Rb26- Enlarging Std Throttle Bodies

[sucram]

It will give gains, but you wont hit 600

5 hours ago, Predator1 said:

bump.

Anyone had this done? either 49 or 50mm? Been pondering what to do while mines in storage. Been thinking of DBW but if I do that, then might as well do this too. Want to retain 6 throttles.

I have a fully build head(CNC, OS valves, 286/278 cams, etc etc still running squish), and a Greddy old school intake with port matching.

Head bench flowed 300cfm. Wondering whether bigger throttles will give me any gains. Makes 563kw.

It will give gains but it wont get you to 600 imo. You will also sacrifice low down and you will be adding extra volume if you port match it, more so if you port the plenum runners depending and how its done

I think the only time youre going to see about 100hp gains is if the car is making well over 1k kw

I do have a spare set that I was going to test with, but i dont actually think there is anything worth getting going larger. I did a fair few S14/S15 and the standard TB is fairly small, I went to 60 and in some cases 64mm which is significant over the stock unit. You can also get some weird ratio changes. I think 20kw will be a stretch on a roller dyno. If I had it apart I might mill the shafts a little and not expect much, I think the stock size is still pretty good for a street/track car even with larger capacity. I might change diameter it if its a drag only car

[LaurelPWR]

On 14/3/2025 at 6:09 PM, Predator1 said:

bump.

Anyone had this done? either 49 or 50mm? Been pondering what to do while mines in storage. Been thinking of DBW but if I do that, then might as well do this too. Want to retain 6 throttles.

I have a fully build head(CNC, OS valves, 286/278 cams, etc etc still running squish), and a Greddy old school intake with port matching.

Head bench flowed 300cfm. Wondering whether bigger throttles will give me any gains. Makes 563kw.

Hey mate, thread dredging hahah

To answer you directly, yes I had my itbs oversized and custom butterflies etc made by EFI Hardware in Sydney back around 2017-2018 if memory serves.

My setup had a vast amount of other changes too though, so no direct comparisons unfortunately.

i went from oem itbs, HPI manifold and internal gate hks 2835 pro S;

to 49mm itbs, cnc port matched Nismo plenum, RB25 Neo lower runner adapter hand port matched, r31house v2 Takoashi MAX equal length twin scroll manifold with twin 38mm tial gates and a FP HTA3076 with 1.01ar twin scroll real hosing.

feom power Fc to a link fury wired by 12volt, r32 gtr fuel tank conversion with frenchys twin 460 / surge tabk fuel hanger combo kit, 1659cc injectors, full 4 inch turbo back stainless exhaust, Z34 6 speed, Tomei 2way with 4.3 final drive and a ton of other shit.

ive posted elsewhere here with complete run down, but here's a graph from its initial tune with no boost control on, 20psi gate springs only and e85.

 

Edited March 15 by LaurelPWR

[LaurelPWR]

On 14/3/2025 at 11:44 PM, sucram said:

It will give gains, but you wont hit 600

It will give gains but it wont get you to 600 imo. You will also sacrifice low down and you will be adding extra volume if you port match it, more so if you port the plenum runners depending and how its done

I think the only time youre going to see about 100hp gains is if the car is making well over 1k kw

I do have a spare set that I was going to test with, but i dont actually think there is anything worth getting going larger. I did a fair few S14/S15 and the standard TB is fairly small, I went to 60 and in some cases 64mm which is significant over the stock unit. You can also get some weird ratio changes. I think 20kw will be a stretch on a roller dyno. If I had it apart I might mill the shafts a little and not expect much, I think the stock size is still pretty good for a street/track car even with larger capacity. I might change diameter it if its a drag only car

The "sacrifice of low end" is now basically irrelevant in 2025.

Turbo/ecu/fabrication/and other tech advancements gives a vast array of options to mitigated any of it.

My setup now is a fantastic example of this.

300whp / 20psi @ 3500rpm from a 600hp turbo setup on a notoriety low reading Dyno Dynamics roller dyno, is almost unmatched on all of SAU and other forums. The only other turbo i've seen that can achieve this is a BW EFR + twin scroll / twin gates etc etc etc...

ps. tuned by Matty Spry at P.I.T.S

as far as getting to the 600 figure, while enlarging your itb's alone probably won't get you there, as Sucram has stated above, it will certainly help. Just do it Predator1 my brother hahaha 🙂

Edited March 21 by LaurelPWR

[Predator1]

It really depends on your setup. Mine is 'accidently' more suited to revving higher. bigger cams, turbo etc. Plus my super long first takes donkey years to come onto boost, so if it will move my power band to the right, maybe i'll pass on this. i have a G45-1500 going on there soon which I suspect might make it more of a fat pig.

[LaurelPWR]

45 minutes ago, Predator1 said:

It really depends on your setup. Mine is 'accidently' more suited to revving higher. bigger cams, turbo etc. Plus my super long first takes donkey years to come onto boost, so if it will move my power band to the right, maybe i'll pass on this. i have a G45-1500 going on there soon which I suspect might make it more of a fat pig.

Yes, at this stage of SAU threads, I  automatically assume that anyone here knows that everything depends on each and every setup and its choice of parts and what an individual wants from the setup.

I think your misunderstanding what I've said, I'm not saying your curve will move to the right..

I'm stating the opposite; that my setup has a much larger turbo than my last one, my current exhaust manifold runners are 2-3x longer than the last one, the intercooler is longer and thicker (hks 103mm thick), the itbs are now oversized from 45mm up to 49mm, and the plenum is now a larger Nismo plenum that's been port matched. As a result, I'm making more power, torque and boost 1000rpm earlier than my last setup.

My curve moved left, in a big way.

You guys with 800-1000+hp turbos should be considering hks v-cam or the other Japanese company Sakura who makes RB25 Neo vct adaptor kits for RB26's.

they all make more power / boost / torque far earlier and higher peak numbers.

Dahtone Racing modifies the Hks v-cam systems to his own specs and he also developed the worlds first and best dbw individual throttle body kits, to complement his V-cam. Every single car is 800-1200+hp and they all produce huge gains in response and peak power. 

Edited March 21 by LaurelPWR

[LaurelPWR]

Just a basic run down of the relevant bolt-ons.

All e85

First graph 406.9whp is HKS GT-RS, HPI manifold, 3" bell mouth dump / 3" straight through fake cat / 3" all the way to the rear and no resonators, nismo 740cc, walbro 460, oem RB26 itb's, power fc, eboost2, Greddy Spec M fmic, poncams, Turbosmart actuator, splitfires, ARD 150amp alternator.

Blue line is same as above but Blitz Dual SBC (before i upgraded to eboost2) and 100cell "high flow" cat.

Red line is after installing eboost2 and the fake 3" cat (it's just a 3" pipe with a cat shell welded onto it). Huge gains in response, as can be seen.

Black dots are a mates sr20 with almost identical setup but uses a garrett 2871 instead oh HKS.

Next grapghn 466.6whp is same as above, but HKS 2835 Pro S with the larger .87ar turbine housing, the special HKS Pro S dump pipe into a HPI split dual 2.5" front pipe and the rest stayed as is.

Red line is after I welded the actuator brackets. (It was originally bolted together and was moving a couple millimetres under load).

Green line is obviously before I welded the bracket, no other changes. 100whp gain at 3750rpm, massive gain for such a simple fix.

Third graph is my current setup, huge changes. 469.7whp

R31house twin scroll manifold, twin TiAl 38mm gates, Forced Performance (USA) HTA3076 with 1.01ar twin scroll rear housing, Link Fury+ and loom by 12volt, PRP trigger kit and r35 coil kit with r35 splitfire coils, HKS step 1 vct staggered cams, new oem lifters and performance springs, 49mm itb's w/port matched Nismo plenum, twin walbro 460 surge tank setup + 1650cc injectors, HKS Type R S13 600x300x103mm fmic, individual egt's, 4" turbo back with no resonators or cat and AES true straight through muffler + 4" butterfly off the dump pipe lower 90° bend.

It's a preliminary tune with no boost control with using 20psi gate springs and 4" butterfly closed. Once body loom and PDM is compete, will go for 30psi tune.

Edited March 28 by LaurelPWR

[LaurelPWR]

*all RB25 s1 if I hadn't said yet, sorry guys haha