850 top end

colin davie · Jun 25, 2023

I have a 850 mk2a which has a close ratio box , Norvil belt drive, clip ons, PW3 cam fitted recently by Norman White ,
when i asked Norman to carry out the work on a perfectly good motor he said you know this is an 850 not a 750..
by this i knew what he was saying that the 750 responds better to tuning than the 850 , whilst i get a fair bit more mid range grunt it strugglrs to pull more than 6000 rpm in top.... what do others rev out to and see at the top end ?
Colin

Fast Eddie · Jun 25, 2023

colin davie said:
I have a 850 mk2a which has a close ratio box , Norvil belt drive, clip ons, PW3 cam fitted recently by Norman White ,
when i asked Norman to carry out the work on a perfectly good motor he said you know this is an 850 not a 750..
by this i knew what he was saying that the 750 responds better to tuning than the 850 , whilst i get a fair bit more mid range grunt it strugglrs to pull more than 6000 rpm in top.... what do others rev out to and see at the top end ?
Colin

6k ish is where peak power is.

I think it was Mick Hemmings (also backed up by Comnoz) who reckoned the 850 was under valved. Not necessarily in stock form perhaps, but it certainly becomes the bottleneck limiting the benefits of tuning.

Big valves are not to be mistaken for big ports BTW.

kommando · Jun 25, 2023

My MK2a will go to 7K in the bottom 2 gears before I run out of legal speed. When I bought it it would not but the previous owner had fitted peashooters and not replaced the main jets, I took out the 230's and replaced them with 260's and the top end came alive. Depending on overall gearing you may never get to 7K in 4th anyway but should do in 3rd. However going beyond 6K on an 850 does not really help as past 5800 the power is dropping.

colin davie · Jun 25, 2023

Meant to add..... the previous owner had bored out the inlet tracts to 32mm saying "it is a standard mod", Norman said NO NO ,,
i have seen talk of 32 to 30mm sleeves available {usa} that improves intake velocity ,,, maybe i am chasing dreams ?

Fast Eddie · Jun 25, 2023

colin davie said:
Meant to add..... the previous owner had bored out the inlet tracts to 32mm saying "it is a standard mod", Norman said NO NO ,,
i have seen talk of 32 to 30mm sleeves available {usa} that improves intake velocity ,,, maybe i am chasing dreams ?

They’re real !

Forum members have used them and reported positively. Easy thing to try.

Intake port sleeves (2) for converting 32mm 850 RH4 heads to 30mm RH10 ports for improved perfomance. Also works for Combat heads.

850 street bike 30mm RH10 heads are prefered because the 30mm ports have a higher flow velocity. Easily convert your 32mm ports to 30mm by simply sliding and gluing in these aluminum sleeves with…

jsmotorsport.com

worntorn · Jun 25, 2023

My stock 850 did pretty well against a tuned 750 and better yet against a stock 750. We always want a bit more though don't we ?

We are a bit like farmers, who don't want much out of life, just whatever is next door to them!

I think Nigel has to be correct, if you really want more from an 850 the valves must be dealt with.
At some point though you run into the problem of an overstressed motor, cracks develop and the trans is also not up to it.
I suppose if one is a careful rider rather than a balls to the wall type you could keep it all together for quite awhile.

Glen

acotrel · Jun 25, 2023

You guys just have to be joking. My 850 has 34mm Mk2 Amals on 30mm ports which are taper 2mm per side for about the first 10mm. It has never had a valve job and I bought it second hand. If I race it, my gearing is close ratio, but overall extremely high.
It is very difficult to keep the revs below 7000, and I usually see 7500 RPM in every upchange.
If you use your bike for road use with silencers, you will never experience similar. You lose at least 1000 RPM off the top.
My cam is almost standard, but advanced 12 degrees to get the 2 into 1 exhaust to work. I use methanol fuel, so heat is not a problem. But with petrol as fuel, separate exhausts and the same motor, you should see 7000 RPM easily. One thing which might stop the motor from revving might be the crank balance factor. Mine has a threaded steel plug fitted into the hole with Loctite. The balance factor is 72% , which is probably what the Atlas motor used to be,.
There was a Triumph Saint 650 in the 1960s which had a light crank - it was as useless as tits on a bull.
What do you think happens when you rev a commando crank which has a 54% balance factor to 7000 RPM ?
It is not designed to do that - it was the first thing I changed when I built my 850 motor.
Isolastics make no difference to what happens inside the motor.

acotrel · Jun 25, 2023

The fastest Triumph motor I ever had was a piece of garbage I picked-up cheap. It had weld on the crank lobes to change the balance factor. I fitted E3134 race cans and it was on 7 to 1 comp. It was an absolute blur. I sold it to buy the short stroked 500cc Triton. - Young and stupid.
Unless you have raced a bit on different bikes with different balance factors, the effect might be unknown. Theoretically, it should not make a difference.

worntorn · Jun 25, 2023

Maybe your tach is wonky.
Factory numbers are max torque of 56 ft lbs at 5000 rpm, peak power @ 5800 rpm for the 850.
Sure they will rev higher but they accelerate harder if you keep it in the powerband, shift into the next gear.

Glen

dobba99 · Jun 26, 2023

Having worked on a Mk3 850 (back in 2016) that had had its engine rebuilt by Norman White, i would take with a big pinch of salt whatever he says. I know this sounds like herasy to Norton folk, but fitting a PW3 Cam WITHOUT asking the owner first, is not good business practise. He also fitted a single mikuni carb to said Mk3 at the same time, so in a single stroke negated any performance gain. Norman and Mick Hemmings went in partnership to get a batch of PW3 cams made and were punting them out to all and sundry and into customers engines with a statement like 'the best road cam ever'
What they should have been asking is 'what are you going to use the bike for?' Are you going to ride it WOT everywhere? or for touring and the like?
A PW3 will take a couple or three horses off the bottom end to put a couple up the top and make the exhaust more noisy.
A higher lift, longer duration cam (like the PW3) will wear the valve gear faster. With the valve train already near its max capability any greater lift/duration is eating into the safety margin.
There is a reason the SS combat cam was dropped and the factory reverted back to the Standard profile (good enough to win the thruxton 500 miler) Reliability

worntorn · Jun 26, 2023

Jim Comstock's dyno showed 6 hp loss in the middle , before and after same bike (850), with a PW3 install. Top end gain was as you say, about 2 bhp. The owner liked the result as the loss in the middle made the top end feel zingy.
Jim also said to plan on a 50 % reduction in valve seat life when using any high lift cam on a Commando.

Glen

Schwany · Jun 26, 2023

Colin,

Some of my delusional thoughts:

You aren't going to make an old classic Norton feel quick up top without destroying the character of the bike and spending a lot of money. My warped opinion is chasing that dream is hardly worth it for a street Norton unless the challenge of doing it is what gives you the greatest satisfaction.

I have a tuned 750, but I don't care much about wearing it out. I probably don't ride it enough to wear it out. Although I could break it without much effort. A few trips to 8500 RPM would do the job. It will wind up a lot more than 6000 RPM. Total waste of money to make that happen. Does my 750 have more top end than a stock 850? Heck if I know. It's lighter weight, so it certainly could.

Dan1950 another member here that has voiced disappointment in the top end on his 850 is trying to get his 850 to break the 6000 RPM barrier. Watch for his posts. I'm starting to wonder what's taking him so long.

The JSM reduction sleeves alone are not going to make a significant difference on the top end, if any at all. They are (and I'm guessing based on tuning a few of my own motorcycles) for improving the low end and midrange torque curve where most people ride. I have not seen any dyno read outs before and after use of the port size reduction sleeves on 32mm ports. It's all just talk at this point. Comnoz probably has some similar useful data, but not with the JSM sleeves.

worntorn · Jun 26, 2023

Once I'm healed up from the cow attack I'll do a before and after using the sleeves on dyno hill.

Glen

Lineslinger · Jun 26, 2023

Is the bovine still in your herd or now in your freezer?

colin davie · Jun 26, 2023

Thank you all for your replies..... pretty much what i was thinking, i wonder if i did fit these sleeves with loctite how easy it would be to remove if no gain was made ... maybe overthinking it all ??

baz · Jun 26, 2023

worntorn said:
Jim Comstock's dyno showed 6 hp loss in the middle , before and after same bike (850), with a PW3 install. Top end gain was as you say, about 2 bhp. The owner liked the result as the loss in the middle made the top end feel zingy.
Jim also said to plan on a 50 % reduction in valve seat life when using any high lift cam on a Commando.

Glen

6 horse's is a big loss in the mid range where I'm guessing most of us ride

Deets55 · Jun 26, 2023

worntorn said:
Once I'm healed up from the cow attack I'll do a before and after using the sleeves on dyno hill.

Glen

There ya go, blaming the cow again!

Hope your healing up.

worntorn · Jun 26, 2023

Healing up quite well thanks
Bad cow gone!

Glen

concours · Jun 26, 2023

worntorn said:
Healing up quite well thanks
Bad cow gone!

Glen

acotrel · Jun 26, 2023

With an 850 motor, a rev limit of 7000 RPM is sane. I think my motor would rev easily and safely to 8000 RPM, which is probably what an Atlas would do, However, there is no need to ever go there and find out. When you achieve an increase in midrange power- to use it , you need to increase the overall gearing. You also need close ratio gears to get maximum acceleration out of your bike. With my bike, it oversteers a lot when I gas it, and it stays more upright. That means I can ride it much faster through corners. When two bikes reach the end of a corner at the same time and one is already going 20 MPH faster then the other, the other one needs a lot more horsepower. With a Commando the throttle response is never good enough, but when you accelerate from a higher cornering speed, you have no trouble staying ahead.
If someone gets in your way in a corner, you have to wait and get past them as they enter the next.
With road bikes, you never use a Commando's full potential and bragging rights are not worth much.
I had to laugh - the last time I raced there were kids saying 'old rider' - it was not me going quick, it was the bike.
Those 1100cc CB750 Hondas absolutely fly down the straights, but in corners they are all out on the ripple strips at full lean, and slow. They are made by Rex Wolfenden, and at one stage there were 14 of them racing in Period 4 Historic.

850 top end

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