Please explain axle ratios to me and their effect on fuel economy. With a Series 60/B500 powertrain, I would like to know the absolute best possible axle ratio for fuel economy. I do not need to tow.
Thanks.
Cheers.
What speed do you plan on travelling? Any mountains in your plans? What does your bus weigh?
Engine tune? Wind resistance? GVW? Rolling resistance? Duty cycle? The same engine may work in one application but not in another.
Back in the day, our old DD fire apparatus was geared very short. The application required maximum acceleration and not much else. 65mph @ 2400..
Same with inner city transit buses? I drove one that would go a max of 45 mph. That was all that was needed. It sounded like max rpm. 2 stroke DD
My old Crown Supercoach Small Cam Cummings ran at 1500 rpm @ 60 mph. The sweet spot. Gearing depends upon many factors and needs. A DD 60 ...
MIGHT, (might) like 1200-1400 rpm IF the application would allow it. Pick a reasonable cruising speed. "Lugging" hurts. "Running Free" wastes fuel.
If one had a 10 speed Roadranger and all the dash gages including boost, pyro, fuel pressure, fuel flow, etc., one could pick the correct gear for the situation.
In a Bus Conversion one may, (may) not have that liberty. One just drives the thing and has fun with it. Pick a cruising speed and enjoy the ride.
Some help here please.
Swadian -
Simply, the larger the axle ratio number, the faster you'll accelerate, but your top speed will be lower.
Example: A transit bus with a 5.36:1 ratio will accelerate to 40 mph faster than the same bus with a 3.33:1, but will top out at 57 mph. The 3.33:1 bus will be slower to 40 mph, but will end up doing 80 mph.
Another way to look at it is that for any given road speed (say 50 mph), the bus with the 5.36:1 ratio's engine will be turning higher rpm than the bus with the 3.33:1 ratio. Obviously, then, the higher rpm engine is going to burn more fuel at the same road speed, thus lower economy.
If you have an S-60/B500, chances are good that the factory engineers specified a rear axle ratio that gives the best compromise between decent performance and acceptable fuel economy.
Of course, the weight of the operator's right shoe has a big influence on fuel economy, too!
FWIW & HTH. . .
;)
The most common ratio for the series 60 with the B500 to utilize both overdrives was 4:30 to 4:56,some Prevost were 4:78.The new generation Allison offers different overdrive ratios you see 3:78 ratios now to utilize the torque range at low RPMs.
I have several friend that tried to use 3:73 gears with the older B500 and the transmissions just hunt between 5th and 6th so all they use is 5th (1st overdrive),my MCI has 4:56 gears with the B500 I am 7.8 at 70 mph.Can you tell us what bus and year model you own
It's a 102DL3. Out here, the roads are straight and flat and the speed limit is 80 mph. I don't use it in the mountains (it bottoms out too easily), so the standard compromise probably doesn't apply. I specifically want it to be able to go 80 mph on the flat as efficiently as possible, so I want the RPM to be as low as possible at 80 mph. Don't call me a speed demon; if the speed limit is 80 and I can safely go 80, I'm going 80.
What is the numerically lowest axle ratio possible for this vehicle? Is it 3.58:1?
What 60 series 11.1 or 12.7 and HP MCI is not big on high HP engines
As asked, there are a bunch of different engines that cover the 102D, all of them are called "Series 60" but the evolving emission controls changed their operating characteristics, and the sweet spot for fuel economy. Two different displacements, 11.1 and 12.7. Later, the 14 litre came along.
And the B500 of that vintage has a minimum rpm required for its lubrication pump, (look it up, but up 1500 -1600 someplace???) and it will not shift to 6th gear without the necessary RPM, defeating the whole exercise...
That RPM is significantly higher than the typical fuel economy RPM of today's engines, so busnuts are going to get into trouble mixing and matching their found treasures.
That said, the S60 of those older days was happy spinning at whatever the B500 needed, they were a good pairing.
4.56 differential went out the factory door in many, many of them.
80 mph in a motorcoach and seeking fuel economy in the same thought, I expect there is little to no wiggle room, it will cost what it costs, with little chance to improve or worsen it with the available stock bits?
happy coaching!
buswarrior
80 mph is not safe in a bus. Too heavy. It may feel good and steady, but stopping it will be tough. What if you blew a tire? 65, 70 is lots.
JC
B500 top 3 gears are 4th-direct 1:1, 5th- .74 overdrive, 6th-.64 overdrive. You want to gear a S60 to cruise at 1400-1600rpm. If you have 12R-22.5 rubber, that is 485rpm (revs per mile). For maximum fuel economy, then you want to be turning 1400 at 80mph-or 1050 at 60mph. Divide by .64 and that gives 1,640 in direct. Divide by the tire 485 and that give 3.38 ratio.
How much does your bus weigh? Do you pull a towed? At 3.38, startability could be a factor (slow off the line, hard to start up a big hill).
Personally, I would gear for running at 1600 @ 80. That works out to be more like a 3.90 rear ratio, that will give exactly 1614rpm at 80mph. Good Luck, TomC
I'm not going to do 80 mph all the time, only when it is legal and I deem it safe in my best judgment to do so. I want the ability to go 80 mph.
The vehicle weight is 35,100 lb, GVWR 48,000 lb. I don't tow and I don't have slides. Tires are 315/80R22.5, diameter 42.3". Engine is a Detroit Diesel 6067BK28 12.7L rated at 430hp. I understand fuel economy will be poor at high speeds, but I'm looking to save whatever fuel I can. Again, I usually only run flat routes.
a quick search turned up this 2005 brochure.
You want the HP/Torque graph for the engine in question, same as on the second page, or it is all a guessing game.
http://www.schneidertrucks.com/pdf/DetroitSeries60EngineSpecs.pdf (http://www.schneidertrucks.com/pdf/DetroitSeries60EngineSpecs.pdf)
Notice on this example, there's maybe 475 HP of the advertised 515 available at 1500 rpm.
If you are starting with a 375 HP 11.1 litre S60....
In the old days, the trucker types said it only took around 280 HP to keep 80 000 lbs moving well on the highway, the rest was just profit blowing out the stacks as they roared up the hills. Must be something to it, witness the fuel economy the 4104 owners enjoy with their smaller HP?
Anyway, the engine has to have the power available at the rpm you are designing towards, or you will be unhappy with the performance.
Interesting exercise, keep going!
happy coaching!
buswarrior
The 4000 and 4000MH transmission are more forgiving on lubrication than a B500,the B Allison were for bus use and that is the only place you find those,very high dollars to rebuild or replace.
A friggn rebuilt ECU cost $3200.00 (I know that for sure) and you are probably going to need one special programed to except the higher gearing you want and IF you can find one or somebody to build it.There is a lot more to it besides the just gearing,MCI,Detroit and Allison engineers knew what the were doing IMO you are not going to save enough in fuel cost to offset the cost,why not just have the ECU's programed for 80 MPH mine will top out at over 80 mph
I must have the slow version b/c I have to use a sun dial to time how long it takes to get up to speed. However, it feels like the sweet spot is around 68.
According to this BSFC (Brake Specific Fuel Consumption) chart, https://www.nap.edu/read/12258/chapter/5#36 (https://www.nap.edu/read/12258/chapter/5#36) that engine (a 1994 440 BHP 12.7 L Series 60)has a sweet spot of .312 lbs/BHP HR (lbs of fuel used to produce a Brake Horsepower for an hour) between 1400 and about 1675 rpm, producing between 360 bhp and 420 bhp. That is at full throttle, so part throttle at those engine speeds will produce less horsepower and use less fuel. But this shows the engine is quite efficient at those engine speeds. To use this kind of chart you need to know your load, so how many HP you need to produce to move your bus, at whatever weight you carry, at whatever speed you choose. This chart was for an 80Klb tractor trailer running at 65 mph geared for 1500 rpm, the yellow dot, and needing around 200 hp to do so.
Now lets guess a bit. Your bus is lighter than a tractor trailer but has about the same drag. Lets pretend it takes the same 200 hp to cruise at 65 mph. It will need around twice the hp to run at 80 mph, so around 400 hp. That chart shows (and it's for a 1994 440hp 12.7 litre Series 60, as it happens) that you need around 1650 rpm to develop 400 hp, so if you want to run 80 mph, you should gear for 1650 or maybe 1700 rpm. If you can find out what the actual hp/mph requirement for a bus is, you might be able to tweak that. It's interesting to note that you get the best economy out of the engine by gearing it so you can run within a window of RPM and as close to full throttle as possible. Small engines working very hard will usually beat a big engine not working hard at all, in efficiency. But big engines have the ability to do a lot more work in a shorter period of time, accelerate to speed faster, etc. so it can come out in the wash.
Brian
According to his serial number he has a good engine that can be upgraded for more power W=12.7L K= DDEC 111 or lV B= premium built engine 28= bus engine 1991 or later FWIW
Something else to consider when you're going 80mph is your tires are rated for a maximum speed of 75.
Bob
Quote from: Swadian on June 26, 2017, 07:51:47 AM
I'm not going to do 80 mph all the time, only when it is legal and I deem it safe in my best judgment to do so. I want the ability to go 80 mph.
The vehicle weight is 35,100 lb, GVWR 48,000 lb. I don't tow and I don't have slides. Tires are 315/80R22.5, diameter 42.3". Engine is a Detroit Diesel 6067BK28 12.7L rated at 430hp. I understand fuel economy will be poor at high speeds, but I'm looking to save whatever fuel I can. Again, I usually only run flat routes.
save your 80 for 1%-2% grades...down hill..
But, do tell, where is 80 mph in a bus legal ?
Quote from: eagle19952 on June 26, 2017, 02:26:39 PM
save your 80 for 1%-2% grades...down hill..
But, do tell, where is 80 mph in a bus legal ?
In Nevada, South Dakota, and possibly Utah and Wyoming.
Usually, I'm at 75 mph; again, I would rarely actually go 80 mph. However, I'd like to save fuel at both 75 and 80.
Quote from: eagle19952 on June 26, 2017, 02:26:39 PM
save your 80 for 1%-2% grades...down hill..
But, do tell, where is 80 mph in a bus legal ?
From El Paso to Junction Tx on I 10 some of the turn pikes are 85 mph and they do run the speed limit it's scary to have a 50,000 bus pass you loaded with tourist when you are driving 70 to 75 mph
Hum I think this is the guy with Wang that is starting a bus line to run between Phoenix and Reno not for sure.I did see a gofund account for a start up bus line there in May ,he has a bus blog also if it is same guy real passionate about buses and bus travel.
Only occasional 80mph, I would gear with 3.90. 315/80-R22.5 are also 485rpm tires (overall diameter and rolling diameter are different). Then with 1450lb/ft torque and figure 40,000lbs (for future stuff) you'll be fine.
Startability is figured- first take your maximum weight and multiply it by 10.7 and put that into memory. Then take your maximum engine torque (on a manual would be your clutch engagement torque) 1450 multiply by first gear 3.51 multiply by torque converter ratio-which for a 1450 torque engine would be 1.62 (2450 maximum torque converter divided by engine torque) multiply by rear ratio of 3.9, multiply by tire rotation of 485 then divide by the memory weight factor and you'll get 36.4% startability! (Freightliner figured 22% startability for off road trucks and at least 18% startability for on road trucks). You'll have no problems. Good Luck, TomC
Allison won't approve 3:90 for a B500 he will need a outlaw to reprogram it because Allison will lock out 6th gear with that ratio for lubrication purposes if he goes there
I have a 99 DL3 with 12.7, and gear was changed to 3.73. It works great, no problems starting, changes to 6th gear at about 65 mph. It gets great mileage on the highway, and is our first coach out for long out of state trips, often running Biloxi trips from our bus shop in south Florida on a weekly basis.
Interestingly the actual in the field fuel mileage is not THAT much better, I see larger variances between different buses. On a standard coach with a 4.30 ratio, we average about 6.5 mpg on the highway, where this bus will get 6.8-7.5 regularly. What I do notice is that the weight will change the mileage more on this bus, when running light it gets MUCH better mileage, but on a trip loaded with people and LOTS of luggage, the mileage won't be much better.
The bus seems overall very happy with this set up, and I may change some more of them. It brings the shift point to a higher speed, but the engine tranny seem happy, they just think they are running slower road speed then they really are. When mountain climbing it just down shifts earlier, but is very happy climbing in 5th, or even 4th.
Quote from: luvrbus on June 26, 2017, 03:18:34 PM
Hum I think this is the guy with Wang that is starting a bus line to run between Phoenix and Reno not for sure.I did see a gofund account for a start up bus line there in May ,he has a bus blog also if it is same guy real passionate about buses and bus travel.
Mr. Wang is my friend. He's the one looking to buy a bus from Charlotte, but he can't find a mechanic to inspect it.
Quote from: AdamWalkup on June 27, 2017, 08:52:32 AM
I have a 99 DL3 with 12.7, and gear was changed to 3.73. It works great, no problems starting, changes to 6th gear at about 65 mph. It gets great mileage on the highway, and is our first coach out for long out of state trips, often running Biloxi trips from our bus shop in south Florida on a weekly basis.
Interestingly the actual in the field fuel mileage is not THAT much better, I see larger variances between different buses. On a standard coach with a 4.30 ratio, we average about 6.5 mpg on the highway, where this bus will get 6.8-7.5 regularly. What I do notice is that the weight will change the mileage more on this bus, when running light it gets MUCH better mileage, but on a trip loaded with people and LOTS of luggage, the mileage won't be much better.
The bus seems overall very happy with this set up, and I may change some more of them. It brings the shift point to a higher speed, but the engine tranny seem happy, they just think they are running slower road speed then they really are. When mountain climbing it just down shifts earlier, but is very happy climbing in 5th, or even 4th.
Considering this, I think I'm gonna replace the current 4.30 with a 3.73. Prevost had a demo X3-45 with 3.58. I'm not sure if I can go much higher (lower numerically) than 3.73 with this particular unit. I'd go all the way to 3.21 if I could.
you can go to 3.21, I have a 3.21 unit on the floor in our shop, and a friend has a 99 DL that was changed to 3.21. That ratio is way to high, and will hunt for 6th all the time, not only is it hard on the transmission, it is obnoxious for the riders feeling it shift on every overpass. With our 3.73 I think we are turning about 1350 at 70mph, which is close to perfect.
That Prevost he is talking about has a D13 Volvo engine 1100 rpm peak torque the most popular ratio is 3:58 to 3:91 for that bus around here in AZ with the generation V World 4000 transmission they don't hunt because the generation V controls the engine.
The ones with the 12 speed I shift use a 2:50 gear.I plan on using the Allison 10 speed if fuel goes back to 4 bucks a gal I already have the 10 speed Allison.From all the info I can find the 12.7 will yield you better fuel mileage set @ 510 hp so that is where I am at and will see if not back to 470 hp
Can I update the transmission to the latest Gen V to go to a higher gear without hunting?
Also, can I go any higher than 3.73 with the existing gear ratio without hunting?
I have no idea but you would need the lasted electronics on the engine,this is conversion RV board most of us are loaded and towing a trailer or vehicle traveling around 60 to 65 mph and you are a charter bus running as fast as you can from point A to B huge difference for revenue that I understand.I know 3:73 gears in a RV at the speeds we drive it will drive you nuts hunting between 5th and 6th gears BTDT
I agree, I run the 3.73 in one of our charter buses, because they cruise 95% of the time with cruise set on 70mph. In my motorhome I'm happy with the standard 4.30, since I tend to run a little slower, and the bus is so heavy, and I often have a 10K trailer behind. I would also not use the 3.73 if I was in a mountain area, but it does work wonderful on the flat lands of the south, in charter operations.
I believe you have confused me with Mr. Wang, but indeed, we are not towing. We will talk to a professional mechanic about putting in 3.73, 3.58, or 3.21.
Quote from: Swadian on June 28, 2017, 04:23:53 PM
I believe you have confused me with Mr. Wang, but indeed, we are not towing. We will talk to a professional mechanic about putting in 3.73, 3.58, or 3.21.
define professional ;D
Quote from: Swadian on June 26, 2017, 02:40:33 PM
I'd like to save fuel at both 75 and 80.
Sounds mutually exclusive to me since the best way to improve fuel mileage is to reduce the power required. The relationship between speed vs required power is not linear.
If a gear change does improve fuel mileage, its gonna take a LOT of miles to save enough to pay for the change.
Just sayin . . . . .
BTW, I can roll at 80 in mine, but 65 to 70 is much easier on me physically.
YMMV
Quote from: eagle19952 on June 28, 2017, 07:42:06 PM
define professional ;D
Someone who knows more about motorcoaches that you do.
Quote from: Swadian on June 29, 2017, 06:02:44 AM
Someone who knows more about motorcoaches that you do.
Good luck with that ! ;D
Hope you get better than my mom, when she asked if the car was fixed, the old mechanic said -- " It will be fine as long as nobody complains "
Sadly, I have found that my needs are so unique & specific that most mechanics can't relate or have different definitions for success than I do. So, I have to either fix it myself, or do without. :(
This is an engineering/design issue, a professional mechanic's training is to replace broken parts.
The mess a busnut gets into at the hands of "professional mechanics" is a major reason these Boards exist.
Re-design is NOT what they were trained to do.
You need a driveline engineer and/or those belt and suspender types who shoulda been an engineer...
happy coaching!
buswarrior
I'm wondering what the stopping distance is for a bus full of people and baggage doing 80 MPH? I think one would need rather more than the recommended 3 seconds of space behind the vehicle ahead. I've been in buses doing 80 or more (I still distinctly remember being in an old AEC Reliance coming down a long straight grade at almost 100 MPH), and while it's a thrill it hardly inspires confidence in the MO of the operating company! There's good reason that all the better buses in Mexico are strictly limited to 95 KMH (just under 60 MPH), and they all have TPMS and reinflation systems. A front blowout at 80 MPH on a heavily-loaded bus would not be pretty - hot road surface is a significant contributor to tire failure rates. Look what happened to the ACF Brills that used to run across Texas at 80 MPH - tire blowouts had very nasty consequences for them.
John
Axle ratios are very important to commercial vehicles that travel over 120,000mi a year. Just squeezing 1/10 of a mile per gallon more can mean an extra $700 in the pocket of the owner.
With our driving, it isn't that important. You must weigh the difference between changing the rear axle ratio, and the fuel savings.
I wanted to change the rear ratio on my truck from 3.55 to 2.7, but the cost was too much. Besides, I usually drive around 60, which with my gearing is 1690rpm. At 2100, I have a top cruise speed of just shy of 75. For bursts of speed, I can rev to 2300 for a top speed of 81, which is more than fast enough. Good Luck, TomC
i guess you missed my cryptic point :) there are plenty here whose advice is worthy. beyond that, taking the words and running them past the horse would serve you better.
you might should ask for a list of horses.
i'll start.
Allison.
Freightliner
MCI
Prevost
all have knowledge of what gears move what weight relative to wishful thinking.
Many companies have their vehicle speed governed, to save fuel, and for safety. Two that I have worked for: Swift at 60 mph. With 40 000 tractors (now that they have merged with Knight), that is a big saving in fuel cost. And Transportation Charter Services of Orange CA, with 20 buses also out of Calgary serving western Canada, at 68 mph.
JC
I found this old sheet from Allison.
I took a photo of the Gear ratio information, just in case it isn't totally irrelevant.
Never mind. Moment of idiocy even given this topic is not a strong suit. Apologies.
Quote from: daddysgirl on June 29, 2017, 10:22:17 AMI found this old sheet from Allison. I took a photo of the Gear ratio information, just in case it isn't totally irrelevant.
Never mind. Moment of idiocy even given this topic is not a strong suit. Apologies.
Actually ... in my (fairly worthless) opinion, this info is actually pretty valuable to have. Not everyone will need it or use it even if they do, but I think that it needs to be out there.
(Back to the long-time discussion of how much "forums" maybe used as reference in the future and how much , if any, we need to assure that info here is correct and useful.)
Quote from: buswarrior on June 29, 2017, 06:51:08 AM
This is an engineering/design issue, a professional mechanic's training is to replace broken parts.
The mess a busnut gets into at the hands of "professional mechanics" is a major reason these Boards exist.
Re-design is NOT what they were trained to do.
You need a driveline engineer and/or those belt and suspender types who shoulda been an engineer...
happy coaching!
buswarrior
Well said!
And people often confuse the two (especially in the UK and its infuriating).
Professional mechanic! kindly here retired, licensed, certified, government tested.
I worked directly under Engineers my whole career, specifically on their assigned fleets thereby myself being "the responsible one". Please dont forget the certifications required for the pay/"carrots".
When the million dollar projects shuts down, guess who is the gettr done person! And who is called. There are caculations needed and that my freinds is where engineers shine. I as the professional mechanic have to have the ability to understand the "parts" you mention yet easily rely of my talents to produce the product or machinery. Let me help you to understand my talents can move your project forward. Or you can call me in a month to ask for assistance in the same. Btdt
Please take this respectfully,
yet be clear of the understanding what a "Real professional mechanic is"
Or quit dam callin 8)
Im Floyd and off my soapbox
In engineering- this is how efficient gearing is worked out:
You look at the vehicle- specifically the engine-speed-load map and see where the vehicle will 'sit' during the typical vehicles duty cycle. The slide below shows a truck with a 13 litre engine. The star on the engine- speed load map shows where it would 'sit' at 60 mph and 1100 rpm. This particular 13 litre engine is most efficient between 1000 and 1100 rpm. However 'where it sits' will change depending on cross winds and road gradients and other road loads so these need to be factored in.
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fi273.photobucket.com%2Falbums%2Fjj229%2FMarquisRex%2FNavistar%2520Truck_zps2y4yuqsb.png&hash=29eb51dd8bea536dde9508f6a675cbf84ebba978) (http://s273.photobucket.com/user/MarquisRex/media/Navistar%20Truck_zps2y4yuqsb.png.html)
For a bus the duty cycle is different compared to a HD Truck:
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fi273.photobucket.com%2Falbums%2Fjj229%2FMarquisRex%2FBSFC_zpsbb541325.png&hash=a0238cfaa81987bacefd5e06cb26ae7326b3cf49) (http://s273.photobucket.com/user/MarquisRex/media/BSFC_zpsbb541325.png.html)
You will notice that the typical duty cycle for a bus is quite different to a line haul HD truck- both are still predominantly high load but the bus application is more scattered where as the line haul is focused or concentrated more along that 'central band'.
This is some calculations I did- comparing my MC5 duty-cycle-loading vs my Jaguar XJR (first pic is my bus and second is my Jag). I had to make some assumptions for frontal area and drag coefficient for the bus). I put both vehicles through the well known US FTP 75 Federal cycle.
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fi273.photobucket.com%2Falbums%2Fjj229%2FMarquisRex%2F2017-06-29_zpshofod0zv.png&hash=987754e54c87534f02abbfcc23f3e6bd43ae1ed3) (http://s273.photobucket.com/user/MarquisRex/media/2017-06-29_zpshofod0zv.png.html)
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fi273.photobucket.com%2Falbums%2Fjj229%2FMarquisRex%2F2017-06-29%25201_zpsp0peejjm.png&hash=c9cb01b6e56be6843f3622cbdfa486273e3980b4) (http://s273.photobucket.com/user/MarquisRex/media/2017-06-29%201_zpsp0peejjm.png.html)
You will notice that the XJR or almost any automotive application is relatively low load and engine speed- its running less than half its load capability. The bus, like the earlier plot is scattered all over the speed load map and predominantly high load. This is why cooling is critical on a commercial vehicle.
And finally- one needs to match the road load vehicle duty cycle to the engine characteristics.
This last plot shows an assortment of truck engine BSFC or engine fuel efficiency curves along the full load lug curve. The 8v71 is in there , against a Cummins engine, a Maxxforce 13 and a DT466 . The 8v71 more efficient point is higher up in the rev range for a HD engine. You need to rev the engine to get it to be efficient. This is predominantly due to the way the blower and scavenging is set up. It makes things complicated. I don't think the 8v92 is quite as 'peaky' as this.
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fi273.photobucket.com%2Falbums%2Fjj229%2FMarquisRex%2FBSFCs_zpsa068ddec.png&hash=60bc797c089f7ae00c2caf798c6b142f8ffb5314) (http://s273.photobucket.com/user/MarquisRex/media/BSFCs_zpsa068ddec.png.html)
I hope this helps
Quote from: Dreadnought on June 29, 2017, 08:11:09 PM
In engineering- this is how efficient gearing is worked out:
I hope this helps
:) good answer :)
Quote from: Oonrahnjay on June 29, 2017, 01:49:33 PM
Actually ... in my (fairly worthless) opinion, this info is actually pretty valuable to have. Not everyone will need it or use it even if they do, but I think that it needs to be out there.
(Back to the long-time discussion of how much "forums" maybe used as reference in the future and how much , if any, we need to assure that info here is correct and useful.)
Well, alrighty.
My moment of brainlessness maybe wasn't as bad as I thought. Here ya go.
When you do less than 5,000mi a year (like the vast majority of us) fuel mileage really isn't that important. I know if Ford still made their 1,000cu/in V-8 that they put in tanks in WWII, I would seriously consider using in my bus. Gasoline engines now are as efficient as Diesels (with variable cam timing, direct injection, etc)-the only difference is the BTU content of the fuel which makes Diesels still 20% more fuel efficient than gasoline.
I posted the BSFC chart for the Series 60 circa 1994 earlier, it shows similar info to what Dreadnaught posted as far as a central sweet spot for economy of that engine. I found the last chart posted very interesting - it shows how dramatically efficiency falls off if you try to run the engine too slow. BSFC really peaks high at low rpm, falls dramatically in the design range rpm and peaks again when running at higher rpm reflective of the increase in power you get as you run the engine faster.