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Old 04-09-2013, 17:04   #16
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Re: Perkins 4/108 high speed/low speed

If you want to know how much horsepower your 4108 is putting out, it uses about one gallon for every 20 horsepower each hour (as do all diesels). Thus, if you are averaging 0.6 gallons per hour, you have an average output of 12 horsepower.

The 50 hp rating for the engine was very optimistic, as no one in their right mind would run it at that high an rpm. It is realistically a 40 hp engine, and should be propped for no more than 3000 at WOT, and run at about 2200 (max torque). The slower you run it the quieter it will be and the less oil it will leak. The Walker Airsep can quiet it down quite a bit and cut down the oil leakage from the seals.

The heat exchangers are on the small side, and most of the 4108's tend to overheat if you run at WOT, regardless of rpm.

There is a lot of misunderstanding of engine performance curves in this thread. Read

Deciphering Your Engine's Performance Curve | Yachting Magazine

and see the 4108 curve at Perkins 4-108 Power Curve
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Old 04-09-2013, 17:15   #17
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Re: Perkins 4/108 high speed/low speed

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The 50 hp rating for the engine was very optimistic, as no one in their right mind would run it at that high a rpm
That is like a car rated at 200 HP, but only idiots would run at full throttle and harvest the 200 hp continuously.

I have used all 51 HP of the Perkins a few times for emergencies only and it sure works, plenty of power available if you have the right size prop.
Some guys over-prop their boats so bad the full HP can not be obtained, or the 4-108 is installed in boats too big for the engine.

I ran a Jouet 42 with a 4-108, seemed a good combo and so was my CSY 33.

(The CSY 44s came with the 4-154 rated at 62 HP, perhaps a bit on the light side)

The highest time 4-108 I heard about was 15,000 hrs on a CSY 37 in St. Croix, even at those hrs the engine was still running fine.
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Old 04-09-2013, 18:48   #18
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Re: Perkins 4/108 high speed/low speed

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Originally Posted by donradcliffe View Post
The Walker Airsep can quiet it down quite a bit and cut down the oil leakage from the seals.
Hope this isn't too much a hijack, but how does one install one of these devices? I've done some research, and found the source, but can't see how they're installed. How difficult is it, especially for a non-mechanic like me. And which one fits the 4.108? It's not clear to me from the Walker website.
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Old 09-09-2013, 18:22   #19
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Re: Perkins 4/108 high speed/low speed

I have a Perkins 4-107 ... almost the same as a 4-108. I'm by no means passing myself off as an expert. I've been involved with 4 different boats with this engine. All have Borg Warner Velvet drive C-71 transmission. Three of them are short 1:1 drive while mine is the longer 1.91:1 version. They can run frim 1:1 to 2.91:1. So naturally the engine speed would vary depending on which gear ratio you have. Also, the prop pitch would change depending not only on boat weight but the gear ratio turning it. Mine is 19500 lbs and I have a 2 blade, 16 inch prop. I don't know the pitch. That's original from Spencer Boat Works. It originally had a 4 cylinder Mitsubisi engine.
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