Rb26 Inlet Manifold And Adaptation To Rb25 Heads .

[Jmaac]

rb26dett intake manifold and itb's on a rb20 head

http://forums.nicoclub.com/zerothread/307325

[Kaido_RR]

blitz kit arrived today.

really is quite simple how they solve the ACC value issue, looks like they also solve the VCT issue, but i will confirmed 100% once i put the kit on

[Behind Horizon]

OK, i do already have that story behind me and i want to share my experience with you guys.

I got my adapter plate ( i think it's not the blitz one) from tokage racing, the shipment and support was very good.

However lets come to the bad things:

- Ports from the adaptor plate dont match up with the rb25det ports as the ones from the rb25det are bigger

- The included gasket was crap and was leaking, i had to replace it with a custom made

- No relocation kit for the VCT Solenoid was included (but i was knowing that)

Here you can see the RB25DET Inlet Manifold gasket, as you can see the RB25DET ports are a lot bigger. Also it isn't matching up with the RB26 water bridge from the other side.

That's why you have to port and polish the whole adapter which looks like that when it's finished:

The next problem was the VCT solenoid as the water bridge can't be mounted when its installed,

so you have to relocate it and mount it outside of the engine, that lot have to be connected

up with braided lines first but i think you can imagine how that works.

The oil which is leaving the vct solenoid will be routed to the sump!. http://www.rb30.com/rb25-6.htm

One other problem is that the injectors are not spraying the fuel in the way they should spray it as

the original angle will be wrong due to the thickness of the adapter plate as the injectors are sitting to far behind from the valves.

Because of this you have to fit special Bosch injectors with a matching spraying angle, also you have to remanufacture the holes for the injectors in the water bridge and bore another angle in it. However, Bosch injectors wont fit if you dont have custom made adapters for them also.

As i have a full programmable aftermarket ecu, i will reuse the oem RB26DET AAC-Valve.

If you have more interest in that conversion just check out my webspace. However i would recommend 95% of you to just buy a

RB26DETT head instead for going the way i went.

Edited March 25, 2008 by Behind Horizon

[discopotato03]

Yes that seems to be the complete solution . I'm beginning to think that using the complete 26 head and inlet may be a good transition from 26 25 (26 upper/25 lower) to 26/30 .

It now remains to be seen what can be done to get the 20/25DET exhaust manifold on the other side of the 26 head ....

I spoke to SK a while back about having extra bosses welded to the exhaust manifold side (head) so that either pattern manifolds could be catered for . Even an adapter plate should not be too difficult .

Cheers A .

[Roy]

Yeh, but then you don tget VCT if you go GTR head.

[Kaido_RR]

ok after getting some help with the blitz instruction, the kit required a section of the gtr throttle body section to be cut and welded, also the chamber where the injectors is needs to be sanded down

[Behind Horizon]

Yeh, but then you don tget VCT if you go GTR head.

Yeah, the VCT is very important for me, my personal oppinion ist that the RB25 Head is better (newer design, bigger ports, better flowing) then the RB26 head as long as you keep your revs under 8k.

When youre starting to smash a very large turbo on the engine which still is making power over 8k (T04Z?) you should choose the RB26 head due to the solid lifters.

[robots]

ok after getting some help with the blitz instruction, the kit required a section of the gtr throttle body section to be cut and welded, also the chamber where the injectors is needs to be sanded down

Can you post some pics of your Blitz adpater kit ? Would be really usefull.

[discopotato03]

Roy losing the VCT many not be the issue many think it is once you change the throttling arrangement .

I tend to look at the RB26 and RB25 inlet tracts as complete sets and I think differences such as VCT/non VCT have a lot to do with what controls air into the system - throttle plates .

To me RB25's have your typical single throttle valve into plenum type inlet manifold system which is cheap to manufacture and works well enough for MR MRS and street rat in standardish form .

I believe the VCT on RB25's is about low down torque driveability and emissions in an engine with soft cams and a plenum manifold that allows all the cylinders to communicate south of the throttle valve .

When you have the ability to change a cams phasing on a DOHC engine you can open and close the degree of overlap (phase in the 4 stroke cycle where all valves are open at the same time - well exhausts closing after inlets start to open) .

At very low revs having stuff all overlap means the cylinders have greater trapping efficiency - this is about sealing the charge in the cylinders when the revs are low and the charge air is moving slowly and will flow backwards given half a chance . Once the revs are up a little the air (which has mass and therefore momentum) travels more easily in the desired direction and the cam phasing can be closed up to increase the valve overlap for improved scavanging .

The easy way to tell if an engine like this has long period cams is how it idles , if more aggressive cams open the valves earlier and close them later the cylinders trapping efficiency is reduced at low revs and the reversions in the inlet tract give that chaff cutter idle effect .

I don't think its a secret that RB26's were all about performance and homologation for race cars .

Once an engine grows a throttle valve per cylinder - throttling restrictions drop markedly , this shows up as much better cylinder filling so the dynamic or effective compression ratio is higher . My thinking is that while an RB25DET has a static CR of 9:1 the dynamic (effective) CR may not be any greater than an RB26 which has a static CR of 8.5:1 .

Another huge benefit of a throttle per cylinder set up is that the throttle plates actually block much of the reversion effect of healthy performance cams so the plenum air supply is much less affected than it would be if the throttle was a single one at the front RB25 style .

The multi throttle system always allowed you to run more aggressive cam profiles than the single throttle plenum manifold and have much better manners at the same trime . ALL true performance engines have multi throttle inlet manifolds and good examples are RB26's and the GTiR type SR20's . Some of the later REX engines are like this and no doubt many exotic European ones .

To have VCT RB25 style I think is a cheaper way to have some say in cylinder filling and emissions and drivability .

They obviously went a bit further with the RB25 Neo's variable cam timing and this gave them the opportunity to increase performance with different cam profiles and a larger turbine housing and not lose the emissions and drivability , they actually had lower emissions than the R33 version from memory .

Fuel consumption doesn't necessarily have to take a great hit with the 26 top end . GTR's may be a bit hard on the juice but so many things about them are aimed at performance that around town they toss it in - particularly if you must have that rush every time you open the throttle .

Out of time cheers A .

Edited March 27, 2008 by discopotato03

[bubba]

May be slightly off topic but, what are the benefits of a larger throttle body on a single entry plenum in a forced induction application?

I am going to use a GTR plenum chamber but without the multiple throttles, instead using a single at the front.. will just be running the standard throttle body for now but am wondering what upgrading to a larger item will do..

[r33-d-rift]

May be slightly off topic but, what are the benefits of a larger throttle body on a single entry plenum in a forced induction application?

I am going to use a GTR plenum chamber but without the multiple throttles, instead using a single at the front.. will just be running the standard throttle body for now but am wondering what upgrading to a larger item will do..

throttle response will not be as good thats 1 reason y gtr is so responsive

[Rhinorebel]

Car is on the road but still in the run-in tune. Give it a week or two and I will have all the restrictions removed, the oil changed and put it on the dyno. Already impressed just with the audible difference it makes!! There is some differences noticeable with power but it has been so long since I drove it that I am not sure yet. Feels good though.

[bumble_bee]

so what did people do about ther VCT? relocate? or modify a bracket?

[discopotato03]

Thank you everyone for your input to a hopefully ongoing thread .

Arguably what we seem to have learned is :

1) Yes the RB26 six throttle inlet manifold/plenum can be adapted to an RB25DET Cylinder head .

2) With spacer/adapter plates fitted the RB26 mount injectors won't spray in the right direction (too low ?) .

3) Even with the injector mounts corrected (milled) you miss out many RB26 head features .

4) R33/34 VCT can create problems .

5) RB26 heads have different exhaust manifold stud pattern .

Next thread will be fitting RB26 head to RB25 VCT type block/in R33/with R33 electricals .

Cheers A .

[GTRgeoff]

I looked long and hard at that 26/25 setup as a solution after fitting twin turbos and wanting the TBs on the 25 head, but in the end I went on to a straight up forged bottom end 26/30 with 9.0:1CR in the GTS25t.

Still TT but running a single Z32 AFM, and doesn't need at IAC setup at all. Made 160rwkw with no turbo boost running through it. Yet to tune but it only cost about 7K all up done by myself and paying a premium on some parts due to time limitation and late change of setup. I'd do it again, and will when the GTR need a rebuild.

[edit] engine is 26/30 not 25/30

[discopotato03]

It was suggested to me this afternoon that the most practical way to get the six throttles on an Rb25DET head would be to cut the RB25 manifolds lower section just beyond the injector/rail mounts , then weld on a flat plate ported drilled and tapped for the RB26's 3 twin throttle sections and plenum chamber .

The plusses would be retaining the Rb25's water outlet + log or bridge and the std fuel rail/injectors/injector loom .

The IAC and TPS wiring would have to be altered to suit their different locations . Ah and the native VVT .

Can someone confirm if R33 GTR IAC and TPS units are electrically same as R33 GTS25T - S2 ?

Where it's going to be different for each individual is the budget and where they want to end up engine wise .

I like the idea of one MAF AFM , one turbo and basically native electricals so the plate and bolt idea is probably about as straightforward as its going to get .

Like most people my pockets are not bottomless and I don't want to be building an expensive pretend R33 GTR .

So (this week) the plan is to look at modifying the R33 25DET head and investigating modifying the 25DETs lower manifold section .

Cheers A .

Edited July 23, 2008 by discopotato03

[Adriano]

Unfortunately disco, this isnt really an option without significant welding/reprofiling of the intake port. If you look at the manifold, by the time you have to cut it back behind the injectors, the two middle runners are about 15mm lower than the other 4.

[discopotato03]

One of the members here was showing me an S 1.5's lower section yesterday and I didn't notice any difference in the "heights" of the runners . That lower section had a casting line a little beyond where the std fuel rail mounts were and I was wondering if that was the point to think about cutting one .

I need to find cheaply a std 33 25DET lower inlet manifold section to mock up .

Do the members here think there is enough interest to consider a small production run of such a component if it can be made to work . I don't want to make any money out of this but if someone set up jigs to reproduce them it would be a bit more cost effective down the track .

I like this idea because it allows people with 33 25DET's to reuse more of what they already have and the finished result should look like the factory would have had it .

It's never going to be the same as a complete RB26 top end , all I really want is the throttle response and for it to look factory . I can live with the hydraulic valve train and VVT , poncams would be enough .

Cheers A .

[Adriano]

Ive got a cut NEO inlet manifold. I can take some photos of it and post ehm up if you like showing the difference in port height. I also recall the port spacing will be wrong, as the runners dont come straight out of the head, some turn in, others out. Nothing that cant be fixed with a tig though. PS disco, what state do you live in?

[1400r]

got this today =)

unfortunately due to exchange rates I can't afford to keep it