not to throw another wrench or stir the pot. but what of your thoughts of true twins? one on each head like that jaguar sled pulling car. i think they are running k27 or k17 turbos on there with a p pump. obviously this for sled pulling only, but what the applications for this for everyday use?
An issue that many don’t consider when looking at true twins is that you are taking the exhaust energy and dividing it in half between the chargers. Not only that, you are dealing with less exhaust pulses or events per charger.
Both of these play a major role in driving your turbocharger(s).
Also, when a second charger is added you now have more rotating mass that has to be brought up to a very significant RPM in order to produce pressure.
In compounds the smaller charger is receiving all of the available exhaust energy. Then the larger(atmospheric) charger is receiving all of the exhaust flow still, but a little less energy do to thermal loss(time/distance of piping/driving of the first charger all impact this). However there is still plenty of energy left to drive the larger charger and you also have all of the airflow going through it.
In “twins” each charger sees have of the drive energy. Neither sees all of the energy. In order for the chargers to spool well, they have to be sized quite small, due to the amount of exhaust energy that our engines can produce(as well as our limitations on airflow due to head design). Unfortunately you cannot run tiny shafts in chargers on a diesel and make them live.
The mass of the shafts as well as the compressor wheels, and exhaust turbine wheels, all adds up.
In order to have a turbocharger that is built robust enough to live in a diesel application, a single charger is a better fit for most(if not all) diesel applications than “Twins”.
As Wes was discussing earlier any charger has certain airflow characteristics. Smaller equals faster spool up, but a limited top end number. Larger takes more energy to drive and spools slower, but has a higher overall capacity.
If you go too large on any turbo charger (single,twins, compounds, triples, etc...) then you may not have enough exhaust energy to ever get on top of the charger. Meaning you may never get the setup fully spooled.
Compounds gained a ton of popularity and for good reason. You can put a small enough charger on to get good bottom end power while being able to run a larger atomospheric charger to provide enough airflow for higher rpm power.
Keep in mind that on a set of compounds it is still a compromise. I will use The k series turbos in this example for since I quoted a gentleman asking about them.
The following example is working under the assumption that when running the compounds or the k31 as a single, that the overall fueling is the same.
The compound setup will spool slower than a single k27 due to needing to draw air through the k31 and due to not having a free exhaust path behind the k27. However it will spool dramatically faster than a k31 as a single.
Also the compounds will have a slightly lower overall power than a single k31. This is due to pumping losses, exhaust energy losses and the restriction that the k27 now places between the k31 compressor and the engine.
Compounds must be matched in a way. Meaning you can’t run a tiny small charger and a huge large charger. They must work together. We see this in Cummins/Duramax/ModernPowerstroke world quite often. I will have to let Wes or one of the more knowledgeable IDI guys here tell you what they have seen work on these engines. But using the number he provided concerning fueling and airflow requirements we could math it out.
The following is from working on Cummins compound setups. So take it with a grain of salt.
For a towing setup on a mechanically injected Cummins we often use a k27 as the small charger and a BW 475 as the larger charger.
As we go up in power/fueling (competition trucks not towing)we start looking towards other combos 363/475, 464/480, 472/491 and so on. If we mismatched the combo then the overall performance would suffer.
Keep in mind I am talking in generalities here. There are individual cases that can be looked at.