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Old 03-09-2013, 12:05   #1
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Question Perkins 4/108 high speed/low speed

I was told recently that the Perkins 4/108 had two versions, one being standard the other being a "high speed" version. I never heard of this before and am wondering if anyone here knows if this is true or if maybe the owner was referring to the gearing of the transmission and was confused.
It all started in a conversation concerning props, his boat and mine have 4/108's and his prop was quite different in pitch and size than mine despite the fact the the two boats are similar in size, weight, hull design. He stated that the prop guy who properly sized his prop (when the original failed to perform well) told him he had a "high speed" version of the 4/108 so it required a different pitch and size prop. No problems with the one he has on it now since it performs well but the explanation didn't make sense. Was he just a little confused or were there really two different versions? All the 4/108's I've been familiar with were known as low speed torque monsters with long lives, it's a new one on me.
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Old 03-09-2013, 12:21   #2
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Re: Perkins 4/108 high speed/low speed

I have a Perkins 4.108 I'm pretty sure they are all the same bore and stroke. Any difference in prop RPM would be due to the transmission reduction ratio.
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Old 03-09-2013, 12:23   #3
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Re: Perkins 4/108 high speed/low speed

There is some confusion and i dont know, but have seen them rated from ~40-51 hp... which may be basically an rpm "derating" thing....?
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Old 03-09-2013, 12:36   #4
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Re: Perkins 4/108 high speed/low speed

Torque is an actual number, horsepower is a calculated number based on torque and RPM so increased engine speed would technically get you more HP, depending on what RPM the torque peak was achieved at.
The different HP numbers for the same model engine might point toward that.
Anyone out there with the definitive answer?
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Old 03-09-2013, 13:03   #5
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Re: Perkins 4/108 high speed/low speed

What you are looking for are the differences between the Highline the Lowline and the new Perkins 4-108m. All 4-108s but different configurations. If you have a manual they will explain the different HP that can be achieved. Also over propping a motor will govern HP. A prop calculator will solve that. For example my Perkins has a Borg Warner C71. That is turning a 15" rh 12 which is a certain pitch that allows me 3500 rpms. It would turn more but because of clearance that's how mine works best. My motor never runs @ 3500rpms it likes 2000rpms to cruise.
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Old 03-09-2013, 14:30   #6
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Re: Perkins 4/108 high speed/low speed

after looking through my perkins 4-108 shop manual i've found absolutely no reference to a high speed or low speed version. the shop manual shows various hp/torque ratings at different rpm's. as explained above, the higher the rpm's, the higher the hp/torque. the 50hp rating that is so often quoted is only achieved at fairly high rpm's, around 3400 i think, much higher than i would ever want to run it.

i normally cruise at 1700 rpm, which moves my 37' 20000lb cutter at about 5.5 knots in calm water. according to the shop manual i'm getting about 35 hp. in some situations i sometimes push the throttle up to 2000 - 2200 rpm, but only very briefly. i can't imagine running the engine continuosly above 1700 rpm or so.

that leaves the prop. i'm not sure why his prop guy told him to run his engine at such a high rpm, but i think i'd seek out another prop guy. would you normally drive your car flat out?
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Old 03-09-2013, 15:10   #7
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Re: Perkins 4/108 high speed/low speed

Quote:
Originally Posted by onestepcsy37 View Post
i normally cruise at 1700 rpm, which moves my 37' 20000lb cutter at about 5.5 knots in calm water. according to the shop manual i'm getting about 35 hp. in some situations i sometimes push the throttle up to 2000 - 2200 rpm, but only very briefly. i can't imagine running the engine continuosly above 1700 rpm or so.

that leaves the prop. i'm not sure why his prop guy told him to run his engine at such a high rpm, but i think i'd seek out another prop guy. would you normally drive your car flat out?
No answers here, but I do have a 4.108 which is apparently a 40 hp engine. I used to run it at around 1700 rpms, but all the diesel guys keep telling me I should be up around 2200. I now try and get up to at least 2000, but I really don't like the way it sounds -- not really bad, but a lot louder. And my fuel economy starts to fall rather dramatically with these higher RPMs.

BTW, at 1700 it pushes our 37' full-keel, 28,000# boat at around 5.5 knots as well. 2000 gets me to around 6.2-6.5. My hull speed is around 7.2.
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Old 03-09-2013, 15:36   #8
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Re: Perkins 4/108 high speed/low speed

The 4-108 was rated from the factory at 51 HP.
Max RPM is 4000 and max continuous 3000.
Some experts recommend running at 70% of Max Continues and that translates to 2100, which Is what I have been running for years with no problems at all and an average fuel consumption of 0.6 gallons per hour. (20,000 cutter)
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Old 03-09-2013, 15:37   #9
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Re: Perkins 4/108 high speed/low speed

All the noise you hear is coming out of the intake. Try an air filter it will quiet it down a lot.
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Old 04-09-2013, 06:34   #10
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Re: Perkins 4/108 high speed/low speed

Thanks for the feedback. From what I've gathered here and on a couple other diesel related sites seems to say that the HP was directly related to the operating RPM, which was a bit optimistic from the manufacturer. To achieve the claimed 51 HP you would have to run it at 4000 RPM, which wouldn't make for a very long life. Most people tend to run between 1800 - 2200 RPM with some bursts up to 3200 RPM, mostly limited by cooling issues. It seems the 4000 RPM is not possible with the exhaust flow available from the marine exhaust manifold restrictions, so, long story short, it appears the motor is really closer to 35-40 HP in most applications at the speeds it's being run at.
I have experience with other manufacturers diesels and have rebuilt a few over the years but have not owned a boat with a Perkins previously, the information I've received here and on a couple diesel oriented sites was extremely helpful. On the other hand the parts availability is great and the rebuild kits are extremely affordable compared to some other marine diesel manufacturers, if you can do it yourself it's not too costly.
For the most part the prop sizing issue seems to be directly related to the transmission ratio and boat size/displacement of the vessel the motor is installed in. The information seems to make sense once you absorb it all. It looks like my new ride is a little under powered at 34,000 lbs, but hey, it's a sailboat. I think there's a cruising chute in my future......
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Old 04-09-2013, 09:26   #11
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Re: Perkins 4/108 high speed/low speed

lifeoreilly57 -

there are some larger but closely related perkins diesels from that era. i know that the 4-154 is basically the 4-108 with more 'real' horsepower. a friend of mine has the six cylinder version - i think it's the 6-236 - in his 34 foot trawler, and i think that engine was used in the csy 44.

my 4-108 is now 34 years old, never been rebuilt, and still purrs along. don't know what i'd replace it with if i ever had to.
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Old 04-09-2013, 09:56   #12
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Re: Perkins 4/108 high speed/low speed

I'm not on boat right now so going from memory. My manual had three different versions of the 4-108 much like Redcoat discribes. Mine is a "new" 4-108 and is rated at 51hp at around 3600 rpm continuous rating. The intermitent rating is 4000rpm and maybe that is where the 51 hp comes from? There is a hp graph vs rpm and also a torque graph. In gear, I can not achieve the 3600 rpm so I'm over proped. The boat is a Tayana 37 and displaces over 25000 lbs. I cruise at around 6 knots doing 1900 rpm and can achieve 7 knots in calm water at 2300 rpm which is WOT. My tach needed recalibration and the results given are based on using a photo tach to recalibrate. The boat came new in 1985 with a 17 by 11 fixed prop which I replaced prior to launch with a 17 inch 3 bladed max prop which is still doing fine. I've experimented with pitch adjustment throughout the years and over the past 10 or so years I'm using a 14 inch pitch. The tranny is a Borg Warner and is 2.1 to 1 ratio.
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Old 04-09-2013, 10:59   #13
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Re: Perkins 4/108 high speed/low speed

Quote:
Originally Posted by CSY Man View Post
The 4-108 was rated from the factory at 51 HP.
Max RPM is 4000 and max continuous 3000.
Some experts recommend running at 70% of Max Continues and that translates to 2100, which Is what I have been running for years with no problems at all and an average fuel consumption of 0.6 gallons per hour. (20,000 cutter)
My CT 44 (roughly 30,000 lbs) had the Perkins 4-108 in it with a max prop and small hurth transmission. I assume it was the "high rpm" engine as was advertised at 51 HP. (Hp/rpm is a rating game.... for commercial use it's rated lower and lasts longer) It was pitched to max out at about 2800 rpm and my favorite cruise speed was 2300-2400. The engine pushed the boat at 8.25 knots hull speed reliably and in most all conditions, currents, and chop in the PNW without a hitch. The overall fuel consumption for the time I owned the boat was .65 gallons per hour.
That engine should be pushing the 37 foot Irwin reliably and at hull speed.
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Old 04-09-2013, 13:54   #14
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Re: Perkins 4/108 high speed/low speed

Mine is in a Cheoy Lee 47 CC which has an advertised weight of 34,000lbs but is probably closer to 36,000 with a modified full keel and skeg hung rudder. If I get 6.5 knots I'll be happy.
It has a bout 3400 hours on it currently and seems to run pretty smoothly, I will be rebuilding it in about 3 years though since we plan to shove off in 4 years and spend an extended time cruising. I want to run it a year after the rebuild just to make sure.
Any idea on how many hours you can get out of a properly maintained 4-108?
Anyone make a silencer intake filter for them? The motor is not to loud but the intake definitely has a fair amount of roar, it currently has a pretty standard screen filter intake that does nothing to reduce the intake noise.
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Old 04-09-2013, 16:24   #15
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Re: Perkins 4/108 high speed/low speed

a well maintained one should go 10k hours if run at reasonable rpms in it's life. Of course... you just never know with things mechanical. My 44 ft boat was probably realistically about the same weight as your Cheoy Lee.... but had a 39 ft waterline.
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