What Differences Internally R33/r34Det Short Blocks ?

[discopotato03]

Hi all , I've seen the question asked often about chamber and piston volume differences in R33 and R34 RB25DET engines .

From reading about engines in the late 90s I know there was a push for LEV or low emission cars and ULEV for ultra low emissions approaching the 00s or naughties . Mechanically I think it involved differences in power steering pumps and belt drives and possibly lower drag alternators . Often VCT and valve timing changed and engine management was set up differently particularly for cold starting . By that stage most engines already had cooling systems with bi level thermostats and big bypasses to help them warm up quickly .

I just ran a couple of searches to try and find out more about specific chamber and piston volumes in the R34 Neo RB25DET but couldn't find anything conclusive ie pics and exact measurements .

Wiki reckons the chambers are different , smaller , and Nissan compensated with RB26 spec connecting rods which sounds odd . To have a piston further down the bore at top dead center doesn't sound like a positive to me , dunno .

I did find a thread at an American site where a fella tried using an RB26 crank in a 25 block with 25 pistons and not surprisingly the pistons ran proud of the blocks deck . Should have seen that one comming .

The thing is this fella eventually mentions a difference in chamber volume from 33 to 34 as being 63cc and 51.5cc .

It would be really good if someones whos been inside a stock standard RB25 Neo can tell us what they actually have in the chambers/in the piston crowns/if the rods are different length wise .

Assuming the chambers are smaller ie 51.5cc it starts to make the cheap RB30 conversion look good and may even get the static CR up around 9.5 instead of the 8.2 or whatever you get with an R33 head on an NA RB30 short block .

Anyone know the real story ?

A .

[KiwiRS4T]

This is from a piston supplier:

The Neo head is 51cc & the normal rb25 head is 62.5cc

I believe the block for the neo is the same as for the R33 block but that's not based on actual observation.

[tridentt150v]

And to add to Disco's questions

Are the conrod lengths the same

Are the pistons the same

ie either a lower gudgeon pin height in the piston or a shorter conrod? or are they the same and the squish area is 18.4% less ie much closer tolerances?

[GTSBoy]

Well, the conrods are RB26 items and the pistons have the gudgeon height right to get the crowns in the right place. How much more do you need to know?

[JonnoHR31]

If they were the same then you wouldn't get:

yes you would, the 26 crank has a longer stroke than the 25, hence simply having a 26 crank in a 25 block with 25 everything else doesnt work.

not 100% sure but from what i've seen on here over the years the rods are all the same length (all rb25 and 26s), with the neo getting the stronger (but not longer) 26 rods. the neo has a smaller chamber with a different piston design to allow for it, which was probably a better squish design, hence why getting aftermarket neo pistons in the right compression ratio isnt always easy.

[tridentt150v]

OK, thanks Jonno, explained.

So in essence the head volume is less but the piston shape negates this, so the final piston/chamber combo volume at TDC may well be the same.

[discopotato03]

Ok , so how much more do I want .

I wanted to know the details in case there was anything people can work to their advantage .

If I was going to do an RB25 head again I would go with a Neo one because of its smaller chambers and non hydraulic buckets . Really anything that potentially has better quench and resists detonation is worth having , Nissan must have had good reason to make these changes and it looks like emissions/consumption reasons to me .

A .

[GTSBoy]

Well, in that case you don't want to go digging through aftermarket piston catalogues looking for things that are pretty much not there. You just need to get an old RB25 piston and a Neo piston in hand and see that they are different and where they are different (which is pretty much crown height and shape), realise that you most likely can't really mix and match with anything else that'sout there in RB25 aftermarket land and just go get some CPs or Arias or whatever floats your boat custom made like everyone else who has had to do so. It's either that or use some non-turbo RB25 pistons (possibly with some extra valve relief), remove the turbo and run on E85 or meth or LPG.