Turbochargers
Turbochargers
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Turbos are another area where everyone and their grandma has an opinion. So here is mine, STOP USING OVERSIZED CHINESE TURBOS! One area where the 30+ year old Nissan engineering cannot keep up with is modern turbo technology. When new, the RB20 was equipped with the worlds first ceramic turbocharger. Since then modern materials and designs have turned that turbo into a fossil. My advice is to figure out what you want out of your engine then buy a quality modern turbo to fit within those needs. I personally would rather buy a slightly smaller turbo, but use it to its full potential than buy a huge turbo I'll never be physically able to use at its limit. RB20s are small engines and it is super easy to buy a turbo that is simply too big. A big turbo will have tons of turbo lag and make the car very lazy to drive when not in boost. Buy all the proper supporting mods, do some research and you will not be disappointed. There are tons of great bolt on options available!
On this page all of my references will be for Garrett turbos and Garrett sizing as their website is easy to find information on and they make great products. For all my calculations and compressor maps I have referenced the Garrett website:
OEM RB25 Turbo on the left compared to a Garrett GTX2860R Gen II. Almost identical external dimensions and capable of ~400 hp. Modern tech is so good.
Things to Consider
Things to Consider
Turbo flange size is the first thing to consider. RB20s come stock with a T3 flange and I'd stick to that. GTSR manifolds come with a T3 flange and the race cars still made 400+ hp. I think a T4 is too big as a RB20 may struggle to spool most T4 turbo options. On a properly sized and shaped manifold a T4 twin scroll can work, but in my opinion just stick to a T3 or get a V band (and v band manifold).
Another thing to consider are the exhaust housing size. Around a ~.65-.75 A/R seems to kind of be the sweet spot for most turbo options. This is almost entirely dependent on what turbo you pick, but in general this A/R range is what works best on a RB20.
The key to making power in turbocharged cars is keeping the intake air temps down. When a turbo compresses air it heats up. Running a turbo in its max efficiency range for as much as possible and having a proper intercooler set up are two of the best ways to keep IAT's low and extract as much power as possible out of your engine.
Remember your turbo is only a single piece to an entire build. Many other things will need to be upgraded that can positively or negatively effect the turbos performance! Exhaust pipe size, intercooler size and location, injectors, cams and boost control are just a few more things to consider when upgrading your turbo.
Lets Do Some Math!
Lets Do Some Math!
Remember the engine air flow table from the Manifolds page? Lets apply that to some compressor maps and figure out some turbo sizes!
I have picked 3 common boost target pressures and plotted them on several different turbo compressor maps to find out what it looks like. There will be a ramp up in boost pressure as rpm increases. It will not be a flat line as shown but more of a curve as rpm increases boost increases. A good boost control set up will produce a horizontal flat line, but that is not realistic over the entire rpm range. Your VE and temperature will also change as RPM changes, but 90% is a good conservative estimate for most of the RPM range.
Click Here to Watch a Quick Video on How to Read Compressor Maps
Turbo Recommendations
Turbo Recommendations
For more power in an RB20 the Garrett GTX28 series are a well proven solution. G25 series are just a GTX on steroids and will have amazing spool characteristics. Many companies make the same sized bolt on turbos as Garrett so feel free to shop around for the one that suits you. Just remember you get what you pay for and I would not recommend saving money on a turbo. A bad turbo choice can ruin a good build!
**These flow numbers are theoretical. Turbo performance depends on many factors such as intake and exhaust manifold design so please consider this as guide not a fact. **
Left Dot is 3500 rpm - Right Dot is 7500 rpm
RED - 12 psi Estimated Engine Air Flow
BLUE - 16 psi Estimated Engine Air Flow
Yellow - 22 psi Estimated Engine Air Flow
★ - Estimated Peak power (6500 rpm)
Garrett GT Turbos
Garrett GT Turbos
GT2860R Compressor Map
GT2860R Compressor Map
300 - 350 hp
Very Responsive
This turbo is still a great option even though it is getting old. You will hit its power limits pretty quickly. This turbo will have great response and good midrange efficiency, but falls flat on its face at high rpm. This turbo isn't really suited to run more than 16-18psi and caps out at ~325-350 hp.
GT2871R Compressor Map
GT2871R Compressor Map
350 - 400 hp
Will have some lag
This turbo is a good choice for a higher hp and higher rpm build. The peak power is dead center in the max efficiently zone with more power to be had past 7500 rpm. It will have a lazy spool at mid rpm, but will jam at high rpm. This turbo is a bit of a mismatch for an RB20 because the exhaust wheel is pretty big and an RB20 will struggle to spool it effectively. Having upgraded cams or more efficient manifolds will allow the engine to flow even more air so more power can be extracted out of this turbo even at lower boost pressures.
GT3071R Compressor Map
GT3071R Compressor Map
400 - 450 hp
Will have a lot of lag
This turbo is a good choice for a higher hp and higher rpm build. The peak power is dead center in the max efficiently zone with more power to be had past 7500 rpm. It will have a lazy spool at mid rpm, but will jam at high rpm. This turbo is a bit of a mismatch for an RB20 because the exhaust wheel is pretty big and an RB20 will struggle to spool it effectively. Having upgraded cams or more efficient manifolds will allow the engine to flow even more air so more power can be extracted out of this turbo even at lower boost pressures.
Garrett GTX Turbos
Garrett GTX Turbos
GTX2860R Gen II Compressor Map
GTX2860R Gen II Compressor Map
350 - 400 hp
Extremely Responsive
Assuming the Air Flow is like the table, you can see how the 3 separate boost target pressures go right through the most efficient part of the compressor map. This part of the compressor map is where the turbo is the most efficient and delivers the coolest air to the engine. Since most driving will be done in the 4000-6500 rpm range this turbo is as perfect as it gets for a responsive set up with fantastic peak potential power of ~400 hp @ 21 psi.
GTX2867R Gen II Compressor Map
GTX2867R Gen II Compressor Map
375 - 425 hp
Very Responsive
This is a fantastic choice for a turbo. It will have a very healthy midrange and make plenty of peak power with room for more.
GTX3071R Gen II Compressor Map
GTX3071R Gen II Compressor Map
400 - 450+ hp
Will have a lot of lag
In my opinion this is the biggest turbo limit for an RB20. This turbo will make as much power as you could ever want, but it will have a lot of turbo lag. The GTX turbos are known to have great spool characteristics, but unless you have a lot of supporting mods to compensate, this turbo will have alot of lag.
Garrett G Series Turbos
Garrett G Series Turbos
G Series G25-550 Compressor Map
G Series G25-550 Compressor Map
350 - 400 hp
Very Responsive
This turbo is a great choice for a solid street car. The peak power is dead center in the max efficiently zone with more power to be had past 7500 rpm. It will have a very responsive spool when paired with a properly sized exhaust housing.
G Series G25-660 Compressor Map
G Series G25-660 Compressor Map
375 - 450+ hp
Neutral Response
This turbo is a great choice for a high hp and higher rpm build. The G25 is proven itself to be a great fit for RB20 engines and the 660 flows more than the GTX's of similar size. Big power without compromising the spool too much.
Garrett G Series II Turbos
Garrett G Series II Turbos
G Series II G25-585 Compressor Map
G Series II G25-585 Compressor Map
350 - 400+ hp
Responsive
The latest and greatest from Garrett. At the time of writing this there is no real world RB20 data to confirm the estimated spool and power levels, but this turbo might be the best paring on paper. I hope some people test and report back. This turbo might be the cheat code for all future RB20 builds.
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