart the car, the light goes away and everything functions normally. Has anyone experienced this before? Where should I start troubleshooting this issue? Thanks in advance!
    • I'm back from the dyno - again! I went looking for someone who knew LS's and had a roller dyno, to see how it shaped up compared to everything else and confirm the powerband really is peaking where Mr Mamo says it should. TLDR: The dyno result I got this time definitely had the shape of how it feels on the road and finally 'makes sense'. Also we had a bit more time to play with timing on the dyno, it turns out the common practice in LS is to lower the timing around peak torque and restore it to max after. So given a car was on the dyno and mostly dialled in already, it was time for tweaking. Luis at APS is definitely knowledgable when it came to this and had overlays ready to go and was happy to share. If you map out your cylinder airmass you start seeing graphs that look a LOT like the engine's torque curve. The good thing also is if you map out your timing curve when you're avoiding knock... this curve very much looks like the inverse of the airmass curve. The result? Well it's another 10.7kw/14hp kw from where I drove it in at. Pretty much everywhere, too. As to how much this car actually makes in Hub Dyno numbers, American Dyno numbers, or Mainline dyno numbers, I say I don't know and it's gone up ~25kw since I started tinkering lol. It IS interesting how the shorter ratio gears I have aren't scaled right on this dyno - 6840RPM is 199KMH, not 175KMH. I have also seen other printouts here with cars with less mods at much higher "kmh" for their RPM due Commodores having 3.45's or longer (!) rear diff ratios maxing out 4th gear which is the 1:1 gear on the T56. Does this matter? No, not really. The real answer is go to the strip and see what it traps, but: I guess I should have gone last Sunday...
×
×
  • Create New...