I need someone who actually KNOWS alot about RB motors to help me out, I have heard that rb20 and rb25 blocks are exactly the same, the only difference is the boar and stroke, so if i took an rb20 and boared it to fit the rb25s pistons and put rb25 rods in there, then i would have an rb25 right? because the cranks are supposed to be the same. or if i wanted to go the cheapo way i could just take the stock rb20 and replace the rods with rb25 rods and and have like an rb22 or something right?
Also, what about the heads? i hear they are different but are they interchangable?
RB20DET specs:
No. and arrangement of cylinders = 6, in-line, vertical
Combustion chamber type = Hemi-spherical
Valve arrangement = Overhead valve type
Camshaft arrangement = Double Overhead Camshaft
Total displacement cc = 1998cc
Bore x Stroke mm = 78.0 x 69.7
Compression = 8.5:1
Bhp = 214bhp at 6600rpm at flywheel
torque = 185 lb/ft
Engine
Type RB25DET - Turbo
Valvetrain dohc 4-valve/cyl
Displacement 2498
Bore & Stroke [mm] 86.0 x 71.7
Compression Ratio 9.0:1
Max. Power [Bhp at rpm] 250 at 6400
Max. Torque [lb-ft (Nm) at rpm] 217 (294) at 4800
Bhp/Liter 100
The theory around changing the parts over is sound enough. BUT by the time you have spent all the money, time and effort to put it together you may aswell have just gone and bought and RB25DET engine anyway.
Just my opinion but i would not waste my time going through all of that even though its possible.
Hey thanks dudes! for your input! Your right it would be just as expensive and alot more of a pain in the ass to convert a 20 into a 25 instead of just buying a 25. BUT.... if you did convert the 20 you would have a bad ass bottom end that you could shove 20+ pounds of boost into that would kick the bottom end "pun intended" of any stock rb25. you goda think about that to. I do know now that the heads ARE alot different, major upgrades were made when the 25 came along, in other words the rb20 head sucks!
Hey thanks dudes! for your input! Your right it would be just as expensive and alot more of a pain in the ass to convert a 20 into a 25 instead of just buying a 25. BUT.... if you did convert the 20 you would have a bad ass bottom end that you could shove 20+ pounds of boost into that would kick the bottom end "pun intended" of any stock rb25. you goda think about that to. I do know now that the heads ARE alot different, major upgrades were made when the 25 came along, in other words the rb20 head sucks!
It really depends what you want... For any decent power level the RB25 really should be pulled down and have a set of pistons thrown in it.
If you go the RB20 running an RB25 or RB26 crank (2.3 & 2.4ltrs respectively) you have a nice fresh motor that as you say 'you would have a bad ass bottom end that you could shove 20+ pounds of boost into'.
This is one reason I went the RB30DET, I couldn't bring myself to spending so much $$ on a second hand RB25DET when I knew it really should be pulled down and have pistons thrown in it as my goals are ~300rwkw. Sure 300rwkw can be done on a stock motor but I don't like pushing my luck. Especially when you invest so much $$ in to a car. You don't want to be investing more when you could have got away with doing it right the first time.
So I grabbed a $100 RB30 bottom end rebuilt it with nice bits, ported/polished/valve deshroud an rb25de head from an R32 and dropped it in.
If you have money to burn go the rb25 route if not build a tough rb20det and shove 20 odd psi worth of boost down its throat.
As i'm sure you know but just to make sure... The RB25 and Rb26 rods are the same length, the RB26 rods are simply tougher. Its the crank that determines the stroke and depending where you are situated can be picked up for pretty damn cheap.
Now just to track down a set of pistons to suite the rb20's bore size and pin height required. :P
rb30 with a 25 head? hmm... interesting, i heard the rb30s sucked so i havent really looked at them, 3.0 litres would be nice tho, are they pretty cheap? what about parts for them? i havent seen alot around. thanks for your input you have been most helpful.
A friend of mine did the rb30det on a budge, cheap $50 rb30 270,000km bottom end that was in good nick, rb25 head. Dropped on a gt35r (machined .82 diesel int. gate turbine house) on to the stock exhaust manifold, injectors, ecu and made 312rwkw on 18psi.
Since he slapped the motor together he's done 40,000km's, still running strong. :P
They are indeed very tough motors, the longer rods improve the rod/stroke ratio creating less side load (which is what breaks rods).
THe ONLY issue with the rb30 is apparent harmonics around 7500rpm with the std skyline balancer, this is rectified by the use of an aftermarket balancer.
I know of another RB30 that is still running the sohc head, that runs a gt40r and pushes 370rwkw on the stock bottom end. Still going strong for the last 6months that I know of.
The down fall of all rb's is not their lack of strength its poor tuning.
Early RB's do have a weakness with their crank oil pump drive, if you notice its not the full width of the oil pumps internal gear, this eventually breaks. Very common in rb26's.
An easy fix is to use a jun style crank collar. I believe nissan fixed this flaw in the S2 R33 RB25det's and R33 RB26's.
Essentially they really are a tough motor and parts can be sourced easier than that of theimprted motors. The RB30 here in Aus was released in a few cars, the Nissan Patrol, Nissan R31 skyline in N/A and the Holden GM VL Commodore in both N/A and turbo, they have been around since the late 80's so parts are plenty full. Here at least. :P
Recently one bloke has machined and destroked an rb30 crank in order to fit it in to an rb26/25 block, used rb26 rods and custom pistons to achieve 2.9ltrs. It has no issues flicking to 8k but has ran out of fuel so until larger injectors go in to push past 400rwkw I won't hear much more from it.
Last edited by Cubes : Jul 17th, 2006 at 12:55 AM.
RB internals, bore, stroke... ect
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