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Author Topic: 6V92TA vs 8V92TA  (Read 4358 times)
rv_safetyman
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Jim Shepherd


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« Reply #15 on: September 11, 2007, 05:22:20 AM »

Wade, you have had some great responses.  I even typed one, only to have it lost in the transmittal Cry  I will try to reassemble some of my thoughts.

First of all, be very careful if the 8V92 is a conversion.  If not done correctly, it will have a pretty fair chance of having heating problems.  If OEM, you will probably like the 8V92 for reasons already mentioned.

Next consideration is mechanical vs electronic.  The board is pretty divided, but my strong opinion is that electronic is much better.  Electronic gives much better engine protection and you don't have to find someone who truly knows how to run the rack on a two stroke.

Next, see if you can stretch your budget a bit and look for a bus with a four stroke (Series 60 or Series 50).  There are RTSs with Series 50 and some MCI also had Series 50 (mostly re-powers). 

John, there are late model buses that have AutoShift (or ZF equivalent) transmissions. 

I love my Series 60 and Eaton AutoShift.  I have a gear for every need (especially important when going down a steep mountain with the Jakes on!!!).  I have averaged a bit less than 8 MPG over 30K miles.  A lot of that was driving over the big "hills" in the west Grin  On this trip I have averaged over 8.6 on a level trip through CO and NE.  This is a very accurate value based on the engine computer/Silverleaf.  My Eagle and toad weigh over 41K.

The only issue with AutoShifts are that they are slower on acceleration.  However, I get to hear the system make 3-4 shifts before I cross an intersection Grin Grin

Just some thoughts

Jim

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Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
Blog:  http://rvsafetyman.blogspot.com/
JohnEd
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« Reply #16 on: September 11, 2007, 08:24:34 AM »

Jim,

What a testimonial Smiley  That newer busses have your setup stock is a moot point to me as they aren't even on my dream list.  Your mpg would be sufficient motive alone.  As would the option of gears for up or down the mountain safely.  Being able to start on ANY hill has got to be worth something in the dream catagory. 

I am confused about your comment that the Autoshift is slower in acceleration.  Can you expound on that?  Three shifts across an intersection sounds fast to me.

Great post from you.

Thanks,

John
« Last Edit: September 11, 2007, 02:44:51 PM by JohnEd » Logged

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rv_safetyman
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« Reply #17 on: September 11, 2007, 04:21:58 PM »

John, most folks think the slower acceleration is due to the time between each shift.  That is not a big issue.

With an automatic transmission, the engine stays "on power" and as a result, the turbo stays spooled up to a major degree.  With the AutoShift, the electronics backs the power off to break the meshing of the gears and then matches the engine and transmission speeds for the next gear.  During this 2-3 second process, the turbo has a chance to "spool down".  The loss of turbo speed and boost are magnified in my application with some pretty long runs from the turbo to charge air cooler and then to the intake manifold that must be brought back up to pressure.

To give you a feeling, my acceleration is about equal to a lightly loaded semi. 

Under most conditions, the acceleration is not an issue at all.  If I need to get the maximum acceleration, I go the the "manual" control and run the engine up against the governor for most of the shifts.  That way, the 500 rpm drop results in the engine being in the 1400-1500 rpm range at completion of the shift and the turbo reacts much quicker.  The typical Eaton software programs the engine to shift at about 1800 under full throttle.

Hope that all makes sense.

Jim 
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Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
Blog:  http://rvsafetyman.blogspot.com/
NJT5047
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« Reply #18 on: September 11, 2007, 06:58:29 PM »

Unless you get really lucky, a nice 8V92/102A3 factory unit is going to be hard to find. 
Keep in mind that the mere presence of an 8V92 doesn't garuntee a large HP engine.  They range from 350 HP up to about 500 HP in a coach.   They all produce a good bit more torque than a 6V92.
An 8V92 will get as low as 3 or 4 MPG if driven up mountainsides at highway speeds...until they get hot.  Which they are know to do with disturbing frequency.  The later "C" model has vastly improved cooling systems for large engines.   
If there is any way possible, save a little more bucks and find a 4 stroke auto "C" or "D" MCI, or maybe a Prevost 4 stroke.  Prevost built more manual transmission equipped buses.  There are a few MCI D models with manual.  I'd definitely buy an automatic. 
Buses with manuals are not geared for close work, or starting on hills.  They are way too tall.  Learning to drive a manual coach may prove to be expensive, and could your spousal component drive it during an emergency?   Anyway...manual coaches are not at all common.
I've got a 6V92 in an MC9 which is similar to what you are looking for...not as wide.  Identical running gear.  Gets upwards to 7 MPG on the flats and runs up and down the interstate at 70 MPH all day. 
Does not climb the mountains...as has been discussed.  Nor will most 8V92s.  They don't exactly scream over the hills...just add one gear up and that's what a larger 2 stroke buys. 
You seem to be pretty close to me in Charlotte (NC?)...I'm just South of Matthews in Indian Trail, NC.
Are you aware that a  Bus Rally is scheduled for Oct 12/13/14?  Appox 3 hours away in Western SC....Cleveland, SC.  Palmetto Cove Campground located about a half hour on the other side of I 26.
May want to drive over and check out the homegrowns!   You'll get some good ideas...and some not-so-good ideas. Wink  I can show you quite a few not-so-good ideas!   If you have the opportunity, Bussin 2008 is also on schedule for Dec.  Arcadia is a very large bus rally.
No matter what you buy, take a good bus mechanic with you!  Look for rust, leaks, drive it enough to rule out heating problems...and just assume that the bus AC won't last long without major $$$ injection. 
Give me a call at 704 650 0235. 
Cheers, JR
« Last Edit: September 11, 2007, 07:07:52 PM by NJT5047 » Logged

JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.”

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NJT5047
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« Reply #19 on: September 11, 2007, 07:28:30 PM »

One more point...I promise!   I saw a post that indicated that any fuel mileage below about 7 MPG would be a sign of problems...not so.  Most 6V92s driven at 70 MPH will see about 6 MPG.  Same's true for 8V92s on flats.
An 8V71 is a different animal altogether.  8V71s will get 2 or 3 MPG better than a 6V or 8V92.   An 8V71 is a much smaller engine that's usually coupled to a manual transmission...not always, but pretty common in GM coaches from where these fuel mileage comparison arise.   GM fuel mileage is one subject...MCI and Prevost 6V and 8V92s are another.   An 8V71N is an excellent motor, but naturally aspirated, it will not compete with turbo 92 series engines.  Nor will the 2 strokes compete with a 60 series. The newer the better...if one can afford the price.
An 8V71 is typically about 270 HP.    6V92TAs start at 270 HP (transit coach engines) and range to 350 HP in buses.  8V92TAs start at 350 HP and go way up from there...whatever the radiators can stand.
GM buses will get much better mileage than a  40' MCI or Prevost. 
Most Class C motorhomes get about 7 MPG too. So what the problem?  Wink
I'm totally done now. 
JR
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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.”

Ayn Rand
RJ
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« Reply #20 on: September 12, 2007, 08:06:56 AM »


An 8V71 is a different animal altogether.  8V71s will get 2 or 3 MPG better than a 6V or 8V92.   



JR -

If you're talking about the 8V71 in a GMC, you're correct.  The 40-foot Buffalos are also about 3,000 lbs lighter than the others, which accounts for a lot of that difference.  4106s are lighter still, which is one reason they were so popular with operators, due to their economical and dependable operating characteristics.  The Scenicruisers, OTOH, return mileage similar to MCIs, since they're heavier.

However, IBME that the 8V71 in an MCI, Prevost or Eagle (all three-axle, forty-foot coaches), gets virtually identical mileage as the 6V92TA, somewhere in the 5.5 - 6.5 mpg range.  BTDT, have the log books. . .

102A3 MCIs with OEM 8V92TAs are out there, probably more common west of the Mississippi due to the Rockies and the Sierras.


I believe Wade was looking at and trying to decide between a 1990 102C3 with the 8V92TA powertrain, vs an 1984 Eagle with the 6V92TA.  My recommendation still stands:  Buy the newer coach.

FWIW & HTH. . .

 Wink

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RJ Long
PD4106-2784 No More
Fresno CA
TomC
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« Reply #21 on: September 12, 2007, 08:17:35 AM »

On my last trip up and back from Mammoth Lakes, Ca I averaged 5.7mpg and was satisfied with that because of my improved performance after turboing. 

Fuel mileage is completely determined by the amount of lead in your shoes.  If an automatic bus followed a manual bus and duplicated the performance, the fuel mileage difference would be very small. Most automatics get worse mileage since most drivers love to mash the pedal since it is so fun to accelerate fast.  Allison has done tests with large fleets, and once the drivers are educated to driving techniques on an automatic, many times the Allison will get better mileage than a manual transmission truck with a bad driver.  I rarely mash my motor from a stop.  With 4.56 rears and the V730, on light acceleration, the transmission will lock up in second at 1600rpm and will drop down to 1100rpm, thats at around 20mph.  So in town, I can be mostly in second lockup between 20 and 45mph.  V drives will always get worse mileage than a T drive, since the transmission has to reverse the rotation of the engine and with the V730, only 3 spds.  My bus is geared that at 2100rpm I'm doing 67mph by my GPS-which is just fine for me.  If you could be at around 1800rpm with the 2 stoke, that's the best for fuel mileage.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #22 on: September 12, 2007, 03:15:35 PM »


An 8V71 is a different animal altogether.  8V71s will get 2 or 3 MPG better than a 6V or 8V92.  



JR -

If you're talking about the 8V71 in a GMC, you're correct.  The 40-foot Buffalos are also about 3,000 lbs lighter than the others, which accounts for a lot of that difference.  4106s are lighter still, which is one reason they were so popular with operators, due to their economical and dependable operating characteristics.  The Scenicruisers, OTOH, return mileage similar to MCIs, since they're heavier.

However, IBME that the 8V71 in an MCI, Prevost or Eagle (all three-axle, forty-foot coaches), gets virtually identical mileage as the 6V92TA, somewhere in the 5.5 - 6.5 mpg range.  BTDT, have the log books. . .

102A3 MCIs with OEM 8V92TAs are out there, probably more common west of the Mississippi due to the Rockies and the Sierras.


I believe Wade was looking at and trying to decide between a 1990 102C3 with the 8V92TA powertrain, vs an 1984 Eagle with the 6V92TA.  My recommendation still stands:  Buy the newer coach.

FWIW & HTH. . . Wink

Yes on the GM fuel mileage, and definitey I agree on the MCI versus the Eagle.  The MCI "C" will make a super conversion.
And, while I'm at it, I'm not at all against 8V92s....wish I had one.  But, when the are retrofitted into dual radiator MCIs, the result is often heating problems.  Expensive to fix, but fixable.  A factory 8V92 unit with the big rads would be a desirable package in my opinion. 
Best, JR
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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.”

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« Reply #23 on: September 12, 2007, 06:52:03 PM »

Gentlemen How would the S50 compare to the above? With the 740 or B500?  Mike
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NJT5047
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« Reply #24 on: September 12, 2007, 07:14:59 PM »

Very desirable.  The later version of an S50 is a great powerplant.  It would be better paired with a B500, but an HT 740 would handle the engine. 
It will get better fuel mileage, has good torque, will outlast a 2 stroke, and is repairable by most current DD mechanics. 
It may not be quite as smooth at idle as a 2 stroke, but with the correct motor mounts, they are plenty smooth enough.  Anthing off idle is smooth.
An S50 would be a better choice.  I would guess that it would be found in a good bit more expensive coach...unless you find an ex-transit.  Transits don't make the best conversions unless you understand what must be altered (and willing to do the mods) to get highway speeds and accept the generally worn condition of the powerplant.  Still, an RTS properly massaged is a great looking, great driving coach.   
OTR coaches  with S50s are getting into 1993 and later, unless it's a retrofit.  Homegrown retros may not be such a deal.  A factory installed S50 would be cool. 
JR
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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.”

Ayn Rand
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« Reply #25 on: September 12, 2007, 08:06:34 PM »

Regardless of the power plant, I will add to the tortoise and hare story. We recently traveled with two other families both of which have chipped Diesel pickups pulling 10,000 pound trailers. Minnesota to Grand Tetons and back again. Our Mc-5 has the 8-71 with N65 injectors 4 speed spicer and a 140 gallon fuel tank. The pickups are way faster off the line and can generally maintain higher speeds than the 70 mph we drove. All three of us started every morning at the exact same time and the 871 powered MC5 always arrived at the destination ahead of the diesel pickups at the end of the day. The pickups lose about 20 minutes every two hours for fuel stops. We didn't have to fuel up until the end of the second day. We still stop for bio breaks but they typically are much shorter in duration.
(I was suprised because I have a chipped diesel pickup that used to pull a 10,000 pound trailer!)
Fred  Cool

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Fred Thomson
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« Reply #26 on: September 13, 2007, 08:04:17 AM »

Following up on Fred's post, he pointed out a good thing about buses.  They have large fuel tanks and can go a long ways before fueling.

On a 4,000 mile trip with an F350 and large travel trailer we had to stop every 3 hours or so for fuel.  We were lucky to get out of a fuel stop in 20 minutes and it was usually 30 minutes or more.  Everybody ran inside to buy food and use the restroom. 

With the bus, I get 7 to 8 MPG and should make it 900 miles easy on a tank of fuel.  We might need to stop for smoke breaks occasionally, but we can stop at a rest area and save a ton of time.  I even figure we can travel a little slower to save on fuel and still make better time than the old F350.
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