Stock 26 Head Vs Modified 25

[WHITER33GTS-T]

I currently have a 25 head on my 3lt. I have had a change of plans and going with a 26 head. My 25 head has been ported, has 256 inlet and 264 ex and made 364rwkw @ 20psi

What sort of power can the stock 26 head flow as I just want to get the car running while trying to work out what cams I should run.

I will be running a 6boost manifold and T51r

Any info would be great

[R31Nismoid]

Much of a muchness between stock heads in overall terms of flow

Just to throw it out there, i had a stock RB25 and made 370rwkw, same as guilt toy

[acsplit]

Ask you don't know shit !!!!

31 # 1

[D_Stirls]

The 25 and 26 port sizes are the same (the 25DE port size is smaller though as they are the same size as the 20 heads), if you were starting from scratch and building a big power head I would have said go for the 26 head as they have a solid setup from factory and therefore would be cheaper than setting up a 25 head with a similar setup but since you already have the 25 head ported then stick with that unless it's a NEO head, as from what i hear your limited to 9.25mm lift due to a lifter gallery with the NEO's

[WHITER33GTS-T]

The 25 and 26 port sizes are the same (the 25DE port size is smaller though as they are the same size as the 20 heads), if you were starting from scratch and building a big power head I would have said go for the 26 head as they have a solid setup from factory and therefore would be cheaper than setting up a 25 head with a similar setup but since you already have the 25 head ported then stick with that unless it's a NEO head, as from what i hear your limited to 9.25mm lift due to a lifter gallery with the NEO's

Ive had a small issue with the lifters on the 25 head at 8500+ and has a slight tick noise. After the car has been sitting the noise has gone until the first rev passed 8000 again. Was told I need to change to solid lifters so just thought a 26 head would be the way to go.

Anyway, I have just sold my Greddy plenum and throttlebody so will be useing the 26 head but just wanted to know how it will go stock or if I should just set it up now and have it ported and big cams...?

Will it be good for 400rwkw stock...? Also im running a T51r and C4 auto. Old setup made 364rwkw @ 20psi but tuning was stopped after it made the tick tick noise. I will fit bigger cams but just want to get it running so I can drive it again. lol

The bottem end was built by Pro Engines

[bubba]

The 25 and 26 port sizes are the same (the 25DE port size is smaller though as they are the same size as the 20 heads), if you were starting from scratch and building a big power head I would have said go for the 26 head as they have a solid setup from factory and therefore would be cheaper than setting up a 25 head with a similar setup but since you already have the 25 head ported then stick with that unless it's a NEO head, as from what i hear your limited to 9.25mm lift due to a lifter gallery with the NEO's

25DE ports are bigger than 20 ports

[GT-RZ]

25DE are larger than the 20's ports... not sure about that 'special edition L2 20' though? .

If you have ever looked in the ports of a 26 you can see there is not all that much gains to be had in terms of porting+polishing. unless you go all the way and get some serious material removed in there. i'v got specs of a 1000hp GTR that pulled 8.7s down the gtr somehwere with a 100% factory head, including stock plenum.

i think the two massive leaf blowers on the side might have give it a kick?

The 26 has slightly larger ports than the 25 also, mainly due to a better shape. if anything id just get the exhaust side polished and leave it be. Just to remove the 'very small' cast marks in there. Wouldn't want to polish the intake side for obvious reasons.

Edited June 13, 2009 by GT-RZ

[R32 Driver]

If your rebuilding the entire head with new plenums etc its possibly better to go with a 25 since they are cheaper

use 26 covers to make it look good

i was advised sub 500rwhp porting wasnt needed on my 26

[R31Nismoid]

^ Yep, my head only has a minor tidy up and the build was put together with 380rwkw in mind if i wanted it @ later stages.

Similar to my 25 head really

[R31Nismoid]

unless it's a NEO head, as from what i hear your limited to 9.25mm lift due to a lifter gallery with the NEO's

NEO is the best of both worlds in some respects.

You get the RB26 solid goodness in the head inself, and you get VCT. win-win

Then consider you can make 380rwkw on stock cams in a breeze (guilt toy 25/30, myself RB25, and every Vic Racepace GTR for RB26s) makes it all the more interesting, but then they have appropriately sized turbos and so on that work well with the stock cam setup

[shanef]

put the biggest size cam in the std 26 head, then next teardown/head off event get it modified. Ideally with that turbo u want to see atleast 270/10.5 cams in that head with a proper port/valve job.

I dare say the ones using near std heads of any sort would have a massive hole down low in the torque/power band.

[WHITER33GTS-T]

Whats the biggest size to drop in...?

I want something that will work with my turbo setup, but I still have nos if needed.

And also want that angry sounding idle :-)

Edited June 14, 2009 by WHITER33GTS-T

[shanef]

Whats the biggest size to drop in...?

I want something that will work with my turbo setup, but I still have nos if needed.

And also want that angry sounding idle :-)

260/9.15 from a jap cam or 264/272/282 8.95 from kelford - dont quote me though

that angry idle is mainly from the port work and os valves. id get the big cams that u want to run down the track, now and just clearance the head for the extra lift. Then when u have time/money do the rest of the headwork later.

[D_Stirls]

25DE ports are bigger than 20 ports

Yep your right, My Bad.

I miss-read this from the PDF doccument.

Inlet Manifold and Plenum

R32/R33 RB25 plenums and inlet manifolds are not interchangeable.

The R32 RB20DET plenum bolts on to the R32 RB25 Inlet manifold.

R32 RB25 & RB26 heads run top feed injectors.

R33 heads run side feed injectors.

The R33 RB25 & GTR ports are slightly larger than the R32 RB25's.

For comparisons sake, a picture of an RB20DET Inlet manifold with a R32

RB25DE gasket over the top. Note the port size difference.

Edited June 14, 2009 by D_Stirls

[Sydneykid]

At the very least I always match port the inlet and exhaust manifolds to the head, the reality is some are just plain terrible. Especially when you have an unknown origin inlet and are sticking it on an unknown cylinder head. An aftermarket exhaust manifold can be worse, they rarely line up perfectly. I have yet to find a used RB26 head that hasn't got at least some bent exhaust valves, it's so easy to touch them on the block when removing that long and heavy cylinder head. It's well known that RB's have mismatched valve sizes, the exhaust valves are simply too small in comparison to the inlet valves. So increasing the size of all of the exhaust valves when replacing the bent ones is a worthwhile progression. Even as little as 0.5 mm larger exhaust valves makes a difference. The use of longer duration exhaust cams (than inlet cams) is a way of masking the too small exhaust valves. Much better to attack the real disease and fix the mismatch in valve sizes.

Keep in mind that boost is simply a measure of resistence to airflow, so if you are after a big boost bragging number from your engine then don't bother porting it. Air flow is what makes horsepower, not boost. Porting it will simply allow the same airflow, hence horsepower, at lower boost or more airflow, hence hosepower, at the same boost.

I don't subscribe to the " no porting until X bhp" theory. Let's face it, if you want more than X bhp then get a bigger turbo and turn up the boost. No matter how bad the ports are, there is always a larger turbo available to pump the air through them. To me what is more relevant with porting is how much response you want, if you don't mind a bit of lag then don't bother getting carried away with porting. If you want/need a responsive engine then making the same power at lower boost is a big advantage. With lower boost you can use more ignition advance, run lower inlet air temps, leaner A/F ratios, higher static compression, tune safer in a smaller window, etc.

In summary, porting is more relevant to response not horsepower.

Cheers

Gary

[bubba]

Yep your right, My Bad.

I miss-read this from the PDF doccument.

No offence mate but I wasn't just spouting drivel I had read on the forums, I have had both heads sitting side by side and run the 25DE head on my 30

[ST240]

I'm switching from the 25DE head to the 26 head this winter mainly because I'm sick of the crossover pipe cluttering up my engine bay, and a custom plenum is $$$ compared to a 26 head. Surely the 26 head doesn't UNDERPERFORM vs the 25DE head in any regard does it? The only place I could think of there a remote possibility would be low and mid range TQ... Is there?

[KiwiRS4T]

RB26 head stock is obviously better than a 25 stock head but I don't see it being cheaper. I got a Greddy copy FF plenum for the 25 head on my RB30 for $300 new and about $30 for the adapter to use my stock throttle body. RB26 heads in good nick are not that cheap and you will need a new exhaust setup and new injectors etc - but you will have a good setup when its done.

[WHITER33GTS-T]

I ditched the 25 head and went with a modded 26 setup. Went from 364rwkw @ 17psi (25 head) to 420rwkw @ 19psi (26 head)

[ST240]

I ditched the 25 head and went with a modded 26 setup. Went from 364rwkw @ 17psi (25 head) to 420rwkw @ 19psi (26 head)

Interesting result... Were both at the same rpm?