8v71 rpm

[Dietrichfarms]

I have a 78 mci cruisader with 92 k original miles on the bus and engine . PO told me he drove it at 2800-2900 rpms "be sure not to lug it he said". Unit has 6 sp Allison auto transmission. I have always hear 2100 as max rpm for 8v71. I have had it run up to 2600 once while I wasn't paying attention never hit the govenor.  Drove bus home from Oregon to texas 2100 rpm runs out to 63-64 mph. Just need to know if need to run harder, check, govenor, or keep driving on!

[bevans6]

The highest no-load governed speed should be around 2450, that is what my military 8V-71T is set at.  That gives you a loaded speed of around 2300 rpm as the governed maximum.  2100 no-load was common for bus engines.  I would question your tachometer first, and then look further.  2100 rpm at 64 mph with a six speed sounds odd, what ratio is your sixth gear?

Brian

[Boomer]

I wouldn't recommend continual use over 2100, especially with Jakes activated.  Could you tell us who purchased the bus new or a little history on it?  Rare to see one with so few miles.  Not too many of the 6 speeds around either.  Like Brian said, you might check the rpm with a hand held tach to verify the actual no load.

Boomer

[luvrbus]

It will live a short life at 2800 that is a marine setting the truckers some ran 2800 and they didn't last, 2500 rpm no load is ok that where I set most for a little extra in the gear range ? is the the transmission a HT 70  or electronic

[RJ]

NAME?? -

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Now, on to you question, and one from me:

Most shops set the 8V71 in a bus for 2150 no-load, which translates to 2100 loaded.  2450 is considered the setting for "fire apparatus" and military operations.  You can set yours to 2450 if you like, but it shortens the lifespan.

Like Brian, I'd question the tach first.  Might want to get that calibrated.

64 mph @ 2100 tells me one of two things - either the instrumentation is off (use a gps to confirm speedo), or the coach has the optional 4.11:1 rear axle in it.  99% of that vintage MCIs had a 3.70:1 ratio, they'll run 70+.  Some operators ordered the lower ratio because most of their work was mountainous.  Easiest way to figure out what you've got is to get one rear wheel set off the ground and count driveshaft revolutions.

A 1978 MC-8 with only 92K seems odd to me, also. . . unless it's a conversion coach that was converted from a new shell and never saw revenue service.  Unless you've got physical paperwork evidence to support the low mileage from new, I'd suspect that the speedometer or hub-meter have been changed - possibly the last time the powertrain was rebuilt, which wasn't uncommon.

A six-speed sounds odd also in this vintage coach.  Many newbies unfamiliar with buses mistake the torque converter locking up as a gear shift, when in reality it isn't.  What does it say on the shift tower?  1-2-3-4-D-N-R?

Again, welcome aboard, there's a wealth of information here, not to mention some colorful characters!

FWIW & HTH. . .

[luvrbus]

2150 rpm no load won't give you 2100 rpm loaded fwiw it takes 2250 no load for 2100 rpm and fwiw the 53 series can go up to 3200 rpm with 25:1 compression  2500 no load will not hurt a 71 series

[Boomer]

It could have the 6 speed hydro, some of those found their way into 8's.

[TomC]

Yes you could twist the 8V-71 up to 2,800. The other question you should ask is if your bus has a gear driven Delco 50DN alternator. The 50DN is rated to 6,500rpm continuous and 7,000rpm intermittant. At a 1:2.75 geared overdrive, that works out to be 2,363rpm continuous and 2,545rpm intermittant. I have my engine set at 2,500 no load. I have cruised at 2,100rpm when I was in a hurry (66mph). But usually cruise around 1,800-1,900.

In commercial fisherman use, 1,800rpm was maximum continuous and most ran the engine around 1,500rpm-where the engine lasted a very long time. Faster you run the engine, the faster it wears out. Good Luck, TomC

[luvrbus]

The extra rpms are nice when you need those no one is telling you to run at top governed speed I see you ran at 2100 rpm on the way home that is good

[Dietrichfarms]

I bought this bus in Yreka Ca, the original owner purchased new and had excecutive motor coach corp convert. Bus was owned most of its life by Stedham Trucking in northern Ca used for personal use only. Correction to original post frame has 192 k on it engine complete rebuild with all new parts, crank, cams jugs everything new. Have the parts list and numbers on the rebuild. Only 92k since then. Transmission is a 750 Allison 6 speed auto.
Bus serial # S13296
Owner Daniel Dietrich
Blanket, Texas
Just finishing interior redo, new led lighting thru out, new blinds, wood floors and granite counters. Ready to hit the road!
I will check speedo against gps next trip out.
I usually run her 185-1900 and enjoy the ride. Plus she gets 8 mpg there.

[bevans6]

If you have normal sized tires (typical 12R-22.5) and the normal 3.7 differential ratio, 1800 rpm gets you 60 mph.  2100 rpm is 70 mph.  2400 is 80 mph, and 2700 rpm is 90 mph.  300 rpm per 10 mph, basically.  The HT750 Allison is a 4 speed auto with locking torque converter, and a 1:1 top gear ratio.

Brian

[luvrbus]

The HT 750 is a 5 speed transmission fwiw

[bevans6]

Indeed it is a 5 speed, typo on my part, I was focused on the top gear ratio being 1:1 and not an overdrive.

Brian

[Lin]

we also keep the no load set at 2500 rpm.  I don't run it that way, but the power is there if needed once in a while.

[Boomer]

I have met Larry Stidham (nice guy) and know the bus.  I don't think he ran it much and it had good care.  Nice looking rig.