Transmission stuff-lifted from "MC-9 / 8V71 Woes"

[buswarrior]

Let's pull this out here where we can continue without dragging the original thread way off topic.

I'll post the related bits and then we can continue.

happy coaching!
buswarrior

[buswarrior]

Lin typed:

John and RJ,

I would like some clarification on this lugging thing.  My 8v71/Spicer combo when running 2000 rpm in 1st, meshes perfectly to about 1100 rpm in 2nd.  Hence, it would seem that one must run at lower rpm when building speed.  I don't remember exactly now, but the rpm differential becomes narrower as you climb the gears.  2000 rpm in 3rd is just over 1400 rpm in 4th.  So I guess the point is that one should not try to cruise or climb below 1600rpm in a 2 stroke.  Am I missing something here.



RJ makes some words:


BrianS -

If you have a HT-740, it is a four-speed gearbox, not a five.  What you're experiencing as a "shift" is, in reality, the torque converter locking up.

IIRC, the sequence is 1C - 2C - 2L - 3L - 4L, where C = converter unlocked and L = Locked converter.  (I may have the lock-up sequence wrong, if so, me bad!)

Running around town, if you just leave the shifter in 3rd, you'll avoid a lot of the hunting back 'n forth between gears.  Obviously, once you hit the highway, you upshift it to D.


Belfert -

IIRC, you've got a S-60 mated to your B500, right?  The torque curve on the 60s is vastly different than the two-strokes, so it can be lugged down a lot lower rpm-wise before downshifts are necessary.

That being said, however, it is still advantageous to downshift manually when pulling grades to keep the engine near the top peak of the torque curve.  By doing so, you also keep it in a range that helps cooling, too.


Mike (Scarecrow) -

Change the oil to Straight 40wt, CF-2, 1% or less sulfated ash.  Chevron Delo 100, Shell Rotella T, Exxon XD-3 and Texaco Ursa are three common and popular brands.  Available thru a local petroleum distributor, comes six gallon jugs to a case, in five gallon pails, or 55 gallon drums.  Takes approximately seven gallons for a complete oil/filter change on your MCI.

Don't fire up the engine simply to "warm it up" without driving it.  Great way to build up sludge & gunk.  Far better to simply start it, and as soon as the air pressure is up to 120, drive it off.  And then drive it enough to bring everything up to operating temps.

Not sure where you're located (hint - put your city/state in your signature line), but yes, a several hour run to Quartzsite the end of next month at 65 - 70 mph will blow a lot of the gunk out, especially with the correct oil in the 8V.  By the time you get to Q, it may be running nice and clean, so you may not need as much hand-holding!



JohnEd -

Entertaining and informative post, and, regarding the Big Buck Dudes and their Prevosts, probably "right on".

Greyhound was the first non-transit bus company to really put the Allisons to the test in revenue service, back in the early '70's.  Once they found out that the slight fuel mileage penalty was less than the cost of rebuilding clutches, the handwriting was on the wall.

Sadly, there's a whole generation of drivers out there that have no clue how to drive a stick-shift coach, they just stab 'n steer.  And even then, they have no clue about manually shifting the automatic for certain conditions.  Sad. . .

Time marches on, I guess.

FWIW & HTH. . .

Wink



RJ makes some more words:



Lin -

First, the gear ratios in the MCI Spicer are slightly different than the Spicers in the GMCs, as is the rear axle ratio, but the powertrain engineer's results are pretty close to the same for each.

In stock "bus" trim, the 8V71 was set up for 275 hp @ 2100 rpm and 770 ftlbs of torque @ 1200 rpm.  Standard cam timing, N60 injectors.  GM Spicer transmission 4th ratio 1:1, bevel gear ratio 0.808, rear axle ratio 4.125:1 (overall = 3.333:1)  In the case of the 4106, 4107 and 4108 models, they were set up for 60 mph @ 1650 rpm using tires that turn 495 revs per mile.

Somewhere I have a HP/Torque graph for this set up, not sure if it's electronic or hard copy buried in a binder.  Anyway, IIRC, the torque builds rather quickly, peaking at 1200, then staying fairly flat until about 1800, where it begins to fall off, but gets "passed" (if you will) by the HP curve, which peaks at 2100.

So, the old adage applies:  Torque gets you going, HP gets you top speed.

Now, to translate that into real world language, yes, you start at the lower end of the rpm/torque range as you're building speed, and at the upper end of the rpm/torque/hp curve to maintain road speed.

Wish I could find that graph, but IIRC, there's a "sweet spot" around 1700 -1800 or so where the torque curve and the HP curve meet/cross.  This is the point where the engine just hums along happily all day long, running on the flat or pulling a grade.

Have you noticed, by chance, that the simple four-speed in our coaches really is well suited for our (relatively) light-weight vehicle, based on it's original design criteria (revenue service)?  First gets you going.  Second is used for those city blocks and residential neighborhoods where the speed limit is 30 or under.  Third is perfect for the suburban four lane routes with 40 - 45 mph speed limits, and finally, fourth gets you comfortable on the interstate.  And that "tall" reverse?  Hey, 99.9% of the time, it was used for backing out of a flat stall at the local bus depot, not trying to wiggle into an uphill campsite at Oh Dark Thirty.

A final thought on lugging.  With the two-stroke Detroit, the general consensus is that if you're pulling a grade and you see black smoke out the exhaust, back out of the throttle.  If you can still maintain your road speed, you're ok.  But if you back off the throttle to clear the smoke, and your road speed starts falling off, then it's time to downshift.  You want to keep the engine in that "sweet spot" on a partial throttle, where if you step on it, you'll get some acceleration, but it will still pull w/o having it floored & smoking black.

(The problem with the non-electronic Allisons, btw, is that when driven w/o manual control by the driver while pulling a grade, they will often downshift too late, bringing the engine so low on the torque curve that it cannot recover, and another downshift becomes a necessity.  And at that point, should it not downshift again, guaranteed you're going to have an overheated engine.  I believe the DDEC/ATEC combination is smarter, perhaps Buswarrior can elaborate more on this.)

Clear as mud?

FWIW & HTH. . .




buswarrior typed: 


I suppose from this point in history looking back...

The Allisons, 740/730 etc and their controls were very simple, mostly/all mechanical, some/most/all having no decision making capacity. The revs reached a certain value, or with an ATEC, some consideration for throttle position, and it shifted, whether that was good for anyone or anything or not. And they all shifted differently.

Now, put into perspective that the alternative was a 4 or 5 speed stick shift.

All of them mated to an excellent, for its day, engine, which did not know enough to meter the fuel as the load increased, it just poured the coals on like its mechanical bits were designed to do, all the way to suicide, if allowed. Again, no decision making.

And let's stop kidding ourselves, drivers then were no different than drivers today. Get away with whatever you can, and skills were all over the map. Some with skills and pride, some that should have been put in jail. The only difference is we have more time watching from then to today to get jaded and more time to forget the details and romanticize the bits we liked.

In revenue service, nobody cares, just get the thing down the road. Did it go far enough between breakdowns that we are still profitable? And the profit in the transportation business was a lot fatter in those days, so more abuse could be afforded... perhaps a reason these things are more thought of today?

Anyhow... leaving a 740 to downshift on its own climbing a hill has a bad habit of producing a much harsher shift or bang, which is less than nice to the passengers. And as RJ noted, by waiting for a deeper RPM drop before shifting, the RPM in the next lower gear is already down from peak HP. Your terminal velocity going over the top may be slower.

The Allison did at least downshift, which made it impossible for a careless or unskilled driver to refuse to downshift and damage the engine with huge piston temps as it lugged down. In that regard alone, the Allison paid for itself. Even at every corner, making the turn on the roll and carelessly leaving it in the now too high a gear that was being used on the approach and forcing the engine to smoke its way out of sub idle rpm takes its toll.

I am always suspicious of readings on dashboard gauges, and put little stock in their info. In fleet service, a matched set of 10 coaches will show you different RPM and speeds and temps and pressures for any given condition. and, your automobile is the same, the gauges are all over the map, you just happen to own one example.

Who has the gear ratios for the Spicer 4 and 5 speed transmissions? With those, we can work out the relative readings you want to see on your gauges, if they are at least measuring consistently, if not accurately.  IIRC, the 5 speed would drop from someplace 2100 to 1500 on each shift, but that was long ago, before I paid proper attention to these things.

happy coaching!
buswarrior


bryanhes types:

BW,

Here are the gear ratios on a Spicer 4-speed Model 7145VK.

Angle Drive Gears- 0.808 to 1
1st- 4.28 to 1
2nd- 2.50 to 1
3rd- 1.50 to 1
4th- 1.00 to 1
Reverse- 3.56 to 1

This is out of my maintenance manual X-6814.

HTH,
Bryan


Lin types:


I must say that I disagree about the 4 speed Spicer being a good application to even old Greyhound Service-- not in mountain/hill country anyway.  I think of it as another initial cost compromise made at the expense of good design.  True, the high reverse was okay for getting out of bus stations, but really I doubt that they had any reason to need to back up at 15 mph.  For hills, they really should have had a granny gear for starting on grades.  Imagine being in stop-and-go traffic on the Grapevine due to an accident up the road.  What would be the MPC (miles per clutch).  I realize that the pros could handle these things a lot better than I, but the physics of getting started on a 6% to 8% grade with a 4.25 gear ratio is just not friendly.  Having once got stuck on a hill that my 8v71 just could not handle with that gear ratio, I am particularly paranoid about it happening again.  I see it as a 5 speed being about ideal with the extra gear being below the current 1st.  The other forward gears could be exactly the same.  Reverse could about match the new first.  Thanks


buswarrior types:


Ok, with these GM transmission ratios,

The 1-2 shift is a 41.6% change, so drops from 2100 to 1226
The 2-3 shift is a 40% change, so drops from 2100 to 1260
The 3-4 shift is a 33.3% change, so drops from 2100 to 1400

So, there is effectively no overlap of gears for a given road speed, except a bunch of mph between 3 and 4. Just like the owners of these coaches tell us!

Or, said another way, road speed dictates which gear you have to use.

And this also shows what might be wrong if you advance time/bigger injector an 8V71 in a GM, moving peak torque up to 1400 rpm from standard timed 1200 RPM, you drop the engine below the power on shifts 1-2 and 2-3, as well as the bottom of all 4 gears is in the toilet a lot longer, road speed wise.

You may have also heard GM owners discussing hill climbing, particularly the 4104 owners, comment they have to give up on 3rd gear, and drop road speed way back to grab 2nd and plow up the hill against the governor with lots of available power, but not enough power to pull with 3rd. They would love to have a gear in the middle, and the numbers show that.

Keeping the technology in perspective, remember that the multi-speed transmissions of the same day required multiple stick shifts to control the auxiliary boxes, and all manner of wild shift patterns and order of using which stick into which position.

The simple 4 speed would have been well regarded, and cut a wide swath across the drivability needs of a line run motorcoach. And these auxiliaries had reliability issues.. the old men I have known who drove those, may remember those days fondly, but do not want to drive those beats anymore!

Once you have equipment, there is a business need to observe the inertia of "run what you know", keep spec'ing the same thing.  There was the bonus of the higher HP of the 8V71 helping hide the wide gear spacing, and the driver's had become familiar with the 4 speed and what to do to stay employed.

Now, fast forward to today's technology, the 18 speed transmission of today (and the top 8 of a 13speed) are spaced  approximately 17% apart, giving a trucker a lovely amount of choice as to which gear to use for a given road speed. Under a 2100 rpm downshift, the 18 speed would only drop 350 rpm or so.

Has anyone looked at whether any of those lovely 6 speed transmissions in the medium duty trucks would fit behind the bevel gears on a GM?

And who has the ratio for the 5 speed, so we can show the incremental improvement over time?

happy coaching!
buswarrior

[buswarrior]

So, may we have the ratios for the MCI 4 speed Spicer, and for the 5 speed Spicer?

Thanks!

happy coaching!
buswarrior

[DaveG]

Boy, nice work BW!

[Lin]

MC 5a Spicer:
1st=4.25
2nd=2.31
3rd=1.41
4th=1.00
rev=4.34

[bevans6]

My 5C spicer, per the book:

1st - 4.30
2nd - 2.33
3rd - 1.36
4th - 1.00
rev - 6.00

differential - 3.70

So first is within a spit, as is second, third is a little taller and so closer to fourth.  I calculate the rev drops to 1554 from 2100 on a 3/4 shift.  I also calculate speeds in gears at 2100 as follows:

1st - 16 mph
2nd - 29 mph
3rd - 50 mph
4th - 68.5 mph
(using a 40 inch tire diameter and a formula I found somewhere...)

Interesting that the GM has an overall gear ratio of 3.33, which is 11% taller.  that would make a difference starting on a hill in first gear, I would think, and the rev's are lower at highway speeds  as well, about 200 rpm at 60 mph.