Random turbo musings...
The wastegate on my turbo appears stuck as I've mentioned before, and I'm going to do something about it. I think this weekend I may try to pull the turbo and start looking at things, figuring out what I want to do. It doesn't look like it's a particularly difficult removal all things considered. I've had both the exhaust and intake sides off of the turbo at various points and the shaft has seemed to spin freely without binding.
The stock turbo is sized the way stock turbos are/were normally sized in the 2000ish era on diesels. Something that tends to be biased towards quick spool and responsiveness, decent low end, and probably undersized on the top end. It looks like it's most efficient in the 1500 RPM range on the engine, and I tend to cruise in the 1800-2100 RPM range on the highway. This same turbo is used on 3126s from my rating (330 HP) all the way down to 250 HP. I haven't been able to find a compressor map for this exact turbo, but from what I can gather, I think that at 250 HP it would probably be fine, but at 330 HP it's being pushed a bit more than it should. I've tended to push it a bit on the harder side anyway.
With the turbo off comes the question of what to do with it. While I've played with turbos in the past, I've never done anything with the turbo on a diesel I've owned, and have been contemplating what I want to do with this one. My goal isn't to make more power, but I would like to see lower induction air temps and EGTs while this thing is working hard (which is most of the time).
Newer design turbos exist, but it seems that the same turbo design in a newer form tends to end up being slightly oversized. This is an S300G, and going to the newest variant (S300SX-E) and looking at the compressor maps, at high boost conditions the compressor map seems to be fairly close to optimal efficiency (which on that turbo ends up being in the 77-78% range). My concern is that at the low end, this may result in the turbo essentially being too large, unable to work well at those lower airflows. I don't want to put a turbo on that ends up killing responsiveness (especially at higher altitudes) and ultimately increases EGTs since boost lowers EGTs on a diesel.
I think I could have this turbo rebuilt perhaps with a newer style compressor wheel (maybe newer style turbine wheel too), which would probably improve efficiency and help lower induction air temps.
The 3126/C7 doesn't have much out there for aftermarket options. One company produces an upgraded exhaust manifold and turbo (PDI). It's pretty expensive at around $4500 for the turbo, manifold, and install kit. I also don't like the fact that they end up providing a non-wastegated turbo. I know this is a common practice on diesels, but I'm not a fan. The 3126 ECU does have an overboost fuel cut-off (something I'm running into now with the stuck wastegate) and for that alone, I think it would be an issue.
So then comes another more excessive option - which of course then has immediate appeal: compound turbos. This is something that you typically see on various race diesels where you need the combined pressure ratios to get boost levels that are desired. But, especially prior to variable geometry turbos, it's something that exists/existed to help optimize the turbo capabilities for low and high end ranges. On Cummins Rams, it's common to see compound turbo setups where the stock turbo is left in place, and a large turbo is added on the atmospheric side. With the two turbos working together, you end up with a setup that's able to produce higher boost levels at the same airflow/fueling, the end result being lower EGTs, at least so goes the theory. The more I look into it the more I see a lot of people who specifically like this setup for towing, although granted most of them are also doing significant horsepower upgrades (something I'm not doing).
There's plenty of room in the bus engine compartment with some level of fabrication required, of course. A Detroit Diesel Series 60 K31 turbo is readily available, not all that expensive, wastegated (something else I like) and often used on 5.9/6.7 Cummins as an atmospheric side turbo, and that seems like it could be a good option if I wanted to do the fab work to make that happen. Basically it would involve modifying some intake/exhaust plumbing (not horribly, though) and then making the adapter/mount to attach the K31 turbo to the S300G turbo on the exhaust side. That part would be the most significant work of it.
I figure once I get this turbo off and can inspect things I may get a better idea of what I want to do and can then decide on a path forward. But I'm thinking the path will probably end up being having this turbo rebuilt/gone through with what upgrades and improvements I can do, and then maybe do something further if I decide more would make sense to do.
