Power Characteristics

I am enjoying using my Mk3 these days after a gap between that and my previous Combat.
I have found the Combat to be really nice and responsive, whereas the 850 has a large amount of torque that is really good for two up and immediately powering on.
When reading the Duckworth Commando book, there is a good illustration of the power and torque characteristics of the two different power units, which shows that they are at the two ends of the scale.
I have read here, that there are lots of people have modified there 850's (828's), which I do not intend to do, however my question is, has anybody specifically created an 850 Combat eg, 10-1 compression and 2S camshaft?
If so what was the end result?
Perhaps the best of both worlds!

My layman’s understanding is that the inlet valve is too small to work properly on an 850, so it will hinder any intended characteristics of a bigger cam.

To make an 850 Combat you need to add big valves to your list above.

Personally, I love the idea! It’s kinda what I had in mind when I hot rodded my own 850. That was 10.5:1, JS#1 cam, big valves, FCR carbs, etc. This transformed the engines characteristics completely. More power everywhere but particularly above 3,000rpm, at which point it went like a scalded cat! Maximum power was between 3 and 6,500rpm, there was no real advantage to going above that.

I think Ashleigh from Queensland has an 850 with a combat cam fitted. The size of the inlet valve should not make any difference to what the cam does. If the timings are more radical and there is more lift, you usually get a change in the location of the power band and more power both below and above it. The trouble is that top end can come at the expense of mid-range and you cannot rev many Commandos above 7000 RPM. I bought a 2S cam a while ago and have not fitted it. It looks as though it has far too much lift. I use a slightly modified 850 cam and a 2 into 1 exhaust. I have excellent pulling power from 3000 RPM - right up and through the top. What will limit the top is any restriction whatsoever in the exhaust system.
If you think in terms of two stroke tuning - when you raise the exhaust port and the transfers, you move the power band upwards.

I built my 850 Featherbed/Commando hot rod to Combat specs back in the 80s, my stock cam was built up and grind to the 2S cam specs by Ivan Tighe here in Brisbane, my head was shaved and all port work done and a bit more, I am still running my stock valves and my carbies where jetted for the hot cam as well my crank was balanced at 72% for the Featherbed frame, I am still running this same set up today.
The hot cam comes into play just above 4k RPMs and will keep reving freely well over 8k RPMs if you let it but you just got to watch how far you push it or it will explode, but the cam performs so well compared to the stock cam and will get up there a lot quicker, I am also running a Joe Hunt Maggie that gives a very big spark and a open Lochart oil cooler.
This was a everyday rider till about 6 years ago and is now semi retired just like me but I still take it out and is a fun bike to ride with heap of torque and top end power, it comfortable to ride around town at 40 mph in top gear and does it smoothly but its when you open the throttle up and when the cam kicks in is when the fun begins, I run stock gearing and has no troubles lifting the front end from first to second without trying, when I first built it I have pulled so amazing wheelies on it and have powered the wheelies right through 3rd gear, this is such a well balanced bike and super light.
I cruises best between 65 to 90 mph on the highway but its best performance is when up in the ranges on tight twisty roads where the whole bike and hot motor comes into its own, it pulls like a train and handles like its on rails.
My hot motor with the Combat specs plus a bit more here and there has been a very reliable set up and has never let me down motor wise, my motor has been rebuilt only 3 times but the last time was only to replace the crank cases because of a few hairline cracks around the main oil seal, I have done well over 160k mile on this bike and has given a few modern bikes a run for their money and has surprised a lot of their owners.
My new Triumph 1200 Thruxton is now my main bike but I still love riding my hot rod Norton and it gets the most attention where ever I go, but after 38 years riding it with this set up it showing its age, but that's the way I like it.
If you want to build your 850 to Combat specs, then I say go for it as it was the best thing I did to liven up my stock motor, do it right and balance the crank you can't go wrong.

Ashley

160k is truly impressive Ashley. You should retire that mag and send it to the US for fitment on my motorcycle.

Scout63 said:

160k is truly impressive Ashley. You should retire that mag and send it to the US for fitment on my motorcycle.

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The Joe Hunt is modern and been on the Norton for 8 years before that I ran a Boyar EI for 33 years, the JH is expensive but worth every cent it throws out a great spark.

Depends on what you want I suppose.
It's good to remember that the stock cam profile was designed by Doug Hele as a race cam.
After using it to power the little 500 around the IOM at over 100 mph, a record for pushrod type, he fitted it to the new 650ss, which also broke speed records.
At the same time it gives you a tractable engine and a valve train that isn't getting the s**t slammed out of it.
Jim Comstock knows the finer points of Norton cams very well and he has gone back to stock profile, which says a lot.
I recall that he figured the stock profile beat all others for midrange power, the best part of the Norton twin.


Glen

I had an 850 in my cafe racer that had a ported head, big valves, lightened rockers, 0.070 milled head, 2S cam, and sucking through a pair of 34mm Mikuni's. Pretty much the Stage 1 mods from the 1973 Norton service release. It was significantly quicker than my 750 MkV Roadster or my 850 Interstate which is stock except for a 2S cam. Not very happy around town as it liked to rev and didn't like being below 2500 rpm. By contrast, the Interstate with a stock RH10 head, 32 mm Amals, and the 2S cam is very tractable with good torque, however the 750 MkV with the RH6 head (9.7:1) and stock cam feels a little quicker. YMMV.

worntorn said:

Depends on what you want I suppose.
It's good to remember that the stock cam profile was designed by Doug Hele as a race cam.
After using it to power the little 500 around the IOM at over 100 mph, a record for pushrod type, he fitted it to the new 650ss, which also broke speed records.
At the same time it gives you a tractable engine and a valve train that isn't getting the s**t slammed out of it.
Jim Comstock knows the finer points of Norton cams very well and he has gone back to stock profile, which says a lot.
I recall that he figured the stock profile beat all others for midrange power, the best part of the Norton twin.


Glen

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My 850 cam has been modified to give slightly more lift. When I looked at the timings they resembled an E3134 Triumph race kit cam. That should be hot enough for anyone. If you cannot rev the motor higher to get more power, you need to improve midrange (torque). That means small inlet ports, different exhaust system and adjustment of the cam position to suit. - And higher overall gearing and a different gearbox. Then ride the bike in a way which suits the power characteristics.

The Combat engine seems to have been a good attempt to get good power by conventional means. Unfortunately, the 750 motor's construction won't cop that. An 850 motor might be stronger, but I doubt it. Most of you guys are probably using petrol - methanol is gentler on motors, even though you get more power.

My 2S cam works best mid range from about 4k rpms and up when it kicks in, any lower revs its just like the stock cam, this cam has been in since the early 80s and still running my stock lifters but they have had a surface grind, I also run cocktail shakers open mufflers with very min baffles, just enough to give that bit of back pressure, when down low revs its quite but open it up it puts out a good sound of its own, my mates can pick my bike from the exhaust sound it makes, I been running these similar mufflers since the 80s and they perform very well with my motor, gets the gasses out very quickly and they are short and upswept.
I built my motor from the stage 1 mods from the performance section of my old workshop manual, but went one step further with my crank balancing for the Featherbed frame.

Ashley

I have nothing to compare it to and only 200 miles under my belt, but my stock Combat feels pretty strong from about 3500. I haven’t really gotten it over 5000 since I’m breaking it in but it definitely has more power than I will ever need. Pipes are Interstate low pipes and much louder ( more open) than I expected. AN ham can and filter. I agree that the 2s cam is great.

You'll love whacking the throttle open in 2nd gear and feeling the front end get light while hanging on for dear life.

Despite all the cams that came after 1961, the fact Norton went back to the profile late in the Commando life says a lot. I like it.
There was a another cam in 1963 that was developed to use the 3'' radius, double chamfer, drilled bucket type tappets they also designed. Not sure what happened to that design though.

I was reading an old magazine the other day in which somebody said they would never fit a 5 speed gearbox to a Commando. If you want rapid acceleration from a Commando, close ratios are essential
If you improve the torque characteristic of your motor, often you will not notice any difference on acceleration, until you raise the overall gearing. When you raise the overall gearing the acceleration rate can increase until it reaches a certain point - then it becomes slower if you continue to raise the overall gearing. If the steps between the gears are close, the revs don't drop so far as you change up, so the motor has less distance to recover and stays near the top of the torque curve.