Mating Rb26dett Throttle Plenum On Rb25det (neo) Head

[GTS4WD]

Hey guys

Papa bear and I have a NEO 25DET in a "ballin’ on a budget" 32 GTST project car.

Latest quest was to get the most out of the NEO engine with the best technical solution. This involves fitting a front mount intercooler with a front facing plenum.

We weren’t prepared to spend $1300 odd for a Greddy style intake plenum or buy some cheap replica. I personally didn’t like the idea of cutting off the throttle body, welding up the hole and attaching the TB on the front of the 25 intake manifold.

Mating an RB26DETT plenum to the NEO head is a far better option over single throttle FFP - individual throttle bodies provide faster response, even air flow and filling of cylinders etc.

We purchased a GTR front mount cooler with factory piping and the complete GTR plenum kit.

Problem is, the stud patterns don’t match up.

Adapter plates can be found or manufactured to mate the 26 intake manifold to a 25 head. Blitz offered one for over $1K. Steep, and no longer available. Also, moving the plenum away from the head creates other issues.

Thinking outside the box, the old man decided to modify the head and eliminate the need for an adaptor plate to suit the 26 stud pattern. This involves plugging some of the holes and building-up areas on the intake side through TIG welding, followed by drilling and threading new holes to suit the RB26 plenum.

As far as I know this has never been done before, not to a NEO head anyway. So, I thought we might write about it.

Photos will come shortly, and the job should be completed this month, so keep checking this post.

Cheers guys!

Any input/comments are greatly appreciated

[Kaido_RR]

good luck,

just remember you need to use the rb25 ACC value, as it has 2 plugs on it and the gtr one only has 1.

you also need to mod the throttle cable, use gtr fuel rail + injectors, wire in a gtr resistance pack and get gtr injector plugs.

Also you need to mod the water outlet on the gtr plenum as it hits the VVT; best way to do this it remove the water outlet on the rb25 plenum (just cut it off) and weld it onto the gtr plenum.

i've put a gtr plenum on my 25 (non-neo) using the blitz kit plus changing afew things along the way to fit better

Edited December 2, 2008 by [Michael]

[GTS4WD]

Ok.

So.

These are the stock RB25DET intake ports and stud pattern:

And now this is the head once the un-needed intake manifold stud holes welded up.

And the the welds built up around where the new manifold stud holes need to go.

NOTE: the RB25DET intake ports are slightly diffrent to the 26 ones. And some had to be built up too

From here, its going to be machined flat. Then new holes drilled and threaded.

More photos to come. More details to come too.

Now, has anything like this ever been done? In Australia? Iam assuming our crazy Japanese counterparts have done something like this.

I have searched everywhere. And have only seen the plates.

If anyone knows of anything done like this done please let me know. Or else the Old man is going to claim as being the first to do so!

[discopotato03]

And my 33 heads just been reworked and reassembled ...

Very interested to see how this pans out , provided you can get the water outlet on the 26 manifold to clear the VCT solenoid (is neo one dimensionally same as R33 ?) it should fly - work .

Multi throttle VCT 25 head , Borat would say "E niiiice" .

A .

[666DAN]

Saying that though, some really well made plenums have popped up on ebay just recently, from Thailand *waits for flaming*....they're NEO specific with the correct AAC flange ~$875 delivered meant to be JUN copies.

search "NISSAN SKYLINE GTT NEO RB25 R34 RB25DET INTAKE MANIFOLD"

[TriniGT]

Any pics of the individual throttle over the intake side? This is may be an option for me, did you use the RB26 Intake Manifold Gasket as a reference to know what needed building up and what needed blanking as well? Good work thus far and let me know when you have any progress on it as I should have an entire throttle and twin setup off and RB26 today. Seeing as the RB25DET Neo has solids and GTR rods I may stick with this option as most people have persuaded me to keep my Neo head being that it is newer and not swap the head for the RB26 non-VCT one. I am tihnking to get a Neo head thought for this project and do everything external and just swap heads when done. Keep the updates coming.

Edited December 22, 2008 by TriniGT

[STATUS]

Saying that though, some really well made plenums have popped up on ebay just recently, from Thailand *waits for flaming*....they're NEO specific with the correct AAC flange ~$875 delivered meant to be JUN copies.

search "NISSAN SKYLINE GTT NEO RB25 R34 RB25DET INTAKE MANIFOLD"

yeah im pretty sure thats whats on wheezys rb20 and it worked VERY well. Ive also used one on a turbo 20V Ke70.

Edited December 24, 2008 by URAS

[Nismo_Boy]

shanef on here has a kit he sells...

[GTS4WD]

Hoookay. So.

The face has been machined flat.

New holes have been drilled and threaded. Looks a little like this:

Slight modification was needed for the water outlet from the intake manifold to clear the VCT. a couple of pieces of alloy and a little welding later, the results are:

BO!

In terms of injectors our intentions are to use the NEO ones.

But we have no clue about the finer details about what we will be doing. No doubt we will be adding details shortly about how to adapt the 25 injectors to the mani.

Edited December 29, 2008 by GTS4WD

[TriniGT]

Great looks like the best option for me and that way I can keep my VCT and not have to use the RB26 head. Any insider info that may come in handy when I am doing it, top job by the way? I guess if you wanna run twin turbo the same applies, wels shut and build up and throw the two twin manis on for the twin setup. May actually do that when the head comes off same time but not weld the original setup shut as yet. Thanks again. Can wait to see the outcome.

[discopotato03]

The exhaust side should be much easier because no existing RB25 stud holes need to be filled in .

Other people think and I agree that Nissan changed the RB26 exhaust stud pattern to stop people using RB26 bits on RB20/25's .

Looking forward to seeing it all running , cheers A .

[rob82]

Hey guys

Papa bear and I have a NEO 25DET in a "ballin’ on a budget" 32 GTST project car.

Latest quest was to get the most out of the NEO engine with the best technical solution. This involves fitting a front mount intercooler with a front facing plenum.

We weren’t prepared to spend $1300 odd for a Greddy style intake plenum or buy some cheap replica. I personally didn’t like the idea of cutting off the throttle body, welding up the hole and attaching the TB on the front of the 25 intake manifold.

Mating an RB26DETT plenum to the NEO head is a far better option over single throttle FFP - individual throttle bodies provide faster response, even air flow and filling of cylinders etc.

We purchased a GTR front mount cooler with factory piping and the complete GTR plenum kit.

Problem is, the stud patterns don’t match up.

Adapter plates can be found or manufactured to mate the 26 intake manifold to a 25 head. Blitz offered one for over $1K. Steep, and no longer available. Also, moving the plenum away from the head creates other issues.

Thinking outside the box, the old man decided to modify the head and eliminate the need for an adaptor plate to suit the 26 stud pattern. This involves plugging some of the holes and building-up areas on the intake side through TIG welding, followed by drilling and threading new holes to suit the RB26 plenum.

As far as I know this has never been done before, not to a NEO head anyway. So, I thought we might write about it.

Photos will come shortly, and the job should be completed this month, so keep checking this post.

Cheers guys!

Any input/comments are greatly appreciated

Sorry Guys but I fail to see the point of doing this modification. The factory rb25 intake manifold is good for at least 500rwhp worth of flow as I have seen. It will also give best cylinder distribution and will also create the highest port velocity due to the long runner length - which will result in a better midrange, earlier boost potential and not to mention better fuel economy.

I have dyno sheets of an aspirated XR6 with a TO4Z turbo conversion which I tuned a couple of months ago - as some of you are aware the aspirated XR6's have a dual length intake manifold to promote bottom end torque. The dyno sheet shows the difference between the long runner and short runner in a turbo situation and I can telll you the difference through the midrage is huge not to mention the rpm at which the desired boost level was acheived. If you interested I could find dig it up and post it.

[Bliksem]

I think the main reason for this is to use the ITB which will give you superior response. In addition better flow over the standard cross over plenum

[psi]

I have a big single turbo, jam that on a little engine like the RB25 and you dont get midrange. We all hear what you are saying, but to some of us, that midrange torque is not really an issue...

I have driven a few GTR's with similar power to my 25, and they ALL kill my engine in throatle response. Seeing as my head is still at the machine shop, this mod looks like the perfect thing to do! Kill a bunch of issues while everything is apart and go for broke!

Thanks for the idea, PLEASE keep posting up pics and pit falls!

[wolverine]

If you interested I could find dig it up and post it.

i am interested.

[Adriano]

I am also interested. In regards to the rb25 manifold with 500rwhp, did you then replace it with a different one without gain, or are you assuming that it wasnt causing a restriction?

[rob82]

I am also interested. In regards to the rb25 manifold with 500rwhp, did you then replace it with a different one without gain, or are you assuming that it wasnt causing a restriction?

I'll grab those dyno sheets when I go back to work.

In regards to the rb25 manifold - I was only mentioning that I have seen that amount of flow through the standard manifold - that car went on to run 129mph in a VL so the horsepower numbers are on par with MPH.

[TriniGT]

I was lucky enough to pick up a Neo head today, it was scrapped missing the cams, cam caps, and RB25 cam covers and the VTC solenoid. I dropped off the entire intake side and looked everything over. The only thing I am concerned about is that I see two exhaust valves halfway open and they have the lifter and springs attached. The block was left open with no covers and lots of dust. I took it home and used the compressor to blast some air and clear up some of the dust, I left the head/valves and lifters soaking with WD40, hopefully things may loosen up. Anyone have an idea as to why the valves may be open, may something was resting on it and caused the spring to compress and then it becoming caked with crud.

I am going to be doing the same thing but what I decided is to take the adapter for the individual throttles and shape the entry to that of the Neo as the Neo has a wider intake opening and leaving the RB26 intake with just enough to get by without leaking. I picked up the Neo head for $150USD, wasn't too bad. How do you'll go about removing the lifters, they are pretty snug in there at the moment?

Keep the updates coming as it has indeed inspired me.

Jason.

[GTS4WD]

ok.. Pitfall number 1

Strip the head BEFORE you send it off to get tiged!

We has massive problems stripping it once the head was hit with all the heat. There is a butt load swaf floating around through the head that needs to be cleaned before putting it back together.

In terms of taking the buckets out, if you can poke them out from the valve face side. If not try log nose pliers. But you need to have very soft hands if your going to use them..

[R.I.P.S NZ]

We've done the 26 itb's and plenim conversion on quite a few RB25s and its actually not very hard, just a little time consuming.

I'd definatly NOT be welding the head to fill up holes and add material for new holes, your asking for trouble with lifters and cam tunnels.

I can't really see the point in going to all the time and trouble unless you just want to have a go for the hell of it.

By the time you sort out the manifold that has to bolt to the top of the plenim base, sort out the injectors, rail, water and air etc it definatly won't be a financially benificial excersize but I do take my hat off to you for havin a go and going about it in a different way, some of my best ideas have come after I've been told something is a waste of time or won't work.

Best of luck to you,

Robbie