I was asked this question awhile ago in a PM and didn't really have time to answer it as completely as I would have liked to. The question was referring to the PDI twin turbo system for a CAT engine that was installed on the winning truck at PDI's dyno contest this year. The second part of the question had to deal with the fact that PDI also installed a water to air intercooler between the two turbos......why?


Twin turbos will give you a larger working RPM range of the engine. This truck produced 1216 Hp to the tires, to achieve this power at the tires, the engine produced roughly 1425 Hp at the flywheel at max power.

A single turbo on this same engine to produce this power would be as large as the big turbo in the twin set up and have a limited working RPM range of the engine. What I mean by this is the turbo would be lazy to respond in the lower working RPM range of the engine. A single turbo of this size at 1375 or lower engine RPM while at cruising speeds with little or no engine load wouldn't respond once the driver started up a hill and throttled in on the engine. Also at these cruising RPM the EGT's would have a tendency to run high at low boost conditions under a light engine load.

To give the engine a wider RPM range to work in, the twin turbo system has a small turbo to respond at low RPM and light work load conditions. The large turbo supplies the air to feed the larger air requirements of the high Hp engine in the mid to higher RPM range of the engine while under a full load.

PDI twin turbo system that is more of a Staged compound system. The small turbo is bolted directly to the exhaust manifold. The larger turbo is remote mounted to a stand with the exhaust gases to turn it coming through the small turbo first. In mounting the small turbo this way you have faster spool up of the small turbo at low engine RPM and low boost conditions to turn it quickly for a faster throttle response. Once the RPM of the engine increase and the large turbo starts spooling up, the small turbo is really along for the ride.........its not doing much in adding air to the engine. It does help the larger turbo move air with the fact it is turning. This effect helps slow both turbos down in RPM for the volume of air being used by the engine versus turbo size. So you would be able to size the big turbo slightly on the small size(very slight) for the Hp being produced when you would compare it to a single turbo engines turbo requirements for the same Hp results.

Now the intake side of this system is a little different.

The small turbo that is bolted to the exhaust manifold again is going directly to the air to air aftercooler mounted in front of the engine and from here to the engine. Again the reasoning behind this is to have quicker throttle response at no to low boost conditions.........the shorter the piping is the faster the engine gets the air.......the faster the response is. The large turbo again doesn't come to life until the mid RPM range of the engine. It draws the air in from the air cleaners and compresses it and feeds it through the small turbo. This turbo also produces the high boost to support the higher Hp levels. As boost increases the air coming from this turbo is hot and needs to be cooled off for maximum power gains and lower EGTs'. This is the reason to mount a water to air intercooler between these two turbos. At 60 psi of boost the air temperature leaving the turbos is 600*F.......with temperatures this high it becomes very hard to cool them off with just an air to air aftercooler mounted in front of the radiator before the air enters the engine. The larger turbo is feeding air through the small turbo, yes it helps compound the air to raise the boost pressure. But the amount of heat generated during this event isn't that high. Most of the heat introduced into the air is coming from the large turbo at max boost. So installing the water to air intercooler between the two turbos doesn't cause a problem and it does drop the air temperature before the air enter the air to air aftercooler.


By mounting the large turbo away from the engine you don't hurt Hp or throttle response. Once the RPM and work load increases high enough for the large turbo to be producing big boost the small turbo is dropping off in it power capabilities to feed the engine air. This is why I called it a Staged system as the stage 1(small) turbo is out of breath, the stage 2(large) turbo is running strong supplying air to the engine. The net effect of having two turbo is you do slow both of them down at the peak Hp for a long turbo life for the air volume being produced.

Do I personally like a twin set up? No, but I'm ole fashioned. Do I see the benefits of doing it this way? YES! Would I change something in this system? Yes, I'd use a wastegated turbine housing on the small turbo to reduce Exhaust Gas Back Pressure in the exhaust manifold and lower boost in the intake manifold for the same volume of air flow into the engine. This would decrease throttle response slightly in the lower RPM range of the engine, but it would increase Hp at the flywheel without adding additional fuel. I think the reason PDI does it this way is because they have dealt with so many truckers crossing the high mountain passes of the Rockies that they like to try to compensate for the high elevation with a smaller turbine housing to drive the turbo harder. you have to remember that as elevation increase the air is thinner or less dense and the turbo has to work very hard to draw air into it. A smaller turbine housing spools the turbo quicker to compensate

Originally Posted by Les2

I got a dumb question or maybe I misunderstood it, but what keeps the smaller turbo from over spooling and over heating? Now I assume the big turbo keeps it from getting to hot by pulling the exhaust thru it faster?

Do you have a pic of how this is set up? I'm pretty sure I understand it, but curious.

Ok on reading it again I got it.

After thinking some more.... It would suck if you blew a turbo cause no matter what one you blow its gonna take out the other.

if i understand kurt, it is just free wheeling after a certain point so its not working to produce boost. kind of like coasting down the road your not straining the motor like on a pull and your motor isnt making hp so it will cool down some.