Rb30/26... Should I Go Aftermarket Plenum Or Stock 6 Throttles?

[den001]

i have a 30/26 with pretty much everything done to it, very close to full assembled, and am not sure if i should uprgade the intake plenum to aftermarket wit a big single throttle or just keep the stock 26 system....

ive heard the 6 throttles have more cross sectional area and flow more than 1 100mm throttle but am still unaware what i should do....

should i go hypertune?? greddy bolt on ?? custom or stock???

thanks

denis

[discopotato03]

The factory RB26 inlet system is the tried and proven one and for the life of me I cannot fathom why people want to remove it . It does have more throttle area it does provide better throttle response and it does make the thing less of a cammy cranky pig if you do use long period cams .

If people really think a 100mm or larger hole is required to get the air in under pressure then it'd be a simple matter to enlarge the inlet to a GTR style plenum and retain its multiple throttle plates as per std .

My logic may be faulty but I look at it like this .

If you measure the compressor housing outlets and combine the cross sectional areas this is what the turbo/s are breathing out of .

Now lets just say that your brave and fit a pair of GT3076R's/GT3037's with a little less than 51 mm outlets . Now area of a circle is Pi x the radius squared or 3.14159 x 25.4 squared = 2026.8 mm square - times 2 (two outlets) = 4053.6 .

The cross sectional area of a 100mm hole (Pi x 50 squared) = 7854mm square .

It's not quite double but its getting close . Other things do have their say like for example a 100mm throttle body won't pass as much air as a 100mm hole because there are losses around the plate and shaft .

Big NA engines need big throttle body's for a very good reason - all they have is atmospheric pressure to charge their cylinders so lots of capacity being charged with modest pressure requires a bloody big hole to let the air in .

Once you force induce an engine the situation changes because the charge air density is (well should be) higher so hopefully a greater weight (mass) of oxygen is going in per cc of charge air .

Ratbag rip tear and bust Jap 11000 rpm drag RB26's are a totally different story and totally inappropriate on anything less .

Your calls but if Gibson Motorsport got real good power using that system then I reckon so can 90% of the rest of us .

The Grp A cars also had to put up with inlet restrictors and most seem to agree that if they hadn't Holden and Whored might have not bothered running at all .

Your call .

[den001]

thank you for the detailed response.. the formula you have enlightnend me has helped me alot....

cheers discopotato

[shanef]

my only let down with the std plenum is the leanout on the rear couple cylinders, i'd much prefer a plenum that flows near on 100% even to all 6 cylinders

[rev210]

nismo unit is spot on. Just $$$$$

[discopotato03]

Shanef I believe that's more a plenum chamber issue than a multiple throttle one , I also wonder how these single throttle manifolds fair on that score .

If someone with the fabrication facilities and abilities were prepared to muck around with a couple of RB26 outer or plenum sections surely something could be made to work .

If it were me (and I'm no gas flow expert) I'd consider making the plumbing /inlet /plenum volume greater than std so the incoming airs velocity was lower and possibly less likely to have that ram air uneven pressure at the back of the manifold . Maybe this is too deep and a simple computer adjustment (increase) of a few extra percent over the front 4-5 cylinders fueling could be done if the difference is known . SK ?

Cheers A .

[Sydneykid]

my only let down with the std plenum is the leanout on the rear couple cylinders, i'd much prefer a plenum that flows near on 100% even to all 6 cylinders

Up to around 600 bhp they seem to work just fine, over that and I agree with Rev210, Nismo is the go. That's why Mines and several others use them.

Cheers

Gart

[paulr33]

depending on the ecu you can always tune cyl6 a few % richer under injection control

the powerfc supports this method

[Full-Race Geoff]

Shanef I believe that's more a plenum chamber issue than a multiple throttle one , I also wonder how these single throttle manifolds fair on that score .

If someone with the fabrication facilities and abilities were prepared to muck around with a couple of RB26 outer or plenum sections surely something could be made to work .

If it were me (and I'm no gas flow expert) I'd consider making the plumbing /inlet /plenum volume greater than std so the incoming airs velocity was lower and possibly less likely to have that ram air uneven pressure at the back of the manifold . Maybe this is too deep and a simple computer adjustment (increase) of a few extra percent over the front 4-5 cylinders fueling could be done if the difference is known . SK ?

Cheers A .

adrian -- ill have to send you pics of the modified hypertune plenum in my car. it retains the ITBs. i think they will make them like this now

[discopotato03]

Thanks Geoff .

Cheers A .

[bubba]

Has anyone got any actual flow figures for a standard GTR plenum? wondering exactly what the difference is in the last 1-2 cylinders and if there's anything I can do while fabricating to even it out somewhat?

[700HP-GTR33]

depending on the ecu you can always tune cyl6 a few % richer under injection control

the powerfc supports this method

+1

My tuner after seeing two of my spark plugs were nearly burn't out on the tips increased fuel to both these cylinders.

6 was increased the most.

Stick to your standard plenum and fuel rail.....

[KISIN]

Up to around 600 bhp they seem to work just fine, over that and I agree with Rev210, Nismo is the go. That's why Mines and several others use them.

Cheers

Gart

adrian -- ill have to send you pics of the modified hypertune plenum in my car. it retains the ITBs. i think they will make them like this now

I was just about to ask SK what he thinks about the Hypertune, but now, change it to, how does the hypertune compare with the NISMO item ?

[bubba]

+1

My tuner after seeing two of my spark plugs were nearly burn't out on the tips increased fuel to both these cylinders.

6 was increased the most.

Stick to your standard plenum and fuel rail.....

I will be keeping my standard fuel rail and the 550cc injectors I've got in there but have no other option than to go forward facing plenum (space constraints). Will be using a GTR plenum chamber mated to my stock runners (via a flange welded to the cut runners) so I can retain the current injector setup.

Any idea what % increase your tuner used on cyl's 5 & 6?

Edited April 9, 2008 by bubba

[Adriano]

I use 1%(5) and 2.5%(6)

[DCIEVE]

in the short time ive been logging egt's, 4 seems to run hottest with no other correction... any one else had similar experiences? A friend with a similar egt setup also saw 4 running hottest and needed a fair amount of fuel to compensate on the dyno (mine hasnt seen the dyno yet)

[rb26s13]

...

Edited April 9, 2008 by rb26s13

[R31Nismoid]

depending on the ecu you can always tune cyl6 a few % richer under injection control

the powerfc supports this method

in addition get the 6 new injectors flow tested first.

The higher flowing 2 of the 6 you put in the rear as there is always one or two that flow a tad more

[GeeTR]

IMO, Trimming works well for safety... not power though.

You can richen the last few cyl's but can you trim IGN per cyl to account for the longer, richer burn time...

Nismo item, or modifying the rear runners would be best.

Edited April 10, 2008 by GeeTR

[bubba]

How would you suggest modifying the rear two runners? port them out slightly? try to increase some volume in the plenum behind them? (I did notice that the last runner looks almost like an afterthought..)