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Hello Everyone Chev R here from South Africa.

I am in need from some of the guys that have done this before. I have checked a few threads on turbo that guys suggested and has been been covered.

My need currently is this I am building a Rb30/25 and have got my current turbo specs as follows:

Ar 70 compressor housing

Compressor wheel 58mm

Exhaust housing Ar68

Exhaust wheel 64mm

Now some of the guys I know in the drag seen here advise that the turbo will be perfect down low but will probably choke towards limiter maybe 6000rpm. What is your take on this please need some guidance🤯

I will be limiting rev range to 7000.

One of the guys I know offered me a turbonetics 7868. I will attach specs as per my research.

What I am after is is my turbo going to.run out of steam higher in the rev range. If someone has had a similar turbo set up?

If so would the turbonetics be a good second option as this is being offered to me at a good price. 

Car will be used for street and once the covid pandemic passes as the race track from time to time.

I would appreciate your honest inputs on this please🙏

Thank you kindly Brian ( Chev R)Screenshot_20200531-104334_Chrome.thumb.jpg.3711b4707a6f4862baa0415df14cbc0f.jpg



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Do not go to the RB30 dyno results forum, that's a trap and will lead you astray. 

I'll now teach you the four step process to correctly picking a turbo. 

Step 1. Look around your house and find the biggest dinner plate you own. 

Step 2. Bring the dinner plate to your local turbo supply store. 

Step 3. ???

Step 4. Purchase a turbo that is bigger then the dinner plate. 

  • Haha 3

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You want at least an 84mm turbo, maybe even a 91. Big one from the EFR range, or Garrett G series.

What is your power target?

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Hi GTSBoy the idea is to start of with 500hp and gradually increase. Car will be used on the street.

Sub assembly is freshly built but not with forged internals.

If I may ask what do you mean by 84mm? Is this 84mm on the compressor side or exhaust side?

The Turbonetics measures 78mm on the top of the wheel and about 90mm at the bottom .

Exhaust measures 68mm on the top of the wheel and 78mm at the bottom of the wheel.

Comes std with a Ar96 exhaust housing T4

So the intent is still for the car to be responsive and not to have too much lag.

Maybe to start boosting at 3-3500rpm.

Hope this helps

Sorry if I did not understand your response too well.

Murray thanks for your response I gather what you mean

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Here is an EFR 83mm turbo, the 8374. https://www.full-race.com/store/borg-warner-efr/turbos-efr-series/borgwarner-efr-8374-turbo-2/

The bottom end of that turbo's range is in the 500HP (engine) region. Can make a lot more.

Here is an EFR 91mm turbo, the 9180. http://www.turbos.borgwarner.com/files/pdf/dimensional_reference/efr/EFR%209180-C.pdf

Obviously the 9180 is ~20% bigger than the 8374.

These things work at really high pressure ratios.


I just pitched you big turbo sizes because you're talking about a 3L twin cam. It is a decent sized engine that can definitely drive a big turbo. But if you don't think you will be needing to go near 800+ engine powers, then smaller options make sense.

There are many smaller EFRs. The Garret G series has lots of options too. The latest gen GTX turbos are still worth a look. Even BW SX turbos are good for more crude (and a bit cheaper) applications.

One thing you will have to pay attention to is the inlet and outlet style and size on the exhaust side (no point buying a T4 flanged turbo if you have a T3 manifold, for example), and whether you need to add one or more big external wastegates. Another thing to think abotu is that a proper twin-scroll turbine inlet and manifold is the very best idea. Not all of these turbos offer all these things at all sizes. You will have to pick what is most important to you and live with the compromises.

You could also get a Turbo from Hypergear here in Oz. He will have something that will do the job for you. Very "offbrand", but decent gear nonetheless. Much better than that horrible Turbonetics thing.


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Hi GTSBoy intake not of all the info and really appreciate all the guidelines. Very detailed.

Will have to decide where I would like to be. 

Just had a thought I can possibly also upgrade my turbo to a 62mm inducer and 84mm exducer on the compressor wheel to a billet 7 blade or 11 blade. Will have to machine my compressor housing to fit.

And leave the turbine as is as it measures about 64mm inducer and 74mm exducer with Ar68 housing.

Once again thanks for the responses helps to have a clearer path of where I want to go.

Will also research some other turbo options on the Garrett range.🙏

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Long time reader of SAU just created my account so can learn and share easier with users here.

Hi Brian, to be clear of your current turbo, it has 58 mm inducer (smaller diameter) compressor and 64 mm exducer (smaller diameter) turbine?

If your turbine has around 74 mm inducer (bigger diameter) then you seem to have a T4 P-Trim turbine wheel, which was used in Garret and HKS T04Z and were popular for RB26 and 2JZ in the 2000's. Its old technology but can work well as its still used in Garrett new GTW3884 (update of Garrett T04R/Z), and you can see from the turbine map at Garrett website that it has 73% peak turbine efficiency, better than newer GT35 GT40 turbines which has the closest flow rate to T4 P trim turbine.

T04R/Zs have 66-67 mm inducer 84 mm exducer compressor good for 750 hp, while GTW3884s have 62-64-67 mm inducer compressor options rated by Garrett up to 800-900=950 hp.

If you can source billet 62-67 mm inducer 84 mm exducer compressor then you will make an aftermarket wheeled GTW3884. Just make sure to order compressor wheel with the same bore size as your turbine shaft as there are small shaft and big shaft versions of T4 P trim turbine.

Turbonetics 7868 is a bigger turbo, it uses Turbonetics better flowing version of Garrett T4 Q trim turbine, one size up from a T4 P trim turbine. For your target of 500 hp a 7868 turbine is overkill as the T4 P Trim turbine has made over 800 whp when used in GTW 3884 67 mm inducer compressor. So iI suggest its better to use your current turbo assuming it has a T4 P trim turbine and update it with a billet compressor (bigger compressor inducer means lazier response but higher max hp).

If you want to optimise response you can also get a T4 twin scroll turbine housings for T4 P trim turbine, you can google AGP turbo and ATP turbo from USA for example to source those housings. In my experience converting to a twin scroll set up even with a single wastegate exhaust manifold similar to a 6 Boost RB25 manifold it gives quicker spool up with no loss of top end max hp. Hope this helps.


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