Upgrading a 750cc Commando engine to an 850cc

This 750 got some poke:

A good friend of mine converted his 750 Commando to a 850 way back in the 1980 but after a year or so he sold it and bought a 74 850 Commando, it can be done but the cases need to be opened up to fit the 850 barrels he was always mucking around with Norton's him and his younger brother who got me hooked on Norton's and Featherbeds, but the 750 Commando's seem to better hot up than the 850 but the 850s seem to seal up better and keep the oil inside instead of outside lol.

Most of the comments about which is faster, quicker, pulls harder, etc. leave out an important piece of info. What front sprocket is installed. A Combat with a 19-tooth front sprocket is obviously going to feel different than an 850 with a 21-tooth front sprocket.

I once had two bikes for sale, a Combat and an 850 with the scenario above. Both Black Roadsters completely rebuilt and beautiful, both for the same price. I guy came planning to buy one (his first vintage British bike). He took the 850 for a test and came back smiling. Then he took the Combat and came back saying he would take it even before shutting the bike off. He was shocked how much more "powerful" the Combat was.

The factory knew their market when they sent all of those bikes out with 19 tooth sprockets fitted. No doubt it also helped to bring on some early engine demolition, but it sure was fun while it lasted!

My 850 has had a 19 tooth sprocket since new and the engine has never demolition in the 46 years of ownership and its been rode hard for most of its life as well a everyday rider till semi retirement 7 years ago.

I was thinking of the Combats as delivered. They liked to rev anyway and the 19 tooth made it even easier to do so.
My cousin's new Combat lasted 3 weeks and 1500 miles of hooligan riding from new. After that it had a broken con rod sticking out of a set of broken cases.
Never saw it again.

Glen

Tornado said:

Have heard the 750's are a bit peppier than their bigger, heavier brothers. Yes more torque on the 8, but that comes at a cost of slower spool up.

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Torque figures quoted earlier in this thread are just very optimistic NV dyno readings off the crankshaft. Measured rear wheel torque figures by CycleWorld were about 44 ft-lb for the Combat 750 and about 47 ft-lb for the 828cc. The standard 750 (8.9:1 CR) will probably deliver 42-43 ft-lb.
If peppiness is desired, a shortstroke 750 is the answer!

-Knut

Nothing wrong with crankshaft figures as long as they are stated as such.
As far as I know, all manufacturers use crank figures.
A lot of internet dyno numbers are given as RWHP when in fact they are corrected to SAE J1349 or another common correction. That creates a lot of misinformation. SAE J1349 converts the rear wheel result to the larger crankshaft number, plus it corrects for temp and humidity.
Mick Duckworth supplied the SAE torque numbers. From memory, the Combat was a wee bit higher than the standard 750 and also hit it's max torque (48 ft.lbs?) at a higher rpm, which would account for the extra 5 HP at crank.
J1349 uses 15% for friction loss, so aside from the temp and humidity corrections, the Cycle World 850 at 47 rwtq= 54 ft lbs crank, not a lot different than the factory number. If it happened to be a warm day for the Cycle World test, then the numbers are about spot on.


Glen

Yes, a lot of member measuring about the 2 configurations. The 850 motor has the potential of similar performance of a Combat spec 750, but it will need help of a better cam and a compression ratio of at least 9:1. The cases and barrels are more durable and a RH4 head preferred. Countershaft sprocket matters only for your style of riding. You want quick, go lower, fast go higher.
Ultimately, the 850 crankshaft is heavier, so throttle response in midrange not as quick as a 750. It can be lightened or just
use a 750 crankshaft unless you are utilizing a MK3.
I guess for this exercise, why are you thinking about a 850?
A 750 doesn't need optimization while a 850 would to compete with 750 performance.
As an owner of both, I do not agree with the statement of an 850 out performing a 750 with mid range roll on .
Both of mine are 5 speed, and of course I'm going to downshift, probably down 2 gears if I really want to pass in traffic quickly.
Currently, the 750 is quicker...
Planned upgrades on the 850 of better cam an CR will close the gap, I'm sure.

Back to the original question. Sure, you can go to a 850, just buy one and don't try to create one out of a 750.
But then there's the debate about of head angle of the steering between the two.....
Personally, the quicker steering of a 750 is appreciated more by me. But that's because I appreciate twisties more than high speed riding.

One of these days I'll run my friend's 750 up dyno hill.
That would be interesting!

Glen

Derek Wilson said:

Not really that easy at all: The barrels fasten to the crankcase in a different pattern due to through bolts from the top deck, and the head bolt pattern changes as a consequence as well.

This is a 30,000' summary - I am sure others will fill in more details.

Whether you could resize a set of 750 sleeves/barrels to take a set of 850 pistons is maybe a better approach, but I am not sure of the feasibility of that.

Bigger question would be why would you want to? A good working 750 is no slouch...

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Why has nobody pointed out the error here?

The barrels do not 'fasten to the crankcase in a different pattern due to through bolts from the top deck'. I assume top deck means barrel to head mating face?

But anyway, the barrels fasten with exactly the same pattern to the crankcases, 750 or 850! The through bolts use the threaded holes in the crankcases previously used by the 750 base studs!

(on some 750 engines you may need to gouge out some metal from the crankcase mouth, but not all. And it doesn't need to be that pretty!)

However, the head bolt pattern is of course different. That is not insurmountable, Steve Maney used to prefer 750 heads as a base for his porting work and moved the outer 4 bolt holes from 750 to 850 pattern using inserts!

Or you can use an 850 head.

So, theoretically, the job can be done with just new barrels and pistons.

Historically, I went the other way once and fitted a '72 750 top end on a MKIII bottom end for a weekend whilst I was having the 850 head fixed!.

Today, I run a 750 short stroke, so the barrels and head are 850 anyway, to go 850 I 'only' need to change the crank! But because I use JSM rods and pistons, I would also need some more rods.

SteveA said:

Why has nobody pointed out the error here?

The barrels do not 'fasten to the crankcase in a different pattern due to through bolts from the top deck'. I assume top deck means barrel to head mating face?

But anyway, the barrels fasten with exactly the same pattern to the crankcases, 750 or 850! The through bolts use the threaded holes in the crankcases previously used by the 750 base studs!

(on some 750 engines you may need to gouge out some metal from the crankcase mouth, but not all. And it doesn't need to be that pretty!)

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Hey SteveA,

My bad - thanks for calling me out and berating me, Twitter style!! LOL

I will not make that mistake again. Consider your job done.

A friend of mine put an 850 top end on her 750
She wanted a bit more grunt pulling a sidecar
I remember looking at the drive side crankcase after she had hogged it out to fit the 850 barrel
It did look a bit thin to me but it never gave any trouble
I had a job to keep up with her on my t140 e

Derek Wilson said:

Hey SteveA,

My bad - thanks for calling me out and berating me, Twitter style!! LOL

I will not make that mistake again. Consider your job done.

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OOPS! Correcting the error, yes, I wanted to do that.....berate you? Not my intention, sorry.

But, maybe there was a reason Twitter banned me? I will be more careful in future too!

SteveA said:

OOPS! Correcting the error, yes, I wanted to do that.....berate you? Not my intention, sorry.

But, maybe there was a reason Twitter banned me? I will be more careful in future too!

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No worries - just yanking your chain!!

I had something clarified for me - everyday is a school day!! Cheers!!

Anecdotal comment rather than something I can give you actual figures for.

Way back in the '70s when discussing the comparison of 750 and 850 HP figures.

A Norton employee told me that on the Thruxton dyno, at 7000rpm 750 and 850 motors of otherwise the same specification were about equal in delivered power.

However, at 4000rpm the 850 would make 20 HP more.

I think that explains the comparison, and I have never had any real reason to doubt it.

Remember, a 750 Combat and an 850 are not at the same specification, so you need to skim the head and fit a 2S to the 850 for that comparison.

Deckard said:

........I appreciate twisties more than high speed riding.

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I appreciate high speed twisties more than anything else!

SteveA said:

Anecdotal comment rather than something I can give you actual figures for.

Way back in the '70s when discussing the comparison of 750 and 850 HP figures.

A Norton employee told me that on the Thruxton dyno, at 7000rpm 750 and 850 motors of otherwise the same specification were about equal in delivered power.

However, at 4000rpm the 850 would make 20 HP more.

I think that explains the comparison, and I have never had any real reason to doubt it.

Remember, a 750 Combat and an 850 are not at the same specification, so you need to skim the head and fit a 2S to the 850 for that comparison.

Click to expand...

"20hp" more at 4000 rpm?
That's a big difference for 83cc

baz said:

"20hp" more at 4000 rpm?

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That's what the man said, although we discuss torque at lower rpms, the motor is still making HP.

We use torque to do comparisons at lower rpm mainly because that is where peak torque is, and we compare peak torque figures.

I told you it was anecdotal.

HP=torque x rev/5252

So as the 850 produces peak torque at lower revs than the 750 then it will also produce more HP than a 750 at the same lower revs, at the top end the 750 especially the short stroke will perform better.

For a racing bike where you are only using the higher rev range then the 750 has more potential to perform, for a road bike, where lower revs are the typical usage band, then the 850 will feel the more powerful engine.

Horses for courses, decide the type of riding you will do and then pick the engine.

Not sure the 850 is 20hp better at lower revs, a dyno comparison would confirm. And this is comparing Norton engine to Norton engine, a 750 is still a high torque engine compared to others, the 850 just has even more.