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Author Topic: 8V71 advantages over 6V92  (Read 5117 times)
JohnEd
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« on: September 19, 2007, 12:04:45 AM »

I recently had an enlightening pm with TomC.  He has a 8V71 that puts out 375HP.  I didn't think that a 71 would be reliable at that power and I don't know where I got that erronious opinion.  I was wondering how many others were 8V71 "averse" in considering a coach for purchase and I would like to hear from those that are and what their rational is for that opinion.  Also, and mainly, I would like to know what the advantages are to owning a 71 series as opposed to a 92.

One advantage I recently stumbled onto is that the 71 can use automotive antifreeze.  That sia cost saving, if it is true.  Anybody know for sure?

I thought low power was an issue but apparantly the 8V71 puts out 318hp in natural asperation with the large injectors.  That figure goes to 400HP with a turbo and aftercooler and I think it also needs to have low compression turbo spec pistons.  400 is more than enuff for my money and even if the life span is reduced I won't ever wear out a fresh engine.  Does this sound correct?

Another post stated that the 71 was a lighter engine with a lighter crank, etc.  This would naturally lead to greater efficiency and more potential MPG.  Is the 71 more econ.?

Are parts more or less expensive for the 71?

Interested in what anyone might be able to contribute on this topic.

Thanks

John
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Songman
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« Reply #1 on: September 19, 2007, 12:38:40 AM »

I don't know all that much about diesel engines, but Don Fairchild does. He says his favorite combo is an 8V71 with a turbo added. That is what he is going to build to go in the Eagle for me. I've asked quite a few knowledgeable people and they agreed. Seems like Don said something about the way air is brought into the cylinders in the 71 is much better. I think maybe they are easier to cool too. Some people who know more will chime in at a reasonable hour I am sure. I'm up late working on a T-shirt design.
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belfert
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« Reply #2 on: September 19, 2007, 04:54:05 AM »

Anybody who chooses one engine over another because the coolant is less expensive is in the wrong hobby.  I didn't know the V92 series takes special coolant.  I know my Series 60 just uses the green coolant.  I have been purchasing Shellzone coolant.

An 8V71 may get better mileage, but how much money are you going to spend to turbo it to get performance?  TomC spent I think $18,000 on his engine, tranny, and cooling system, but he had it done professionally.  DIY would certainly be less money.

It would seem to me that a 6V92 or 8V92 is a better choice if you want a turbo unless you already have an 8V71.
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edroelle
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« Reply #3 on: September 19, 2007, 06:19:29 AM »

1.  Brian - Does Shellzone fit DD requirements?  "just ...green coolant" does not.
The following I wrote on the MCI BB holds for Series 60 (most disagree it is required for 71 series).

There is actually a specific antifreeze that you must use with Detroit Diesels -
just like there are specific oils. You do not want to use the standard ethylene
glycol.

http://www.axlealliance.com/public/brochures/7se298.pdf

Detroit Diesel sells their brand. I used Final Charge at about $12.50 per
gallon - full strength.

http://www.finalcharge.com/

Dilute with distilled water at $.75 per gallon.


2.  There was an article in Bus Trader (I think) about various DD 2 strokes.  It stated that the 671 is probably the most reliable, followed by the 8V71, and then the 92 series.  When I repowered my MCI 8, I installed an 8V71T, basically for ease of replacing an 8V71, AND reliability.  If a good 6V92T or 8V92T were available, I would NOT shy away from.

Ed Roelle
Flint, MI
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JohnEd
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« Reply #4 on: September 19, 2007, 08:59:16 AM »

Belfert,

I don't think the correct language was used in the question about the antifreeze you are using.  It should have read: If you don't use the correct antifreeze you will DESTROY your engine.  Did I mention that you will DESTROY your engine if you use automotive stuff?  I was up at the DD shop a while back and they had a cylinder on the counter for show and tell that came out of an engine that had had auto stuff run in it and the wet sleeve(cylinder) was eaten away like a piece of swiss cheeze or a sponge.  The mech said "it only took XXX months to do that" but I don't recall the number but I do recall my jaw dropped even further.  If you are trying to save a buck on coolant you are in the wrong hobby ( little joke there, LOL).

In my question I said that the 8V71 had come back into the equation for the power in my future bus due to my being enlightened by the board.  In this last post I learned that the 71 is "easier to cool" and that is a significant point.  Not a hinge point but definately worth noting by me.  Just look at the posts on that issue and the money that has been spent by really smart people.

The antifreeze issue is not just a matter of saving a buck.  I think that knowing that the engine condition cannot have been compromized by  using the wrong stuff is of some small value.  Short of catastrophic failure, I don't think you can tell if it is damaged by using the wrong coolant.  These aren't pivotal issues but they do get considered by me and you add them all up you get a different pic.

Comments like "TomC spent $12K building the 71 of his dreams", make a dent in my reasoning.  These guys not only have the resource but the knowledge/experience to make good decisions.  Me thinks maybe nostalgia might have played a small part but they wouldn't shoot themselves in the foot.

Sooooo, while I am not making it a requirement, I am considering the 71 as the engine for my future bus and they are thousands cheaper, usually.

Imagine, a guy that would make $30 worth of antifreeze his deciding point on purchasing a bus and that same guy having the faculty to link words together into a coherant sentence.  The mind boggles.  LOL lots.

Be happy,

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
—Pla
Dallas
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« Reply #5 on: September 19, 2007, 09:24:41 AM »

Quote
Another post stated that the 71 was a lighter engine with a lighter crank, etc.  This would naturally lead to greater efficiency and more potential MPG.  Is the 71 more econ.?

Are parts more or less expensive for the 71?

Interested in what anyone might be able to contribute on this topic.
[/i][/b]

Actually, an 8V71T weighs more than a 8V92T by around 100lbs, however, an 8V71NA only weighs about 85 pounds less than an 8V92T. The 8V71 is also 1" shorter than a 8V92T.
One caveat: This is true only in certain setups. Yours may be different.

I have heard, but don't know it for a fact, that the exhaust valves on an 8V71 being smaller, aspirate better than the 92, allowing more efficiency.

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Green-Hornet
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« Reply #6 on: September 19, 2007, 09:32:44 AM »

Seeing as there was a coolant question in here too, it was my understanding that the 6v had some cooloing issuses. If they overheated much, they would blow the o-ring, like the space shuttle. The 8v doesn't use the same seals. Huh
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belfert
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« Reply #7 on: September 19, 2007, 09:41:11 AM »

The Shell Zone coolant I bought says it meets the DDC 7se298 standard which is the coolant selection ducument for the MBE engines and the Series 50/60 engines.  I also bought this coolant at a Freightliner dealer that services the Series 60 so I would hope they are selling the right coolant.

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uncle ned
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« Reply #8 on: September 19, 2007, 12:18:39 PM »

Don Fairchild praises 8v71's  but his new gillag has a 8v92.

He is a knowledge and great guy. Really helped me understand some of my cooling problems on my 6v92.I am going to do several of them before Kyles non-rally to see if they helped.The best help comes from someone that understands a fellow "hot rodder".

these are thethings you get from the rallys. the knowledge floating around them is amazing.
the 3 day spent camped beside him was very enlightening on thing Detroit. I had Bryce, Dallas, and Don to learn from.
Hope he considers me a friend. Because he really impressed me on his knowledge.

uncle Ned

ps. he has beautiful bus by the way
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4104's forever
6v92 v730
Huggy Bear
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« Reply #9 on: September 19, 2007, 01:54:38 PM »

One hears all sorts of things about the differences (if any!) between the 6V92T and the 8V71T like bottom ends, sleeve type, bearing life, rpm limits, service life, hop up capability, etc., etc..

About the only thing I would/could be concerned about would be the totally sweet sound such a Detroit would make late at night all alone on the highway.  I think the 8V beats the 6V hands down.  Smiley Smiley
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TomC
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« Reply #10 on: September 19, 2007, 04:29:08 PM »

The 71 series is the only big engine I know of that uses dry liners.  With the 92 using car type antifreeze, if you don't monitor your acid level, then yes it will eat through in a sort period of time.  There was a Series 60 in the shop that went through liners in 220,000 miles.  The main advantage is that the 71 series with its' dry liners has a lot more cast iron meat that won't rust out or react as fast with bad antifreeze.  All engines including the 71 should be tested for acidity levels.  I ran standard Prestone antifreeze in my 8V-92TA when I had the truck, and my first overhaul was at 500,000 miles.  I could have used the liners over again, but wanted the more up to date that used less oil.  It is also a good idea to have a water filter that adds Nacool to the system.  Then you don't have to think about it, just at oil change time.

Granted my turboing job with the V730 overhaul was about $17,000, but now I have an engine I like to drive and will be reliable for many years to come.  Personally, when I buy another bus or truck for my next motorhome, it WILL have the correct power train in it at purchase time.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #11 on: September 19, 2007, 05:02:54 PM »


  Ok, now I am confused (go figure)

   I thought there were 2 style of liners for the 71 series and the wet upper was the type one should
 use if one was going to add a turbo.

  Am I confused about the 71's or 71's vs. 92's


   Just a clarification question

Skip
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RJ
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« Reply #12 on: September 19, 2007, 08:18:52 PM »

Skip -

71 Series are dry liners, 92 Series are wet liners, with O-Ring at top.  Clear as mud?


John -

Some real-world experience for you, from my charter days.  At that time (excluding the GMCs, which were all 8V71s), we had MCIs with 8V71N (275 hp), 8V71T (350 hp), 6V92TAC (275hp) and 8V92TAC (400 hp) engines, all bolted to HT-754 Allisons.  Spent many an hour behind the wheel of all the various combinations, so here's sort of a "Readers Digest" observation.  (Remember, if we were going to Reno, Tahoe, Vegas, LA or San Francisco, we had to climb 6% grades to get out of the valley.)  Anway:

The 8V71Ns were quicker "off the line" to about 30 mph.  This was offset by having to shift down earlier when pulling grades, and often plugging along at 30 - 35 mph in 2nd gear at altitude.  Also used the least amount of oil and was the easiest to keep cool. . .

Only the 8V92TAC coach would out-pull the 8V71T in the mountains, and often would get better fuel mileage, due to it's greater torque and ability to stay in a higher gear longer.  But the 8V71T would out accelerate this one initially to about 30 mph, primarily because of the 92's annoying throttle delay (to keep the tree-huggers happy), which the 71T didn't have.  Was easier to keep cool than the beast motor, too.

When running together, the 8V71T and the 6V92TAC would get almost identical fuel mileage. but the 71 would pull ahead of the 92 on those slight grades - the ones where torque, not HP, helps to maintain your speed.  (Good example is the shallow climb out of Bakersfield to the foot of the Grapevine SB on 99/I-5.)  The six was also down 75 hp from the eight, suspect if they were both set up for 350 hp there probably wouldn't be much difference in their performance - except for the cursed throttle delay on the 92s.

If I couldn't snag the 8V92 for a run, I would do my best to land one of the 8V71T-equipped units.  For me, they were my favorites among the MCI fleet.  (Of course, the 4106 I now own, plus the 4905s, were the favorite GMCs.)  The 8V71T is a good engine, I know why TomC likes his.

Now - change of scenery:

At the transit property I moved to after my charter days, we had 8V71Ns and 6V92TACs bolted to V-730s.  In transit operation, the 71s held up far better than the 92s, often going 75K > 150K more before overhaul.  The trick to getting longer mileage out of the 92s, according to the shop, was to replace the main and connecting rod bearings at the 100K-mile mark.  Now, transit bus operation is the most severe duty you can subject a powertrain, it's amazing they hold up as well as they do.  Obviously, this is considerably different than the RV service we subject our vehicles to.  (Some folk think what we do is MORE severe, due to lots of sitting. . .)

318 is the truck engine, and is actually 304 hp.  To get this out of the naturally aspirated 8V71, you've got to advance the timing and go to N65 injectors.  All fine and dandy, but this also shifts the torque curve up by a couple hundred rpm - not a problem with a 13-speed 18-wheeler, but can become annoying with a 4-speed bus (auto or stick).

IMHO, the MOST dependable, durable two-stroke engine for a 40-foot bus is the N60-equipped, standard-timed 8V71N that puts out 275 hp, followed by the 8V71T putting out 350 hp.  For me, the 92s are a toss-up, unless I wanted a fuel-burning monster motor, in which case I'd go with a 500 hp 8V92TA.  Couldn't keep it cool, but it would go fast and suck fuel at an equally dizzying rate!

Oh, and I cannot forget the virtually unbreakable inline 6-71, too.  Darned things will run forever if you keep them cool and change the oil regularly (that which doesn't leak out, anyway).

FWIW & HTH. . .

 Wink





« Last Edit: September 19, 2007, 10:14:12 PM by Russ » Logged

RJ Long
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« Reply #13 on: September 19, 2007, 09:17:17 PM »

The last time that we pulled Snoqualmie Pass EB on I90, I had to back out of our 8V71's throttle in third gear when clearing the top at over 50 mph. And our coach weighs 27,000 lbs. No overheating, either.

That leaves me perfactly happy with our standard shift setup.

For what it's worth.

Tom Caffrey
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Tom Caffrey PD4106-2576
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JohnEd
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« Reply #14 on: September 19, 2007, 11:44:32 PM »

Thank you Tom Caf.  See, now when I started this thread I was willing to accept a 71 instead of a 92 and now I am looking for a 71 equipped bus as my first choice.  And they are more common....I think.

This has been a great and illuminating trip.  Thank you all so very much.

Brian,

The operative word in your last post was "hope".  There ain't no hope...really...none.  Sincerity is subject to proof.  I spent 20 years tracking and checking people that were entertained by screwing the "Gummint" or were lazy or were trying to make a killing or simply were not all that expert despite having a position.  Defence contractors!  I also met and worked with some of the absolutely finest...  My position description, as a project manager, had a statement that I added after a year on the job.  It read "The incumbent must work on a daily basis with individuals whose goals and objectives are dimetrically opposed to those of the incumbent."  My boss asked from what professional series I had taken that statement, and my answer: " From the position description for a pcychiatric nurse".   He just grunted his approval.  Verify!  If you possibly can.  The profit motive can wreck havoc with your personal finances.  I think that is what makes this board the absolute pleasure it is.  The motivation is pure and all simply want to help and there is no profit involved.  With my best intensions.

John
« Last Edit: September 20, 2007, 11:08:07 AM by JohnEd » Logged

"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
—Pla
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