Relationship between no-load high idle and full load rpm

[bevans6]

I've been reading everything I can on mechanically governed two-strokes and I haven't found a discussion of this.  My "tank engine" is supposed to have a no-load governed top speed (that I have seen referred to as "high idle" of 2450.  In the same specification it says maximum loaded speed no more than 2300 rpm.  What I would like to understand is - what is the effect of load on the max rpm that the governor allows?

Thanks, Brian

[Geoff]

Okay, you have a no-load high of rpm of 2450 (kinda high, in trucks it is usually 2250).  Under full load, that 2450 should drop down to 2300 after the allowed 150 rpm governor droop.  Governor droop is set by the governor gap, and under full load it goes from .0015 to zero, and all other rack and governor play/settings should be bottomed out. 

Not the easiest concept to grasp...

[lostagain]

I would think that the only accurate way to measure and set max engine speed is with no load. Any load subsequently applied to the engine would not be consistant, therefore not easily measurable. For example, going down the highway as fast as it will go in top gear depends on the vehicle weight, friction, wind resistance, slope up or down, etc, and changes constantly. Unless you could apply a known, constant load on an engine on a dyno, to get a reliable value. Using no load to set the rpm is just more practical.

JC

[bevans6]

That makes a lot of sense to me, and clears up the "what the heck is governor droop" question too, actually.  You can easily set the no-load, but applying some load - doesn't have to be fully loaded to max, just a decent load, should do as you say and take up the slack in everything in the governor, and reduce the max rpm.

The no-load and max rpm is the spec on the engine, I have measured the no-load at 2425, and I may well reduce it a bit.  On the other hand, I can keep the rpm's in an appropriate place with my right foot, too.  I will ponder this a bit more...

brian

[azdieselman]

I had typed a lengthy response, but it diassapeared. The short answer is the balance point in the governor, the position of the weights and springs will determine the rack position, which determines fuel delivery. Fuel delivery controls power output and rpm. It really isn't that simple, there's alot more that goes into rack position vesus load, but that's why there's an rpm difference.

[artvonne]

  I remember reading all that governor droop high idle high loaded rpm crud a while back, but I seemed to be forgetting it as fast as I read it. Or so I thought.

  I do recall reading that governor clearance spoken of above was real critical, or you could have a runaway. Then I remember you talking about setting the high idle and Clifford and friends starting talking about blowing up engines.

  Definetly not something for the untrained or unguided to monkey with.

[lostagain]

Paul,

That is what the manual is for. It is clearly explained step by step under the "tune up" section. I have tuned up both the 4-71 and the 6V92 several times, and I am not a mechanic. But I can do it at least as well as the apprentice who did it once for me under the distant supervision of an older mech. at $125/hr DD shop.

JC

[artvonne]

  Thats kinda what I meant. Things make much more sense when your actually looking at them AND have a manual AND are attempting to read it.

[Bill B /bus]

All of the above written about load and no load RPM can be reduced to - figure on a 150 RPM drop for no load to load RPM. I do think that 2450 is a touch high. I've seen 6-71's at that no load RPM. But 8-71's. How about no more than 2300 RPM no load. That leaves you at 2150 RPM loaded. Or about 78 MPH.  Well really somewhere between 75-80 MPH depending on tire size.

Bill

[luvrbus]

I always set the 8v71 @ 2450 no load a DD dealer has no problem setting one at 2450 rpm either

good luck

[lostagain]

The DD dealer in Cranbrook BC reluctantly set my 4-71 at 2300 and would not go any higher. They would've rather have left it at 2100. I set it at 2500 after I overhauled it. But that is an inline, they can spin faster than a V.

JC

[luvrbus]

Must be a Canadian thing JC Smith's, Pacific,Valley, WW Williams and Stewart and Stevenson they don't mind doing it they will set one at 2450 rpm in a heart beat 

good luck

[artvonne]

  Why was there so much rumor years ago about the RPM of Detoits being regulated by law? Ive heard for almost 40 years that 2100 was max legal in trucks, but military and boats could run higher. Did Detroits RPM used to be regulated??

[RJ]

Okay, you have a no-load high of rpm of 2450 (kinda high, in trucks it is usually 2250).  Under full load, that 2450 should drop down to 2300 after the allowed 150 rpm governor droop.  Governor droop is set by the governor gap, and under full load it goes from .0015 to zero, and all other rack and governor play/settings should be bottomed out. 

Not the easiest concept to grasp...

Geoff -

I read your post twice, then went and dug out my DD manuals and read the "tune-up" section, twice.  Came back in, re-read your post and "the light came on!"  Ah ha!

Thanks for your explanation, I understand the concept much better now.

I trust you're enjoying AZ more than the Bay Area, right?

[artvonne]

 So .015" is 150 rpm across the board/rpm range?