Max sustainable RPM?

milfordite

milfordite

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Here's a question for the masses. A long time ago in Cycle magazine, a fellow wrote in and asked "At what rpm can I run my Honda for sustained periods?" Cycle asked Honda, they replied "At redline". Obviously the engine would wear faster, but you could run it at redline all day without it blowing up. This assumes the engine was in good shape to begin with I would think. So, at what RPM could you run , say, an 850 engine in good shape for sustained periods of time? I asked this at a rally tech session once and no one wanted to commit to a number. Just curious what you all think? Standard engine, factory parts. I'm thinking 5,000 rpm all day would be pretty safe? I run mine at 4-4,200 all day long on a trip. 70-75mph.
 
When I first got my Commando I ran it at 6400 in top gear for about 4 or 5 minutes.
At the end of that run the rpm dropped to 6200 then 5800, then I looked down and saw lots of oil everywhere. The studs in the head had pulled.
But at least the crankcases and crankshaft held!

I have since run mine at 5000 -5500 for much longer periods without any problem.
I don't think these are a greatly durable engine when it comes to high rpm (if you can call 6500 high rpm), but I can only guess at where the disaster point is.
The guys that tried racing with the original engines probably had a good idea.
Doug McCrea did a lot of that and lived thru at least one disaster.

Glen
 
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If I recall correctly, Jim Comstock said you can run a Commando engine all day long at 5k.
 
I think there's more to be considered than just a number for the question. Rider weight and girth, number of front sprocket teeth, compression, ambient temperature, humidity, with or without a passenger, and probably a bunch of other things.

You can have an engine loafing at 5k rpm or one straining at 5k.
 
In a road test of the 850 MK2a by Dave Minton in '74, I think Norton advised 5,900 rpm as maximum for sustained cruising. I happily do that on mine when I can get away with it, but the only Norton bits in the 920 engine fitted to my 850, are the oil pump, timing gear, & timing cover. I would expect a standard crank to make a bid for freedom if trying the above on a regular basis.
 
I asked this same question on a classic mini forum. Only response I got back was "you can run it as high as you want until it blows up".
I guess by that logic, only absolutely safe rpm is zero. ;-)
 
A quicker test would be to take it to max RPM. I've had my 750 up to 8200 RPM. But its not stock.
 
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If your crank is balanced at 72% of the reciprocating weight, you can safely rev your motor at 7000 RPM forever, But if it is balanced at 54% - not so long.
Isolastics DO NOT protect your crankcases. Think about what you are doing with that heavy crank.
BELIEF has a very strange effect on the humasn brain.
If you pray to God, your crankcases might last longer. But you must use the right words.
THe problem in rebalancing the Commando crank lies in the way you fill the hole in the counter weight with steel. The only really safe way is to either fit a billet crank or a different flywheel - ( Andy Molnar )..
I would not under any circumstances drill holes in the fly wheel. I think it is cast iron.
The Cammando design with the isolastics and the low balance factor works well in a road bike qhich is not used for high speed stuff, They are obviously not intened to be used for that. Just ride it sensibly or buy a modern bike.
 
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jseng1 said:
A quicker test would be to take it to max RPM. I've had my 750 up to 8200 RPM.
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I have taken my stock 750 to 7500 rpm on a couple of occasions
So at least I know it'll do it without flying apart
But that was in the lower gears
I definitely wouldn't want to sustain it
 
jseng1 said:
A quicker test would be to take it to max RPM. I've had my 750 up to 8200 RPM.
Click to expand...
My 850 feels as though it would rev safely to 8000 RPM, but I would never take it there. If your motor explodes while you are riding the bike, you will almost certainly crash. 7000 RPM with a Commando engine is fast enough for anybody. It pulls much higher geasring than most other motors, and with a close box, it is a real whiz. With road bikes, you do not really use the motor to its fullest extent. What really amazes me, is just how good the 850 motor really is. I never believed in it, but it is actually competitive against the really fast stuff in classic racing.
There is nothing eeally trick about my 850 motor. There are lots of things which could be done to it, but there is no need. Changing the balance factor is enough to make it fast. The rest is tuning.
 
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baz said:
I have taken my stock 750 to 7500 rpm on a couple of occasions
So at least I know it'll do it without flying apart
But that was in the lower gears
I definitely wouldn't want to sustain it
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7500 is the practical limit and pushing it hard for a stock 750 and yes those RPMS are for lower gears. 8000 and above is unreasonable and above the powercurve of a long stroke. I only did it because I was testing components befor marketing.
 
Commando advertising brochures for 1971 and 1972 quote the "Max cruising r.p.m." as 7,000. The ones for 1973 and 1974 quote the "Maximum r.p.m. continuous cruising" as 5,900. By 1975 the brochures are no longer giving this information, only saying that max power is at 5,900 rpm, and prior to 1971 they only give a max rpm (with no mention of cruising) as 7,000. My memory says that somewhere in Norton's service notes, tuning notes, or other documentation, they specifically state that prolonged running above 5,800 (or maybe it was 5,900) rpm is detrimental to the engine, but I haven't been able to find it in the amount of time I've been willing to spend searching.

Ken
 
lcrken said:
Commando advertising brochures for 1971 and 1972 quote the "Max cruising r.p.m." as 7,000. The ones for 1973 and 1974 quote the "Maximum r.p.m. continuous cruising" as 5,900. By 1975 the brochures are no longer giving this information, only saying that max power is at 5,900 rpm, and prior to 1971 they only give a max rpm (with no mention of cruising) as 7,000. My memory says that somewhere in Norton's service notes, tuning notes, or other documentation, they specifically state that prolonged running above 5,800 (or maybe it was 5,900) rpm is detrimental to the engine, but I haven't been able to find it in the amount of time I've been willing to spend searching.

Ken
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Interesting that the maximum safe rpm was going down as the engines were made stronger.
I guess they had to face reality.
They were living in dream world with the Combat design and the result was a rude awakening.
Much the same thing happened to Vincent with the Picador drone military engine.
The military wanted an engine that could sustain 60 bhp for 4 hours minimum.
That's easy says Vincent, " We have the 55 hp Black Shadow engine. With a tiny bit of tweaking it will do 60 bhp easily.
And everyone knows it's indestructible, the bottom end is built like the Forth Bridge."
The Black Shadow engine on dyno made 48 bhp, not the advertised 55.
It blew up after 7 minutes of running on the dyno, nothing salvageable from the wreckage of the engine.

"Something must have been wrong with that engine, bring another one and hook it up"

After 11 minutes, same result, now two destroyed engines. Gee this is going well!
Not long after that they realized that a complete redesign would be needed to get 60 bhp (crankshaft) sustainable for four hours.
They did eventually get there.

Glen
 
O.P. was standard engine, standard parts. If 5,900 is max power and from memory max torque is achieved at slightly less, 5,900 it is for sustained operation…Otherwise risk for no return 🤔
 
worntorn said:
Interesting that the maximum safe rpm was going down as the engines were made stronger.
I guess they had to face reality.
They were living in dream world with the Combat design and the result was a rude awakening.
Much the same thing happened to Vincent with the Picador drone military engine.
The military wanted an engine that could sustain 60 bhp for 4 hours minimum.
That's easy says Vincent, " We have the 55 hp Black Shadow engine. With a tiny bit of tweaking it will do 60 bhp easily.
And everyone knows it's indestructible, the bottom end is built like the Forth Bridge."
The Black Shadow engine on dyno made 48 bhp, not the advertised 55.
It blew up after 7 minutes of running on the dyno, nothing salvageable from the wreckage of the engine.

"Something must have been wrong with that engine, bring another one and hook it up"

After 11 minutes, same result, now two destroyed engines. Gee this is going well!
Not long after that they realized that a complete redesign would be needed to get 60 bhp (crankshaft) sustainable for four hours.
They did eventually get there.

Glen
Click to expand...
I'd like to read more about that!
 
Matchless said:
In a road test of the 850 MK2a by Dave Minton in '74, I think Norton advised 5,900 rpm as maximum for sustained cruising. I happily do that on mine when I can get away with it, but the only Norton bits in the 920 engine fitted to my 850, are the oil pump, timing gear, & timing cover. I would expect a standard crank to make a bid for freedom if trying the above on a regular basis.
Click to expand...
MichaelD said:
I want some lager valves on mine too. But I wouldn't want to combine lager with sustained high speed running, that would just magnify the risk 😳
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My Panhead stroker always ran "better" with lager.
 
Max sustainable speed depends on how long the body can withstand the head wind on the body and comfort of the rider,. with stock gearing and the work done to my motor my Norton happily sits on 75 to 90 all day without any problems but our speed limits, speed cameras, radars on cop cars and big fines will prevent you sitting on those speeds, as well comfort of the rider and using a lot more fuel, wear and tear they all add up.
My stock 850 Commando valve bounced at 6500 RPM my hot motor will rev freely past 7500 RPM if you let it but that's the danger zone and when things go bang, but as I say depends on how long you can comfortably sustain those RPMs or speed.
But they will comfortably cruise all day between 5000/5500 RPM but would you.
 
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