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Author Topic: MPG  (Read 3640 times)
Blacksheep
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MPG
« on: June 04, 2008, 08:30:19 PM »

With the recent and ongoing rediculous fuel prices, a friend has chosen to do a motor/trans swap. He presently has an MCI equipped with a 8V71 and stick. Claims to get 7-8 mpg. He purchased an 8V92 nopn ddec, and auto trans for the swap. He was told this is not a good idea due to his mpg will drop drastically even as low as 4 to 4 1/2 mpg. Does that sound right? It is a turboed DD 8V92. He is kind of taken back now that all the work and money seems like a waste from info he recently received. The 8V71 is probably 318hp where the 92 is 450hp. He knows going in, the auto trans will cause less mpg and the larger motor will lose some but down to 4? He has common sense and knows how not to dog the motor and race it but any help in deciding on what is best is greatly appreciated so he can get started or not. One way or another the trans is going in so in case his wife HAS to drive, she can!

Thanks

BS
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bobsw
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« Reply #1 on: June 04, 2008, 09:22:26 PM »

I have a 8-92 t with an auto trans and I averaged 6 mpg over about 6ooo miles.
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Don4107
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« Reply #2 on: June 04, 2008, 09:37:29 PM »

If the auto is going in anyway and the 8V92 is mated to it I would switch if the 8V92 is in same or better condition.   One property of like designed diesel engines is that if you ask the same hp from different displacements, within reason,  the fuel consumption will be close as long as both engines are in comparable condition. 

If on the other hand he can't resist using the added hp, it would be VERY hard for me to resist, he will consume more $$$.  So if he goes up a hill using the same hp that the 8V71 made, the loss will be from the auto.  If he puts his foot in it, well......

Don't forget the need for extra cooling both for the extra displacement and for the auto.

Good luck whatever he does,
Don 4107
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Don 4107 Eastern Washington
1975 MCI 5B
1966 GM PD 4107 for sale
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tekebird
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« Reply #3 on: June 04, 2008, 09:53:10 PM »

my last trip- in my A3 with the 8-92T  6.5 MPG doing 80 for about 1200 miles
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TomC
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« Reply #4 on: June 05, 2008, 12:00:00 AM »

If you want genuine fuel mileage increase, then get away from the 2 strokers.  The best, and popular engine to use is the Cummins ISM with World transmission.  They are in just about every trash truck and many buses.  With this setup, you could get 400hp and close to 10mpg driven properly.  I wouldn't waste my time on installing another 2 stroke engine-they are just not close to the efficiency of the new 4 stokers with electronic injection.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
Sean
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« Reply #5 on: June 05, 2008, 01:22:58 AM »

Ace,

Ive got a 475hp 8V92TA DDEC-II mated to a four-speed (ATEC 748).

We weigh 47,000 and have a terrible ratio on the diff (4.73), and we still have averaged 6.3 mpg over about 90,000 miles.

We've slowed down about 10% lately due to fuel costs, so I expect that number to go up, perhaps to as much as 7mpg.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com
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tekebird
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« Reply #6 on: June 05, 2008, 02:27:48 AM »

unless you drive like 100k miles a year no repower is cost effective.

just adjust your driving style.
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Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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« Reply #7 on: June 05, 2008, 03:55:44 AM »

Same here,

6.5 mpg towing, 7.5 without...

I think it will come down to his rear axel gearing. What is he equiped with?

Nick-
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luvrbus
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« Reply #8 on: June 05, 2008, 06:41:28 AM »

Ace,I have a 8v92 435 hp with the 740 and average 7.5 to 7.9 doing 68 to 70 mph with 3:36 gearing around 1800 rpm.The most important thing the guy can do is have someone that is a DD man make sure he has a by-pass blower, the right hot housing for the turbo  right injectors and properly adjusted and the 8v92 will get  greater fuel mileage than you thought possible.I am still working on 10mpg (no luck yet)Check with these guys on the board that know these engines there are several here that know what can be done for fuel economy with 8v92
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TomC
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« Reply #9 on: June 05, 2008, 07:44:23 AM »

If you want maximum fuel mileage in a 8V-92TA mechanical, reduce your injectors to 75's that will make 375hp, but also you can run with your foot in it without hurting the engine.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
buswarrior
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« Reply #10 on: June 05, 2008, 08:12:42 AM »

Notice the gear ratio!!!

Changing to a taller differential gear is the common key to the ones who achieve better fuel economy with an 8V92 swap.

Get the cruise revs down, the 8V92 has the guts to pull the taller ratio.

I'm with Tom, as much as the storied history of the DD 2 strokes, and their great noises, is lovely.... the modern 4 stroke torque, efficiency and finding knowledgeable service moving forward must be considered.

Side note: as noted, a busnut will not recover the costs of the swap in saved fuel in this life time. Swap the driveline for other reasons, the small improvement in fuel costs, if you get any, is juts a little bonus.

happy coaching!
buswarrior
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« Reply #11 on: June 05, 2008, 08:28:45 AM »

Even a non-modern 4 stroke kicks ar$... all of us Crown owners routinely get 10mpg, mine with it's 1969 engine, no matter what the gearing (I've had from 5:29 ro 4:11 in mine and it's always been 10mpg regardless).  Not that it'd be easy for you to change but everyone is right about the 4x mileage thing....
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« Reply #12 on: June 05, 2008, 08:32:36 AM »

Ace;

What gear ratio does the bus have. What tire size is he running. How is the engine configured. He will get better mpg because of the turbo, and the added tq on take off and in the hills. give me the serial # and model off the block and I will try to help. My 8V92 TA DDEC III 525HP HT4060 trans ( double over drive ) 4:56 gears 12R22.5 tires weight 34000 lbs I average 8.7 mpg 65-70 mph If your friend wants to go with the 92 he needs to go with the big radiators and speed up the fan drive. Also if there is room remove the after cooler and add a charge air cooler.

Make sure the engine has the double double oil cooler and add a aux cooler to the trans.

hope this helps

Don

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luvrbus
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« Reply #13 on: June 05, 2008, 08:34:17 AM »

 Boggiethecat,  you probably have a 10 speed roadranger in that bus also no 740
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kyle4501
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« Reply #14 on: June 05, 2008, 09:25:23 AM »

Crown coaches also have less frontal area & the rounded fronts of Crowns & GMs do help reduce wind drag.

It'd be a tough call for me, if the current set up is working well & not causing any problems.

But - If there are some 'issues' with the clutch, linkage etc.  .  .  . I'd opt for doing the swap & if he finds his foot is too heavy, he can replace the injectors to drop the thirst of the beast.

The turbo & automatic can make driving easier, but some don't like the harshness of the shifts of some automatics.
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