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Old 05-10-2012, 06:12 AM   #21
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"Why did the Cat 3208 become so instantly popular 30 years ago in over the road use, it replaced 6-71s."

My guess is they had no use for the power the DD was rated at , usually 230hp /2100rpm for the 6-71 so the wimpy (perhaps 160HP regardless of "factory fiction") was enough for their work, and the smaller , better matched engine did better.

The 4-71 might have been a better initial choice than the 6-71.

OTR use for Cat 3208 is Skoolies ,or garbage trucks , not long haul trucks .

Floored every 8 seconds , indeed a more powerful engine will cost more fuel than a throwaway wimp.

As Skoolies are replaced on some federal dime , by years of service , the 5x longer service life of a DD was of no useful value.

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Old 05-10-2012, 06:44 AM   #22
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Quote:
Originally Posted by FF View Post
"Why did the Cat 3208 become so instantly popular 30 years ago in over the road use, it replaced 6-71s."

My guess is they had no use for the power the DD was rated at , usually 230hp /2100rpm for the 6-71 so the wimpy (perhaps 160HP regardless of "factory fiction") was enough for their work, and the smaller , better matched engine did better.

The 4-71 might have been a better initial choice than the 6-71.

OTR use for Cat 3208 is Skoolies ,or garbage trucks , not long haul trucks .

Floored every 8 seconds , indeed a more powerful engine will cost more fuel than a throwaway wimp.

As Skoolies are replaced on some federal dime , by years of service , the 5x longer service life of a DD was of no useful value.

FF
Yeah...those POS 3208's were useless....after decades of work...rebuilt...another decade of work and another rebuild...they are probably gonna get tossed after the next vehicle gets junked that they gave their third decade of service in....
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Old 05-11-2012, 08:49 PM   #23
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I've got a Cat 3406b for the main and a 2-53 DD light plant. Both motors are relative dinosaurs compared with the fancy Electronic jobs of today.They are simple, stout, easily repaired,and parts are available world-wide. Removing and replacing them would cost way more than I would ever save in fuel with the new engines. Maybe when Diesel hits the ten dollar range I will reconsider.
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Old 05-12-2012, 12:38 AM   #24
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My first car was a cherry Model A. It was solid, stout and I could get parts for it anywhere I traveled. Then along came a great Volvo with dual SU downdraft carbs. I could get parts for it too anywhere I traveled. I loved my vehicles of the moment, as I have done with all my boats. But time marches on with each of my oldies replaced with something a little better and newer. It is called progress, fleeting at times, but none the less grinding ahead. Even old stodgy GM saw the handwriting on the walls and 30 years ago prepared for shutting down 2 stroke production. As with my beloved Model A and Volvo, DDs are well past their prime. Still love all of 'em though.

Diesel at $10 you say 2dsea, try Europe it is nearly there. If you were to listen to our Energy Czar, the US would be better off at $10 too.
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Old 05-12-2012, 06:12 AM   #25
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Even old stodgy GM saw the handwriting on the walls and 30 years ago prepared for shutting down 2 stroke production.

The GM decision was forced by the Air Police .

It is interesting to contemplate where the DD 2 stroke would be NOW.

AS some of the most efficient engines in the world are ship 2 strokes , weather ship tech would be in our boat engines is unknown.

A 60% efficient boat motor would be a huge improvement .

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Old 05-17-2012, 06:22 AM   #26
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Detroit's die slowly , what more can be useful on a cruiser ,than an engine that will bring you home with a couple of dead cylinders?

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Old 05-17-2012, 06:58 AM   #27
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Pulled off-topic posts and restored to public forums. Thanks for your patience.
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Old 05-17-2012, 10:28 AM   #28
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2 strokes over scavenge.
So do 4 strokes, particularly supercharged 4 strokes. Valve overlap allows complete scavenging and cools exhaust valves which improves engine life.


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That is they exhaust more than their exhaust and tend to leave less air in the chamber when the compression stroke begins. This leaves less air in chamber to compress so less pressure is present at TDC. Also this extra air cools the cylinder causing thermal loss and less efficiency.
No, the exhaust valves close first and that is where the “super” in supercharging comes from. The cylinder pressure goes above atmospheric.

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If the blower pumps more air into the chamber than it can use it's energy lost. And if the blower pumps less than is required or needed less pressure will result.
See the comment above. It is not ‘energy lost” it is energy expended to clear the cylinder of exhaust gases and thereby increase the power available and the overall efficiency of the cycle. That "extra" air permits more fuel to be burned than if the engine relied on any other type of scavenging, and more fuel burned equals more power.

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Everything is not always right in a 2 stroke. There is always some hot oxygen starved air remaining in the cylinder when the compression starts.
Virtually none, and far less than a normally aspirated 4 stroke.


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And the blower is required to pump a bit of excess air out into the exhaust ...... a waste of energy.
See my first comment. Extending engine life and increasing efficiency is not a waste of anything.

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Lats but by no means least is that the plower takes gobs of power to run.
That is the “cost of doing business.” A fuel injection pump and a coolant pump does too but the overall benefits provide very high returns. A mechanical surpercharger provides benefits not even mentioned in this thread. The performance and history of the DD 2-strokes is witness to the fact that the benefits far outweigh what some people mistakenly believe are detriments.

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The above applies only to DD type diesels w poppet exhaust valves.
Which perform cylinder scavenging exactly the same as large slow-speed 2 stroke diesels which achieve thermal efficiency in the low to mid 50 percent range at full power. The only difference is the large slow-speed engines use turbochargers and have a single very large exhaust valve.



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Old 05-17-2012, 11:53 AM   #29
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Rick,
Nice to see you back here on the forum. Few people take the time and go to the trouble to explain complicated mechanical things.

The DD is 15 to 25% less efficient than other engines and as far as I know that's a given.

And it would seem to me that loss is associated w the blower. The efficiency of the blower seems not to be in question since GM had many many years to change to another type but did'nt. So I see that it must be something that the blower does or does not do that causes the loss.

Now if there is no fault in the contents of post #28 will the real cause of the DDs lack of efficiency please step forward. Put another way if post 28 is true why is'nt the DD the most fuel efficient in the world.

I would like to see sunchaser in on this too.
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Old 05-17-2012, 05:51 PM   #30
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Actuall Efficiency Numbers from my boat

2400 rpm 124 hp 6.85 gallons perhour 18.1 hp per gallon of fuel
These are actual tested numbers. it doesn't sound like This engine is anywhere near 25% less efficient that the 4 strokes of the period. Do you think the common knowledge is another urban legend?
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Old 05-17-2012, 06:49 PM   #31
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Scary,
Where do you come up w the 124hp?

Are you looking on a chart that tells how much power that engine develops at 2400rpm?

"urban legend" Haha It's what I hear and read. I talked to a man on BoatDiesel that said he thought that Detroit Diesels (NA) are about the same as an old Lehman....(16hp per gal per hr). The fellow talked like he was very knowledgable and said that the boat I was looking at w a 4-53 would burn about exactly as much fuel as a Lehman in the same boat at the same speed. I think the DD would suffer a bit more at low loading though. Sounds like you run yours at about 60 to 65% load so you're probably doing better than most DDs.

But compared to a newer Cummins or Deere an excellent but old 6-71 should be about 25% less efficient. Don't you think?
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Old 05-17-2012, 09:23 PM   #32
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The Hp numbers come from the Detroit engine manual

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Scary,
Where do you come up w the 124hp?

Are you looking on a chart that tells how much power that engine develops at 2400rpm?

"urban legend" Haha It's what I hear and read. I talked to a man on BoatDiesel that said he thought that Detroit Diesels (NA) are about the same as an old Lehman....(16hp per gal per hr). The fellow talked like he was very knowledgable and said that the boat I was looking at w a 4-53 would burn about exactly as much fuel as a Lehman in the same boat at the same speed. I think the DD would suffer a bit more at low loading though. Sounds like you run yours at about 60 to 65% load so you're probably doing better than most DDs.

But compared to a newer Cummins or Deere an excellent but old 6-71 should be about 25% less efficient. Don't you think?
The fuel burn comes from my boat and digital tachometers and flow scans. There is also a boat review in Motor Boating and Sailing published in 1976 testing a sister ship validates my data almost exactly. Back to my original question which was if there was a difference between the old DD two strokes and four strokes of that era. It it sounds like not much. No doubt any of the new Electronically controlled turbo charged inter cooled engines will provide improved economy. It is interesting to note they actually built some turbo charge electronically controlled 6/53's probably for the military personnel carriers and the airline luggage haulers.
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Old 05-18-2012, 06:33 AM   #33
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"Do you think the common knowledge is another urban legend?"

Are we back at Global Warming?

The Consensus is......
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Old 05-18-2012, 07:03 AM   #34
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Fred... This thread is no longer in OTDE. Thanks.
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Old 05-18-2012, 07:40 AM   #35
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... will the real cause of the DDs lack of efficiency please step forward.
Mythology. Probably created by people whose previous knowledge of 2-stroke engines was limited to the hideously inefficient and dirty loop scavenged lawn mowers of the day.

It has become a favorite past time to trash DDs "just 'cause." For many reasons they are a fun and easy target with few defenders other than users but if you are going to claim 25 percent worse efficiency then maybe there should be some data to back that up.

I suggest comparing BSFC of 2-stroke DDs and other engines of similar power. Keep in mind that BSFC is a moving target that varies with load.
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Old 05-18-2012, 10:19 AM   #36
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"Keep in mind that BSFC is a moving target that varies with load."

And the higher the load the higher the efficiency on diesels..

DD's biggest problem was with folks that do not understand that a real engine is rated for cont operation on the pin, its max loading.

Under loading a DD , esp using an engine built for 180 to 230hp CONT as a 60 hp loafer IS a poor concept.

In most cases folks would be far happier believing DD , load a 71 series to 20 to 30 hp per cylinder and live long and prosper.

I have seen Displacement boats with a 6-71 , when the 3-71 would have been a far better choice.

Even when wide open a 6-71 would only add 1K or 1 1/2K to the speed of a displacement hull, so the "get home in bad weather " is pure nonsense.
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Old 05-18-2012, 11:08 AM   #37
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Another aspect that is not mentioned may relate to the noise a DD 2-stroke makes.

A fair number of posters have mentioned in threads in this forum that going to high idle (full throttle at no load) is a frightening proposition. They are not comfortable with the noise and being close to a machine that makes that kind of unfamiliar noise. Because of the type of blower used, A DD has earned the nickname "screaming Jimmy" and when compared to a 4 stroke turning the same rpm, that sound intimidates some people.

Unfortunately those same folks believe the engine is about to destroy itself and so surely must be very inefficient if it converts so much fuel to noise. Do you think the manufacturers of competing 4-stroke engines went out of their way to correct the myths that surround the legendary Jimmy?

Look where DDs were most popular, look at the throttle response those applications required to be successful. Ever wonder why "boom boats" almost univerally use Detroits?
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Old 05-18-2012, 12:31 PM   #38
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Frequently yachtsmen look at their power graph and see that their engine develops X amount of power at that rpm and assume that when they are cruising at that rpm that's what their engine is developing or as some would say "putt'in out". Not even close to true. An engine at 75% of max rated rpm is probably "putt'in out" well below 50% of rated power. I mention this as many boaters are under the impression that 50% of rpm = 50% of power. Far from reality but a common misconception.

Just say'in for the benefit of non gear heads.
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Old 05-18-2012, 01:45 PM   #39
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This has turned into a very interesting discussion

My last boat was Bayliner 4788 With Cummins 330b's mechanical engine controls. I just spent a hour on Cummin's web sight reviewing 330B hp, prop load hp, fuel consumption, and calculated hp per gal per hour. It averages 18.5 hp gal per hour. A little better than the 453 Detroit at 17-18. When hatteras built the LRC series we were in a period of time when we had a fuel shortage and they were looking to maximize range and economy. All of these boat. with the exception of the 42 were underpowered when compared to the semi planing hulls. The 48 had the 453's the 58 came with 471/or 671 and the 65 with 671's. The 58's actually needed the 671's on the west coast to power though the rough conditions. All of these are operating at 60-80% at cruising speed. My tractors and trucks that had Detroit's all seemed to running at near max output , especially my dump truck. Power skiffs that purse seiners use to pull nets seem to like the Detroit's as well.
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Old 05-18-2012, 03:25 PM   #40
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Well Scary I see the difference is smaller than I thought. Perhaps I should go look at that boat w the 4-53 again. No time for that now though. And a DD in Willy would need to be a 1-53 and I do,nt think it was made. 2-53 would be about double the power Willy needs but glad to hear the DDs are more fuel efficient than I thought. Perhaps I,ll have one one day.

PS...If the 4-53 boat had a 3-53 I,d be more interested.
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