Upgrading a 750cc Commando engine to an 850cc

It'd be interesting to know the facts because if 83cc nets an Extra 20hp at 4000rpm then imagine how much more hp 175cc would give you from a 920cc conversion?

As this thread demonstrates, aren't there just too many important variables: cams, heads, carbs, silencers, exhaust headers, air box, starter weight... to decide which is inherently the best motor.

I prefer the slightly punchier sound of 750s, but needed an electric start. I mostly enjoy riding at 2500 to 5000 rpm and don't want to use the bike on track. That pointed me to a MkIII. Different priorities would have been a different choice, I guess. My MkIII has aftermarket silencers and K&N, so sounds nice too, but not quite as hard as the earlier bikes.

If only I could have one of each (& twice the free time)

The crankshaft torque figures in Norman White's new book are
Standard 750 - 48 ft lbs @ 5000 rpm
Standard 850 - 56 ft lbs @ 5000 rpm
Combat 49 ft lbs @ 6000 rpm.

The standard 750 BHP at 5000 is 46.6
The 850 is 53.3 BHP at 5000, an extra 6.7 bhp.
The big white Norton book gives the same torque figure for the 850 at 5000 ( max torque point) and also states that max power is at 5800 rpm, although they don't provide a number.

Glen

kommando said:

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.

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I'm not sure either, like I said, it is what the man said! Anectdotal.

At the time he had just left employment as a development engineer at Norton. Whatever I knew at the time, he knew far more about combustion engines than the young aircraft avionics technician, me, did.

However, way back then, in the same Rickman frame I ride today I installed an 850 full of Thruxton parts. It was basically a 750 short stroke head, pistons and cam with a longer stroke and standard rods. That motor revved to 6800. Hard to compare over 35 years apart, but it pulled like a train coming out of corners....and all the way down the straight! What the mid-range power did do was make it very easy to ride, you could be in the wrong gear and really not lose much at all.

Today I have a short stroke 750, which we will say revs to 7500, get it right, and I suspect there is little difference to corner exit between those 2 motors, get it wrong, however, and you are searching for gears as quick as you can.

So I would say the 'potential' of the 850 in the hands of an 'amateur' is higher. (and a 920 higher still, 1007 I think is better left to the likes of Gary Thwaites, unless you just like breaking transmissions)

The best riders should not be getting it wrong, but they can still appreciate the benefits.

Dave Croxford recently repeated that in his opinion the best bike he ever rode at Brands was a '74 space frame with the 850 fitted (which they used when not restricted to 750cc for F750.)

Peter Williams we think of as more of a precise rider (engineer!) So he may have had a different opinion.

worntorn said:

The crankshaft torque figures in Norman White's new book are
Standard 750 - 48 ft lbs @ 5000 rpm
Standard 850 - 56 ft lbs @ 5000 rpm
Combat 49 ft lbs @ 6000 rpm.

The standard 750 BHP at 5000 is 46.6
The 850 is 53.3 BHP at 5000, an extra 6.7 bhp.
The big white Norton book gives the same torque figure for the 850 at 5000 ( max torque point) and also states that max power is at 5800 rpm, although they don't provide a number.

Glen

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I was always under the belief of the following crankshaft HP numbers:

Std 750: 55 hp
Combat 750: 65 hp
850: 60 hp

If this is the case, it works out well with the numbers that Glen listed:

750 Peak Torque - 48 ft.lb @ 5000 rpm for 45.7 hp
750 Peak Power - 41.3 ft.lb @ 7000 rpm for 55 hp

750C Peak Torque - 49 ft.lb @ 6000 rpm for 56 hp
750C Peak Power - 48.8 ft.lb @ 7000 rpm for 65 hp

850 Peak Torque - 56.0 ft.lb @ 5000 rpm for 53.3 hp
850 Peak Power - 54.4 ft.lb @ 5800 rpm for 60.0 hp

These numbers would definitely fall in line with my experience with the 750 Combat and the 850.

Some background that may not be obvious to everyone.

Racers are driven by race classes, defined by capacity. Back in the early '70s there was F750. But the majority of Nortons were raced in club and national races that were typically 501 to 1000, and the 'unlimited' class typically actually limited 175 or 251 to 1300.

So 750 or 850 Nortons raced in the same classes, of course the number of 750s exceeded the number of 850s for quite a while, certainly up to '77/'78, by which time the competition was mounted on 350 and 750 2 strokes, and the coming 1000cc Formula 1 Kawasakis and Suzukis! I spent a lot of my races trying to compete with the 350TZs which in most hands were the quickest thing available to around a short circuit.

Today classes vary, but the UK has F750 as a class, and 1300 classics, very similar in Belgium. Here in France they rationalized and there is only a 900 class available, so not even open to 920s. Several 750 and 920 riders are thinking 850 is the way to go.

I chose to build a 750 in part because I wanted a short stroke and in part to fit 750 classes across Europe, and I now live in a country which doesn't have a 750 class! C'est la vie!

Derek Wilson said:

I was always under the belief of the following crankshaft HP numbers:

Std 750: 55 hp
Combat 750: 65 hp
850: 60 hp

If this is the case, it works out well with the numbers that Glen listed:

750 Peak Torque - 48 ft.lb @ 5000 rpm for 45.7 hp
750 Peak Power - 41.3 ft.lb @ 7000 rpm for 55 hp

750C Peak Torque - 49 ft.lb @ 6000 rpm for 56 hp
750C Peak Power - 48.8 ft.lb @ 7000 rpm for 65 hp

850 Peak Torque - 56.0 ft.lb @ 5000 rpm for 53.3 hp
850 Peak Power - 54.4 ft.lb @ 5800 rpm for 60.0 hp

These numbers would definitely fall in line with my experience with the 750 Combat and the 850.

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Build an 850 'Combat' and rev to 6800, what do you reckon?

I reckon the limiting factor would be the inlet valve size, the inlet tract and carbs. So you may need to address that before you actually get to 6800.

If the 650 ss has it right with the 1.41" valve then the 750 is also about right with 1.5".
The 828 needs roughly 1.6" to maintain an inlet valve to displacement ratio similar to the 650 and 750.
I guess that's why the 850 does great in the middle but doesn't exactly scream toward the top end.
There is a fix!

Glen

worntorn said:

If the 650 ss has it right with the 1.41" valve then the 750 is also about right with 1.5".
The 828 needs roughly 1.6" to maintain an inlet valve to displacement ratio similar to the 650 and 750.
I guess that's why the 850 does great in the middle but doesn't exactly scream toward the top end.
There is a fix!

Glen

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You may have gotten to the root of why (as far as I know) Mick Hemmings only sold one Fullauto head that he didn't fit big valves to! Mine!

But he wanted to! For the 750, I still think I was right. But if I went 89mm stroke, maybe not.

I truth I am just messing around with this thread, but the fact is there are 2 types of basic 'Commando' motors 89mm, 750 and 850.

(of course there are several other configurations, but they are in the minority, even the 920 and short stroke 750)

Each delivers different things. If you are in a position to 'choose', you need to understand what you want from it!

Converting a 750 to an 850 will only give you what you want if you want a slower revving max mid range power experience.

SteveA said:

Build an 850 'Combat' and rev to 6800, what do you reckon?

I reckon the limiting factor would be the inlet valve size, the inlet tract and carbs. So you may need to address that before you actually get to 6800.

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My 850 has 9.5:1 pistons and a 312a cam with 4" radius lifters. JC ported FA head with +1.5mm valves, conical springs and lightened rockers. Amal Premiers and a Maney replica exhaust. No issues spinning to 7k with plenty of oomph. I have to watch it so I don't spin right past

gortnipper said:

My 850 has 9.5:1 pistons and a 312a cam with 4" radius lifters. JC ported FA head with +1.5mm valves, conical springs and lightened rockers. Amal Premiers and a Maney replica exhaust. No issues spinning to 7k with plenty of oomph. I have to watch it so I don't spin right past

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That's sort of Combat ++

But I think it confirms Glen's/my point!

baz said:

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

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I converted my 750 to 920 in the mid 1980's. To get the extra diameter for the 850 or 920 barrels, the crankcase mouth has to be widened by the appropriate increase in diameter, and it results in the left rear crankcase mouth becoming very thin as Baz has said above. Roger at RGM predicted that the crankcases at the "thin" point would eventually crack, but they never did and I did ride it quite hard. Maybe I was just lucky as was Baz's friend.
This bike is now owned by a friend of mine and has been dormant for a couple of decades and it still remains as a 920.
Pete Lovell converted the cylinder head pattern, but I don't know if he is still working? Steve Maney still does some work when he's in the UK. Maybe he's the man to go to for this work now?

Pete Lovell is still going strong …

Reggie said:

I converted my 750 to 920 in the mid 1980's. To get the extra diameter for the 850 or 920 barrels, the crankcase mouth has to be widened by the appropriate increase in diameter, and it results in the left rear crankcase mouth becoming very thin as Baz has said above. Roger at RGM predicted that the crankcases at the "thin" point would eventually crack, but they never did and I did ride it quite hard. Maybe I was just lucky as was Baz's friend.
This bike is now owned by a friend of mine and has been dormant for a couple of decades and it still remains as a 920.
Pete Lovell converted the cylinder head pattern, but I don't know if he is still working? Steve Maney still does some work when he's in the UK. Maybe he's the man to go to for this work now?

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Yep I can attest to the fact that it got ridden very hard
It was also tugging a sidecar , usually with her German shepherd sitting in there with a jumper on !!!!!

baz said:

Yep I can attest to the fact that it got ridden very hard
It was also tugging a sidecar , usually with her German shepherd sitting in there with a jumper on !!!!!

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Found this photo

baz said:

Found this photo

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How many miles did the 920 cover?
Any 920 issues with it?

Glen

worntorn said:

How many miles did the 920 cover?
Any 920 issues with it?

Glen

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It wasn't a 920 it was an early 750 bottom end fitted with an 850 top end
I remember looking at how thin the crank case looked on the drive side
No issues with the engine as far as I can remember
The back wheel/swingarm would perform some stunts tho

Anyone know what the weight difference between a 750 and 850, stock cranks are?

To me the biggest factor on how a bike performs is how it is geared. I had two 750's, one with a 19 tooth sprocket and one with a 21. They were like two different bikes. The 19 was much more punchy off the line but seemed like it was wound tight at 70. The 21 was loafing along at 70 but was just adequate off the line. Maybe a 20 would be the ideal on a 750, but I never tried it. My 850 has a 21 and I think the higher torque pulls that sprocket size better off the line and still cruises at 70 with plenty of reserve.

If I had a 750 Commando, I would keep it as a 750, rebalance the crank, so it can rev higher without destruction, and upgrade the gearbox. A Manx gear cluster with a Commando first gear, would probably make a 750 faster than any normal 850 - the cost is about $700 for the gear cluster. It depends on how you intend to ride the bike. All this stuff about bigger motors and higher compression ratios is nonsense. A race cam gives more power right through the rev range, even below the revs at which the cam begins to work. But a Commando cam is pretty hot already. In any case, a standard exhaust system limits what a race cam can do.
When you modify a motor, all the changes do not usually add up numerically to give a faster motor. You can fantasise, but reality bites. It is easy to build a race bike which would scare you shitless. My first race bike was like that. It turned me into an instant dud, but it taught me how to survive.