4107 thoughts

[artvonne]

  I have less than half a bus length before the grade. I do not have 15 bus lengths. I may have to completely re-think all this. There would be no purpose to back up and re try. If it cant make it up the first time, more tries wont change anything. Either it can make a 20% grade or it cant. Sounds like it cant.

  Everything I have read and discussed over the years has been that 4107's could range between 9 and 11 mpg.  There are some posts/threads on this site that claim that, but maybe they are fudging numbers. 7-8 is pretty awful, and something else I will have to re-think carefully. I would be curious how you drove these, if you drove to reach best economy, or to make time. How fast did you generally run on the interstate? I remember when they were out on the road alot in my younger days, most were hard to keep up with as they were rolling 75-80 mph and some more than that. 

[RJ]

Art -

The company I worked for had the engines governed at 1900 rpm max, compared to the usual 2150.  This translated to a top speed of 70 mph.

However, every coach also had a "tattletale" (tachograph - google it if you don't know what they are), the drivers had to turn in the discs after every trip.  Those caught running over 65 mph for any length of time were disciplined and/or fired.  Period.

This was also back in the days of the idiotic 55 mph National Speed Limit, so the vast majority of runs were made at 60, give or take a couple mph.

I'm also in Central CA.  Every trip we take to anywhere, be it SF or LA, Tahoe or Vegas, requires us to climb 6% grades to get out of this valley.  70 miles of 4, 5 & 6% grades going to Reno, NV, on I-80 out of Sacramento, for example.  Or the 6% on the Grapevine (I-5) to LA.  That kills fuel mileage when you have to plug along in 2nd gear for miles on end.  From Barstow to Vegas on I-15, besides the notorious Baker Grade, there are miles of subtle grades where the coach won't pull it in fourth, but will sit easily on the governor in 3rd, again, killing the fuel mileage.

(Let me rephrase that last sentence - the bus would pull those subtle grades in 4th, but you'd spend a bunch of time sitting along the highway with an overheated engine to do so.)

That's my real-world revenue-service experience.  Those who operated these in the prairie states, where it's miles and miles of flat land as far as the eye can see, probably have different fuel mileage figures.

I do know a fellow who likes to say he gets 13 mpg with his 4905:  6 in town and 7 on the highway.  6 + 7 = 13 mpg!   

Be aware that a 4107/8, stripped to a shell, probably weighs more than your Bounder, fully loaded.  We're talking 20,000 lbs, empty/stripped.

Your driveway is really the biggest challenge you have.  If your Bounder has problems with it, the bus is going to be worse: heavier, lower ground clearance, manual gearbox, less low-end torque.   

Continue doing your homework.  If you can find a 4107/8 in your neighborhood, TomC's suggestion of bringing it by your house to try getting it up the driveway before buying one is a good idea.

Good Luck!

FWIW & HTH. . .

[artvonne]

  I spoke too soon maybe. I took several measurments to be able to give you guys a better picture.

  The steepest part of the drive is 23%, which occurs about 150 feet up from the bottom. Beginning down at the road I have about one bus length (40 feet) to accelerate from almost a dead stop before the grade starts, which increases to 18% within another 40 feet. (IOW, From a dead stop I will be on an 18% grade within two bus legths). It stays at 18% for about 50 feet and then increases to 23%. It is only 23% for about 50 feet farther, at which point the grade immediately levels off to 11% and remains at that grade for another 1400 feet up to my house. Backing down would not be fun. If I can keep accelerating up the 18% part of the grade, I think I should be able to carry enough momentum to clear the 23% part and then im home free. If you guys dont think it can make the 18% part, then its never going to make it over the hump.

  I know the 4107 is about 20k empty. What I dont know is what it will weigh rigged out. But I do plan to build it very light. I used to build pickup campers. Light ones had 3/4" x 1 1/2" lumber walls and cabinets with 1/8" ply. Heavy ones used 2x4's with 1/4 ply. The light 11 footers four people could carry, the heavy 9 footers 10 people couldnt pick up. Ive read of some bus conversions with the inside sheeted out with 1/4 ply. Oak cabinets. Granite counters and floors. Some of these no doubt exceed their original GVWR. I have planned to build this to keep weight down as low as possible.

[artvonne]

  7-8 mpg in mountains is pretty reasonable, I could live with that. The bounder does about 6-9 on level road, 5 and worse in heavy grades. Most of my travels will be midwestern flatland on US highways with littlle interstate. From what I have researched the 8-71 likes to run at around 1200-1400 to achieve best fuel economy and I would try to size the tires to run at that engine speed at about 55 mph. Does this sound reasonable?? I know a lot of the bus drivers drive foot to the floor, but I would probably nurse it up to speed as much as possible as long as I didnt have traffic behind me. Ive nursed over 50 mpg out of my 190 diesel Mercedes driving that way.

  My dream for over 10 years is to someday drive to Alaska. I dont know if it will ever happen, but if it does it would be awesome to do it in a bus. Using the toad to help push it up the driveway would make quite a difference and is a wonderful idea I hadnt thought of. Its a 4x4 Jeep Cherokee, so we could put it into low range and give it the boot. I think that problem may be solved??

 

[RJ]

Art -

GVWR is an issue primarily with stick 'n staple Winnibuggies.  Many exceed their ratings after the water tank's been filled, fuel's been topped off, and mom & dad have loaded up their "stuff" for the trip.  Often dangerously so.

Not the case with most bus conversions (except for the rolling whorehouse models with granite everywhere and fourteen slide-outs).  The vast majority of GMC 35-foot conversions weigh in between 25K - 27K ready to roll.  The coach has a 32K GVWR, so you've normally got 5 - 7K worth of margin.

If you're going to strip it to a shell, THE most important thing you can do is insulate it well - spray foam is the best, hands down.  Light weight and extremely efficient.  Why do you think every refer trailer pounding the Interstates are insulated with spray foam?  Slightly more expensive up front, but payback is much greater.  Oh, and it makes a HUGE difference in the noise level inside this big steel/aluminum drum.

Since you were involved with building camper shells, you know that 1/8" ply is NOT going to hold up as well as 1/4", especially for walls.  1/8" is commonly used here for ceilings, due to the greater flexibility w/o any real loading.  These beasts do flex going down the highway, the better you build it, the better it will withstand the road.

Based on my experience, I'd say an automatic-equipped GMC coach would struggle getting up your driveway with the info you've provided, but would make it - barely.  If the engine had a turbo, boosting hp to 350/370 hp over the stock 275, it would pull the hill better.

No go with a manual box though - they're geared too high.  And sorry, no other manual gearbox is available for a V-drive coach.  Turbo wouldn't help here, you don't have room to get the rpm up high enough for the boost to assist.  At least without smoking the clutch - a real no-no in a bus!   

Pushing with the toad might work, but it might also destroy the tow bar in the process.  And depending on how the hitch is mounted to the coach, it might do some strange things to the engine cradle, too - ouch!  (The bumper is NOT mounted to the coach frame - the coach doesn't have a frame!  The engine cradle, which the bumper is attached to, literally hangs from the roof of the bus.)

1200 - 1400 are the torque peaks for the 8V71, depending on whether it's standard or advanced timing.  This rpm range is not the "sweet spot" of the engine - it's the range that will get you into overheating trouble if you run for extended periods with a load on the engine. 

Quickly. 

VERY quickly with ambient air temps above 85^o or so.

A little GM engineering history here for you:  The powertrain of the GM highway coach, from the pre-WWII Yellow 719s and 743s on, was designed around tires that turn 495 revs per mile, with a rear axle ratio of 4.125:1 for the 35-foot models, and 4.375:1 for the later 40-footers.  Remember that magic number - 495.  It's important when you're sizing tires for a GM.

Six-cylinder models got a bevel gear ratio of 1:1, whereas the eight cylinder buses got an overdrive 0.808 with the manual.  With the 1:1 4th gear in the Spicer, the overall final drive ratio for the sixes is 4.125:1, the eights come in at either 3.333:1 (35') or 3.535:1 (40').

For the 4106/7/8 with the 8V71 / 4-spd manual, this results in 60 mph at 1650 rpm - just under the sweet spot of 1700.  The forty footers run 100 rpm higher at the same road speed, due to the lower rear axle ratio.

Remember, this is with tires that turn 495 revs per mile.  Buy taller tires (that turn less than 495), and you'll turn fewer engine rpm at the same road speed.  Buy shorter tires (turning more than 495 rpm), and you get the opposite effect.  The closer they are to 495, the closer you'll be to OEM performance.

For example, Bridgestone makes a drive tire (M726EL) that turns 470 rpm.  That tire will give you 65 mph at 1650 rpm - 5 mph faster than stock.  But you're lugging the engine if you try to run 55 mph with it - you're down at 1400.  Good way to burn up a motor.

HOWEVER -

That same tire, if the coach has been converted to a V-730 automatic, gives you 60 mph @ 1650 rpm - exactly the same as OEM with the manual box.  A lot of GM owners run the larger tire to compensate somewhat for the lower bevel gearing in the automatic.

Must add a caveat here:  The taller tires, while they'll help with the fuel mileage out on the highway, will also make it harder to get up your driveway - especially if you still have a stick shift!

One other point:  The two-stroke Detroit climbs grades best on a partial throttle between 1700 - 1900 rpm.  You should be able to accelerate somewhat if you add more throttle while in this range.  Black smoke means you've got your foot in it too far, back out.  If you do and your coach maintains it's speed, you were wasting fuel and creating unnecessary heat.  If you do and the coach starts slowing, it's time to grab the next lower gear.  Very simple.   Follow that guideline and it would be extremely rare to overheat a GMC (providing the cooling system has been maintained properly).

I've covered a lot with this post, might take a few minutes to digest.  TomC possibly can assist with the different torque and grade capability of the turbocharged 350 hp engine, compared to the naturally-aspirated 275 hp stock engine.  Just to give you some more data to ponder, of course.

FWIW & HTH. . .

[Ed Hackenbruch]

After reading all of this 2 thoughts come to mind.  It sounds like instead of trying to get up your driveway with a bus, you would be a lot better off to find a place close to home where you could store it/work on it.....or move!   Second, besides the other advice against pushing it with your jeep, if your driveway is paved you do not want to have your jeep in 4 wheel drive.....unless you want to start replacing transfer cases, etc.    Good luck with this, hope you can figure out something that will work out for you. Ps. i have a 5A with an 8V71 and an auto tranny and get between 7-8 mpg....mostly right around 7.25 mpg. I am in the western US.... Wa. in the summers and Az in the winters, so i have a few passes to go over no matter how i go.

[zubzub]

how often are you going to go up the drive?  If it's rarely I am leaning more towards a pull from your toad instead of a push.   If you will only be using it every so often might be worth the trouble.  It's not like the toad is pulling the whole weoght just helping to get it going.  Once i actually get mine going in the revs it feels pretty strong.

[buswarrior]

hmmm...?

Who be playing the busnuts?

happy coaching!
buswarrior

[steve wardwell]

23% + bus = no fun at all !  Although you must have one hell of a view)

[artvonne]

  Ive got to say im getting a bit bummed hearing some of this, I really would want to get the bus up here if its at all possible.  But I am a bit hard headed and dont take no for an answer real easy. So I have some more thoughts and have attempted to apply some math to the problem. This comes about from another discussion with a guy trying to get an MCI up an even steeper and longer driveway in NC with a switchback in the middle!

   I have a power curve chart for an 8-71 272 HP which shows torque to be about 760 lb ft at 1000 rpm. 4107 has an overdrive bevel gear at .808, a 4.28:1 first gear reduction, and a 4.125 final drive. I dont know what torque would be available at lower rpm, but can possibly extrapolate 700 lb ft at 800 rpm from looking at the graph. 11-20R tires show to be about 41.5 inches tall, for a radius of 1.72 feet. So I assume roughly 700 lb ft of torque from 800 - 1000 rpm. The bevel gears drops torque to 565 lb ft, first gear multiplies it 4.28 times to 2420 lb ft, the final drive multiplies it 4.125 times to 9985 lb ft at the axles. Divided by a 1.72  tire radius, this puts 5805 lb of force to the tire treads, providing they dont lose traction. Of course this all assumes the motor is producing rated power.

  On an 18% grade with the bus weighing 28000 (I'm pushing the numbers up for the worst case), I would have to provide (28000 x 18%) 5040 pounds of force to keep the bus from moving downhill. With 5800 pounds of traction it should keep moving upward if its already doing so. If my weight is lower I will have even more acceleration. If the engine speeds up it makes more torque, I will have even more acceleration. 
 
  Obviously this is marginal performance, and I dont know what the bus rigged out will weigh. You guys pegged it pretty close that it might not be possible, and the math shows it is truly marginal. So now to ask what other options are possible. I do NOT want an automatic. Is it possible to change the bevel gear to the earlier 1:1 ratio? Obviously the bus would be lower geared, but I would get an impressive jump in torque into the gearbox. That 5800 pounds of traction goes up to almost 7200 pounds with a 1:1 bevel. With that much tractive force the 23% part of the grade shouldnt even slow me down. (28000 x 23% =  6440). It wouldnt have the much top end but it would sure climb hills!

  I am well aware the bumper is fastened to the engine carriage, and would certainly strengthen it well before attempting to push it too hard, and the tow bar is well strong enough. I would hope that anyone pulling a load behind a bus (or anything for that matter) would have their hitch built to withstand the stress of pulling whatever they hook to it. Hitches should be rated higher than whats hooked to them, in all three axis. IOW, the hitch on the bus should be capable of withstanding forces in excess of the load. In any case, a 3800 pound jeep would never be able to muster more than its weight in traction, and on this grade I doubt it could provide more than 3000 pounds traction, and so could probably only provide a push of 500 to 1000 pounds to the bumper. That would probably be all id need to make it, but I would be much happier if the bus could pull its own weight up.

[TomC]

With the above figures, your startability would be 21.7%-not enough to maintain a 23% grade-sorry.  Good Luck, TomC

[pvcces]

Art, one of the reasons that you will find trouble trying to get peak mileage out of the Detroit two stroke is because they are not very efficient at light loads.

I have tried a full tank of fuel running 55 on I5 along the west coast; I got 9 mpg. We typically get right near 10 mpg running 60 to 65. However, when the speed is picked up to 73, our mileage only drops to 9.5 mpg.

A couple of times, we fought some serious headwinds, and I thought that the mileage would be really be bad, but it wasn't. When we went exploring in King's Canyon, driving anywhere from 35 to 50 and climbing and dropping thousands of feet, we got 8.5 mpg.

Driving slow, a four stroke could have done a lot better than our two stroke, but the efficiency while heavily loaded is probably close to a lot of four strokes. This is one of the reasons that you see repeated references about slamming the door on your hand before driving one of these old girls. Being shy with the throttle doesn't get you a whole lot.

However, if you can get it to the point that it is usually working just hard enough, you get pretty good results.

Our coach is a stock 4106 weighing in at a bit over 27,000 lbs. We climb a number of grades at 20% and a bit over. I believe that our coach would climb the driveway that you describe.

To do this, I would get the clutch out all the way as soon as possible at the bottom of the grade and floor the throttle as the speed came up to 15 miles per hour and I would keep it floored against the governor until the grade leveled out. If it didn't pick up enough speed before it hit the steep part of the grade, I would stop and creep back down.

If you don't have DD3s or spring brakes, I would not consider climbing the grade at all. If you messed up and stalled on the grade, the engine could start up backwards; it couldn't help you slow down while backing down the grade. A hand brake alone would not be safe.

Your mileage may vary. Use the above information with care.

Good luck!

Tom Caffrey

[artvonne]

  Tom

  I dont need to start out on a grade, I would start my run on almost level ground. I have about 40 or 50 feet to accelerate, then  it increases to 18% in a bus length. It holds at 185 for about 70 feet where it then increases to 23% for about another 40 feet.  Then it immediately drops to 11%. I may be able to rework the entrance to get more of a run, but probably the best I can hope for is another 40 or 50 feet of run before the grade starts.

  How fast can a bus like this accelerate in 50 feet on level ground? If it can make 10 mph dont you think id have enough momentum and power to get it up over that hump?

  A far more radical alternative is to reslope the entire driveway, then it would be about 14% all the way. But the magnitude of work would be enormous.

  Possible to put a two speed axle in one of these? What about the 4104 bevel gears? Does the 5 speed spicer have a lower first gear? Is there a lower (higher numerically) axle ratio? Any other possibilities??

[artvonne]

If you don't have DD3s or spring brakes, I would not consider climbing the grade at all. If you messed up and stalled on the grade, the engine could start up backwards; it couldn't help you slow down while backing down the grade. A hand brake alone would not be safe.

Your mileage may vary. Use the above information with care.

Good luck!

Tom Caffrey

Now there is a fun thought, full throttle reverse backing down a steep driveway. 

[RoyJ]

Artvonne,

The only thing you left out in your equation, is a 15% drivetrain loss for a typical manual transmission. Your 5800 lb tractive effort has just gone down to 5220 lbs  The 23% part would definitely be very difficult! This is assuming you're at sea level; if you're high up, it only gets worse...

Is your Jeep a 4.0L? If so, it should make enough power to spin all 4 tires in 4Lo. Tires have around .9 coefficient in the radial direction, so 3000 lbs sound about right. But I'm confused how you got a 500 - 1000 lbs net thrust? Wouldn't 3000 subtract (3700 x 23%) be arund 2200 lbs?

Regarding a 5 speed Spicer, I'm not too sure what GM uses, but the 5 spd Spicer in my Prevost has a 6.1 (or 6.6 maybe) 1st gear. As a result, there're rarely any hills my bus can't climb.

Another solution is to change out the tall tires to a low profile 22.5, such as 275/70, which is 38" compared to 42.5" for a 11r20 tire (same as 12r22.5). Nets you roughly 11% increase in thrust, but that 23% section would still be hairy!