for those that care to read.
RHB31's have at least 6 different variations (and probably a ton more). The variations are OE specified, so that the particular compressor and turbine are sized correctly to work with the OE application.
For instance an RHB31 is also on an M59. I have a couple of those. But it makes zero boost on a D722 (NA D722). Sure change the turbine housing right? Tried. The turbine wheel is larger than the smaller housing I got online (OE housing by the way). SO now I'd have had to change the shaft assembly, seals, bearings, o-ring, etc. Grand total if I got the parts retail is about $150 (and that was over a year ago, so probably $300+ now IF you can even get them)
I see this stuff all the time with automotive and powersports. Guys buy a generic turbocharger and toss it on, sure it makes 2x more power and they're happy with that. In under a year they're griping about ventilated blocks or wrecked cars or whatever. So sure you can absolutely make 20 psi, but what's the outlet temp? What's the shaft speed? Is the turbo rated for that speed? Is the shaft assembly and/or wheel(s) gonna come apart? What's the EGT? Whats the drive pressure?
How's turbocharging work (or forced induction in general)? It uses a device (turbo or supercharger) to increase the amount of air that is going into the engine. Anytime you increase pressure, you also increase heat. With that, your EGT can go sky high. Or it might not. Depends on the turbo, engine, inlet temp, and a bunch of other things. So for instance you make 10 psi with an outlet temp of 100 degrees F and a drive pressure of 10 psi. That's considered pretty good. Or, another turbocharger may make the 20 psi but at 250 deg F with a drive pressure of 40 psi. #1 is going to make way more power and a lot more reliably every single time.
So on any piston engine, the pressurized air does more than jsut stuff more air into the engine. Remember the engine has to ingest air on it's own without a turbo, via pressure drop on the inlet stroke. This takes crankshaft power to pull the piston down the hole against the rings, viscous drag, and in this discussion it also has to overcome in some cases a throttle blade which is restricting air movement, which I call parasitic loss. The turbocharger forces some air in under pressure which helps force the piston down the bore, which takes a lot of the parasitic loss away during the inlet stroke. At 0 psi boost, a turbo engine will make more power than a naturally aspirated engine which is also at 0 psi boost and 0 vacuum. But there's JUST the very beginning. Enter: drive pressure....
Remember the intake stroke is only 25% of the engine's workings. We also have an exhaust stroke. That's where the piston is forcing spent gases out of the engine. The turbocharger is in the way creating a restriction to flow which equals pressure. It takes more force to push those gases out on a turbo engine, thus it backs up and causes what I call drive pressure (the actual pressure inside the intake manifold). Remember the spent gases are hot and they can contaminate the intake charge. Additionally a lot of times the intake and exhaust valves are both open at the same time for a short period, any exhaust pressure will back up. It is inert and can't burn again and the burn does the work via expansion.
On a gas engine you have an intake tract, and most of the time the fuel injectors are placed at or near the end of the tract, but before the intake valves. In doing this, the fuel actually cools the intake valves, and when the valve(s) are open, the air/fuel mix (cool) helps cool the piston crowns and combustion chambers. On a diesel, you have air. You compress air and then really close to TDC fuel is injected into a prechamber (on most kubota's) where it almost instantly ignites, then the expanding gases are forced out of the prechamber and into the cylinder where the work is done. There is no fuel to cool the valves and piston crowns. Thus, drive pressure (hot) further heats the valves and crowns and also becomes a parasitic loss on the exhaust stroke. This is where guys can get into trouble not knowing what's actually going on. Most Kubota engines were specifically designed for atmospheric aspiration, and then there were some that were specifically designed for turbocharging. What's the difference? Grab your illustrated parts books, and compare the the differences. I'm too lazy today.
Going through this with my 2.3T swapped Mustang. Makes roughly 295hp with the stock turbo at 22 psi (TB0341). BTW stock boost on the SVO is around 15 psi, with a very restrictive intake (I cured that, and upped the boost). Calculating the airflow via mass airflow sensor, the stock TB0341 is spinning roughly 137,000 RPM at full boost and max engine speed. It is out of it's efficiency island at that point. Discharge temp at the IC inlet is 210, out of the IC at 132 is as high as I saw. So I called around and found out that the Borg EFR series of turbochargers are a much newer design and much more effiicient. So I bought one. Bolt-on for the most part (drain line was a little different). At 22 psi, it's making 346hp at the same 6000 RPM, outlet temp 172 and the air going into the engine is around 85. If I am lugging the motor, the TB0341 sends the boost gauge needle above zero at 1800 RPM, but its' about 2600 rpm before it's at full boost. The EFR? It is past zero at any hit of the throttle down to 1200 RPM (too scared to go any lower), fully lights quicker (2300 which honestly is too soon), is smoother (doesn't "hit" like an old 2 stroke dirt bike), quieter, and drive pressure is down from 51 psi to 27 which is significant and IMO impressive. Keeping all that in mind, a diesel doesn't respond quite that well as a gas burner does. Not without being able to get the right amount of fuel into the engine at the right time and actually be able to burn it. This is where the shape of the head, the type of "chamber", the shape of the piston crown, the intake and exhaust port designs, manifold, exhaust pipe/muffler, intake designs all come into play. Indirect injected diesels won't make the power that a direct injected engine will, but indirect's are quieter where direct's are louder but also extremely sensitive to piston/chamber shape/design. Mind you this is all in reference to single shot injectors, not common rail electronic. Electronic stuff opens up a whole other world in diesels. Now you can easily change timing, quantity, and a bunch of other things a LOT easier, too easy in fact, you can burn them down in no time flat. I've done it (at a training facility nonetheless....and if any of you reading this remember it from Fort Worth in about 2006, you just know what happened...LOL).
Indirect injected diesels in larger engines went away years ago in favor of more modern direct injected engines, that also use electronic control to run it. Indirects with mechanical injection are super limited in what can be done with them. And most Kubota's are just that, indirect, mechanical, and designed originally for atmospheric aspiration. The turbo engines, as said above, were designed that way from the get-go. So you take a D722 that had 16.5 hp using a manifold pressure of about 15 psi (atmospheric) and then you compare it to a D722-T which had a manifold pressure of roughly 21 psi, and made 21hp....but what we don't know is what the internal differences were? Even I don't know, because the D722-T was not a commonly available engine, and I ain't got no parts diagrams to compare parts numbers. Then we compare big trucks to Kubota's. Trucks have been electronic/direct for decades and "turning them up" was a simple matter of throwing more fuel to it (to an extent) at the right time. And they respond pretty well because the tuners can easily change it, and inexpensively. On mechanicals you change nozzles, pumps, pump timing, and a bunch of other stuff...not nearly as inexpensive or easy. Thus, again, IMO it's not cost feasible to take a N/A kubota engine and turbocharge it UNLESS you're in the hills and you just MUST have the power. Then you have to weigh the cost vs reward. Can that money be put toward paying the house or car off? Or do you have disposable income to play with?
