Fitting Twin Turbos

Written by Ivan on 24/07/2013. Posted in Vehicle modifications

To make life a little easier towing my 5th wheel on mountain roads I decided I neede a little more power so, a couple of months ago I fiited bigger injectors to my Cummins which are rated to give me an extra 110 horsepower.  To take advantage of this extra fuelling I knew I would need more air if I am going to keep the EGT’s in check so, I planned to fit a second turbo.  To facilitate this, the exhaust manifold has to be turned upside down and the oil filter needs to be moved from the stock location otherwise it will be extremely difficult to change once the second turbo is fitted below the first.  The top turbo is the modified factory HX35 while the lower on is a new, ported and tuned K31.

This first picture shows the manifold replaced inverted and the oil filter in the stock location.  It also shows the new, larger heat exchanger for the Veg. Oil system.  More power means more fuel flow so I figured I would need more heat transfer to the Veg. Oil.  Aside from this item the origional system remains unchanged and has served without problems for 217,000 kilometers.

The next step is to move the oil filter from it’s factory position which will be virtually inaccessable when the twins are mounted.  A very nice kit from Pac Brake makes this a simple task.

Here are the two turbo’s side by side on the bench.  The K31 is the larger of the two on the left.

The next step is to lay the lower turbo (K31) in the engine bay, fit the hot pipe that connects it to the upper turbo (HX35) and then fit the upper turbo.

At the same time, the EGT gauge sensor is relocated from the lower to the upper side of the manifold.  This is not necessary for operating reasons but it provides more clearance room to work.

This picture shows the “cold pipe” connection between the two turbos and gives some idea of the final relative positions.

The black flanged “hot” pipe at the rear left of the K31 fits to the rear left of the HX35 which will raise and rotate the lower turbo to line up with the “cold pipe”.

After re-routing the cab heater return hose (the black pipe running parallel to the manifold) and re-positioning the automatic transmission fluid dipstick (the yellow handle), the K31 was attached to the HX35 and bolted home.

Oil feeds and drains connected and the compressor housings “clocked” to align outlets for the cold pipe.  Coolant lines to cab heater and heat exchanger re-routed and reconnected.

Hot pipe lagged, silicon boots clamped, HX35 inter-cooler outlet positioned relative to inter-cooler inlet pipe which I will now start manufacturing.  Then comes the air inlet pipe to the K31 and finally, the

exhaust system.

Inter-cooler pipe connected, Air Intake elbow attached to the K31





Twins 10

Air Filter positioned to determine length of intake pipe.

Twins 11

The job complete!!

Twins 12

I drove the truck and 5th wheel to Washington State in the USA for an annual diesel truck meet where I was able to run it on a dynomometer to measure the rear wheel horsepower.


Despite a leaking boot on the inter-cooler pipe, which I later fixed, the truck produced 48 pounds of boost and 418 horsepower.  With the leak fixed the boost gauge now reads 60 pounds which should put the final horsepower figure close to 500.  I could certainly feel a significant improvement in towing capability through the mountains with lots more “pull” and low EGT’s. and no increase in fuel consumption

It was a lot of work but very well worth the effort.



Leave a Reply



Trackback from your site.