8V71NA versus 6V92TA

[TomC]

To know whether or not you can turbo a 8V-71NA, you need to pull at least one of the pistons to see if it is a two piece piston and what rings you have.  I lucked out in that although my pistons are the high compression 18.7 to one pistons, they are two piece and have the tighter transit type piston rings that will seal turbocharging boost.  I only went from N65's to 9G75's-and with a Series 60 12.7 liter waste gated turbocharger to keep the turbo boost down to only 15psi.  Then too-I installed an air to air intercooler in front of the radiator and bypass blower valve.  You can just keep the same injectors and install what is called a smoke turbo-which will keep your sea level performance the same at altitude.  Good Luck, TomC

[Don Fairchild]

Or you can get it over to luvrbus and for a sum larger than the national dept we will pull the engine and built you a hot rod 8V71TA @ 425HP. That will get you up the hill and if you keep your foot out of it on the flats will get decent miliage.

Don

[RoyJ]

Can someone check my math one last time? A Spicer manual trans in an MCI-5, 1st gear ratio is 4.25:1, rear axle ratio is 3.7:1? Standard tires approx 495 revs per mile, gives a tire radius of 1.7 feet?  900 ft lb's tq from turboed 871 with n60s X 4.25 X 3.70 = 14,152 ft lbs tq at rear axle? /1.7 foot tire radius delivers 8240 pounds of tractive force to the rear wheels?? A 22% grade requires 22% of the vehicles weight in traction? At 27000 pounds (high estimate MCI-5) this Bus would require 6160 pounds of traction at the rear wheels??? Am I correct to assume then, that on paper this Bus could climb a 22% grade in first with over a ton of tractive force in reserve power??

  So if my math is correct, it looks like I could go up to nearly 30K pounds with stock power (740 tq) and still climb it??

Paul, I know you didn't agree with me the last time, but I will stand firm and say you must take transmission losses into account when calculating torque.

%torque loss = %hp loss, for any geartrain, or else the math simply won't work. We'll assume 12% loss.

Since the bus is rolling, you also need to add rolling resistence. For a typical tire which we can assume .02 coefficient, giving 540 lbs of resistence.

Your net trust now becomes: 8240*.88=7251 lbs, and your total resistence becomes: 6160 + 540 = 6700 lbs

So regardless, you'll make it, but it may be closer than you origionally thought.

[buswarrior]

Yup, one day when I got the ca$h,

I'm going to get Don to build me a motor.

A fine, fine, motor.

Until then, don't worry, be...

happy coaching!
buswarrior

[Barn Owl]

How about this:

An automatic transmission has the benifit of torque multiplying:

"The torque converter portion has the ability to multiply torque from the engine. The impeller (sometimes called the pump) has specially curved vanes and is driven by the engine's crankshaft. The turbine also has specially curved vanes and is connected to the input shaft of the transmission. Adding a third element, the stator (also called the reactor), gives the assembly the capability it's named for. The stator has vanes and is mounted on a one-way clutch, to allow it to freewheel in only one direction. The stator assembly is located between the impeller and turbine and redirects oil that bounces back off the turbine. The force of the redirected oil assists in rotating the turbine, resulting in torque multiplication. When the impeller's speed is high and turbine's speed is low, torque can be multiplied by as much as 2:1. When the impeller's speed and the turbine's speed are about the same, torque can be transferred at almost 1:1."

http://www.carcare.org/Auto_Transmission/torque_converter.shtml

HTH

[TomC]

V730 with 8V-71 has a 2.53 ratio torque converter.  So with the 1.77 to one first x 2.53 torque converter, you have a 4.478 first, then like my bus with the 4.56 rear end ratio-has 20.42 to one overall starting gear.  Good Luck, TomC

[artvonne]

  Its been my understanding a V730 wouldnt fit an MC5 with an 8V71?

  Nobody said but I dont think a 7 speed will either. Anyhoo, I'll just pretend its the lil engine that could and pray.....if I can find a Bus, that is

[Barn Owl]

Its been my understanding a V730 wouldnt fit an MC5 with an 8V71?Huh

Yes, you are correct, it is for left hand transverse mounted setups. I think a 4106 with a v730 would make it up your driveway easier than a MC5. Gut feeling biased opinion only, no proof. One day I will remember to check the grade on my road. It is very steep and long (7/10ths of a mile, various grades).

[artvonne]

Oh silly me, V is for "V"drive, duh. 

[RJ]

Paul -

On the MC-5A and B, you get your choice of:

8V71 + 4-spd #8844 Spicer

8V71 + MT644/645 Allison (very rare OEM)


MCI made some changes to the rear of the chassis on the 5Cs, so your options are a little different:

6V71 + HT740 Allison

8V71 + #8844 Spicer 4-spd manual

8V71 + MC644/645 Allison (very rare OEM)

or the hot rod

6V92TA + HT740 (not OEM, Tim "The Tool Man" Allen upgrade)

This last combo can also be installed in the A & B.

FWIW & HTH. . .

[bevans6]

If you are calculating driveline efficiency, you can use a number like 12% loss and be pretty close.  I use 8% to 10% for the loss in the differential hypoid gear, which is a sliding tooth gear design, and 1% to 2% for each helical gear pair in the train.  In first gear, an MC5 with a Spicer has two helical gear pairs in the transmission, and two in the drop box, so the loss will be between 4% and 8% plus 8% to 10% for the diff, so a total of between 12% and 18%. 

I don't know anything about driveline efficiency in an automatic transmission in first gear, though.

Brian

[artvonne]

Paul, I know you didn't agree with me the last time, but I will stand firm and say you must take transmission losses into account when calculating torque.

Your net trust now becomes: 8240*.88=7251 lbs, and your total resistence becomes: 6160 + 540 = 6700 lbs

So regardless, you'll make it, but it may be closer than you origionally thought.

  While I may not completely agree with you, I do hold out the possibility im wrong. I figure im near the ragged edge, you just wanna shove me closer to the edge and make me more cautious. Certainly no harm in that and I appreciate the help. 

  The snows about gone, the melt water is about ran off, time to start grading my runaway truck ramp.

[RoyJ]

I don't know anything about driveline efficiency in an automatic transmission in first gear, though.

Brian

Brian, I would think that in 1st gear, converter unlocked, the overall efficiency would be super low, as a lot of power is wasted as heat in the turbines. But efficiency won't matter, because torque multiplication is more important in this case. I read modern ZF autos have an "in gear" efficiency of 97%. This is just the planetary gears though, not including converter and hydraulic pump.

[RoyJ]

While I may not completely agree with you, I do hold out the possibility im wrong. I figure im near the ragged edge, you just wanna shove me closer to the edge and make me more cautious. Certainly no harm in that and I appreciate the help. 

  The snows about gone, the melt water is about ran off, time to start grading my runaway truck ramp.

You know Paul, as a worst case scenario solution, you can deflate your rear tires, effectively cutting the rolling radius to 13 inches or so, and go up with ease.

Not a recommended daily procedure though

[artvonne]

To know whether or not you can turbo a 8V-71NA, you need to pull at least one of the pistons to see if it is a two piece piston and what rings you have. 

  Cant they be inspected through the side access covers?