Rb26 Head On A Rb25 Block

[Simon-S14]

just to rehash and old thread.. neo's use shim over bucket solid lifters where as RB26 use shim under bucket design.. i have had 2 customer cars lately with neo's that ave spat shims out and destroyed heads with high rpm track use and tomei drop in cams. I run the same cams in my drift car and havnt had an issue with shims spitting but it makes you wonder.

[DVS JEZ]

are the customers heads built? or standard just with cams dropped in

[Simon-S14]

stock with tomei type B poncams

one of them was my mate andrew from AM performance, did track days at winton and mallala and then boom just slipped a shim out.

[JustinP]

Hmmm hope I will be OK! I am putting in the Type B Poncams and uprated valve springs at the moment.

stock with tomei type B poncams

one of them was my mate andrew from AM performance, did track days at winton and mallala and then boom just slipped a shim out.

[Rolls]

edit: simon beat me to it

Edited February 8, 2012 by Rolls

[vorse]

So to confuse the matter further can you take the neo 26 head and stick it on a non neo 26

eg r34 gtr head on a 32 gtr

[Rolls]

no such thing as a neo rb26

[JonnoHR31]

neo is r34 rb20 and rb25 only, the 26 didnt change internally from 33 to 34.

but to answer your question, yes you can swap the heads

[045HOW]

So.... To utilize every sweet perk we can find someone should build a heavily ported 3.4L RB34DETT with V-Cam and 26 Plenum

[MRHD66]

just to rehash and old thread.. neo's use shim over bucket solid lifters where as RB26 use shim under bucket design.. i have had 2 customer cars lately with neo's that ave spat shims out and destroyed heads with high rpm track use and tomei drop in cams. I run the same cams in my drift car and havnt had an issue with shims spitting but it makes you wonder.

so your saying the valve/cam/bucket/shimmy design is better in the RB26 head ?

[GTRsean]

Maybe they didn't check valve clearances before/after fitting new cams...

I think the bucket over shim is better for high RPM and bigger cams.

Toyota guys always complain about their shim over bucket crap... easier to change shims though, cams don't have to come out.

[Dobz]

The only way a shim over bucket design would spit a shim is if the valve springs weren't the right pressure for the amount of rpm and boost the engine is running. Having clearances at the big end of the tolerance would make it worse.

Edited November 5, 2012 by Dobz

[discopotato03]

I was speaking to SK recently about RB26 manifold flange differences and his reasoning sounds logical .

So I'm told Nissans earlier experiences with R30 and R31 Skylines was that all the boy racers in Japan bought the more mundane versions and bolted on the good gear from the high performance varients - because everything bolted on or plugged into the poverty pack versions .

He reckons Nissan didn't want smarties tossing GTR bits on GTSTs so they made GTR bits different enough to not fit without major re engineering .

RB25s could very easily have been the same water passage and manifold pattern wise but that was obviously not Nissans intention .

Head casting wise I'm not sure if the R33 or R34 RB26s were any different to R32s and I suppose if they got by performance and emissions wise it probably wasn't justifyable to develop a better head .

It sounds to me like the Neo RB20 and RB25s were the last of these RB engines to see any developement at all and that was probably aimed at meeting later more restrictive emissions standards and having a bit more usable torque as well .

My best guess is that the RB 25 Neo head would be better than an RB26 one if you could get a six throttle inlet system on it . Also it may pay to see if the 25 Neo uses the same length and height valve guides and same diameter bucket bores because RB26 valves and buckets may fit straight in .

I reckon small as in compact chambers are good chambers particularly in a 4 valves per cylinder head because you don't have that silly plateau on the piston crown to make the static CR acceptable . A compact chamber with a flat or slightly dished piston should have better quench characteristics and if the head was from a late ULEV era car you'd expect it to have better water jacketing to better cool hot spots , think lean = hot and detonation prone , and lessen ignition retard from trying to control detonation .

From memory on paper an RB25 Neo cranked out 206 Kw where I think early RB26s on paper had 209 kw , they used an 82 deg C thermostat as well .

I think by the time R34s came to life GTRs were old news and Nissan weren't so concerned with the copy cat issue . They obviously found ways to get almost GTR power with one turbo one throttle and that little ole R34 GTt SMIC . Lots of subtle differences in the Neo RB25DET obviously helped like a few Kw here a few Nm there . Ultimately a much cheaper engine to make than an RB26 .

Gut feeling is that there is more to the humble Neo turbo head and valve train than meets the eye .

A .

Edited November 6, 2012 by discopotato03

[MJTru]

About the porting of the head is a big waist of money. We must understand a few things first. The first thing is cash disposal, and how much you have to spend..

To kill this it took our shop 5 motors and hours of testing. I will start of by mentioning that velocity is the wholly grail to making power. We took a standard RB26 and ported and polished it and tested it at 8000 rpms. I can't seem to locate the result sheets at the moment, but I will go on. After having the same motor stripped of its head we then installed a standard 26 head, and many would argue that the porting job was done wrong. The point is velocity please remember we are trying to kill this myth. At the same rpm we noticed that the standard head made an easy 48 more hp. We then were a little confused. What I said, “Half the cars out there with ported heads will lose power”. This can’t be the motor made more power on a standard GTR head???????

We then went on to do more test to find out what is going on, a whole new rb26 motor arrived. Velocity: velocity. We came to the conclusion that the air had slowed down, and in order to make more power from a ported head we would have to raise the RPM of the motor, but is if really that or just the extra rpm??? So the test continued.

Well in short, we contacted a fellow race shops and most were lost to. The bottom line is if you lower velocity you must raise rpm to get the same atomization of the fuel as standard head, and it is way more effective to go for more velocity than to open up the port to a bigger diameter within reason. By opening it up you slow the air down, and one for one our test shows that it is better to just clean up the factory casting and valve seat areas, than to open it up, unless you are going to raise the RPM of the motor, and still after testing the same motor at higher rpms with standard head still made more power. For this Myth to work you need to build a vehicle specific application like only drag or drift. Then you would need all the supporting mods. I always wondered why so much hype went into port and polishing if this is the case, not to mention we are forcing air in. In my head it would yield a better result if we were to look at making it more efficient through many avenues available to make a more efficient motor, like the turbo, cooling of charged air fuel burn rate, that’s a good one, and so on.

So why haven’t many shops know this, just that too much research and development. That is 1 reason, and they just kill their income, or they just don’t have a clue.

Hope I help mate

Edited November 6, 2012 by MJTru

[MJTru]

as for the 26 why do the japanese only mainly use the 26 and no 25 parts, the block is thicker in some areas, and i heard that the metal composit is different. i can remember reading this in an artical many years ago. i dont know how tru this is, i believe a 26 head flow why more air as well as the aluminum composit. can anyone verify this?????

[Adriano]

So by your way of thinking, every rb26 with a ported head revving to 8000 is down on power, strap a t88 on the side and i assure you the results will be different, velocity is proportional to volume, not boost.

48hp seems a lot, how much power was it making, if a stocker was making 300hp at the treads, surely porting hte head didnt reduce this to 250?

[MJTru]

I understand that but, this is the answer you will get from most because they either have a ported out head, or have to swallow the pill of wanting to do it. lol.. All variables were kept within a 2 % range nothing was ever over that. I never mentioned boost we were looking for one isolated even not different turbo’s. But I know what you saying. Man it was porting vs velocity just that within reason.

you know the answer to you last question. it took me a while to come to terms with what we were finding out. Its like when scientist found out that alumimum pots oxidies with viniger and salts, and cause cancer in the 90's they can't stop the world from stopping producing it right way, but hide it and, force companies to fix the composits, still to this day were are dieing of cancer more and more before it slows down

sorry for that exsample. but people just dont want to face the truth.

Edited November 6, 2012 by MJTru

[discopotato03]

Yep I go with velocity over volume and with a bit of research you tend to find that often high rev race engines don't necessarily have huge ports .

Going back a bit but Cosworth BDAs didn't have big ports but they could turn quite high revs with bigish cams and induction .

Don't want to go too deep but I see an engine as a piston pump and the velocities and pressures are critical to have the gasses coming in one side at the right velocity and out the other at the right velocity . The pressure balance becomes critical particularly with forced induction because you are attempting to force a given engine to breath and perform like a larger one - usually without larger engine gas paths . It also has to have acceptable performance in un boosted and boosted states which is a big ask .

With RB26 heads I'm guessing the design didn't change much if at all and nissan got more std performance from them with different turbos possibly cams and management .

I don't recall seeing that R34 GTt heads have larger exhaust valves than earlier 25 and 26 heads , although valve ratio size exhaust to inlet 25s/26s appear to be a little short changed on exhaust valve size than the ideal formula . I reckon this was done to help with emissions (EGR effect) and possibly help spool std turbos on engines that were not large capacity wise for the weight of the car they had to drag around . It is really easy to fit 0.5mm oversize exhaust valves into RB25/26 heads and the gains don't seem to be compromised with any negative aspects .

Manufacturers are not stupid and mostly have good reasons for everything they do .

A lot has changed engine design wise since the late 1980s and change has mostly been driven by emissions consumption and manufacturing economics . Had Nissan designed the RB26 from scratch in the late 90s it probably would have had a smaller bore and a longer stroke or more likely more capacity . I think single or twin VCT would have gotten a serious look in too .

One day someone will find a way to make or remanufacture the inner section of the RB26s inlet manifold to suit RB25 heads , that will be the day when the potential performance difference between the two disapears .

Much as I like the I6 layout the trend has been V6s almost certainly for packaging reasons .

A .

Edited November 6, 2012 by discopotato03

[GTSBoy]

There's a lot of waffle going on in here now.

People who actually know how to port know that you size the cross sectional area of the port to suit your overall engine power (and hence per port flow) so that you get the right velocity to enhance cylinder fill (trapping) in a given rpm range. After that you decide if you need to lift the port (or otherwise move it) to improve flow into the bowl. Then you shape the bowl and guide to create the flow behavior you are looking for. This may (and usually does) include trying to suppress separation of the flow off the short turn. Even the type of valve seat cut you choose will be related to and relate back to the port shaping. And none of this should be done without reference to the cam profile that is intended to be used. If you are using long duration and high lift profiles, then the port flow at low valve lifts will not be as important as at high lift. But if you have small cams then a bigger proportion of the valve opening event is spent with the valve close to the seat and so low lift flow becomes very important. The porting required to satisfy each of these cases will be different.

The smart guys bring a lot of science (even if it is poorly phrased in "mechanic speak") to this work. But they also need to bring a lot of intuition and experience to it as well. They have to have a feel for the air wants to do when confronted with a certain geometry to flow through, and what to do to change that to what they would like to see it do.

[MJTru]

well we were planing on contacting Tom Geller from Toyotas F1 team to put some rb heads in a liquid dynamic flow bench. Or some sort of high tech stuff he had mention to me. He went on to mention that these tools are needed to properly determine all these geometry issues you mentioned. well he lost me there, but it was not feasible for us to do this. being a close friends for year we had some conversations over dinner a couple of times and he went on to say that most porter just do guessing game, and waist money over time, given that they learn and get better results than the guys who just dont know the science behind the whole thing. He went on to say that the tools that are at toyota disposal made the guessing game more managable, wow i didnt think i would be getting into this so deep. were as you spend years grinding ports and basically learn tricks from one head to another you cant really atest to what digree and where your power is beeing gained.

we then parted and felt as if any more from him i would have to give him a resume to join the team. lol we still speak once in a while.

I always say you pay for a piece of mind or pay to win races either way you going to pay. So i spend most of my time making the money and everything esle can then follow

good stuff guys

thanks