8V71 Governor Adjustment

[Fredward]

I've searched and found a couple of references but have not found what I'm looking for. My 871 with 4speed spicer idles about 450 RPM according to my mechanical handheld gauge. Wide open no load is about 1750. She goes about 75 mph which is fine. But I'd like the option of winding it out to 2150 or so. I've had the rack run but the engine was on a dolly at the time so he couldn't adjust speed.

How do I adjust idle and wide open speeds on this engine? (I see where Tom C says it will handle up to 2800 RPM but I'd be happy with 2150). 

Fred

[makemineatwostroke]

Fred, you need to have your handheld tach calibrated to make sure it is accurate before trying to adjusting the governor idling and no load speed those handheld tachs can get you into trouble and also be careful where you take the reading (check the manual for the tach)most are off the tach drive. I believe TomC said he had seen them up to 2800 but I doubt if his is set at 2800   have a good day

[Stan]

I am suspicious of your speed readings. It is not normal for a bus with a 4 speed Spicer to go 75 MPH at 1750 RPM. Do the numbers with your tire size and drive axle ratio. Your old Spicer should be 1:1 in high gear.

[Melbo]

My four speed spicer with an 8V71 went about 73 mph top speed -- close enough to 75 if you had a taller tire it seem you would go 75 easy enough.

Melbo

[Fredward]

First of all, wouldn't I just measure the engine RPM in the dimple on the end of the crankshaft? (I still have the bumper removed).

Secondly, it has 12R-22.5 on it which are 495 rpm tires I think. The book says the axle ratio is 3:36 or 3:70. I'm guessing 3:36 because it was delivered to Greyhound in Texas. 4th gear is 1:1. So at 1750 engine RPM the axle is turning 583 RPM - right?

Stan, are you saying that 75mph at 1750 is slow or fast?

Fred (not Einstein)

[makemineatwostroke]

fred,with 12r 22.5 a 3.36 gear a 1.1 transmission at 1750 rpm you should be around 65 mph and for the handheld tach not knowing what the ratio is on it I don't know what to tell you, check it by using a cordless drill  the rpm of the drill  to rpm reading on your tach if they are the same it is a 1.1 drive and the end of the crank will work

[Fredward]

Hmmmmm. I checked my handheld on the PTO of my 4020 John Deere. It should have read 540 rpm at 2100 engine rpm and it reads about 500 rpm. So it is reading about 10% slow. So if it says 1750 it really means 1925. And 1925 = 71 mph correct? If I figure your math correctly, 2150 rpm would yield about 78 mph. (2150x.037) So maybe 2150 unloaded would be about 1925 rpm under load.

Sound right?

[makemineatwostroke]

You , got it now Fred but never trust the hand held tach to many different variations of wheels and ratios

[Stan]

Fred: If you want another SW hand held tach, I have one on the shelf I haven't used for years. The new electronic tachs are now so cheap ($35.00) and so accurate and can tolerate a drop on the concrete floor that you should just abandon the mechanical tach.

Likewise, if you want an accurate tach on the dash, get one that counts the teeth on the flywheel and uses a digital or digital to analog meter.

[buswarrior]

If you want the fun tools, optical hand held tach is the way to go.

A little piece of that silver reflective tape on the edge of the crank is what big transit does here.

happy coaching!
buswarrior

[TomC]

12R-22.5 are generally around 485rpm.  With 3.36 that works out to be 2037rpm at 75mph.  Me's thinks your hand held is not reading right.  Idle should be 550-650rpm with a manual and 650-750rpm with an automatic.  I have my top rpm set at 2400rpm.  It's fun to talk about the guys that turned the 8V-71's to 2800rpm (don't even try that with a 92 series-they don't like much over 2300rpm), but I've always been conservative with truck and bus engines.  While I could have put in 90 injectors in my 8V-71, I stayed with 75's; and my Caterpillar 3406B was factory set at 400hp.  When it comes to moving big heavy vehicles, I think of the engine as a tug boat-you just want it to run and run for years on end.  Good Luck, TomC

[Fredward]

A thread like this would not be complete without asking a "theory of operation" question. If an 8V71 is topping out at 2100 rpm no load; is there any advantage other than higher top speed, to increasing the max no load speed to perhaps 2300?

Anytime the throttle pedal is fully depressed; if the engine isn't at max rpm, the governor goes wide open to get the engine to the desired throttle setting and then once that new throttle setting is achieved, the governor reduces fuel to maintain that speed. So does increasing the max no load rpm improve performance or just increase the top speed?

[Dallas]

Good Question Fred...

There is as far as I'm concerned, no real gain in turning the governor up. You actually reduce the life of the engine when you raise the RPM's.
The Torque rating doesn't go up, the HP rating may go up, depending on the engine set up, but then the ability for weak springs to let the valves float goes up also. so does the tendency for the bearing to scrape the crankshaft. Remember... Bearings should never touch the shaft.. the oil is the cushion between them.
You will also notice that you lose a lot in fuel mileage the faster you run your engine beyond the "Sweet Spot">>> THAT is the point where torque and horse power cross on a chart..

Good Luck,

Dallas

A thread like this would not be complete without asking a "theory of operation" question. If an 8V71 is topping out at 2100 rpm no load; is there any advantage other than higher top speed, to increasing the max no load speed to perhaps 2300?

Anytime the throttle pedal is fully depressed; if the engine isn't at max rpm, the governor goes wide open to get the engine to the desired throttle setting and then once that new throttle setting is achieved, the governor reduces fuel to maintain that speed. So does increasing the max no load rpm improve performance or just increase the top speed?

[buswarrior]

One advantage to raising the governed speed, is more overlap between gears.

Those who stick it in Drive, move on to the next post.

If you know how to exploit it, the flexibility to have choice between a higher or lower gear for that few more mph comes in handy.

A little more mph at the top of 2nd and the top of 3rd gear would be really handy for tooling around on the secondary roads.

happy coaching!
buswarrior

[Fredward]

Buswarrior,
You stated what I am thinking. Its not like I would run her wide open all day trying to get from point A to point B. I'm not looking to hasten an overhaul. To me thats not what bussing is about. But having a little more flexibility or snoose when I want it might be nice. The fact that it would do 80 mph means that 73 mph is just that much more comfortable. Actually, if I were planning to keep it long term I'd look more into turbo'ing it as Tom C suggests.

My plan is to sell it and get a stainless one. Mine, in case you didn't notice, has a roof raise and the whole thing is re-skinned with steel and aluminum. Looks real nice but not bus-like enough for my tastes.
Fred