Turbo slection

Dark winter evenings in Norway often results in ideas for things to do in the workshop. This year the idea of fitting a turbo to a Shovelhead from 1983 started to materialize. I searched for turbo calculator on the Internet and found a couple good and hopefully reliable ones. Next was to understand the turbo maps in order to select the right turbo for my application.

The calculators and other advices resulted in the following assumptions:


Estimated boost pressure:     0,8 barg
Estimated air flow:                 11-13 lbs/minute
Estimated effect with turbo:   90-100 Hp

Turbo Map GARRETT GT1548

Based on the above turbo map a GARRETT 1548 was chosen and ordered... What a bummer..
1st learning never buy a turbo before you have decided whether to build a draw thru configuration or a blow thru. My initial plan was to build a draw thru configuration but this can not be done with any of the GT turbos from GARRETT. If you are planning for a build with draw thru configuration make sure that you buy a turbo with carbon seals. Newer turbos are made with piston seals which are not suitable for draw thru configuration.

Draw thru configuration:
This is most likely the easiest way to put a turbo on a bike with a carburetor but not the most efficient. Nearly all new turbos are made with piston seals as a default – some older models can be re-built to carbon seal. As I see it piston seals are regarded as an improvement towards the carbons seals as long as not used in a draw thru configuration. My conclusion is that a turbo with piston seals can NOT be used in a draw thru configuration. And Why? It is quite logic if I had thought about it before. When the throttle is open the condition for the turbo is the same for draw thru as blow thru. When you close the throttle the turbo continue to rotate in order to maintain the turbo pressure but the closed throttle does not let air in. The turbo then creates a vacuum on the compressor side which subsequently suck the oil out from the bearings ending in blown seals, bearings and turbo…
Blow thru configuration:
This appears to be more efficient due to 2 reasons. A mixture of petrol and air has a higher density than only air and the turbo have to work “harder” to maintain boost pressure. When the petrol air mixture goes through the turbo the air fuel gets “separated” which is not ideal for the efficiency. Still there is not a common stand to this by the turbo freaks on the web. The biggest challenge for this configuration is to get a carburetor to withstand the pressure and the jets to be fit for purpose. I have found few examples that standard carburetors can be rebuilt for a turbo engine.
I now have identified a Dellorto 40mm carburetor that is turbo sealed an suitable for boost up to 30 psi. If the supplier can convince me that they can deliver with the correct jets I will change to blow thru configuration.

Another item under lessons learned is that newer turbos is prepared for  water-cooling around the bearings in order to increase the lifetime. I have started to search for a small radiator and an electrical centrifugal pump in order to make a cooling system for the turbo bearings.

The following parts are now chosen:

GARRETT GT1548 turbo
S&S heads prepared for more valve lift
New cylinders
Wiseco forged pistons
Crane 1-0108 camshaft
Crane racing ignition module
Dellorto DHLA40 turbo sealed up to 30 PSIIn addition a small radiator and an electrical pump is bought in order to build a cooling system for the turbo.