Convert 750 to 850

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Sounds like a trick question to me.
Yes, anyone does know the difference, just ask her for yourself.
100 or thereabouts as well as some decals and I believe the 850 only has one question mark is what I was told.
Parts difference will vary according to similarities and also will to some varying degree depend on model and vintage.
Sorry, I had to, I'm bored and mischievous. If you have a more specific question one of the lads will be able to offer assistance no doubt.
All the best.
 
About 73 cc.

The 850 cases are bored larger for the larger cylinder liners. Late 750 cases may be large enough already.
Pistons and cylinders and hold down hardware.
Cylinder head bolt pattern is larger on the 850.
Crankshaft bobweigh is heavier on an 850.

Stock 750 will usually outrun a stock 850 due to the 850's lower compression ratio.
850 makes a bit more midrange torque and is more durable. Jim
 
An 850 with it's low compression will run on regular fuel and is more durable.

If you want an 850 to be faster then you will need to raise the compression up to the 750 level. Jim
 
Here's an entry of mine from a while back on crankcase/barrel/piston/head configurations using components available to me at the time. I've since bought 750 and 850 bikes and a nearly complete 750 engine--my engine work since this original message has confirmed the relative merits outlined below:


Many thanks to Rip et al. for empirical, compelling evidence making the argument for using 850 cases where feasible. If I wanted to leave the 850 bottom end undisturbed I could probably use my almost clapped-out spare 750 barrel with .060" or .080" oversize forged pistons to compensate somewhat for the heavier 850 crank in terms of the balance factor. Potential combinations that would make sense for me:

1. 750 crank and Combat cam in original 750 cases wth Combat head (on bike at present)

2. 850 crank and standard cam in 850 cases with RH10 850 head (current spare engine)

3. 850 crank and Combat cam in 850 cases with skimmed-down RH4 850 head (and cam tunnel opened up)

4. 850 or 750 crank and Combat cam in 850 cases with bored-out 750 barrel, heavier pistons and pins and Combat or spare 750 head

All I need is a second Combat cam and oversize pistons to cobble together any of the above configurations over a long weekend. If someone has experience running 750 barrels on 850 cases please let me know. Given that the Combat bottom end has been practically bulletproof since I put in Superblends in '72, the current inventory may just suffice for the rest of my miserable little life. Thanks again to everyone who weighed in on this.


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, '74 RH10 850
 
said:
About 73 cc.
Click to expand...

Actually, the difference is about 83cc
(745cc vs 828cc, for stock engines).

The 850 is much torquier, by about 10%,
as you would expect, since the engine is over 10% bigger.

Opethiselps.
 
Rohan said:
said:
About 73 cc.
Click to expand...

Actually, the difference is about 83cc
(745cc vs 828cc, for stock engines).

>>must be why my math teacher hated me<<

The 850 is much torquier, by about 10%,
as you would expect, since the engine is over 10% bigger.

>>I have never seen 10% more peak torque on the dyno from an 850. <<

Opethiselps.
Click to expand...
 
redFireNorton said:
Does anyone know what the difference between the 750 and 850?? and parts needed??
thanks
Click to expand...

To put together a 850 , the easyist way wuld be to get a complete 850 Engine . :mrgreen:
for a 72 on machine, the earlier ones arnt the same , so woulnt be 850 spec when done .

Guy in NZ raced sidecar speedway with 850 , 2S cam , and 1500 Cortina pistons for 920cc
with no problems regards reliability .Gives the 920 cc. sleeves needed to go larger .
 
What sort of torque do you see from the different engines ?

The specs quote Torque @ 5000 rpm as 48 lb./ft for the 750, the Combat at 49lb/ft, and the 850 at 56 lb./ft.
Thats more than a quoted 10% increase ?
 
Rohan said:
What sort of torque do you see from the different engines ?

The specs quote Torque @ 5000 rpm as 48 lb./ft for the 750, the Combat at 49lb/ft, and the 850 at 56 lb./ft.
Thats more than a quoted 10% increase ?
Click to expand...

I have not located a stock 750 report but I did find a stock 850 with a peak torque of 50.8 corrected to sea level. I have a high compression hot rod 750 report with 52.2 and my high compression 880 at 55.9. Jim
 
Doesn't sound like you have tested a stock 750 against a stock 850 for torque ?
So the factory claims could be correct ? (You would hope so !!).

A low rpm roll-on of a 750 against an 850, with same gearing and same weight rider, should see the 850 walk away from the 750.
Thats what the factory intended with the 850, anyway..

Higher rpms will be different...
 
Rohan said:
Doesn't sound like you have tested a stock 750 against a stock 850 for torque ?
So the factory claims could be correct ? (You would hope so !!).

A low rpm roll-on of a 750 against an 850, with same gearing and same weight rider, should see the 850 walk away from the 750.
Thats what the factory intended with the 850, anyway..

Higher rpms will be different...
Click to expand...

I have been dyno testing Nortons for over 20 years. I have dynoed many stock 750's however I have not dynoed one since I upgraded the dyno operating system a little over a year ago, so many of the dyno reports from years back are no longer available without attempting to read an old hard drive.

I would doubt that a stock 850 would pull away from a stock 750 at any speed. At least not any that I have ridden. The additional compression ratio of a 750 makes up for the 83ccs. They come out pretty close. The factories intentions and advertising do not always agree with reality. Jim
 
comnoz said:
I would doubt that a stock 850 would pull away from a stock 750 at any speed. At least not any that I have ridden. The additional compression ratio of a 750 makes up for the 83ccs. They come out pretty close. The factories intentions and advertising do not always agree with reality. Jim
Click to expand...

Well then, it would be interesting to see the torque curves of a stock 750 vs 850 then, wouldn't it. ?
On the brochure torque claims, at low rpm the 850 should have it over the 750.
Unless the factory was fudging the figures somehow ?

I've only owned 850s, and they are pretty strong in the low rpm range.
The 750 I've ridden was pretty revvy, chalk and cheese really....
 
I have merger experience hopping from stock 750 standard, 750 Combat and fairly early 850's, one ES. The 850 seemed to pull stronger with less throttle at first than the 750 but then didn't pull any harder while the 750 increased their pull until made me cry uncle. I've followed an ancient 850 rider in my familiar paths in the Ozarks where the speed changes ranged 35-65 in the leaning parts, I was hard pressed to keep up with just his normal on purpose cruise speed. He could enter corners about as fast but could zoom out of them with that 850 short spurts of grunt. I could catch up and pass in the opens but was more neck and neck strain. I felt the 850 had the advantage in the short blip zoom ups, like to whip out to pass a car but not all that extra fast by time to pull in, while the 750 could pull almost as well but mainly pulled longer in each gear, so could also zoom past the just over 850. These were basic factory issue not anything special done inside. As some of us like to zoom up a bit longer higher than 850 allows I pick 750 as my power plant of choice. In legal ranges and in city traffic and short light to lights the 850 may be better suited.
 
Rohan said:
comnoz said:
I would doubt that a stock 850 would pull away from a stock 750 at any speed. At least not any that I have ridden. The additional compression ratio of a 750 makes up for the 83ccs. They come out pretty close. The factories intentions and advertising do not always agree with reality. Jim
Click to expand...

Well then, it would be interesting to see the torque curves of a stock 750 vs 850 then, wouldn't it. ?
On the brochure torque claims, at low rpm the 850 should have it over the 750.
Unless the factory was fudging the figures somehow ?

I've only owned 850s, and they are pretty strong in the low rpm range.
The 750 I've ridden was pretty revvy, chalk and cheese really....
Click to expand...

I suspect the torque at 3000 rpm is a little higher on an 850 than a 750. I know the peak torque up there around 5000 is a little higher on a 750 than it is on an 850. If I get some time to spend on the dyno computer I will see if I can find something that is close to stock from the old hard drive. I am not sure if the files are usable with the new system. It might be easier to get one of the guys down from Denver for a rip on the dyno if anyone has a stock 750 anymore. I have 5- 750s but unfortunately they are all in boxes. Jim
 
I often wondered why with only an 11% increase in capacity the 830 is often quoted as having amazing torque compared to the 750. Certainly the average 830 will pull better than my 750 with over modded head. There is no way even with earlier 21T gearing that my 750 would pull well from 30mph and 1650 rpm in top gear. BTW has anyone seen this curve, attributed to John Favill and printed in Mick Duckworth's book from a few years ago. It must have been some engine to produce max torque at only 3150 rpm against a std 750 at 6000, and it matches the 750 at 6k. For sure it doesn't match Norton's specifications in the workshop manual.
Convert 750 to 850
 
The chart is in line with my seat-of-the-pants experience with the two E-Bay choppers and my mega-mile Combat--the RH10 850 with .040"-over pistons and 32mm Amals is almost as quick as my breathed-upon Combat despite the heavier engine and perhaps twenty extra pounds of forks and bling--it may have a Combat cam. I also find that the heavier engine just never stalls at stoplights or coming off a tollway run--it's impressive


Tim Kraakevik
kraakevik@voyager.net
 
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