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Mar 31, 2020

ITB vs Single Throttle : Individual Throttle Bodies : RB26dett

One of three sets of 2 individual throttle plates for an RB26dett
As far as RB26's and Skyline GT-R's go, I consider myself a purist.  I like keeping them closer to stockish systems, unless I really think  the systems are an issue. For instance, I like twin turbos, I don't mind Hicas. I like running the stock airbox. Even something like the factory intercooler is a good part.

However when it comes down to operation, and flow, I think the ITB are actually an issue.
  1. Leaks
  2. Complexity
  3. Restriction to flow
DO NOT CLEAN OEM THROTTLE BODIES, unless you have a sealer like Tomei Throttle coat to ensure sealing. If you clean the sticky mess off the inside, once you get it all back together, the idle will be high, with no real way to get it to idle lower.  That is without taking it all apart and adding something to seal up the blade edges.

The OEM paper gaskets are thin, and leak fairly easily. We like the Tomei gaskets, they are metal, and will not fail like the stock gaskets. There is one long gasket against the head, and one set of two for each side of the throttle plates, or 6 of them.

The stock throttle bodies are 45 mm.  The math for area is:

1590.43 mm2 each x 6 = 9542.58 mm2

However a large portion of that space is taken up by the shaft, throttle plate, and the screws.  The shaft is 10 mm thick.  The throttle plate itself is 2 mm thick.  The screws protrude into the airstream.

Flow through a throttle body paper
Airflow bottlenecks - Dsport Magazine
Design and optimization of a throttle body by CFD analysis

RB26 ITB throttle blade. Notice screws. Notice thickness of shaft

45 mm - 10 mm shaft.  Each side has 17.5 mm radius. Or a total of 35 mm radius or
962.11 mm2  x 6  = 5772.66 mm2

I also drew the GTR inlet port and measured the area on the computer. It comes out as 1552mm^2. Now the area of the std GTR throttles with spindle is just 1140mm^2. The 45mm AT throttle on the other hand will come out at 1500mm^2. So is nearly a perfect match area for area for the inlet port. Hopefully the flow tests will show a good improvement, looking at those numbers they should do!

45mm dia = 1590mm^2
10mm spindle 45mm long = 450mm^2
2mm blade 45mm long = 90mm^2

GTR 1590 - 450 = 1140mm^2
AT 1590 - 90 = 1500mm^2

1500 / 1140 = 1.316 (or in other words 131.6% which is a 31.6% increase)

90 mm throttle body like a Q45

6361.72 mm2

with the same 10 mm thick shaft

5026.55 mm2

Airflow around throttle blades and shafts. More from this paper

I think that if you include disturbed airflow above and below a throttle blade, that you actually have more usable area even in a 90mm throttle body.  That is area where air can flow and not tumble or be disturbed by the shaft and blade.   Just some thoughts. Obviously a larger throttle body would flow even more air, if needed, like max effort drag cars.

So had some cool data from Jeff Gin on GTRUSA.

Interesting read, however i myself currently run stock manifold vs a friend of mine with a hypertune manifold with 102mm single throttle, same engine porting cams etc and we both make the same power at the same boost throughout the whole rev range
Actually although peak power may be the same, it looks to me, that the stock throttle body/stock collector car makes more area under the curve, more torque, more of everything than the Hypertune plenum and 102 mm throttle body.

Look how vertical this chart is from 4000-5000 rpm. Has to feel like you are hit by a ton of bricks.  OEM throttle and collector
619 kW peak or about 840 horsepower. 

This setup has the Hypertune plenum and 102 mm throttle body
Looking at a single point, say 8000 rpm you are both about 725 nM. At 8000 rpm, you both are around 610 kW.

If you pick some points in each chart, you will see that the white chart/ OEM TB above is more responsive than the Hypertune setup/102 mm.  They both are nutty in the 4000-5000 range, but 400 nM to 850 nM compared to 300 nM to 750 nM.   Thanks much Jeff, this is some cool data, the kind of thing I was looking for.

Nismo collector on an RB26. The design of this helps to balance airflow in the engine. 

Something else to consider is plenum volume, and its effect on tuning and powerband. Even though many people call the collector a plenum, it isn't. It is pre throttle body, and the only real plenum in an RB26 is a super short runner that is post throttle body. 

Shaft-less throttle blades

Not sure if anyone actually ever did any of these. Reading the thread, it seemed to have some traction, and then died. Cool idea, but never went forward.

On the GT-R UK Forum a member posted up some pictures and thoughts about shaftless throttles for an RB26
The increase in airflow for a 45mm is equivalent to running a 48mm throttle - but as the GTR one is a 10mm shaft, rather than a more usual 8mm shaft, it could be more of a gain. at 48mm you're talking a 15% increase in cross sectional area. But, because the throttle diameter is the same, you're not loosing out on torque - Plus everything else would fit as normal.

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