Jump to content
SAU Community

Recommended Posts

Hello !

I'm building an engine for my Skyline R33.

I want to use the RB26 shaft in the RB26 block.

Configuration:
RB25 S2 engine block
RB25DET NEO cylinder head
Mahle Motorsports RB25DET S2 87.00mm forged pistons
Forged H-Beam connecting rod 119.5mm
Crankshaft RB26DETT

Will this configuration be ok? I'm still thinking about converting the pistons from S2 to NEO pistons using a milling machine. I'm worried about the compression ratio. I already have all the parts except the connecting rods.

Link to comment
https://www.sau.com.au/forums/topic/485183-rb25det-crankshaft-from-rb26/
Share on other sites

You’ll need a 26 piston due to the pin being in a slightly higher position to compensate for the increased stroke, you can then machine the crown to take the compression to where you want it to be 

3 hours ago, Dawid20 said:

I want to use pistons from RB25 but use forged 119.5mm connecting rods and additionally adapt the pistons to the NEO head.

Your pistons will be 1mm below block deck which means less ideal squish/quench so may be easier to knocking.

Stock 25det : 121.5mm rod length + 31.5mm piston height + 35.85mm half stroke = 188.85mm.

Your combo 119.5mm rod length + 31.5mm piston height + 36.85mm half stroke = 187.85mm.

Sell your pistons and buy rb26 pistons, mill down its dome to get proper CR for rb25det neo head, that way you have proper piston to deck clearance/squish and uses common rb25/26 121.5mm rods which also have better rod/stroke ratio than less available shorter 119.5mm rods.

If have to use rb25 piston with 121.5mm rod, pistons will be 1mm above block deck :

121.5mm rod length + 31.5mm piston height + 36.85mm half stroke = 189.85mm

Can use thicker head gaskets to compensate pistons above block deck, cometic has rb25det 87mm head gasket thickness of 1.8mm, 1.9mm, 2mm, 2.5mm, that will work fine with pistons 1mm above block deck :

https://www.cometic.com/applications/automotive/nissan/rb-series/rb25det--152ci25l-i6

  • Like 3
19 minutes ago, silviaz said:

I heard rb25 neo engines use a rb26 crank, is this true? I can't find any info on it and the part numbers are different. Makes me wonder for the neo engine since there are less parts, what do you do if the crank is completely stuffed.

They use rb26 rods, not an rb26 crank. The throw of the crank determines the capacity of the engine (rb25 vs rb26) as the bore diameter is the same

  • Like 1

Here's some previous threads from this very site.

This one didn't resolve very clearly.

 

This one shows the compression situation

One where I piped up with the usual comment, and @r32-25t with his ditto, and some other useful tidbits.

 

If it were me doing it (and I might, if I ever have to build my 25, because I have access to a 26 crank if I want it, despite me previously saying that it's hardly worth the effort), I'd just call up Nitto and say, "Hey, here's my plan, what piston can you magic up for me?", and leave it to them to make a nice forgey with the required combo of pin height and crown volume. They probably use the same blanks for all their RB pistons anyway, and just carve out what needs to be carved out as required.

  • Like 1

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

    • Yeah I suspect even if you hold airmass per cycle/cylinder constant if you get too far away from stock you're still going to have problems running the factory tune within the bounds of the factory load scale. Cams, different displacement/rod ratio, etc. I'm just lucky that the GTIII-SS with wastegate boost + CA compliance cats is pretty much equivalent to stock turbos. When I have actual space I can finally get it tuned and modify the fuel system for flex fuel to 100% handle any detonation concerns when cranking the boost to whatever those dinky turbos can put out.
    • I would say no, why, because my daughter, who also lives in Goulburn, hasn't recommended us going there Pity, as we miss all the German joints around in Sydney, actually, the restaurants are the only thing I really miss about Sydney, and a special mention to Ishibanboshi at Bondi Junction, their Kara-age Don is heart cloggingly deliciousness (always added a special boiled egg...or 2) 😋 
    • Does that German restaurant still exist in the old place out the NW end of Goulburn? When I say "out the NW end of"...I am really being vague. It was 1997 when I was last there, and the only point of reference I can recall is that it was on the opposite side of the main drag from the big merino. And when I say "opposite side of the main drag", I don't mean "on the main drag". It was either a couple of streets back from there, or might have even been out in the sticks a bit further. Was an old farm building or mill or somesuch. And when I say "the big merino" I might actually be thinking of a completely different part of town, because I just looked on maps and the big bugger is not where I remembered him to be! The food was good, consisting largely of various German mystery-meat sausage/loaf things and kartofflen.
    • So while the second sentence is completely correct and the whole point of the conversation, the first sentence bears consideration. If this bloke is just hoping to throw big turbos on and drive it around, because there are no helpful facilities at all in his tropical paradise** then he likely has zero chance of even knowing what the TP is on the last column in the stock maps, let alone know whether the ECU is operating anywhere near it or past it. So the point is very very moot. And, per what I said before, at stock boost on those turbos, you may well be off the end of the map. **I'm just back from Vanuatu, so I know exactly what small Pacific nations can be like wrt paradise without requisite facilities. But it's not even that simple. I put a high flow on my car and had to drive it around without a proper tune because of the lack of opportunity*** to put the bigger AFM and injectors into it to allow it to be tuned. I had to turn the boost down to less than I had before, and back off the boost controller's ramp, because it was exploring parts of the map that it didn't drive in before, and really couldn't access for tuning on the dyno either, and so was pinging. It was still well within the last column, because when I first**** set up the Nistune on the Neo I rescaled all axes of the maps to give some more space to explore. ***Family dyno was broken ****This was 13 years ago, and the TIM thing wasn't a thing then and so TP would definitely grow when pushing past the stock tune's limits.
    • Yep, this bit another local owner. I caught it before putting the transmission back into the car, what I noticed was the pressure plate fingers weren't flat and even. It's more obvious with the pull style clutch because the throwout bearing ring was visibly not flat once everything is put together. Nismo should really update their instructions to call out this specific detail. I'm not even sure the clutch as-shipped orients everything properly.
×
×
  • Create New...