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Bus Discussion => Bus Topics ( click here for quick start! ) => Topic started by: Rick 74 MC-8 on December 30, 2012, 07:59:40 AM

Title: 12v71 & lugging
Post by: Rick 74 MC-8 on December 30, 2012, 07:59:40 AM
My question if lugging is defined as stepping on the throttle and not increasing rpm.  Could the 12v71 with that much torque at low rpm be geared  down to a cruising speed RPM much like 4 strokes without hurting itself?   I'm not thinking of changing to one just curious

                                              Rick 74 MC-8.  8V 71na

Title: Re: 12v71 & lugging
Post by: Utahclaimjumper on December 30, 2012, 08:47:06 AM
All engines reguardless of torque or size have there horsepower & torque numbers rated at an optimum RPM,, operating much out of that RPM range will change those numbers respectivly.>>>Dan

Title: Re: 12v71 & lugging
Post by: TomC on December 30, 2012, 09:07:16 AM
Non turbo Detroit 2 stroke engines (NA) have definite operating rpms by how the engine is tuned and what size injector is used. For instance-crank up the 8V-71 (or 12V-71) with fast gear ratio on the blower using N70 injectors, and your rpm range will be between 1800-2300. Go down to the more normal 65 injectors and the torque will be produced at 1600rpm. The rule of thumb is you can safely go 100rpm lower then the torque rpm. Use 60 or 55 injectors (like what most buses came with) and you have torque produced at 1200rpm-to accommodate only a 4spd transmission.

On my 8V-71 turbo and air to air intercooled, I have 7G75 injectors and I had Don Fairchild time them for optimum fuel economy at 1800rpm. Hence pulling a hill, at 1800 I have no smoke. Get up towards 2000 or down to 1600, it will start to smoke. A very narrow rpm band.
Compare that to the new DD Detroit engines that have their horsepower rated at 1800 with 2050 being governor, and 1100 being torque with easily taking to 950 if you want. Many of the trucks now, I put electronic governors for 1650 max full throttle and stopping at 1750. The new engines like to be run slow-the slower the better (around 1100).

Title: Re: 12v71 & lugging
Post by: wg4t50 on December 30, 2012, 08:00:44 PM
My opinion from Ricks question, I keep the rpm above 1700 when driving, but the lower rpm torque is impressive such as taking an exit loop and driving in direct, just throttle down to about 600 rpm in turn, upon exit just push down on throttle, it will pickup very nice, I do not recommend this as SOP, just a comment when I had other issues going at the time.  Was always amazed how easy it was. One other funny, in 2nd gear, idling around, when motioned to speed it up in a parking lot while forming up for a convoy, I spun the drive wheels, was a total surprise.  That is when I got the flash back to the early 70's and trucks breaking U Joints, drive shafts, ring & pinions while exiting a parkinglot, just big torque off idle.
Dave M

Title: Probably Not And Why
Post by: HB of CJ on December 30, 2012, 09:29:43 PM
The beasts are designed and intended to be running free at higher RPM.  Also, the low end torque, (low and just off idle) of the mighty 12V is strong enough to possibly, (probably) distruct clutches, U-joints, drive shafts, trannys and rear ends.  I once busted a companion flange right into pieces just backing up with a 12V71 mill.  I wasn't my fault...honest!  HB of CJ (old coot) :) :) :)

Title: Re: 12v71 & lugging
Post by: TomC on December 30, 2012, 09:47:56 PM
The low end torque off idle of the 12V-71 and the drivers burning up clutches is the main reason the MC6 was changed to the 8V-71 and Allison. Good Luck, TomC

Title: Re: 12v71 & lugging
Post by: Rick 74 MC-8 on December 30, 2012, 10:13:48 PM
Thanks sounds like a fun engine

                                Rick 74 MC-8

Title: Re: 12v71 & lugging
Post by: wg4t50 on December 31, 2012, 03:30:11 AM
Have had alot of fun passing empty trucks on mountains, some would ask on the CB, what are you running for engine? I tell em a wore out 318, I just know that caused a few bent steering wheels.
Yes, it was a very costly education but the fun factor was way worth every penny, with the advantage of the newer Silver engines with the new castings and imporoved seals and gaskets, the engine way dry and stayed dry.
The best sand boy toy a kid ever had.
Now have the ISM500, 5 speed 4000 Allison, no class and little fun, like the family car, just get in and go.
Damn, I miss that old junk.
Dave M

Title: Re: 12v71 & lugging
Post by: Don Fairchild on December 31, 2012, 10:42:56 AM
   the short answer is yes it can be done with a turbo engine. In Tom's case he needs about three more pounds of boost and his would stay clean and pull better at a lower rpm but that is harder to do with the wastegated turbos, they did not make as many hot housing as they did for the early turbos. you dont want to pull a two stroke down very hard below 1200 rpm they dont have the oil flow and pressure the four strokes do. You can modifi the oil pump. BUT

hope that helps.