Gearing for economy

[viking42]

Hi bus nuts,
       Been reading your jewels of wisdom for some time now and would like some opinions. Courntly have a 4106 with 730 auto with 65 injectors, not advanced timing. Going from 1200rx22.5 to 1100x22.5 dropped about 3-4 of mile per gallon, and black smoke on solid acceleration.the black smoke  has been there since the 65 injectors were installed, 23 years ago, and 125000 miles have gon by. Now that fuel has gone througn the ceiling, i would like to go back to 55 injectors. Would it be worth the expence, can do the mecanical work. Would appreciate responses. Sorry about the spelling my sesretary is not in today. Thanking in advance.

[bevans6]

Well I think this should be in bus topics, not "off topic", if injectors and economy aren't on-topic I don't know what is!

I'm hoping you dropped 3/4 of a mile per gallon, not 3 to 4 miles per gallon...   With the vee drive, and if you're handy with wrenches, it's a little easier to get at the back of the engine and change the timing than with some configurations.  That would give the fuel longer to burn in the cylinder and should help economy.  Switching to the smaller injectors is also possible, obviously, but the expense is going to be around $800 depending on where you get the injectors and what you pay for them.  What I don't know is what the return in fuel mileage will be.  Less power and more full throttle can sometimes mean worse economy than a more powerful engine driven economically.

Hopefully others will have more advice.

Brian

[buswarrior]

Black smoke is unburned fuel, so money out the tailpipe.

If your use of the throttle pedal can make the black smoke go away, drive it like that and save your injector money for fuel.

partial throttle starts and acceleration yield better fuel economy anyway, and reducing the black smoke will help some more.

Sometimes you want the power, and once the 55's are in there, you lose that choice.

Self discipline or smaller injectors, choices, choices...

happy coaching!
buswarrior

[Nellie Wilson]

"Going from 1200rx22.5 to 1100x22.5 dropped about 3-4 of mile per gallon..."

Not sure I understand (so what's new, you say  ). Do you mean your mileage INCREASED with 1100 tires or DECREASED? Either way, I can't figure how that could happen. Can someboy explain? Might be worth swapping my fat (lovable) 315s... that's if the mileage is going the "right" way.

Nellie

[pvcces]

With standard timing, it is really easy to switch back to 55 injectors. Most people who have 65s have advanced timing and that is a chore to switch back.

You have three things costing you fuel:

Your engine is turning too many revolutions per mile;
The V730 is hard on energy; it produces a lot of heat in normal operation;
You are injecting too much fuel.

If you had mentioned what you get with your current configuration, we might be able guess if anything is wrong. We often get 10 mpg, but we have none of the items I mentioned above. However, we have a bad clutch right now, so you might think we don't have it too good.

When I was visiting a DD rebuilder in Ketchikan, he told me that hands down, the 55s were the easiest on fuel. Stock for your rig was 60s.

Good luck.

Tom Caffrey

[TomC]

I believe if you change from the N65's to clean tip N60's, your black smoke will just about disappear at sea level.  When I had brown tag 65's in mine, at sea level it would smoke a bit on full acceleration, but just pull up the accelerator about an inch and it would stop.  Now at high altitudes, that was a different story.  That's why I turbocharged my 8V-71 with 9G75's and don't get any smoke on hard pulls now-plus the extra power is substantially more.  Good Luck, TomC

[RJ]

Viking -

If you were to install the 470 rev/mile drive tire that Bridgestone makes in the 11R24.5 size (M726EL), you'd get back to almost identical gearing as the stock 4106 with the manual box.  That would help your fuel mileage.

N60s were the stock injectors.

Buswarrior's right - quite a power drop with N55s.

Keeping your foot out of the throttle helps the most, and is the least expensive "fix."

FWIW & HTH. . .

[viking42]

Guys and gals;

             To clear up a few things about my post. When i got my coach in 1985 it had the 4 speed spicer and was getting 9 .5 miles per gallion much of the time on trips. with 12x22.5 tires. after installing the 730 auto it dropped to 7.5 mph but was happy with the performance plus the wife would drive then without to many complaints. Couple of years ago after reading of problems with aged tires, checked mine, guess what, 15 years old, they had to go. Tire guy talked me into 11x22 tires, what a difference in mileage. Now down to 6.0 to 6.5 mph. I installed the 65 injectors when i installed the 730. Dont notice much difference in performance,as far as the injectors are concerned, but on hard accelerationshe smokes to much, I admit i have a heavy foot, but with a GREAR MOUNTAIN CLIMBER

[TomC]

Go back to the N60 injectors and you'll be up just about back to your original mileage-like in the 8mpg range.  Good Luck, TomC

[RJ]

Viking -

If you were getting over 9 with the OEM powertrain, you don't have the "mountain climber" gears in the rear axle - you've got the stock 4.125:1, which, btw, is the "tallest" or "fastest" axle ratio available for the V-drive.

However, the three changes you've made to the powertrain since you got the coach have all contributed to your poorer fuel economy.

First - The automatic, by it's nature, uses more fuel than a manual, especially with a lot of city driving.  In addition, the bevel gear ratio is "lower" than the manual gearboxes, so the engine turns more rpm at the same road speed, another fuel economy hit.

With the manual gearbox, the overall effective rear axle ratio is 3.333:1.  With the V-730, that drops to 3.609:1, approximately 8% lower/slower.

Second - The larger injectors, without the additional change of the timing, contributes to the additional fuel burn - more squirt per stroke!

Third - The 11R22.5 tires were the wrong choice, especially since you'd already lost some mileage with the automatic.  The 4106 was designed around tires that turn 495 revs per mile with the manual gearbox.  My guess, if you look it up, you'll find that these 11R22.5s are turning more than 495, again, hampering mileage.  That's why I suggested that 24.5 tire by Bridgestone, to help get you back closer to OEM.

Finally, as I said before, the least expensive way to improve your mileage at the present time is to keep your foot out of it!  I know that's hard, because the 4106 is such a sports car. . . 

FWIW & HTH. . .