RB25DET Intake Plenum change

[Sydneykid]

Hi guys, we have used a Sub Zero plenum on an RB25 head. It replaces only the top half of the inlet manifold, so the standard injectors, rail wiring harness etc all remain in situ. Cost was around $1,400 complete with a 65 mm throttle body.

The RB26 inlet manifold, multiple throttle bodies and inlet plenum don't fit the RB25 head. An adaptor is need for the bolts and water feed to fit. Very tricky to machine out of billet aluminium and really only worth it if you want the multiple throttle bodies for better response. No extra horepower over the Sub Zero style.

If you want the extra response, go to the full RB26 top end.

I saw a RB25DET with the standard inlet manifold and throttle body make 360 rwkw a couple of weeks ago. So I don't believe the longer pipework is a big an issue as it is made out to be. The Gibson R31 GTSR made 460 bhp and it had the standard RB20DET manifold.

Mathematics tells me that the pipe length is not that much of a problem, but pipe diameter can be. I subscribe to the theory that larger diameter pipework contributes to poor throttle response because it lowers the velocity of the air flow. If you measure 2.5" pipework on the flow bench at - 2 bar it shows no resistance at all at flows up to 700 lbs per minute. This is enough airflow for 650 bhp. If you were targeting more BHP than that, then maybe larger diameter pipework would be worthwhile.

Hope that made sense.

[Warpspeed]

Hi wegs.

I have been thinking about this myself. My idea is to try and pick up a cheap (needs rebuild) RB26DETT, these sometimes go for $3,000. If you are lucky the valves and guides are not too bad, and you can put the whole top end onto your RB25.

You can then sell your RB25DET top end + turbo complete, to one of the Commodore guys, You should be able to recoup a decent chunk of that original $3000 outlay.

Likewise with the twin turbos, use them as is, rebuild them, or flog them off, and get something better. If you do the sums, it will almost certainly give you best bang for your buck over the long run.

The advantages of keeping the same RB25 bottom end are the engine number stays the same, and there are no sump related problems, and its a lot less work.

You may even be able to sell the complete RB26 botom end to someone else as well. Or else keep it for a long term future rebuild, and further upgrade.

[Mick]

Someone earlier siad that he got quoted $650 for a cut & shut on the rb25det plenum. My question is, is there more to this job than simply moving the throt body to the front and patching the hole, like maybe something internal ?

Cos if it is just as simple as moving the throt body and patching the hole then you could take it to any welding place and probably get this done for $100 MAX ...

[B-Man]

I got mine done at Hills Morotsport - Less that $1K (I think) includes larget throttle body - Works well.

See pics here: http://www.pbase.com/image/12483260

[Steve]

Brendanf, thats a nice looking plenum. Do you know if they are using the stock runners? Also do you know if the runners are continued inside the plenum or just butt up against the base and inside is a big void? Sorry lots of quesions, but I am intested to know what different shops provide.

Cheers

Steve

EDIT: did they give you any, or have any flow figures?

[B-Man]

Steve, not sure what you mean by runners ??

No flow figures - been meaning to ask that - I'll find out !

Cheers,

[Steve]

The bits that go from the plenum and bolt to the head, and have the injectors attached to em. If you know what I mean

[B-Man]

I understand that the bottom of the std plenumn is chopped off and the new higher voluetric plenum is attached via argon assisted aluminum welding. The fuel rails and injectors are all std (expecpt mine are hi flowed)

[Steve]

Hi Brendanf,

You will be pleased to know Mark quoted an extra 95cfm flow per cylinder from these plenums on a flow bench. They do consturct a bell mouth inside the pleunum that fits against the runners to achieve this, as just bolting them straight up would not be doing any favours. And they all flow evenly, which is also pretty important. Mark states that they put in a bit of R&D to achieve this - which doesnt surprise me considering the results.

All this for $895 fitted, and for an extra $200 will fit an 80mm throttle body. This has to be close to the bargain of the century - I'm getting one!

Thanks for pointing Hills motorsport out - saved me hundreds

[B-Man]

Coolies Steve, ya know I was round at Hills Yesterday and I forgot to ask the questions..... Well you have now which is great. (You sound like you know what your talking about more than me anyhow)

Cheers,

[Steve]

Nah, just did a bit of research as I am looking at upgrading

Steve

[Gradenko]

Originally posted by GTS-t VSPEC

I have a Greddy plenum, cost $1500, and a mate of mine had his throttle body moved to the front of the plenum.

GTS-t VSPEC, do you know where your friend got his cut'n'shut done or if he had it tested for even flow? I'd be interested in pics (or seeing it in person). I'm in the process of doing something similar, so first hand feedback would be great.

[Ender]

Does the rb26dett head fit rb20det bottom end too, or not?

Also I have simply wrapped my intake from plenum to intercooler outlet in insulation and the heat soak was measurably different. (Looks dodgy but...) Even before I put the thermometer on it the pipe stays cold with the insultion but without it is quite hot to touch. Cheap way to earn a few measly Kw. Has anyone else done this...I thought it was a pretty cheap obvious thing to try.

Does someone have the contact details for where sub zero intakes are sold.

Cheers,

Russ

[Sil8ty]

I have nearly finished my manifold.. It is getting welded as i speak.. Will take some pix when i have finished it.. Did take much to change the T/Body to the front... Made intercooler pipe work sooooo much easier hey....

[Steve]

russcb007

Different bore size between RB26 and RB20 for starters, wouldnt have a clue what other problems you would face, but in short, it is not a direct fit.

Subzero have a website, they will supply direct.

Steve

[meggala]

hey russ I can get rb 20 plenums they are quite a bit cheaper than the sub zero ones they are 650

have a look

[Guest MFX_R33]

I got my plenum from MOOD Motorsport. Still retains the factory injector's etc, but it does do away with the factroy idle control, etc, so it required an aftermarket ECU (in my case an EMS from Hills). This cost me $750 including the XF throttle body, and Moods welding is flawless!

Jeff.

[dabigbolf]

MFXr33 that looks awesome.

meggala i ordered the same same plenum to be made for mine too. once polished they look pretty god i reckon. will post a pic of another guys.

[GTS-t VSPEC]

MFX_R33,

The plenum looks good, did you notice the difference in power and torque?

I still need to get my Greddy plenum polished, cause you engine bay looks sweet.

See'ya:burnout:

[dabigbolf]

here we are.