Lugging the engine...

[bevans6]

I understand the 8V-71 needs to be run at a decent rpm if you are trying to pull power out of it, accelerate, run up hills, and so on.  What about the situation where you are kind of drifting along a nice road at 45 - 50 mph?  Governed speed in third is about 50 - 55 mph.  Do you run at low throttle and high rpms in third, or do you shift into fourth, again cruising at very low throttle?  What's best for the engine and for fuel economy?

Brian

[TomC]

On the 8V-71, buses either used 55 injectors (for 255hp and 680lb/ft torque) or 60 injectors (for 280hp and 750lb/ft torque).  The 55 injectors were called economy injectors, and the 60 injectors were called combination injectors, 65's were called performance injectors.  All 71 engines with either 55 or 60 injectors had their maximum torque ratings at a low 1,200rpm-which means you could run the engine 100rpm lower then that and not hurt it. Whereas 71 series with 65 or 70 injectors (non turbocharged) had their torque ratings at 1,600rpm and were intended to be run with at least a 9 or 10 spd so you could keep the rpm's up.
Detroit made generators with both 1,800rpm and 1,200rpm settings in the 71 series.  There is a 12V-71N on Youtube showing it running continuously at 1,200rpm for generating.
With a 4 spd manual, it is advised to just rev it to the governor then shift.  Making note of the speed at which to shift, is where you should also down shift.  To be more vigilant about your engine installing a tachometer will be the most accurate way of engine monitoring.  After I installed my tachometer, I was amazed that my Allison V730 on light acceleration would lockup its' torque converter in second gear and allow the engine to go down as low as 1,100rpm.  It was very telling.  Good Luck, TomC

[Len Silva]

As long as you can step on the throttle and get a response, you are not lugging it.  If you are going so slow that you can step on it and the coach doesn't gain any speed then you should be in a lower gear.

[bevans6]

Ok, but I still don't understand.  The situation is flat road, no wind, light load, I am cruising at minimum throttle, at say 80% of the governed speed in third gear.  I am not trying to accelerate, and acceleration would be lazy in fourth, but I'm not trying to accelerate.  do I leave it at say 1800 - 1900 rpm in third for an hour or two, or do I shift to fourth?  All my four stroke instincts say shift to fourth and cruise at the lower rpm.

Thanks. Brian

[bcaddel]

With our MC7, 8V71, 5 speed automatic, governed at 2350 we often run it in 4th gear and 2300 rpm and 59mph. I drove from Texas to Nevada right on the governor 8 hours a day and the temp seemed to stay in line, MPG was around 6.5 and ran like a top. Now that we have put a few more miles on her (15,000 this year), we are experimenting with going into 5th gear, 1900 rpm around 64 mph. The MPG seem to be about the same or a little better, and we are traveling 5 miles per hour faster, but it is running a little hotter.

Most of what I read says to keep it under 2000 rpm, but my son-in-law who has been a Detroit diesel mechanic for over 20 years keeps telling me to put my foot in it and keep it rev'ed up 2200 to 2300. It doesn't seem to hurt it any and it runs cooler when it is revved up.

I think if you want it to run cooler leave in in the lower gear and revved up, it you shift it into a higher gear and reduce the rpm as mentioned previously just make sure when you step on the throttle it picks up speed right away and that indicates you am not lugging it. The higher gear/lower rpm seems to produce a little better MPG.

[luvrbus]

Brian, you are not going to do any damage to your engine running it a 1200 rpm vs 1800 that is a old wife's tale about running a 2 stroke against the governor all the time it does not hurt to run the engine out of it's torque curve.
Like Len said if you are in 4th and it will pickup speed go for it and save a little fuel.
A non turbo 8v71 will blow black smoke and heat up when it is lugging go with you instincts it's a diesel engine ju800st like the 4 strokes.  
FWIW there was 1000's of 8v71 and 8v92 fuel squeezers engines built that would only turn 1950 rpm max no load and that would give you about 1800 rpm loaded




good luck

[TomC]

Any of the 8V-71's with either 55 or 60 injectors were made to have your foot in it all day.  With these engine, there is no such thing as lugging it, unless you're way down below 1,100rpm.  Partial throttle at lower RPM's is perfectly acceptable-in fact-you should run the engine as low as you can without the engine putting out black smoke.
When I had my 8V-92TA in my truck, Delaney and Ahlf in Bakersfield recommended to up shift at 1800rpm and down shift at 1400rpm for maximum fuel economy.  I ran with two other trucks-one with a Cummins Big Cam 400 and the other with a Caterpillar 3406B 425hp. All three of use were grossing the same-right at 80,000lb, and I out pulled them by about 3-5mph on all grades by driving in the economy range.  Just drive old 2 stroker Detroits-they're extremely hard to hurt.  Good Luck, TomC

[Hi yo silver]

Yeah, just drive it where it sounds right, and keep one eye cocked on the temperature gauge, as my dad would say.  Don't worry, be happy.  My wife swears I always have a silly grin on my face while I'm driving the bus.  LOL
Dennis

[bevans6]

Thanks guys!  Now I get it...   

Brian

[Fredward]

Brian,
The previous replies are of a more scientific nature than mine is going to be. I have an 8V71 4 speed spicer in an MC-5 with N65 injectors. So as Tom says, the injectors are a little big. I also am a four stroke guy; accustomed to staying between 1200 and 1600 RPM. So I naturally tend to shift up when its not loaded heavily and run at lower rpm.

But my experience with over 20,000 miles on the bus is to choose the lower gear/higher RPM combination. By that I mean 1900 - 2000 rpm; not wide open (2100 rpm on mine) It seems to use less oil when I drive it that way and it sounds better. I feel like these engines were designed to work also in a stationary application which would have meant running at or near governed setting all day every day. THats the way we run our farm tractors - right at or close to the governed wide open setting all day long. And those are four strokes.

The 2 cycle Detroit has more air and more fuel and more oil moving through it at 2000 rpm than at 1400 rpm. I'm guessing it relies on all those for cooling and longevity. Its just the opposite of what we learned driving four stroke trucks.

Fred

[gus]

What Len said and what you said, essentially the same thing!!

I do this all the time, the engine isn't working at all.

[HB of CJ]

Another way to think about it is to imagine or estimate how much load and fuel you are giving the engine.  If you are "floating" (old railroad term) you can upshift to the next higher gear and reduce the rpm.....as long as the mill is under very light load.

A long time ago, we drove Fire Engines with hopped up 8V71s that had N90 injectors with a 2500 rpm governed limit.  Even with those powerplants, we could and did routinelly run as low as 1200 rpm.  The key was throttle control.

Think of it as almost but not quite coasting down a very slight hill and you get the idea.  Less fuel means less heat, less load on the enternals.  Get a feel for the coach, look out the mirror for ANY smoke (black) then downshift if needed.

If a downshift is NOT needed, then you can feel comfy running your Detroit at 40 to 45 mph in high gear (4th) AS LONG as you don't accelerate or bury your foot in the pedal.  Listen and learn to love your Detroit baby....she will love you.

However...and this is important, if you are accelerating or pulling a grade under heavy throttle, then your mighty mill will love you best at 2000 rpm...or more.  You will NOT hurt it with high revs AND throttle.  Built for it.  Enjoy.  HB of CJ (old coot)

[TomC]

To compare piston speed from a 2 stroke engine to say a Series 60- both the 71 and 92 series has a 5" piston stroke.  The Series 60 has a 6.3" piston stroke. So if you're "twisting" your 2 stroke engine at 2100rpm, that translates into an rpm as far as piston speed of 1,666rpm for the Series 60.  On the other hand if you twist the Series 60 up to 2100rpm, that would translate to a 2 stroke piston speed of 2,646rpm.  My point-at 2100rpm on a 2 stroke, you can run all day at that speed and not hurt it.  Course as with any engine, the lower the rpm you can get the horsepower out of an engine, the better for fuel mileage since you'll have less friction at the lower rpm's.  Just something to think about.  Good Luck,TomC

[lostagain]

 When I started for Brewster's in '73, we were taught to drive on the governor, and always downshift when you reach the governed speed of the next lower gear, no matter if you're going uphill, downhill or on the flat. (We didn't have Jakes then). 2 stroke DDs are designed and built to run on the governor. You get better oil circulation and pressure, and better cooling. When I overhauled my 4-71 last winter, I was impressed by how big and overbuilt everything is: crank, rods, bearings, etc. That made me feel even better about running it hard. It is governed at 2500 rpm, and I drive it with my foot on the floor at 60mph for hours and hours on the highway. It is happy that way! I drive it on the governor around town too. It was meant that way, and sounds better

It took a while to get used to the Series 60 in the hockey  bus, and not go against the governor.

JC

[luvrbus]

You guys drive the 2 strokes any way you want it is your engine but they were never design to run against the governor all the time and if you think you are building more torque running wide open you need to check the torque will fall off at higher rpms   



good luck