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Turbo upgrade

Getting the best performance from your Isuzu, the in's and out's

Turbo upgrade

Postby dieselguy86 » Mon Dec 24, 2018 1:50 pm

So im finally back on my engine build, 4jb1t, original plan was to switch over to a garrett turbo but wasnt going to be 100% happy with it, so started researching other IHI turbos.

The biggest complaint ive seen is boost dropping off after 3,000rpm and this is why. The stock turbo has a 35mm x 47mm compressor wheel (intake wheel), after 3,000 rpms it goes into what is known as choke. This is a condition in which the engine needs more air than what the wheel can produce. All the other 2.8’s ive built turbos for have atleast a 39mm x 56mm wheel, and that begins to choke also, but not as bad. The best i have found to work is a 41.5 x 56 wheel. I know one person on here has swapped the wheel and housing from another turbo, but being honest, the exhaust wheel is too small for any performance as well.

So i went on a hunt and found an IHI rhf5 vf38 off a subaru legacy gt. Features ball bearings, titanium exhaust wheel/shaft, and twin scroll housing. The inlet flange is for the subie so it will take some fab work to get it down to a t25 pattern i already have on my manifold. Found a 42mm wheel of an isuzu rhf55 turbo that i will use for the induction part. Ill keep this updated as i build, but from my turbo building experience, this should be exactly what these 2.8’s need, with room for more power.
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Re: Turbo upgrade

Postby Sickmind » Tue Dec 25, 2018 4:25 pm

Definitely nice project! I also rebuild one 4JA1 with head, IP from 4JB1t and a small garret turbine from VW.
Why you don''t try IHI VICF RHF5 from Trooper gen. 3 with 4JX1 engines. These engines have a lot of power in a wide range of rpms, from personal experience(i have the both vehicles) I can say 4JX1 have more power than 4JH1. If you need some measurements of the wheels just tell me.
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Re: Turbo upgrade

Postby dieselguy86 » Tue Dec 25, 2018 4:42 pm

I went with that one because it is designed for quick spool, but has larger wheels for more flow. If you could, get the measurements of the VICF turbo, i need somewhere around 41mm on the compressor, and 42mm on the turbine. I may buy a chinese clone turbo just to use the exhaust and intake housings.
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Re: Turbo upgrade

Postby paulthepilot5 » Wed Dec 26, 2018 2:30 pm

Interested to see how this will go. I’m currently running a kinugawa TD04L-15T with just a stock cast wheel, 41/56 I think. It will Maintain about 36psi to about 3600-3700, then drop off after that.
Just about to try a hybrid built kinugawa subaru TD04 with a 46/58 Billet extended tip wheel. Just want to get a dyno run done on the current setup for comparison purposes first.
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Re: Turbo upgrade

Postby dieselguy86 » Wed Dec 26, 2018 3:22 pm

Ill keep everyone posted on it. Ive actually found the parts to build the stock turbo to near the same specs as your current turbo, slightly smaller but would still be a big improvement over stock. Stock is 35/47 compressor, 37/42 turbine. Ill be installing a 39.5/54 compressor, and 40/45 turbine. I have no need for it so Ill post it for sale on here.

While inspecting this turbo, the stock wheel has stress cracks all over the compressor wheel, which means overspeed. The stock turbos must be spinning insane amounts of rpm’s trying to keep up.
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Re: Turbo upgrade

Postby paulthepilot5 » Wed Dec 26, 2018 3:50 pm

dieselguy86 wrote:Ill keep everyone posted on it. Ive actually found the parts to build the stock turbo to near the same specs as your current turbo, slightly smaller but would still be a big improvement over stock. Stock is 35/47 compressor, 37/42 turbine. Ill be installing a 39.5/54 compressor, and 40/45 turbine. I have no need for it so Ill post it for sale on here.

While inspecting this turbo, the stock wheel has stress cracks all over the compressor wheel, which means overspeed. The stock turbos must be spinning insane amounts of rpm’s trying to keep up.


Not supprised about overspeed in the stock turbos, they do perform pretty well for what they are, was able to get around 30psi out of the factory turbo and make around 125kw, probably well outside it’s limits. An upgrade in wheel size in the factory turbo would make a good upgrade
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Re: Turbo upgrade

Postby Sickmind » Thu Dec 27, 2018 1:40 am

I didn't find the shaft to take measurements, but I found the intake and exhaust housings and also the middle part wich needs a rebuild.. so if you gone buy a chines one just for the housings, I can give you mine for free.. just pay the shipments :)
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Re: Turbo upgrade

Postby geeves » Thu Dec 27, 2018 2:07 pm

How far do you intend to go? I know they can do lots more but my old 89 bighorn was still on factory turbo at 240000km and was running 14psi at 1600rpm up to 3500 where it started to drop off. It was still above 12 at 4k. For what was a standard engine it went well but it was never a race truck. It still struggled with heavy loads though. I towed a Hiace van plus 15 foot caravan and 6 Harri Krishna members from a standing start half way up Ngaranga Gorge to the top. Still went over the top at 35kph. Had to take the lot in one go as the caravan had no brakes. The guy from the AA who had been called to help but didnt know what to do was impressed
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: Turbo upgrade

Postby Marsheng » Thu Dec 27, 2018 3:24 pm

I have been researching the same topic. I have an Isuzu Elf truck with a 4JG2 motor. It's a bit flat compared to my Toyota 2.2 Turbo 3c in a lightace. I found a good condition IHI RHF5 but it looks like I need new intake and exhaust manifolds plus a few other bits to make it run. What do I need to do with the motor itself to make this all work ?
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Re: Turbo upgrade

Postby dieselguy86 » Fri Dec 28, 2018 10:45 am

The vf38 turbo came today, definitely NOT ball bearing, but titanium intake wheel and twin scroll housing. Im thinking it will be easier to build a new, divided exhaust manifold.

Some more research shows that a vf36 turbo has a titanium-aluminide turbine (half the weight of the standard turbine) and is really ball bearing. The turbine also has more blades which is great for spool up. So plan now is to swap the titanium compressor from the vf38, to the vf36 center section, then install that into the vf38 intake/exhaust housings. I want the titanium wheel because it will be sucking in 120+°c air from the first turbo.

One thing i found interesting, on the cover of my stock turbo it says its a ihi rhf5, but its actually a rhf4h.
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Re: Turbo upgrade

Postby tonkamu » Mon Dec 31, 2018 5:31 pm

Theres a guy in Auckland who runs a vf38 or vf36 on a 4jg2 at 40 psi with solid front axle. Looks pretty good from the videos he posts, paddles through the sand like a v8 on what look like 35's.
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Re: Turbo upgrade

Postby dieselguy86 » Tue Jan 01, 2019 12:28 am

Well time for a bit of an update i guess. Im now not 100% certain the intake wheel is titanium, but the exhaust wheel is DEFINITELY what is called titanium-aluminide, or Gamma-ti. Weighs half of what the stock exhaust wheel does, its incredibly light, still blows my mind everytime i hold it how light it is.

So my next step is ball bearing, still researching on it. Best i can tell, is that IHI did very little changing when they developed their bb turbos, which is good. It looks like ill be able to convert the vf38 to bb, all i need is the bb center housing, and the bearings.

Im undecided on what to do housing-wise, twin scroll really helps with spool up, but will require me to make a new exhaust manifold. Easy for me to do, but im also trying to think of others if they want to upgrade their turbo. So im on the lookout for other rhf5h turbos with different exhaust housings. Other than that, the vf38 almost seems to be made for the 4jb1/4jg2 size wise. And if i can convert it to bb, youll be able to have great spoolup AND top end power.

More parts coming so ill update then.
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