Big valves or small valves

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'You cannot make a silk purse out of a cow's ear' A commando motor will never be top end and high revving. But massive torque, high overall gearing and close ratios, can make them quick enough. I tried to race with the standard Norton box - it was disgusting, the bike was not quick anywhere. With 4 speeds close ratio, it became a different bike. But what fooled me was the need to raise the overall gearing, if you improve the torque output. If you keep the motor spinning above 5,500 RPM everywhere and race-change up and down, the Commando motor is q1uite quick enough to beat that of many other bikes.
 
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What surprised me, is the last time I raced I had no trouble keeping up with the 1100cc CB750 HONDAS. - I did it easily, and we all use methanol. Those guys have been around for a long time and are very well-practised. I race once in a blue moon. Over the first 25mm, my inlet ports come back from 34mm to 30mm. My cam is near standard 850, but the inlet opens at 65 BTDC - not 53. I think if I was not running methanol, the exhaust pipe would melt. It is completely unrestricted.
 

Dan1950

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With 4 speeds close ratio, it became a different bike. But what fooled me was the need to raise the overall gearing, if you improve the torque output. If you keep the motor spinning above 5,500 RPM everywhere and race-change up and down, the Commando motor is q1uite quick enough to beat that of many other bikes.
The best way to improve the older Harley Big twin acceleration was to go with a taller (lower numerically) 1st gear and bring 3rd back closer to 2nd. Couple those ratio changes with a bigger countershaft sprocket. Those changes allowed the big twin to stretch its long legs without a lot of shifting.
 
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The 500 head below uses 1.5" valves. Its on an all out domi 500 that will be racing against Manxes and they are a lot faster than 119 mph. So stock configuration won't even come close to being competitive. This motor will have components that weren't available back in the day such as lightweight high compression domed pistons, lightweight Carrillo rods with 1-3/4" journals etc to help keep it from exploding. It will have to put out more power than previously available with domni 500s. Getting the exhaust, intake porting configuration and lengths along with the cam and valve train to work together to produce the necessary power will take a lot of development. Whether it succeeds or not won't be due to lack of effort.

270625046_10222076083630742_4789827252318966625_n.jpg


When I and others were racing 750 and larger Nortons we rode against high powered Harleys, Ducatis and jap bikes. We started with hot cams, racing exhausts and bigger carbs. That was only the beginning. It took radical alterations to keep up and win trophies. Bigger valves, wilder cams, radiused cam followers, guillotine carbs and everything under the sun was stuffed into the motors without hesitation because it was either find more power or fall behind.
 
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It would be interesting to know what Dunstall did with the 650ss engine. Did he use larger valves?
His 650 was said to be faster than Manx Nortons.
For road use, I wouldn't alter a 650ss other than to add Superblends and a Newby belt drive. My 650 ss had its ports modified by Herb Becker before I bought it. No complaints , he was careful to just reshape the bowl as he does on the racing Commandos. He didn't change the stock 1.41 " valves.
Racing is definitely a different thing from road use. You aren't expecting those race engines to run 50 or 60 thousand miles between rebuilds. Most racers will happily trade some midrange for an extra few horses at the top.

Glen
 

Fast Eddie

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The term ‘big valves’ itself needs contextualising IMHO…

Would bigger than stock valves help in an otherwise stock 500cc Norton road bike? Doubtful. Ditto the 600 or 650 perhaps.

On a 750 there’s probably a debate to be had either way.

But on the 850 I thought it had been made fairly clear that the stock valve is simply too small. So actually a mild ‘big’ valve job in an 850 isn’t really any such thing, it’s more accurate to call it the ‘correct’ valve !

Comnoz concluded clearly that in the bigger Norton engines, when combined with some port work etc, ‘bigger’ valves provide gains at all rpms.

Meanwhile, back with Jim’s 500, it’s not the 1960s anymore, so to keep up with the Manxes today he’s gonna need close to 60rwhp… more than my (what felt rather rapid) 850 produced in its first round of it’s not insignificant tuning work. It’s a tall order. You’re gonna need every trick up your sleeves Jim ! Can’t wait to hear it on full chat !!
 
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Road use and racing are such different pursuits.
119 mph is not a speed anyone rides at for long on Public roads.
One fellow recently mentioned that his body got very tired and beat up at a continuous 70 mph on his road bike. This is true
He finds it more comfortable at 55 mph.
Other said they routinely ride at 80 mph for long distances.
I do this as well. It's a very good speed to ride at for awhile because when you finally slow down to 65 the whole world goes quiet and calm. It feels great at 65 mph after doing 80.
Do you need bigger valves in a 650 ss to do 80?
Not at all.
Nor does an 850 or 920 need any help there.
But I might sneak some big valves in my spare 920 head, even just to pretend I ride around at 125mph!

Glen
 
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Fast Eddie

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Road use and racing are such different pursuits.
119 mph is not a speed anyone rides at for long on Public roads.
One fellow recently mentioned that his body got very tired and beat up at a continuous 70 mph on his road bike. This is true
He finds it more comfortable at 55 mph.
Other said they routinely ride at 80 mph for long distances.
I do this as well. It's a very good speed to ride at for awhile because when you finally slow down to 65 the whole world goes quiet and calm. It feels great at 65 mph after doing 80.
Do you need bigger valves in a 650 as to do 80?
Not at all.
Nor does an 850 or 920 need any help there.
But I might sneak some big valves in my spare 920 head, even just to pretend I ride around at 125mph!

Glen
Well, being very sincere about this, no one who wants to ride around at 55mph has ANY business with ANY performance tuning on ANY Norton heavy twin!

Difference horses for different jockeys !
 
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Racing IS different to road use. There are two parts to any race circuit - the corners and the straights. If you come around the corners faster, you will be quicker down the straights. With some bikes, if you gear them low, you will be quicker around the corners, but slower towards the ends of the straights. If you gear them high, you will be quicker towards the ends of the straights, but slower around the corners. Bikes which develop their power more at high revs than midrange, usually need to be upright as you come out of corners before you really accelerate. The early two strokes were quicker around most race circuits than the old four strokes, because of their light weight. When using a Commando engine, you need to be able to accelerate hard as you come around corners. So smooth power delivery is essential. Most four-stroke motors which have big inlet ports and radical cams do not deliver power smoothly everywhere.
A modern two stoke motor has been developed to deliver more torque, so they can corner faster. A modern 250cc Yamaha two stroke would probably make any old Commando look stupid on any race track.
If you fit separate pipes with megaphones to your Commando racer, in place of a two into one exhaust system, you will probably be slower around most race circuits.
However what you do on a race circuit does not relate to what you do on public roads. On most race circuits, there are no blind corners and oncoming traffic. And you always know where you are going.
 
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My friend who has a very nice RZ350 Yamaha blew another friend to the weeds at Broadford race circuit when the other guy was riding a 750cc Minnovation Seeley Commando.. Both of them are excellent riders. On any race track, there is always somebody who is faster than you are. But there is nobody who cannot be beaten.
 

maylar

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The 500 head below uses 1.5" valves. Its on an all out domi 500 that will be racing against Manxes and they are a lot faster than 119 mph.
Pffft.. that'll never work. Too many holes :p
 
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Yes, a good start would be to weld the holes back in. I had a Vincent with drilled heads like that. It was supposed to be for racing and the claim was that the increased edge area with all of the holes increased cooling.
The damn thing ran hotter than my regular Vin!
They had reduced the cooling fin area too much.


Glen
 
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To further the discussion, here's a comparison between a Maney Stage 3 head (for a 920) and a standard Combat head.


The Combat head against a Ducati 900ss head (a bit hard to tell, but the valve sizes aren't much different).


I think it's not unreasonable to say the Ducati took over from the Commando as the big racing twin, and also worth bearing in mind that the redline was a 7,500 rpm, so the Desmo bit was hardly an anti-valve bounce thing.
The 750s and 864s all ran 40mm Dell'Ortos
 
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@B+Bogus

Yes the historical take over might be right, but nonetheless based on my experience( with working on such heads up to the Ds1100) those Ducati 2valve heads are so outrageously asthmatic and Imho such a bad design also in terms of port design (bevels next to no SSR, DS ports more of a snake line maze) that it's next to a wonder that they even make the announced Italian HP.
Should get soon a 450 head for scrutinizing, thus can drop some more info soon, if there is interest.

Kind regards Christian
Btw the desmodromic was in later years, regarding my Limited knowledge, chosen to use more aggressive profiles than with comparable spring sizes possibile. At least that was the idea from what I got told.
 
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@B+Bogus

Yes the historical take over might be right, but nonetheless based on my experience( with working on such heads up to the Ds1100) those Ducati 2valve heads are so outrageously asthmatic and Imho such a bad design also in terms of port design (bevels next to no SSR, DS ports more of a snake line maze) that it's next to a wonder that they even make the announced Italian HP.
Should get soon a 450 head for scrutinizing, thus can drop some more info soon, if there is interest.

Kind regards Christian
I'm willing to take your word on this Christian, but in the Real World, even the ancient bevels do go rather well. It seem to be a combination of rock solid handling, frontal area and low weight. At least on my bevel Sport, there's not a wiggle to be felt at Spa, whereas the Commando is, shall we say, somewhat lively in the corners. Even at my rather pedestrian pace, the difference between the two bikes is enormous.
The Ducati is very much a complete package, whereas my Commando is work in progress. It's very satisfying when you can feel that having changed something, it makes a considerable improvement to the bike. I can do things to the Norton engine and frame that I would never contemplate doing on the Ducati for example.
 
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@B+Bogus

Yes the historical take over might be right, but nonetheless based on my experience( with working on such heads up to the Ds1100) those Ducati 2valve heads are so outrageously asthmatic and Imho such a bad design also in terms of port design (bevels next to no SSR, DS ports more of a snake line maze) that it's next to a wonder that they even make the announced Italian HP.
Should get soon a 450 head for scrutinizing, thus can drop some more info soon, if there is interest.

Kind regards Christian
Btw the desmodromic was in later years, regarding my Limited knowledge, chosen to use more aggressive profiles than with comparable spring sizes possibile. At least that was the idea from what I got told.
I'm looking at a 1000DS head right now, and can't agree with your characterisation. If I don't get 100BHP at the back wheel I'm calling it a day... I believe Rutter's bike was making substantially more than this (I just checked and it's 120BHP at the wheel); however, we're getting a little off-topic.
 
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Ducatis are often used in a different way to Commandos. Ducatis are good for thundering around the high line in corners. I do not use my Seeley 850 in that way. If your bike is set up right, you can take a much tighter line and accelerate hard most of the way around corners. An 850 Commando motor is not good as a a high-revving top end motor. I don't believe you can set a Ducati up to accelerate most of the way around corners. The Ducati is better when it is out on the ripple strip at full lean, where outright power counts more. Ducatis usually have much shorter strokes than Commandos. The concept is different. With any bike, it is the complete package which counts. Also, an 850 Commando motor is considerably different from a V twin Harley engine. Just making the Commando ports similar, probably won't do much - The power characteristic is very different.
 
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Most riders do not often grab a great big handfull of throttle when they have just entered a corner. It is more normal to follow the others at full lean on the high line without much throttle and rely on tyres to stay stuck. I did not find out about steering geometry by intent - only by accident. I sensed I could use a lot more gas and found it worked. When the back goes down, my bike steers in the correct direction on a much tighter line, while staying much more upright. Ducatis and Harleys and 1100cc CB750 Hondas do not handle like that, so their required type of power delivery is different.
 
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If your Commando motor had 150 BHP, would you he guaranteed of winning every race you entered ? - 70 BHP and better handling is good enough. Most of the early four cylinder bikes had about 100 BHP. On an reasonably tight circuit, a Z1 Kawasaki is easily beaten, as long as the straights are not too long. But even on a big circuit, a Seeley is better - no flex.. It takes a lot of nerve to ride a Z1 Kawasaki fast at Phillip Island. Ducatis go good there.
 
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