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Jul 8, 2019

Nissan Skyline GT-R Boost Restrictor

The famous or infamous boost restrictor in a Nissan Skyline.  According to the internet experts out there, all you have to do, is pull the restrictor, and instant power. Please understand what things do, before you blindly follow idiots on the internet.  The guys that heard from a friend of a friend, probably have no clue on the ramifications of raising boost pressure.

Stock an R32 GT-R runs about 10 psi of boost. However the stock boost gage reads in torr, or mmHG.  Half way on the stock boost gage is about 3.5 x 100 mmHG or about 6.7 psi of boost. A factory R32 GT-R with the boost restrictor in, should read just above the first mark, or about 10 psi.  If it reads 7, or 700 mmHG, that is closer to about 13.5 psi, and the restrictor has probably been removed.

Above about 1.1 bar (16 psi) the stock RB26 turbos have a tendency to break the exhaust wheel off. People have gone low 12's, high 11's on the stock turbos, but not for long. Stock peak boost R32 is about 0.8(11.5 psi) , R33 0.9(13 psi), R34 1.0 (14.5 psi).

Voltage, Engine Oil Temperature, and Factory boost gauge in an R32 GT-R
In an R32 GT-R, the factory boost control solenoid is mounted on the right shock tower area. The boost solenoid has two lines going to/from it.  One line has a yellow band on it. The yellow band is where the boost restrictor is located.  What the restrictor does, is limit the amount of air that is bypassing the wastegate actuators.

Fuse box, factory boost solenoid, and battery
By removing the restrictor you bypass more air back to the inlet side of the rear turbo, raising boost pressure.  If everything is stock, new, and running perfect, you will see about 14 psi of boost with the restrictor removed.  As a general rule of thumb, every 1 psi of boost, gets you about 15 horsepower.  So by going from 10 psi to 14 psi, you just gained about 60 horsepower. 60 horsepower may not sound like a lot, but going from "280 horsepower" to about 340 horsepower is fairly significant.  In theory should pick up about 6 tenths, or 6 mph in a quarter mile.

The famous boost restrictor on a Nissan Skyline GT-R

Restrictor size. About 1.6 mm. 
RB26 vacuum diagram, updated 2019

Now for the potential non fun parts.  These cars are 25, 27, 30 years old.  The systems are not new. The condition of the fuel pump, fuel injectors, timing, turbochargers, ECU are all unknowns. Any of these items can let go sooner if you up the boost.  There are lots of ROM tuned ECU's out there, that are very aggressive on the timing.  If you are running 91 octane fuel, with aggressive timing, and 14 psi, you have a really good chance of hurting the engine, specifically #6 cylinder.  If you don't hurt the engine, you can knock a ceramic exhaust wheel off.  Unintentional single turbo RB26 are not as fun as twin turbo ones.

Fuel system
Fuel pump
Setting Ignition Timing
Boost control

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