What Are The Possibilities And Who Has Done It

[TriniGT]

Okay so I have made changes to my previous setup and I am looking at using a Neo RB25DET head on an RB26DETT block. I have done most of the work to the head already including adapting the RB26DETT individual throttles and routing of the water neck and what not. I am using stock head internals with new valve guides and an entire head service kit and using the RB26DETT cam covers. Question I am after is will the head be a direct fit on an RB26DETT block for both the water and oil galleries? I am going to be widening the headstud hole on the head to accommodate the RB26DETT headstuds. Plan is to run the entire RB26DETT bottom end and twin turbo it and keep the VCT by drilling the blank on the head to allow the oil to be supplied to the VCT solenoid rather than having an external line supplying it. Also will have the stronger RB26 sump and already have two oil returns for my turbo setup, already have a guy cutting and flikpping the cast iron manifolds for the exhast side on the head. Can anyone that went down a similar route give me some details or knows of someone that has done this and any problems they encountered.

Jason.

[GTRNUR]

Sounds like un-charted territory to me. It would be a lot easier to run the RB26 internals in the RB25 block instead to get the same displacement and not have any assembly issues. Unless your after a means of retaining an AWD block that is.

[Kranker]

mate this is a text book freudian-slip

you are asking about putting a neo head on a RB26 bottom end...but you posted it in the RB30 section

i think deep down you want to put this head on an RB30 bottom end...

go on...u know u want to

[N1GTR]

Moved

[TriniGT]

Sorry didn't realize I had it in the RB30 section. Anyone has thought on this idea. Looking at the twin option and already did the work on the head. Just wanna confirm that I can have the head successfully lodged on the RB26 block and what issues are gonna be encountered besides what I listed above.

[GTRNUR]

Well you know about the VCT issue, so that means external oil line, and a welded or plugged VCT oil feed gallery in the head.

Your next issue is compression ratio. The Neo head has a smaller combustion chamber than the RB26 (64cc ish). So the standard Rb26 pistons with their 15cc domes are going to mean you CR is going to be very high.

Rb25 pistons have a 11cc dome but also have a 31.5mm compression height so they would be sticking out the top of the block.

You cant use RB30 flat tops straight out of the box, as they are a 32mm compressio height, and the compression would still be high.

The solution that is worth investigating is trying out some modified RB30ET pistons. First off they are dished so that will help with the compression ratio. Second, they are a 32mm compression height, but you could have them machined back to 30mm (and that will remove some of the dish), and that will achieve the 0 deck height.

The next option would be to machine the same RB30ET pistons for a positive deck height, so they protrude out the top of the block by 0.5-0.8mm. Then run a 2mm head gasket to achieve a safe piston/head clearance. This way the top of the crown is more intact (only cutting 1.2mm off it), and it will all fit inside the block and under the head still.

On the bright side, the RB30ET pistons are way cheaper than the RB26 stuff, and some 2nd hand pistons to do a trial fit-up with can be found very easily. Cheap as chips or free even.

As for the water gallery and oil return lineups, you will need to ask the guys that have done a NEO head on the RB30 block before to ask what pitfalls they encountered. I believe Spool Imports have done one for starters...

Cheers,

Ian

[TriniGT]

Cheers for the response. I am wondering if I can get a company to make me some custom pistons to my specs, maybe that will help that entire area. I will also investigate into the oil and water gallery positions using a headgasket for both the Neo and the RB26 and see what differs. Thanks again and will check the RB30 guys.

Edited June 18, 2009 by TriniGT

[GTRNUR]

Talk to Steve at quantum racing ([email protected]) or +61 (7) 3290-5911. They are the australian distributor for CP pistons and yes they can provide you with any compression height, dish or dome configuration piston you like. You can just say you want a R34 RB25DET NEO configured piston with a 30mm compression height instead of the 31.5mm compression height and thats your problem solved right there. You can alter the dish/dome at the same time to get the CR you want too. Its always best to set the desired CR by optimizing piston design rather than running a thick head gasket. Aim to use a 1.2mm gasket and you'll be fine.

[TriniGT]

Only thing is that I am in Trinidad, wondering if I can contact a more local person say in the States to do such or just contact your person above? I have a couple questions as I am not a guru with the CR and whatnot. What is thew ideal setup for me to run for either a twin steel wheel N1 setup or a single GT3076R or max a GT35R twin scroll? I want to use an RB26DETT block with Neo head, is the stock bore of 86mm and 8.5CR acceptable with a 1.2mm headgasket? If so is that all I need to let them be able to design a piston for me using stock GTR rods and crank with Jun collar?

[XRATED]

Dude, I'm in South Florida at the moment.... I gotta come down to Trinidad for the weekend. Some of you Island boys shure are crazy about your imports/skylines.

Good luck

[mad082]

i recall reading somewhere that the head studs are different sizes but can't remember which is bigger.

[Adriano]

I would consider looking into fitting vct to an rb26 head, then you could do intake and exhaust cam timing. Use neo cams, rb25 cam pulleys(or evo ones if you want to get tricky). IMO the nissan vct is primative in that its either advanced or ratarded, nothing in between, wheras almost all newer vct setups are true variable control. Thats where the power is.

[GTRNUR]

The VCT systems on the RB25 and NEO heads is sound in design, its the computer that lacks the capability to control it properly and in an advanced fashion.

The early "clever" method used to be to control the VCT valve using the PWM boost control output of an aftermarket engine management system. Something that could vary pulse width based on RPM basicly. Any haltech, wolf ecu could do this. Once the frequency was worked out you can use that to control cam operation at any engine speed in a true variable fashion.

If you want to get really sophisticated with cam control look at the Wolf V5 ECU and its multi-controller outputs. One 3D map with customisable axis's (eg MAP/Load vs RPM). You can then have override 2D maps based on inputs or any other signal, all output to a programable PWM output.

Considering the effort he has put into in making the throttle bodies compatible with the neo head, its worth going the rest of the way and finishing it properly. Custom pistons can get a little pricey but there are other options too. Ive done a lot of research in this area recently with the engine project I am working on. Using off the shelf pistons from other cars allows you to get what you need a lot cheaper. Once you work out your head's combustion chamber volume in CC you can choose a piston to suit your requirements.

Here are the options that I can see that you might want to look further into. These were all honda piston sets and all used a 22m gudgen which would mean machining or possibly special rods for the engine too. I know that some of the stroked versions of the EVO 4G63 engines run a 21mm gudgen and an 86.5-87mm bore along with good dish configurations that might possibly be a closer match for you.

Wiseco Code Bore Comp height Dome/Dish Pin size

K568M87 87mm 30mm -9cc 22mm

K631M87 87mm 30mm -3.5cc 22mm

K622M87 87mm 30mm -21cc 22mm

K544M87 87mm 31mm -14cc 22mm

Edited June 18, 2009 by GTRNUR

[Adriano]

Have you actually tried to cantrol the cam timing variably with the pwm setup, im pretty sure it wouldnt work due to the hydraulic setup. As i said i would look at evo stuff, as the siemens cam pulleys i think are electric.