850 RPM redline

I think this has been covered before but I couldn't find it - I may have even asked it!

The rider's manual lists the 850's "Peak RPM" as 5800 and the 750's "Peak Rpm" as 6500. It then says for both engines, "Do not exceed 7000 RPM."

A tech bulletin (if I remember right) says that max sustained RPM for a 850 is 5900 RPM. I assume that means it can run at 5900 RPM all day long.

So here's my question: IGNORING the HP peak RPM and the "max sustained" RPM, CAN an 850 safely rev to 7k at all without "combatting" itself?

Sure it can, a few times. Don't stand in line with the flying parts though. Jim

I'm no expert, but I slept in a cheap motel more than once and asked any-everyone about what breaks first then second then 3rd most on Commando's.

The thing to keep in mind is the damage danger is like radiation, it accumulates till something dramatic shows up. So you could run an 850 up to valve float and likely get away with it for a while, but who knows how long.

Takes lots of money and time to build any Norton to stand relative hi rpm.
Forces go up exponentially as rpm rise so 7000 rpm is like 4x harsher than 5900.
[just ball park math but level of pensiveness at tach runs upward]

The weakness item is the cast iron flywheel. Best tale of that breaking in non-racer I heard was one that let go at idle in drive way to go through cases into car across the street to richohet through the home picture window and mess up stuff inside home. Steel flywheel is vital for sustained hi rpm. ~$500+ for that then ~$350 for dynamic balancing, not counting the cheeks and bearings.
Off the shelf race level crank, ~$1200-1500 depending on how much heavy metal slugs to fiddle the BF. Higher BF is about universally used in hi rpm Nortons, solid or iso mounted.

Crank tends to turn into a jump rope shape over 6000, with ends tugging on the cam drive dramatically, to point it can unwrap cam tensioner &/or jerk off the oil pump snout. Dragsters preferred the thinner 750 cases to take up some the flex more than fracture. Tends to separate horizontally through the rear 2nd bolt.
Can have cases re-enforced or pay Maney for ~$1800 race cases.

Then long stroke acceleration on pistons rushes up on destruction. Al definitely has a fatigue life - can take max loads fine for a while - till it don't. Norton rods seem Ok up over 8000 rpm but not factory pistons for very long. You've seen Jim's piston steel rod kit ~$1500.

Valve train seems good to 7000 if all is well but reaching its float limit to clash.
Most of ~$1000 to get head up to speed.

This sorta ties into 750 vs 850 thread, if seeking sports bike thrills best go with 750, if normal thrills and long rides 850 is fine, but 750 matches that same way.

On factory 850 I'd follow the manual and cruise all day up around 5000 rpm but know I'm using up engine to sustain over that w/o using up wallet thickness.
Gearing up helps to use torque of 850 rather than rpm to fly freeways.

hobot

I sure would respect what Jim said. He knows more about these motors than just about anyone.

I think it is safe to say that most here would agree that somewhere around 4500rpm is sustainable for prolonged highway cursing. It IS a long stroke air cooled pushrod motor.

I have found that there is really no point revving my 850 past 5500 even for a short burst.

Yep a waste of time and motor on 850's unless brought up to Combat specs and racer quality parts. To me the Cdo went down hill after '72, though late '73 850's may be exception worth spiffing up. Glad I got some bargain 750's before their desirability over 850's gets better known. But even though 750 keep on pulling they are also limited in life time run over 5000 for long. Norton must of known this as they dulled the 850 handling by increased rake for cruiser market, fooling public with reputation of the prior 750's as super bike sporting.

My experience is that for long journeys gear it so that no more than 4200 is used under normal circumstances, at 4200 or thereabouts there's a sweet spot, over 5k it feels like you're killing it (years of experience tells me thats when the problems start) I've got a couple of Commando's in the shed that came to me with 19T gearbox sprokets, they felt harsh and the 70mph at 4500 felt stressfull, if you are racing then thats obviously different, but as the racers will tell you there is a correlation between revs and cash!

I have many times ridden an 850 for extended periods at 5500 to 6000 -over 100 mph for 500 mile stretches. They are pretty tough but as hobot says the flywheel is the weak point and over 7000 is pretty risky. Jim

There has been a few threads recently about setting the main jets. How is anyone going to set them without revving it?
Suppose I ought to stop redlining mine after 30 odd years.

Flo said:

There has been a few threads recently about setting the main jets. How is anyone going to set them without revving it?
Suppose I ought to stop redlining mine after 30 odd years.

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That doesn't sound like fun. Just replace the rods and go for it. Jim

Interesting Hobot.

I would think the 850 is better suited.

The Combat had a higher compression, higher lift cam, 19 tooth sprocket, and originally non superblends.

The 850 also has the barrel through bolted for more strength.

1/4 mile times for standard 750 and 850s were essentially identical.

As far as the handling, one more inch of wheelbase gives the 850 more straight line stability than the 750, without any complaining loss of flick ability to the vast majority of riders, your own expertise excepted of course!

And, with some 35 years after production ended, one would think the secret advantage the 750 has in your opinion would be very well known by now. Desirability seems to be fairly evenly split among owners.

7000 is the alleged redline for both models. The 850 makes good power to 6200, and I bring mine there regularly in the first 2 gears. I have hit the ton for short distances (6400 rpm) and the motor just hums.

I learn a bit more each time this subject comes up. Just plain ole fate in the components production quality and attention to assembly. I sure can't counter all the good points mentioned in 850's just I'm more atuned to the cockier 750's, Combat in particular and race like modified in particular.

I didn't like the 19T on Combat either, perfer 20 or even 21T for the long legged cruise and ups 2nd gear range to 80-90 mph. The 850 ratio 2nd is definitely the way to go instead of lower 750. In reality I hardly ever get into low speed or standing start drag races, what usually happens is we all get up to race speeds pretty together then the real game is on staying near power band in and out of turns pulling ahead in the opens or at least now losing ground.

Would be fun and educational if there was a race event for just Commandos mostly factory set up and see what pecking order works out that way. Pilots would need switching around too as can't ignore skill or bravery to get most from a bike.

Personally I've seen the light no Norton is innately capable of hi rev for long so I've gone the other way to spank sport bikes then harsher yet to make me squeal by pure torque of extra displacement and boost - not to rev much if any over 7000.

Heck the most elite racers are breaking the best parts in the world not much beyond stock red lines, ugh. Vendors need our support to keep hobby going, so get out there and see what breaks first then fix it better and better forever and ever. Can't forget that Commando won endurance races in their time and some can today. I want that too so spending a lot to attempt it.

Hobot, not sure about this but I thought the main reason Norton made 2nd gear just a little higher on the 1973 model line was to insure their bikes imported to the US market would be able to meet the Federal decibel standard where sound was measured at some 3800rpm. The 1973's both the 850s and any odd lot of 750s also imported had to have this gearing to meet this standard. I could be wrong, just going off senior moment memory.

Also, you state that you could pull to some 80-90 mph in second gear. Really? You SURE about that?
Well ok I suppose you are using a much larger sprocket for all out top speed.
What size are you running on your bike? Thought you said you run a 20 or 21 tooth. Must be more teeth than that?

The reason I started this thread was that I'm thinking of dropping the 22T sprocket to a 20T to gain the acceleration - actually, to "REGAIN" some of it. 7400ft MSL here lops around 13 HP out of the engine. BUT, I don't really want to reduce the "no-concern" cruising speed capability any lower than mid 80's. It would seem to me that a 20 would be ok for that - engine RPM around 5400RPM but the top speed would be only around the ton. Life is full of compromises. I don't recall my 750 being so limited - I ran it out to 7k every day in every gear I could traffic-permitting and it seemed far stronger and faster top end.

But I have to remind myself that my 750 lived in Memphis, TN, altitude 337 ft, as opposed to my 850 living in Mexico City, altitude 7400 ft though it will get as low as 6400 in San Miguel de Allende. Also have to remind myself that what I "remember" may not correlate to reality - sort of "the older I get, the faster I was" thing.

Altitude sure kills them. I love it when I take road trips down to near sea level. I am at 5400 ft here at home for about a 16% power loss. Makes the difference between first gear roll on wheelies or not. Jim

Also, you state that you could pull to some 80-90 mph in second gear. Really? You SURE about that?

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Yep, but took at least 7500 to get to 90 mph with almost 21T ratio d/t belt drive + 20T. But I'd gone through pre-Peel 5yrs with much hand holding and race level upgrades.

But what makes this impressive to me was only time I'd rev that high was going faster than fear in btween 10 mph switch backs, in a section officially know as the <Jasper Disaster> about the most concentrated turns per distance in America,
later in the day I got last rod link fitted on front and had already hi centered on dirt mogals and steep ravines in the woods trying it out. I'd have blower on that engine long ago and be done with Peel but for a stuck throttle event to start over again. I know what happens on over rev engine and gear box and just won't do that no more on plane jane Cdo's no matter the displacement.

Knowing street Commando'rs love the higher ratio 2nd for it 35 to 80's mph zings.
Its a performance upgrade for road work not straight line sprints, though I liked it.
Also safer less rear hop on fast down shifts with motor drag, another bad habit I no longer do, but sure was fun while i learned about that in '99.

I feel about 5000 rpm is my limit to ride long at, but fine for as long as I like.
Peel did over 100 mph at 5000 so freeways were about 4500 ease at 80's.
My stock Trixie Combat just feels too damaging to press much but do run her into low 6K's to get to speed up steeps on hi way entry. Then I settle down to legalish rates on her. 90 mph on non linked Commando is about tops for me
both to preserve it and my own comfort zone on un tamed isolastics.

It hurts my bones to watch the real racers wring the hearts out of their engines.

90 mph in second gear is certainly possible with the right gearing. You could just barely do it with a stock primary drive, and easily do it with a belt drive primary with a taller ratio. I did a spreadsheet some years ago with all the different primary drives I knew about, versus rpm, sprocket combos, and speed. All are based on an 18" Avon rear tire. I'm also assuming the later 1.63 second gear set and stock 42T rear sprocket. This is what I get for the two extremes.

Stock 2.19 primary drive and 24T countershaft gives 147.91 mph at 7500 rpm in top gear, and 90.75 mph in second.

Bob Newby 1.75 belt primary and 19T countershaft gives 146.49 mph at 7500 in top gear, and 89.87 mph in second.

In the real world, you'd lose some of that top gear speed to tire slippage, bit very few Commandos will pull that sort of top gear speed anyhow. The only ones I know of are either killer 920 bikes or turbocharged or running on nitrous.

A belt primary drive with ratio somewhere in between would still let you run 90 mph in second at 7500 rpm with the right countershaft sprocket, something between 19T and 24T. For example, if Steve had a 1.82 primary ratio, like the Norman White belt drive, he would get that result with a 20T rear sprocket.

The trouble with that kind of gearing is that you really have to slip the clutch to start from stop on a hill, and you find yourself havng to drop back to third on the highways to pass or maintain speed up hills.

The only place I ever ran that sort of gearing was at Daytona or Bonneville.

Ken

The trouble with that kind of gearing is that you really have to slip the clutch to start from stop on a hill, and you find yourself havng to drop back to third on the highways to pass or maintain speed up hills.

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I had to slip clutch a bit even on level ground for smooth starts, but did feel a real commitment to forward motion on full release. Peel was wonderful geared up, light and powerful enough I'd often try to up shift to 5th over drive on steep climbs at highway speed d/t to sensitive throttle to run right over legal or safe speeds, so wanted temptation removed to more Harley like lugging ease. Peel Deer sold drive was 36/72 ratio with 20T sprocket on 18" 120x80 tire. She could only pull 6000 in 4th.
I hated 3rd ratio, too tall to keep pull going, too low to get over 120 with.
Next Peel will only need 4th - with some clutch slip.

mike996 said:

The reason I started this thread was that I'm thinking of dropping the 22T sprocket to a 20T to gain the acceleration - actually, to "REGAIN" some of it. 7400ft MSL here lops around 13 HP out of the engine.

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Bummer. 13 hp

BUT, I don't really want to reduce the "no-concern" cruising speed capability any lower than mid 80's. It would seem to me that a 20 would be ok for that - engine RPM around 5400RPM but the top speed would be only around the ton.

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Stock gearing for the 850 was 20T, with 112 mph theoretical top end. It comes out to about 16 mph / 1000 rpm in top gear. A healthy 850 will do 85 mph at 5400 rpm all day long.

I run a 20T on my MKII but my personal preference is to keep below 70 mph for long cruises. The combination of wind in the chest and mental fatigue watching for road hazards makes high speeds on the Norton very uncomfortable for me. Maybe if I had a wind screen I'd feel differently.

Flo said:

There has been a few threads recently about setting the main jets. How is anyone going to set them without revving it?
Suppose I ought to stop redlining mine after 30 odd years.

Click to expand...

I went up a couple jet sizes on my '73 850 (to a 280) based on a ride with my then girlfriend (now my wife). There was a big hill (probably 8% grade) on one of my common routes, and 2-up, it required full throttle to maintain just 65/70 mph on it. At that throttle setting, there was an obvious "hole" in the power. To get a feel for what the mainjet is doing, you need to hold full throttle for several seconds and it's not too easy to hold full throttle on a Commando solo on public roads, even on a typical "big hill" that we have around here, you'd be doing more than 90 mph. So, if you can find a big hill and a willing passenger it makes it much easier to see what the mainjet is doing. The best way would be with a dynometer.

-Eric