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Author Topic: MPG  (Read 3699 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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« 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
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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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« 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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« 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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« 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
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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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« 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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« 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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« 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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« Reply #15 on: June 05, 2008, 07:47:32 PM »

boogiethecat
Wondering what motor you have and if its auto trans .
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« Reply #16 on: June 06, 2008, 01:20:40 PM »

BS....me thinks it just may be easier (and more fun!) to teach the wife to drive and shift a crash box.  No big deal.  She can do it....I guess the real question is....does she want to? 

An analogy.  My mom was 17 and was going to ferry B17's and B24's by herself from Canada to England.  Canadian Wackes or Waves.  Met my dad in primary flight school instead.  Wow!  Smiley Smiley Smiley
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« Reply #17 on: June 06, 2008, 02:54:33 PM »

If you know how to drive the Allison, you won't get your back hit with harsh shifts.  If you have an ATEC, then you hardly know it is shifting.  I don't care how hard it shifts, Allisons are a down right joy to drive-no one with an manual transmission could keep up off the line.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #18 on: June 08, 2008, 07:18:20 PM »

Tom C ,could You tell Me the proper way to drive an allison.  My HT70 will loosen fillings when shifting to high.  Any words of wisdom would be much appreciated.  John
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« Reply #19 on: June 08, 2008, 07:25:49 PM »

Tom C ,could You tell Me the proper way to drive an allison.  My HT70 will loosen fillings when shifting to high.  Any words of wisdom would be much appreciated.  John

I will relate that with my operators manual it states that if you maintain full throttle, the Allison will wait to shift higher.  That definitely adds a jolt.  I've seen many on this forum talk about just keeping their foot on the firewall.  To my thinking that is the wrong way to handle the Allison.  It is also bad for mpg and probably none to good for transmission life.

I drive mine easy for good mpg's.  As long as I'm not pushing it to full throttle, you can hardly notice the shifts.
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« Reply #20 on: June 08, 2008, 07:44:11 PM »

We used to have Allisons in our high flotation spreader trucks.  With them if you kept your foot on the firewall they would rattle your fillings when they shifted but if you held it to the firewall until it was almost ready to shift and then fairly abruptly backed out of the power the Allison would shift almost unnoticed and then you could put the power on again.

Somewhere along the way somebody told me that hard shifts were a design feature for an Allison and I shouldn't worry about them.  And I know on those working trucks the guys would come out of a corner in first and hold it to the firewall through three shifts day after day with no apparent ill effects.  We did engines and diffs almost as a maintenance item but I only recall one transmission in 22 years. 
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« Reply #21 on: June 08, 2008, 08:15:46 PM »

It is better for automatic transmissions to shift harshly.  Most automatics don't do so because the owners expect nice smooth shifting.

I had the automatic in my F350 upgraded to handle more power and it shifted very harshly after that.  The rebuilder said that is better for the transmission and I found plenty of others who agreed.
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« Reply #22 on: June 08, 2008, 08:29:28 PM »

thats just plain BS.

an Alison can shift as smooth as any car transmission is it is proprly maintained has the right fluid and you know how to drive it.  I have found that on allisons that have a harsh shift if you understand your gearing and power curve you can manage the shifts via throttle modulation.

as for modifying a trans to allow more power......harsh shifts are a side effect but has nothing at all to do with making it better for the transmission....bunch of poppycock...likely just a poor engineering design on the modification to allow modified shift points.
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« Reply #23 on: June 08, 2008, 09:24:45 PM »

Doug, That is not B.S at all.

the clutch packs will last much longer if they are not slipping when engaged and a harsher faster if you will shift means less slippage hence longer life for the clutch discs at least

the reason for the softer shifts in your car from the factory is simply drivability

people like smooth shifts so the designers gave them smooth shifts


Chris
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« Reply #24 on: June 08, 2008, 11:23:25 PM »

Its called overlap.  Meaning that in the typical auto tranny the next gear clutch pack starts to engage before the previous gear clutch pack has completely released for a nice smooth shift.  If there was no overlap, the engine could rev between shifts if the timing was not instant and perfect.  This of course causes a certain amount of wear, having two clutches engaged at the same time, at least partially.

The run of the mill shift kit for "improving" performance without improved clutches reduces overlap at full throttle to save the clutches and make quicker, firmer shifts.  Some increase pressure for more positive, quicker shifts. 

From what you are saying, it sounds like at least some applications of the Allison work a bit like a hot rodded auto tranny with shift kit.  Hold her wide open and shift firmly, back off and smoother (more overlap) shifts.

When was the Allison developed?  Being a GM product, are they just a turbohydramatic on steroids? 

I hope to just keep stirring the stick on my Spicer for a while.

What ever tranny you have, Good luck.
Don 4107
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Don 4107 Eastern Washington
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« Reply #25 on: June 09, 2008, 05:12:28 AM »

A harsh, teeth rattling shift isn't 'good' on anything. Sure the clutches slip less, but the risk of breaking bands or the like increases. Not to mention the jerking of the motor mounts & beating hell out of the rest of the drive train.  Roll Eyes

I have seen the insides of many a th350 & 400s that suffered from too many harsh shifts. Broken bands, stripped out clutch packs, even a sheared output shaft.  Shocked

I think the reason the big allisons survive so much abuse is a testimony to their conservative ratings.   Cool
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« Reply #26 on: June 09, 2008, 05:37:04 AM »

Hmmm guess all 4 of our Allisons are broken then, because they all shift very smoothly

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« Reply #27 on: June 09, 2008, 08:22:31 AM »

Been a long time since i last posted, anyway this thread caught my eye. I was wondering what mgp i should expect with my bus; 1955 GM Scenicrusier pretty much gutted, converted to 8V92TA mechanical with Allison 754, original rear drive-i think it is a 4.11/1, tires are 11R24.5. I haven't got to drive it much since the drive train conversion due to numerous reasons, major one was layoff. And now, selling house to move to Kansas. Will probably have bus trailered, but will be safest thing to do as tires are at least 8 years old. I will be posting more questions soon.
Thanks,

Mike
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« Reply #28 on: June 09, 2008, 08:58:14 AM »

6 I would guess.

Providing the sidewalls are not all cracked and you can bring yourself to drive 5 and stop every so often to let tires cool I'd drive it if it is drivable
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« Reply #29 on: June 09, 2008, 09:20:06 AM »

Stock rear axle ratio in the scenics is 3.7:1.

Your mileage will depend on the HP your engine is tuned for & how close you are to the peak efficiency (of the motor) while going down the road.

My stock greyhound spec. 8v71 got ~10 on the trip home from Fl.

I'm with Doug, DRIVE IT.  Grin

Put new tires on the steers. There are enough tires on the back that they aren't likely be overloaded. Speed may be something else all together tho . . .
Start out driving, going ~50 ish & have an escort driver to keep an eye on things. . . . If there's a problem, you can stop before it gets big & if you can't fix it, then get it trailered.

If you post the details, you may get a few busnuts to volunteer to help drive.  Grin  Grin  Grin

« Last Edit: June 09, 2008, 09:26:01 AM by kyle4501 » Logged

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« Reply #30 on: June 09, 2008, 09:39:45 AM »

Over the weekend I rented a new (104 miles on it) Uhaul truck (26ft'r) with the GM 8.1 liter gasoline engine and Allison 1000 transmission.  It had unbelievable acceleration-easily out accelerating cars from the signal, and the shifts were quite smooth throughout the spectrum of gas pedal positions (it shifted at 4600rpm at full throttle, yet cruised at 2000rpm at 65 mph, but drank the gasoline like a drunken sailor).  Course, that is an electronically controlled transmission (baby World transmission).  Was a nice truck to drive, but had thee worst ride I've ever experienced in my 33 years of being in the trucking industry.  Good Luck, TomC
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« Reply #31 on: June 09, 2008, 10:33:11 AM »

I would really, REALLY like to drive my Scenicrusier, but i think there are too many unknowns. Every system; air, brakes, steering, fuel, structure, electrial has been worked on, modified, removed & replaced or otherwise messed with. It would take a good month or longer to go thru and test out all the systems enough that i would trust it, not only with my life but the lives of everyone else in my path. Ok, so i'm a little chicken, call me what you want, but safety first.

Then on top of that, we are moving to Kansas and selling our house here in Denver. New carpet, painting, painting, and some more painting and fixing all those little nicks and stuff associated with selling a house. Well, my day is quite full. We just have to have the house on the marker in the next week or so to try and catch families while the school season is out on vacation. And in this marker, who knows...House will probably sell a little better if it didn't have a 40 foot bus out front on blocks.LOL. It's gonna be an interesting summer.

Mike
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1955 GM Scenicrusier, 8V92TA & Allison 754. Totally rewiring all 12v systems and lots of questions.
Dreamscape
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« Reply #32 on: June 09, 2008, 10:40:47 AM »

Sounds like you have your priorities in order. I agree with the possible danger in driving a coach that could cause problems for you, your family and others. Get your house on the market, then put your bus on a trailer and move. Your coach sounds like it needs lots of work, time and money before it is safe to drive anyway.

Sounds like a good plan to me.

Good Luck,

Paul
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