Repower MC9 Conversion from 8V71

[RoyJ]

8 v 71 = aprox 9 ltr   61 cubic inches per ltr  568 inch in 8v71

You forgot one important factor: manifold pressure.

8V71 is 9L @ 0psi, or converting to 4 stroke: 16ish Liter (due to volumetric efficiency factors).

6.7L ISB @ 30 psi = 20ish Liter @ 0psi. Then factor in 3000 rpm vs 2000 rpm, and all of a suddent the little ISB is pumping double the air of an 8V71.

[RoyJ]

Remember that 26,000lbs is the maximum load for a pickup pulling a trailer.  It is very rare for most pickups to be loaded that heavily.

Pound for pound, it takes more hp to move a truck+trailer comobo than a bus, due to much better aero of the bus. A pickpu with 40' toy hauler has a huge frontal area, tons of gaps midway, which causes a lot of turbulance losses.

Yes, it's it rare to see 26,000 lbs pickups outside of hotshoting and RVing, that's why I stressed an ISB would not work for a commercial bus running round the clock.

Keep in mind though, some pickups, like F450 and F550 have GCWR of 33,000 lbs, with an engine (Powerstroke) of even lower duty cycle than an ISB.

[RoyJ]

All that said, you can over-fuel the thing to death and get 400 or even 600 hp from it, for a little while! 

That may be true for an early 12V, but with modern iterations making 350hp stock, you're a DPF and EGR delete, intake, and VERY mild tune away from 400hp. The point I was trying to make, is that an ISB would have just as easy a time making hp, compared to the 8V71, whether 300, 350, or 400 hp.

600hp is an entirely different story. Duty cycle at those levels would be too short even for recreational use.

[Lonnie time to go]

Warning I am in no way a engine person

My thoughts are simple  take a dt466 up the hp and torque
It should last maybe 200.000 miles
Rebuild kits less then $1000 and parts available at more stores.
There is way less weight, length, height etc....

Again no expert but just thinking opps.

Lonnie

[RoyJ]

Roy,
   I don't doubt your word, and if you're right, I'd be very interested to pursue your suggestion.  But I have a few questions:

1.  Is the ISB a 2-stroke?  Why the high rpms?  3000?  That sounds high, even for me, who is used to 2300 rpm cruising speed at 64mph.  I thought THAT was high.  The poor 8v71 sounds like its hurting itself.

2.  10mpg is my dream goal.  Are you just shooting from the hip or is that really possible?

what do the rest of the board readers say about this?

thanks!
Alan

Alan, as an engineer, I like to backup my arguments with numbers and calculations, instead of simply stating something can or can't be done.

First of all, 3000 rpm is the max for an ISB, not cruising. I put cruising at 2000 due to the following:

8V71 w/ N65 injectors makes 800 lb-ft @ roughly 1700rpm, which is typical cruising speed @ 60 mph. 800 * 1700 / 5252 = 259 hp.

6.7 ISB makes 700 lb-ft @ 2000 rpm with mods mentioned. 700 * 2000 / 5252 = 266 hp.

Since the ISB is making just as much hp @ 2000rpm, as an 8V71 does @ 1700rpm, we can conclude the ISB has just as much reserve power left, at cruising, as your current Detroit.

However, if a hill comes up, the ISB can instantly downshift TWO gears, putting it at 3000 rpm, and 400hp, where as the 8V71 is stuck (you can't downshift if already @ 1700 rpm). 400 hp, regardless of torque, will yield MUCH better climbing power than 259 hp.

Lack of torque only applies when you CAN'T downshift to take advantage of gear multiplication.

10 mpg is possible, at 60 mph, based on two observations:

- I routinely get 11 mpg, at 60 mph, in a 30,000 lbs loaded school bus. Coach is slighly heavier, but has better gearing. At hwy speeds, wind drag is the pre-dominant factor.

- pickups hauling 20,000 toy haulers get 10 mpg, when they cruise at 60 mph. Slighly lighter combo, but again, MUCH worse aerodynamics.

[robertglines1]

change rear diff  min $3500 if you can find a 5 plus gear ratio to fit your housing.then extreme cost. Our family pulls diesel  4 wheel drive trucks and yes we make extreme horespower & torque. when they let go they are scrap metal..so the math adds up to making torque and horse power. Fact is cubic inches equals dependable Torque & horsepower. don't have to be on the edge to do it just plodding along..You can do anything! put money in top speed /performance comes out bottom..Fyi mci put turbine in bus in 1979. When Steve got it in the 90's it had already been changed over to a 8V71.

[RoyJ]

change rear diff  min $3500 if you can find a 5 plus gear ratio to fit your housing.then extreme cost. Our family pulls diesel  4 wheel drive trucks and yes we make extreme horespower & torque. when they let go they are scrap metal..so the math adds up to making torque and horse power. Fact is cubic inches equals dependable Torque & horsepower. don't have to be on the edge to do it just plodding along..You can do anything! put money in top speed /performance comes out bottom..Fyi mci put turbine in bus in 1979. When Steve got it in the 90's it had already been changed over to a 8V71.

I don't disagree with that, but "modding" is not black and white, it's grayscale.

Straight pipe, larger intake, slightly advanced timing and additional fuel; is totally different than: built injection pump, huge injectors, compound turbo...

The former gets you a reliable 400hp ISB, the latter gets you an 800hp weekend toy. OEM engineers have a crapload of restrictions that we don't have. These "OEM restrictions" is the only area I feel safe playing in.

[niles500]

When determining torque you must remember that torque (work) is most properly defined as Mass * stroke - FWIW

[RoyJ]

When determining torque you must remember that torque (work) is most properly defined as Mass * stroke - FWIW

Hmm, you lost me on this one. What do you mean by "determining" torque?

Torque is a force measurement, not work; it's force in angular reference. I have no idea where the mass and stroke comes from.

[DMoedave]

This is a pretty good thread. I would really make sure you read the articles that Jim wrote in the magazine. If you take your time and dont jump into the repower and do LOTS of real research you will be $ ahead and alot happier in the long run. Sure the smart thing to do is get another coach with the power you require. But go to a rally and listen and learn from people who have the knowledge and actual experience. We have some antique units and wanted more power,hill climbing,economy etc and were drooling ver the 6V-92's in some of the 04's at Arcadia. We ended up with a 6V-71 and allison and 3.73 gears and its perfect for OUR rig and application. LOTS of people told us we would be sorry and not have enough power or HP etc with the 6V-71. The first thing, it was rebuilt by someone who knew what they were doing. Pay attention to the good advice do lots of research and dont jump into this. I would really check out all the engines down in Arcadia at New Years Rally even if you just fly in. The knowledge is INCREDIBLE. The best part is all the rigs that have things that cant be done in them.

[luvrbus]

As you are a student of the ISB why don't you post a graph of the hp curve and torque curve of the ISB set at 400 hp for these guys compaired to the 8v71TA 350 hp set at 2500 rpm


good luck

[kyle4501]

The manufacturer has to be concerned with more than just duty cycle.
There is the predictability of failure rate that is of great concern. It only takes a few failures to overshadow a good product.

How happy do you think arl will be if his 'freshly tuned to 400 hp' takes a dump after just 40,000 miles?
I know there are no guarantees, but the light duty ISB is definitely more prone to failure than a heavier built engine.

I'm not saying don't do it, but I am saying it ain't as simple as comparing apples & oranges.

Maybe arl would be happy with an ISB simply because he would be able to drive it like a car. . . .

BTW, a 8V71 at 2000 rpm sounds like a 6cyl 4 stroke at 5,333 rpm, No wonder it sounds like it is screaming! 

Clifford is absolutely right. You can't compare motors without looking at the power curves.

[bobofthenorth]

Something to keep in mind is that the raw cost of the engine is only a portion of the cost of the swap.  For my money, if I was going to do a swap I'd want to know that the new engine was going to be a significant improvement over what it replaced.  In our case when the chips were down I went with a rebuild.  I think it cost me about 2/3 of what it would have cost to swap engines.  Maybe in my own shop on my own time I could have done the swap for what it cost me to have Luke do a good rebuild but that would have involved a used engine and a lot of time.  When I was all done I'd have still had a used engine with a somewhat uncertain history.  As Jim's experience should clearly demonstrate that route is not necessarily foolproof.

[RoyJ]

How happy do you think arl will be if his 'freshly tuned to 400 hp' takes a dump after just 40,000 miles?
I know there are no guarantees, but the light duty ISB is definitely more prone to failure than a heavier built engine.

Care to explain why the ISB would take a dump at 40k miles?

I'll list my choice of mods again: DPF & cat delete + larger pipe, EGR + cooler delete, cold air intake (easy on bus), slight timing advance (raise power and NoX levels).

If the ISB, by design, is more prone to failure, then we must have a lot of students late for class every morning... I drive an old Thomas with 1/2 million kilometers, grossing 30,000 lbs for the last 15 years, and still runs like a champ, all on a little 5.9. That's more miles than almost ANY busnut needs.

[belfert]

I am assuming if you are making that many mods to a 6.7 ISB that you must also be messing with the computer.

School buses have always had fairly small motors as ones operating in urban areas rarely get up to highway speed.  I'll bet that ISB in a bus is rated closer to 250 HP than to 400 HP.  Lower rating means longer life.