Adding a turbo to the Perkins 4108 engine

[Jomac]

I know this question has been asked before all over the place, I like to think I have refined the question a little. I have access to very good workshop facilities and have played with turbo's and fitting them to smaller petrol engines with reasonably good results. I have a 37ft Motor Cruiser which is fitted with a low hour and well maintained Perkins 4108 non turbo diesel. Its max output is 51 BHP+ at 4000 RPM and 46 BHP at 3600 RPM 1753 cc (107.4 cubic inches) I have read of some success where people have added a modest turbo and limited the max boost on a non turbo engine such as the older BMC engines. I have a small brand new quality turbo designed for fitting onto smaller high revving engines ranging from 200cc to 1200cc. It has a large diameter input and output bore. I would limit the boost pressure to something around 2 PSI. I have also considered adding a custom water cooled intercooler to help the whole set up. I'm trying to get the base BHP up a bit to help with coastal cruising, not so much the 51 BHP at 4000 RPM but the lower engine speeds. The stock 4108 delivers 23 BHP @ 2000 RPM. A 10% increase I would be happy with, any more would be a bonus. Can anyone think of anything that would scupper this idea? TIA John

 

[makomark]

I don't think it will hurt anything but you likely won't see any significant improvement cruising...

I would suggest that you would see a similar improvement getting the prop "tuned" .

 

[Jomac]

I don't think it will hurt anything but you likely won't see any significant improvement cruising...

I would suggest that you would see a similar improvement getting the prop "tuned" .

This is an area I know does need addressing as the boat is severely underpropped which is hardly surprising knowing its leisure history. I am just going by a couple of engine size calculators that I ran, and both suggested around 60 BHP. The Perkins is 51 @4000 and 32 @3000. I want to do coastal work in calm seas and it's the tides I was concerned about.

 

[makomark]

60 BHP for what - an optimal cruise speed with you hull? FWIW, it is the torque produced that spins the propeller...

I'd say you would be better off getting the prop sized for the engine/gear combination as the first step...guess it all depends on the goal(s) for the project that you are trying to achieve...

for prop tuning, I am familiar with Prop Scan but understand there are a couple of competing products that can achieve similar results...the idea is to get the prop 'close to" its designed shape and to ensure even loading on all blades...you would be surprised how many 'new' props are off.

If you add the turbo, then you will need to get the injector system adjusted to ensure proper a/f ratio...and my gut says it will be much faster and more economical to start with the prop instead of the turbo.

 

[Lepke]

Turbo engines almost always have a lower compression ratio. Increasing the pressure will shorten the life of the rings, sleeves, and valves, probably fast. The 4108 is a nice older engine that is reliable when used normally. They do not hold up well to abuse. Many people have had trouble in warmer waters keeping the engine cool. Most buy a bigger heat exchanger. The continuous 4108 rating I have is 36 hp @ 3000 rpm.
The commercial rating is for hard use and frequent overhauls.
If 37' is the water line length your hull speed is 8 knots. Above hull speed for a displacement vessel means double or more hp. If you want to go faster, buy a longer boat and save your engine.
Vessels I have run, where I knew the hp requirements at different speeds got 25% more speed for double the hp.