750 versus 850 rev-ability

Oooooh! I love it when a Frank Thomas gauntlet gets thrown into the ring.
Over to you Ashley.

I was fortunate to attend the 2011 Bikers Classic meet at Spa in 2011 with Norman Lorton and some tasty Manxes. Luckily for me I was riding in a different class so never crossed tyres in anger against these swift old war horses in the 140miles or so around the track that weekend.

However my modified Mk3s engine performance surprised me. Basically blue printed standard spec with DIY lightly modded RH4 head, lightened rockers, titanium valve spring retainers, Maney steel pushrods, +0.040" JE forged flat top pistons and Carrillo conrods. Valves were standard size Black Diamond with stock AN springs.

Compression was a measured 8.5:1, carbs were 32mm Premiers with long Amal velocity stacks and a custom K&N filter, the only real aftermarket tuning part being a USA Johnson J380 cam. 
Essentially a smoother modern version of the PW3 enabling 0.006" and 0.008" valve clearances rather than the usual 0.011"
The crank was balanced by Bassett Down to offset the heavier Carrillos. Most of these mods were for efficiency and reliability rather than outright power.

I initially kept the revs at 7000rpm but soon noticed it revved easily to 7600 on one delayed shift. Mindful of the fact I was halfway through a 1000 mile return ride fully loaded with panniers, tent and leathers I resolved to stay at 7000.
However on a later lap in the company of a Guzzi 850 cafe racer, a quick glance at the Eliot electronic tach showed 8000rpm with no signs of distress from the engine. I had forgotten to change gear and was still in third on a 100mph sweeper.

I too wait to hear how well similarly modded 9000rpm 750s have gone on such a trip, but not expecting tales of creamy 850 torque everywhere not just low down...

When I bought my Mk3 (forty or more years ago), it had a Smiths speedo and a Vaguelia tacho. I found a T160Trident "NVT" wavy line tacho and had it rebuilt into a Commando studded case...What I hadn't realised was that the Trident didn't have a marked red line and I wanged around for about ten years with the idea in my head that the Commando red line was at 7500...I regularly revved the 850 to 7 1/2 through the gears in those days...it stayed together.

I have pretty well always run with reverse cones and the non-A suffix (750 type) air box.

I've heard that the short stroke 750 wasn't popular as it called for more gear changing.

I got some good advice from Steve Maney when building my engine. He also remarked that 750 and 850s had no trouble revving but didn't necessarily make big hp as a result. Solution for best power and efficiency was to get the fuelling correct for these revs. True for both road or race.

Hogslayers John Gregory and TC Christensen solved this conundrum in true hotrod fashion by adapting automotive Hilborn fuel injection to pour plenty of juice into the fire. End result around 150hp per engine and good reliability without mega high rpm.

Why the hell would you want to red line an engine.

Never done a classic track day Peter? When a Jota comes past you on the back straight at Mallory, you`ve just got to get past the flash B@stard again, which I did, diving under him going into the bend at the end of the straight on the lighter Commando. He never bothered me again. Commandos are quite capable of surprising people on a track. It was showing 90 on the clock at 7000 rpm in 3rd coming up to Gerrards and still wanting more, only my pants wouldn`t take it : ) Standard 850 engine with clip ons and rearsets.

 

Fair comment Alan, I will rephrase my post. Why the hell would you want to red line your road bike. Maybe I have too much mechanical sympathy.

It's not just the fast chaps on trackdays who need to red-line...Those of us who are slow-in and slow-out on bends need to thrash like b*gg*ry to keep up too.

If your group of friends steadfastly maintains a policy of "last one to the pub get's the round in", this is a matter of some importance.

Anywhere in a Commandos power band from idle to redline or beyond is fair game. It was a Superbike in its day, feared throughout the land and I recall most being ridden that way.

A possible source of the revvin' 750s old wives tale was that they were lower geared than 850s as far as I know. An 850 with a similar 19t front sprocket and more torque should comfortably outgun a 750 with both in standard tune.
However I won't be stripping down my primary drive to find out, 850 21t front sprocket the best all round gearing for me to avoid buying the beers.

It is my road bike Peter, 50 something K on the odometer, never had to look at the bottom end, still going strong. Something owners of the new Nortons can only dream of, apparently . . .

Not quite true Alan , Yuichi Yamamoto in Japan has 53K on his 961. Apparently some people seem to biassed against the 961, not me having owned them since 2015. I still remember the problems with my Fastback, Interstate Combat motor and the 850 interstate in the sixties and seventies but there wasn't any internet then so they didn't get a slagging off like the 961 and  being a Norton  man it didn't put me off buying a modern one.
Oh Richard I still maintain there is no reason to red line an old bike, simply because I have too much mechanical sympathy to do that. Maybe try going in slow and coming out faster to save the poor old donk, and your bank balance.  :-)

That's great news Peter, perhaps a pole of 961 members could be conducted to see what percentage are actually perfectly happy with their bike's reliability and we might see them in a whole new light rather than the recent bad press. 
Original Commandos did of course have issues back in the day also well reported in the  press at the time but they all seem to of got sorted eventually,  perhaps dedicated owners will do likewise with the new bikes. I wonder if anyone revs them to the redline?

The answer is simple, stick your bike on a dyno and you'll find out, just like I have and my opposite number has done with his 750. The result I had I queried with Steve Maney and he found the same with the 850. The best way to move the roll point higher up the rev range is to use big valves so the 850 can breath more efficiently, the size of the standard valve has spare capacity for the swept volume in a 750, but is just beyond capacity for the 850. 
I have the traces of the 750 and 850 overlaid, and it proves exactly what Neill was told by Steve Maney, both hit max HP (not the same HP) at around the same time, and carry on revving, but the 750 holds the line longer
Dyno runs are pretty cheap and can tell so much, fuelling response, air fuel ratio, torque and HP. I don't get why more don't do them - you don't need to rev to max if you don't want to. 
As some above have said, its about where the power is useable, some may have noticed the 920 going to auction at the end of the month, it is a bike that AN acquired, we repaired it and tested it in our car park, and it even wheel spins freely in 2nd gear.  

Stan,

Para A - yes.
Para B - yes, and yes he did and so did others, and like the many that pester me every week that they want cylinder heads with more material to re-angle and allow larger valves to be fitted along with porting work. Just like those currently working on 8v heads - why? because they know the bikes need to breath better, once the bike breaths better and revs higher they more area under the curves to work with. 
Para C - Possibly, as I doubt if anyone has done that and dyno'd the bike afterwards to achieve a result. 
Para D - it's not an insertion - fact, take a 750 and 850 and put them on a dyno and you will see it as you'll soon work out where the revs will roll off. You'll get to point where the energy incoming will only spin the crank to a certain rpm limit, the rotating mass now in play, either lighten the crank, rods and pistons or get more energy into the cylinder. 

I will mention you to Steve next time our paths cross, as he said and others know, you can tune a 750 relatively cheaply and easily, to tune a 850, needs more work to get the same comparable increase. We have had several 920's here, and one of those 1007cc bikes, none were quicker than my sensibly set up 850 that runs with FA head, PW4 cam that many said would not work and standard unbalanced pipes and peashooters. However, the latest 920 we have and going to auction is far quicker, but we have not had the engine apart to see why. 

For a good morning out take a friend with a comparable 750 or 850 to a dyno and try it, the results are worth obtaining, and it is good fun, checking to see how quick your carbs meter when changing gear, fuel / air ratio etc. 

the subjective feel of a "free revving" engine is another part of the discussion. For example, my MK3 engine, but with light weight Jim Schmidt pistons, long rods and lighter cam followers  revs very easily, subjectively. 

which auction for the 920 please?

https://www.handh.co.uk/auction/lot/lot-1974-norton-commando-920/?lot=60755&so=0&st=&sto=0&au=542&ef=&et=&ic=False&sd=0&mc=5&art=96&pp=48&pn=1&g=1

I think it's obvious the myth of 850 non revability is simply a red herring as I and others have found so nothing further to add there.

The standard size valves work perfectly in different ways on 750s or 850s. I never believed the 750 was inherently better, but the lower gearing and slightly peakier delivery undoubtedly convinced people otherwise. No denying the 850 torque however and it isn't more expensive to get good power than a 750, many of the same parts are used or similar cost. I favoured the 850 as a better place to start tuning.

Steve Maneys and Paul Dunstalls race bikes operated at a far higher level than our road bikes. However there are things both did which can be of benefit to keen DIY enthusiasts interested in real improvements to everyday bikes 750 or 850, like reliability and efficiency.

Using dyno tests between a 750 and 850 to back the revability debate is also a non starter, it simply compares the results, not how quick each revs. In fact going by your published dyno results I have before me, both bikes peak power is the same 6000rpm±. Both should also reach the 7000 redline so no way is the 850 1000rpm lower. Depending on what the operator feels is adequate, most end the test just past the power band unless you specifically ask them to hit the redline.

The PW4 cam you use is almost certainly unsuitable as others have stated and a standard cam would be a much better option. I followed your posts on Access and apparently it took some work to get it installed and working. However the dyno result was no better than the 64.5k mile, standard 750 tested beside it. Not what I would expect from a healthy 850 having a Fullauto head with optimised porting, an experimental cam and free flowing exhaust. 

Re bigger valves:
I bought a late production Aussie Fullauto head, a dream come true when my RH4 died. I fitted +1.5mm Black Diamond inlet valves to the existing valve train and a standard type Web#312 cam after the excellent J380 was damaged by cam follower disintegration. So along with the mods listed in my first post, this engine is really a blueprinted standard spec 850 carefully assembled and tuned with a big bore exhaust and Peashooters. Ignition is Trispark Classic and 3Ω Dyna mini coil set to 28° Cam fitted to standard marks, no degree wheel required as advised by Jim Comstock who sold me the cam

My goal was to improve low to mid range with more top end and better fuel consumption than the 45-47mpg I was getting. No intention of revving it any higher to achieve this as those AN customers seem intent on doing. After many jetting adjustments on an (unable to be revealed) long stretch of good tarmac, my seat of the pants dyno had the bike revving to 7000rpm in all 5 gears and getting consistent 60-65mpg on long rides. The main jets felt slightly weak but I'd mislaid the richer 270s so dyno test was with 260s.

Time for a dyno run at Dyno Tech Basingstoke, on the now 13,000 mile since rebuild engine to confirm my efforts. Result was a nice curve from 3000 to max power of 55 rear wheel hp at 6300rpm (with flashes to 57-58hp) The torque was even better with a 50- 52lb ft flat line from 3000 to 5800rpm or 50 to 100mph, exactly where a road bike needs it.

The operator was amazed and overlaid the result over other Commando runs he'd done, including Ashley's 850 and 750 plus a couple of Dunstallised Commandos. None came close, notably on the torque curve. I remarked it doesn't feel that powerful on the road just open the throttle and it goes. He said it does that because of the progressive power and torque, no power band as such.
He said fit the 270 mains to smooth up the weak top end and walk away. I eventually found the 270s and fitted them, can feel the difference and still getting 64-65mpg on long trips at the current 27k miles.

No debate in my eyes that bigger inlets properly done on an 850 benefits the whole range of power and torque not just at high revs. How a pure standard bike with properly set up larger inlet valves would work is debatable until someone does it. I suspect it would make a noticeable difference. Over to you Ashley and Stan.

Can we organise a burn up showdown? Anyone got a drag strip to spare? : )

All above very interesting ( what is a "Fullauto" head).  My 850 mk2a when dragged out from under the bench was serviced and ran well enough. I fitted peashooters of better finish and removed the air box in favour of trumpets.  I was amazed how free the motor ran and the torque was present right through the rev range. It was very under geared for this arrangement so I upped the output gear to a 22. Slightly long for town use but I don't do that often. A big difference to the standard set up.
J

Yes a beautiful casting and nicely machined. Is that an offset hemi combustion chamber?  I use a weir disk in my single cylinder exhaust to stop back pressure affecting the fill. It's not used in anger so no comparative runs to verify its effect.  I did a quick search on our forum and found a few interesting postings on the Fullauto. To see the head ready for installation shows the differences and by your account the tangiable benefit.  Thanks again. 

Jon

Hi Jon
No just a normal hemi but squish is offset due to progressive over boring from 500cc I think. Someone here will know the full story. Resulted in what they call a desaxe layout 

https://www.nortonownersclub.org/forum/desaxe-engine

A recent Access post had a guy making stainless 3D printed inserts for Commando RH4 exhaust ports to try and duplicate the Fullauto effect. Similar to your disc but filling in the port floor as well. Looked promising with both 3D exhaust and inlet inserts now available from Jim Schmidt Motorsports.

A friend and I have discovered that some prewar cammy Nortons were built Desaxe. Joe Craig referred to it after the war but it was discovered on 1936 / 37 single cam 500 and 600 motors.
thanks for the info.
Jon