When I started in the industry it was in fire apparatus and they mostly had automatic transmissions. As things got more electronical idle speeds crept up and up, sometimes for no apparent reason. Some particular pieces could be commanded by CANbus to idle lower than they were factory set temporarily, but we of course had no laptop or anything to change it permanently. So with an automatic it pushed real hard against the converter when they were idling at 800+ RPM. 40+ PSI of brake pressure to keep it from driving away and your pump would be at 60+ PSI with no throttle. Not good if your are hooked to a 100 PSI hydrant and you only want 130 or 140 delivered. Mr. Truck Shop, I am talking to you since you seem to be on a roll with the olde tyme facts lately. The old ones idled a lot slower. I seem to remember Detroits at around 450 if the bouncy cable drive tach was to be trusted. Plus there was a lot more "cool factor" with that slow clatter, or growl.
I know what you mean.. I do the inj. pumps on ALL SORTS of engines.. & the low idle speeds are crazy, like you said.. I set one yesterday at 900.. low idle.. ALOT of the big machines get set at 1000 rpms' or better.. The ones that scare me are the gen sets.. the throttle is "supposed to be" locked at WFO.. On a cold start, initial start-up.. that's not good.. I'll set them at a "reasonable" idle speed & put a tag on it, stating "after start-up" w/ engine at operating temp. run idle screw down to wide open position & tighten." I don't have an answer as TO WHY the engine mauf. started doing that to idle speeds.. I GUESS its because of all the "parasitic loads" being put on an engine now.??? LOTS of hydros..
The old IR DrillMasters we had back in the `70's and `90's had 8V-71 Detroits and they did have an over-ride control on the governor that operator could/should use to hold it back on start up. However when set up to drill holes for blasting in the quarries most of the time it was not an easy climb to get up on the machine to reach that over-ride lever. So 95% of the time they cranked up and went right up to 1,800 rpm! Then there is the Cat 773G truck that was new a couple years before I retired. That one had a computer that controlled the cold idle speed. The thing that would scare the crap out of you was when started cold it might be idling a little fast then all of a sudden the computer would decide it needed to warm up things a bit faster. It would shift transmission into gear to put a load on the engine and add a little more RPM! Never got used to walking near that thing with no one in the drivers seat and hearing it go into gear and jump slightly and hear it coming under load.
Most idle speeds on the old NTC , A&B cat plus Salt Water Chevy's were around 500 to 600. Then came the electronic engines and idle speed increased to 650. Then came the emission engines and it went a little higher. With that said some of that was to keep a little more heat in the exhaust for emission engines. But if you read the owners manual they don't want you idling a emission engine because it will plug the particulate filter 35% faster and cause more regen. One point is on a fuel truck with Roper or Blackmer pumps moving gasoline the pto should turn no faster than 450, preferably 400 rpm. Any faster and they will chatter like hell, because the gas has no lube/dry on the veins. So pto pumps are set 10 to 20% under drive to idle speed. The bad thing about the old Cummins PT pump is it seemed to want to climb in rpm with a throttle lock if you were running hydraulics at a high idle. So for most rigs I have the pto set at 10% overdrive that way it will operate most anything at idle. For a fire truck I would think those would have a overdrive pto with automatic transmissions. And olde tyme facts come from old school old fa^t's Truck Shop
What you said about PT pumps not holding speed applies to Detroit limiting speed governors as well. I have changed out several engines to Variable speed governors to keep the loaded speed constant. Cummins also offered a dual governor pump with standard road style governor with a second control lever for constant speed control to run pumps or cargo blowers.
I stumbled on this video. It sounds to me like idle speed might be a little more but the tach seems to say 400. Of course in those days you could turn the screw to whatever you wanted it to be. Code: https://youtu.be/BktbUhX4dh0?t=344
Can't see your video. Like the pumpguy said also,parasitic loads. When idling with A/C on these late engines drop 75 rpm because the cooling fans these days are eight blade and 30" in diameter, they draw 30 hp plus at 1600 rpm. Most trucks years ago only had 6 or 7 blade fans and were 26" diameter. One other thing is with over head cams you don't have the added effect of the crank oiling the cam and followers. Run a Series 60 Detroit with out the valve cover. So a little higher rpm=more oil. Just my 2 cents. Truck Shop
The case X120/230/240 series combines idle around 1000 rpm with the separator off. After a maybe 5 minutes they will drop down to ~700 rpm. No idea why they would set it like that, but I would notice it when I was taking too long changing sections.