Max RPM?

[OneLapper]

If someone can help me determine the max RMPs of my 8V71 I would appreciate the effort.

The factors are as follows:

4106
8V71
4 speed Spicer
tires are new Hanook 12R22.5 (I don't know the rolling diameter)
Top Speed in 3rd is 55 to 57 mph (GPS indicated)

I don't know what the top speed is.  The fastest I've seen is 78 MPH

I have a replacement engine on a crate that I'm installing N65 injectors, advancing the cam timing, running the rack, etc, and I would like to know what the RPMs are of the current engine.  I may be adjusting the RPMs on this replacement engine as it was a NYC transit bus with 7E55 injectors.

Thanks for the help!

[Lin]

There is a variation of opinions on where to set your no load RPM.I do not think I have heard much advocacy for going above 2300 rpm.  Mine is set at 2200 rpm, and I have read of some that advocate several hundred lower.

[buswarrior]

Have some fun with these two speed calculators and decide for yourself how fast you want to spin the engine:

http://www.cwis.net/~mallie/page12.html

http://www.thebouthilliers.com/4106/calcmph.html

The choice is yours. In fire apparatus service, stories are told of 2800 RPM....

Of course, the more you spin the engine, the shorter the lifespan.

happy coaching!
buswarrior

[pvcces]

In modern 8V71s, I would expect a high idle setting of 2250 RPM and a droop of 150 RPM, yielding a top speed in the low eighties. Ours will run right at 83 MPH.

Don't mistake the fast idle switch for the high idle setting. The high idle setting is what you get with the throttle wide open with the bus sitting still. The droop of 150 means that you should be able to get a full rack postion until you get to within 150 RPM of the high idle setting.

At eighty MPH on the flats, I would expect you to be turning about 2150 RPM, burning about 12 GPH, and yielding about 6.5 MPG.

While better performance may be pretty important to you, I would stay with the 7E55s and standard timing, partly because the stock power band is much better suited to the four speed standard transmission. With the 65s and advanced timing, you will need to work at keeping the engine RPM up to protect the engine and it will be unlikely that you will be able to maintain the cooling and fuel economy that the stock setup gives.

According to a longtime Detroit man I know, the 55s give the best fuel economy of any of them in the 71.

For what it's worth.

Tom Caffrey

[JohnEd]

Maybe I NEED a 4106.  So what is your MPG if you just conservatively cruise at 55 or 60?

Thanks,

John

Seasons Greetings to you both...by the way

[OneLapper]

Tom,

I may just leave the engine the way it is.  You're not the first to say that I should try it first, before spending $2k on upgrades. 

Buswarrior, thanks for the links.  I was able to find all of the data and determined that my current engine governs around 2200 rpm.  This engine has N60s and runs better going down the road at 70 mph than at 60, or even 65.  My only concern is that 7E55 injectors just won't have enough HP to tow my 7500 lbs trailer up and down the hills in New England.  I've towed to Indiana and SC and on the flats she cruises just fine at 60-65, but she'll fall flat if there's any type of incline.

For reference, my 4106 with N60s and Spicer will get 10.8 mpg at 65 mph over a 1000 mile drive non-stop.  7.8 mpg same drive at 60 with trailer.  I'm happy with the mileage and I'm willing to sacrifice 2 mpg for more HP if it'll allow my to use a cruise control (not yet installed).

Thanks!

[Lin]

Tom,

With our 3.36 rear end, 2000 rpm in the Spicer's 4th gets us about 71+ mph.  It would seem the 83 mph would require another gear or a higher ratio.

[OneLapper]

Tom,

With our 3.36 rear end, 2000 rpm in the Spicer's 4th gets us about 71+ mph.  It would seem the 83 mph would require another gear or a higher ratio.

Lin, something doesn't compute here.  In my 4106 the stock ring and pinion gear ratio is 4.125 to 1, which is what we would call "short" gearing.  The smaller the number the "taller" the gearing which means that the drive shaft in my bus turns 4.125 times for every 1 revolution of the tire.  In your bus with 3.36 ratio the drive shaft only turns 3.36 times for every one revolution of the tires.  This means that your engine should be turning lower rpms at a given speed compared to my engine.

Mark

[OneLapper]

Lin, just used the first link that Buswarrior provided.  I entered your rearend ratio and the approx tire revolutions for a 12R22.5 tire and your bus should be traveling at 90 mph at 2000 rpm. 

Tell us more about your bus and let's see if we can figure it out.

Mark

[Lin]

Something is wrong there.  Although my tires are 11R22.5, that will not make much of a difference.  Figure 2000 rpm/3.36/500 x 60(to convert miles per minute into mph).  My 4th gear, I believe, is 1:1.  That would give us about 71.42 mph.  Now if I had 12R22.5 tires and a .8 4th gear ratio, as I think some 4106's have, I would be doing 90 mph.  I have tried to verify my speed/rpm relationship with my GPS, and it is pretty close.  Thanks

By the way, I don't think I want to go 90 anyway.

[Jerry Liebler]

Onelapper,
    Let me try to re enforce what Tom said above.  The reason you really  don't want advanced timing is the unavoidable fact that using advanced timing moves the torque peak from 1200 RPM to above 1400 RPM and all the "V" drive transmissions cause the engine to slow from 2100 to 1200 on their last up shift.  This is true for the V730 automatic and the 4 speed manuals.  The best all around performance will be with N60 injectors and standard timing in a "V" drive bus with an 8V71.  Better fuel economy can be had with the smaller injectors but at the expense of some horsepower. Going to N65 injectors and standard timing makes more smoke but actually less power than N60s.  If you want more power with standard timing you'll need a turbo. All turbo'd engines use "standard timing".  Just what must be changed when one adds a turbo is controversial, with opinions ranging from nothing to practically everything.  If you choose to add a turbo you'll be pushing the stock cooling system beyond it's design so you may need to "upgrade" the fan.  Several GM buses have been turbo-ed, run with N75 injectors and cooled successfully with upgraded fans.
Regards
Jerry 4107 1120

[OneLapper]

Something is wrong there.  Although my tires are 11R22.5, that will not make much of a difference.  Figure 2000 rpm/3.36/500 x 60(to convert miles per minute into mph).  My 4th gear, I believe, is 1:1.  That would give us about 71.42 mph.  Now if I had 12R22.5 tires and a .8 4th gear ratio, as I think some 4106's have, I would be doing 90 mph.  I have tried to verify my speed/rpm relationship with my GPS, and it is pretty close.  Thanks

By the way, I don't think I want to go 90 anyway.

Okay, with the above info it sounds like you're right on!

Mark

[OneLapper]

Onelapper,
    Let me try to re enforce what Tom said above.  The reason you really  don't want advanced timing is the unavoidable fact that using advanced timing moves the torque peak from 1200 RPM to above 1400 RPM and all the "V" drive transmissions cause the engine to slow from 2100 to 1200 on their last up shift.  This is true for the V730 automatic and the 4 speed manuals.  The best all around performance will be with N60 injectors and standard timing in a "V" drive bus with an 8V71.  Better fuel economy can be had with the smaller injectors but at the expense of some horsepower. Going to N65 injectors and standard timing makes more smoke but actually less power than N60s.  If you want more power with standard timing you'll need a turbo. All turbo'd engines use "standard timing".  Just what must be changed when one adds a turbo is controversial, with opinions ranging from nothing to practically everything.  If you choose to add a turbo you'll be pushing the stock cooling system beyond it's design so you may need to "upgrade" the fan.  Several GM buses have been turbo-ed, run with N75 injectors and cooled successfully with upgraded fans.
Regards
Jerry 4107 1120

Jerry, it's diffiicult for me to gauge the performance of my current engine because it clearly has a least one bad cylinder.  The other factor is that there is no one clear answer because so many people have some many different setups with varying degrees of success.  I've never had the chance to drive another 4106.  Heck, I've only seen two others, and they were rolling down the road!  Like I said, this replacement engine appears to be a recent rebuild, the compression sounds even when cranking over, it has 7E55 injectors but the original build sheet from DD show it shipped with N60s.  Who knows?  I do know I want to do whatever work I'm going to do to it while it's sitting on the pallet!

Thanks,

Mark

[baker4106]

I have a 4106 and at 2150 rpm it runs just over 80 mph.   My previous 4104 did the same.   Don't know what gear ratio I have but have 12R22.5 tires.   Transmission is the 4 speed spicer.   

[NJT 5573]

If you want to make horsepower with a diesel, you need to retard the timing, not advance it. Pittsburg Diesel has alot of info on how its done at their site. I had the 3:36 gear in my 05 and it loved to run 90 MPH. (pittsburgdiesel.net)