Max RPM?

[Jerry Liebler]

Mark,
     Actually your present engine is doing pretty good, even with the bad cylinder, judging from your fuel economy data.  Don Farchild is the expert on the 8V71s and occasionally is on this board.  Don insists the 8V71 can easily tolerate 2300 RPM.   In setting up your replacement engine, I'd recommend leaving standard timing, installing N60 injectors, at least for now.  If you aren't happy with the performance, you could switch blowers, switch to N75 injectors and add a turbo, drive it a while watching temperature carefully and if needed switch fans all without pulling the engine again.  For what it's worth my bus is a bit taller and heavier but otherwise very similar & I get 8.25 MPG at 65, not towing,  and 8.0 towing a 5,000 pound towed at 60.
Regards
Jerry 4107 1120

[quantum500]

Your address is no good. pittsburgdiesel.net

[RJ]

Mark, Lin, Jerry, Tom, et al -


OK, time for a little refresher on the 4106 powertrain:


Remember, there were an awful lot of slide rules working on this during development, and, since everything's a compromise in some way, shape or form, GM obviously felt that this combination gave the best overall performance for a vehicle designed for revenue service in all types of operating conditions.  Thus:

GM designed the 8V71/4-spd powertrain to run 60 mph @ 1650 rpm using tires that turned 495 revs per mile.  The goal was also to return 10 mpg at that speed using N60 injectors.  When you punch in the appropriate numbers into Mallies's calculator, it shows that the coach should be capable of 76 mph @ 2100 rpm.

Now, some numbers to ponder:

Rear Axle Ratio:  The stock rear axle ratio in the 4106, 4107 & 4108s is 4.125:1

Transmission Ratio:  The stock "high gear" ratio in the manual 4-spd and V-730 automatic is 1:1

Bevel Gear Ratio:  The stock bevel gear ratio for the manual 4-spd is 0.808:1 (the 4104 and earlier models have 1:1).  The bevel gear ratio in the V-730 is 0.875:1.  (Supposedly there are some V-730s out there with a 1.04:1 bevel gear ratio, but this may be an urban myth, as there is not listing in the parts book for them.)

Overall Final Drive Ratio:  This is computed by multiplying the rear axle ratio times the bevel gear ratio times the transmission gear ratio (for each gear, if you want to get all techie).  Thus, for the 4-spd in fourth (1:1), you get (4.125) x (0.808) x (1:1) = 3.333:1.  The V-730's numbers in 3rd (1:1) yield (4.125) x (0.875) x (1:1) = 3.609:1

Thus, for any given engine rpm, with equal-sized tires, the manual gearbox coach will be going faster than the automatic, due to the "taller" overall final drive ratio.  Better fuel mileage, too.

Now, back to Mark's original question:

All of the various literature pieces that I have in my collection regarding the two-stroke Detroit's indicate that the normal setting for maximum rpm is 2150 no-load, and 2100 loaded.  Fire apparatus specs are a little higher, at 2500 loaded, but that's the exception, rather than the rule.  Again, being a compromise, Detroit most likely specifies these settings to give the best overall performance and engine life for a revenue service vehicle.  Since we're now dealing with a non-revenue service situation. . . well, you decide.

The literature also agrees with what Jerry & Tom have said - that advancing the timing shifts the torque peak UP the rpm scale.  With N65 injectors, you gain all of 30 ft/lbs. of torque (800 vs 770), but it's at 1600 rpm, not 1200, like an N60-equipped engine.  This is fine, if you've got a 10- or 13-speed transmission behind the 8V, so you can keep it in the power band.  But with our widely-split 4-spds. . . not so good! 

If you're into the Tim Allen Syndrome (More Power!), the turbocharging with the appropriate injectors is the way to go.  Stock 8V71T with N-75s gives 350 hp and 990 ft/lbs.  If you can find one of the rare 8V71TAs, they produce stock 370 hp w/ 1064 ft/lbs. of torque using 7C75 injectors, with which your coach wouldn't even know the trailer's behind it climbing RockyTop.

With the crate engine you've got sitting on the pallet, it's entirely possible that you could put a low-boost "smoke" turbo on it with N60s "as is" and have a dependable, fairly fuel-efficient powerplant.  Suggest you give Don Fairchild a call, and ask him how to check to see which pistons you've got in the crate engine - that would help determine if you could turbocharge it or not.  Maybe Dale (Songman), if he's following this thread, can ask Don and forward the info, too.  Don's office number is 1-888-473-3626.


Sorry for the long dissertation, but thought it was appropriate to review this info again.

FWIW & HTH. . .

[Lin]

RJ,

I believe that I have C60 injectors.  What is the difference between N60's and these.

[OneLapper]

Lin,

The difference between C injectors and N injectors, as it was explained to me, is that the C injectors were used in city that had smog concerns.  The injector tip design was supposed to reduce black smoke at throttle tip in.  N injectors are the most common used for truck and industrial applications.  7E series were common in coaches for fuel economy.

Mark

[OneLapper]

RJ, thanks for the info.  How would I find a copy of some of the literature you have?  I love reading technical paperwork!

BTW, I did forget to factor in the vee drive bevel gear ratio.  Oh, and the tech manual I have says the 4th gear ration is .808, same as the bevel drive gears.  Is this a typo?  Third is 1.21:1.  If fourth is 1;1, then the ratio split between third and four would be very close.  Which it isn't.

It sounds like a N60s is the best all round injector according to your specs.

Thanks,

Mark

Mark, Lin, Jerry, Tom, et al -


OK, time for a little refresher on the 4106 powertrain:


Overall Final Drive Ratio:  This is computed by multiplying the rear axle ratio times the bevel gear ratio times the transmission gear ratio (for each gear, if you want to get all techie).  Thus, for the 4-spd in fourth (1:1), you get (4.125) x (0.808) x (1:1) = 3.333:1.  The V-730's numbers in 3rd (1:1) yield (4.125) x (0.875) x (1:1) = 3.609:1

[pvcces]

OneLapper, the ratio given is the combined result of high gear and the bevel gear; they just didn't say so. If you look ant the transmission section of your service manual, it should be obvious that there is no overdive within the transmission.

If you think about it for a minute, there would be no reason to put an overdrive inside the main box and every reason not to. The final ratio could be set anywhere they wanted it with just the bevel gear; they wouldn't want to pay for it twice. In any case, the transmission needed to be short, and an added overdrive would add some unwanted length and weight.

I see that RJ's explanation shows a droop of 50 RPM. If that's the case, I've probably forgotten it. I thought it was listed as 150 RPM.

Good luck.

Tom Caffrey