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Old 12-11-2019, 00:39   #1
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Perkins 4.108 - which variant?

Hello,

I read online somewhere that the Perkins 4.108 had varying maximum RPM depending on the variant.

Could I determine the variant (and hence maximum RPM) from the serial number. This is what is stamped on the block:

Engine Serial: ED33211U616924M
65081F

It appears to be Engine Model: 1986 Perkins 4.108M - but I can't work out anything more than that. Other threads suggest max RPM might be 3800 - but again I have read it differs depending on exact variant.

Cheers

Al
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Old 12-11-2019, 00:44   #2
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Re: Perkins 4.108 - which variant?

I believe the Perkins 4108 was originally made with a cooling system that is considered undersized and it was updated at some point down the line to a larger exchanger or something like that.

This may be what you are referring to.

And this is also the extent of my knowledge on the subject, unfortunately.
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Old 12-11-2019, 03:02   #3
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Re: Perkins 4.108 - which variant?

To Identify your engine:
Copy the full 15 or more digits as printed on the engine, excluding any spaces.
https://shop.perkins.com/support/identify-your-engine
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Old 12-11-2019, 05:11   #4
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Re: Perkins 4.108 - which variant?

Quote:
. read online somewhere that the Perkins 4.108 had varying maximum RPM depending on the variant.
My 4-108 had a max RPM of 4000, max continous 3000. I usually cruised at 2100.
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Old 12-11-2019, 05:31   #5
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Re: Perkins 4.108 - which variant?

The max rating is 4000 rpm and the max continuous is 3600. I cruise at 2500 rpm doing 6 plus knots.
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Old 12-11-2019, 07:22   #6
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Re: Perkins 4.108 - which variant?

Quote:
Originally Posted by lancelot9898 View Post
The max rating is 4000 rpm and the max continuous is 3600. I cruise at 2500 rpm doing 6 plus knots.
You must have a different model, my 1979 4-108ís manual said 3000 for max continous.
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Old 12-11-2019, 07:30   #7
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Re: Perkins 4.108 - which variant?

Mine is a 1985 model and the manual refers to it as "new 4108. I'm surprised that the max ratings are the same but yet the continuous ratings are different. I've set the pitch on my max prop so that my WOT rpm is at 3000 so I'm over propped. I was not happy with the pitch setting which allowed 3600 WOT.
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Old 12-11-2019, 07:32   #8
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Re: Perkins 4.108 - which variant?

Always interesting to see the range of RPMs people use. Iíve got a 4108. We cruise at around 1800-2000 RPM, which gets us up to around 6 knots. Boat max. is 7.3ish.

I can achieve full hull speed by going about 2300-2500 RPM, but at this speed our fuel efficiency is cut in 1/2 (or even worse), and the engine feels and sounds like it is working way too hard. If I keep it around 1800 she just seems to purrr (or perhaps growl, like a big old grumpy cat ). Beyond that it starts to feel pained and strained.
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Old 12-11-2019, 07:53   #9
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Re: Perkins 4.108 - which variant?

Hi Mike

I'm using about .75 gal/hr at cruising speed. My hull speed is about the same as yours. (It's a Tayana 37 with almost a 32 ft waterline) And I can achieve hull speed at 3000 on my tach. I'm use to just reading my tach, but it's not accurate so I put a photo gun on it and here are my results so I'm really at a much less rpm that my tack indicates. I'm really over propped.

Tach Reading Actual rpm
1000 830
1500 1120
2000 1500
2500 1890
3000 1954
4000 3000 est
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Old 12-11-2019, 08:02   #10
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Re: Perkins 4.108 - which variant?

Quote:
Originally Posted by lancelot9898 View Post
Hi Mike

I'm using about .75 gal/hr at cruising speed. My hull speed is about the same as yours. (It's a Tayana 37 with almost a 32 ft waterline) And I can achieve hull speed at 3000 on my tach. I'm use to just reading my tach, but it's not accurate so I put a photo gun on it and here are my results so I'm really at a much less rpm that my tack indicates. I'm really over propped.

Tach Reading Actual rpm
1000 830
1500 1120
2000 1500
2500 1890
3000 1954
4000 3000 est
InterestingÖ so you think I might be over-propped? We have a largish 3-blade prop. To be honest, I donít even know how big it is, let alone what the pitch might be.

Our fuel efficiency is similar to yours as long as I keep boat speed to around 6 knots. Beyond this it really drops precipitously. And yes, very similar boats, yours and mine. We might be a bit heavier (~15 tons).
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Old 12-11-2019, 08:24   #11
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Re: Perkins 4.108 - which variant?

Well the conventional wisdom is that you're over propped if you can't reach your continuous design rating on your engine at WOT. That's the definition at least. So I've been over propped for 35 years. My design weight is 23000 pounds but on the travel lift it is very similar to yours at 30000, but then again how well are those lifts calibrated. Plus the max prop with it's flat blades is less efficient in forward vs. a fix prop. I think I looked at the design specs to determine gal/hr at various rpm and it seemed to mess with actual fuel usage. It's been a good engine over the years...knock on wood.
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Old 12-11-2019, 08:31   #12
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Re: Perkins 4.108 - which variant?

Quote:
Originally Posted by lancelot9898 View Post
Mine is a 1985 model and the manual refers to it as "new 4108. I'm surprised that the max ratings are the same but yet the continuous ratings are different. I've set the pitch on my max prop so that my WOT rpm is at 3000 so I'm over propped. I was not happy with the pitch setting which allowed 3600 WOT.

I thought that this was discouraged? Something about unburned fuel and carbon build up, overheating possibilities, etc.
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Old 12-11-2019, 08:56   #13
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Re: Perkins 4.108 - which variant?

If you see black smoke at higher throttle settings, you are over propped, that is the unburned fuel, AFAIK.
Cheers all
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Old 12-11-2019, 09:20   #14
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Re: Perkins 4.108 - which variant?

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Originally Posted by Midnight Son View Post
If you see black smoke at higher throttle settings, you are over propped, that is the unburned fuel, AFAIK.
Cheers all
That is burnt fuel, white is unburnt fuel.
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Old 12-11-2019, 09:25   #15
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Re: Perkins 4.108 - which variant?

Quote:
Originally Posted by lancelot9898 View Post
Well the conventional wisdom is that you're over propped if you can't reach your continuous design rating on your engine at WOT. That's the definition at least. So I've been over propped for 35 years. My design weight is 23000 pounds but on the travel lift it is very similar to yours at 30000, but then again how well are those lifts calibrated. Plus the max prop with it's flat blades is less efficient in forward vs. a fix prop. I think I looked at the design specs to determine gal/hr at various rpm and it seemed to mess with actual fuel usage. It's been a good engine over the years...knock on wood.
Same boat, same engine. Few things before assuming your over propped. What in line filters are you running? This can restrict fuel flow if there are too many or too fine. Have you checked your lift pump, does it work? When and who last did work on your injection pump. The max rpm is set on the pump and depending on the serial # you can have varying max rpm. I think this is where a lot of the confusion comes from. Other side of that is if it was specifically requested to lower the max rpm by an owner at some point regardless of previous settings.
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