Big valves or small valves

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Alan L said:
I think I was there. I seem to recall Peter Stevens Motorcycle Shop ran a very nice Triumph T140V that year.
we had just bought one and it was good to see it going around although not really competitive
al
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At about that time Peter Allen was riding Frank Mussetts 750 triple and Bonneville 650. He would probably lapped slightly faster then the guys in that production race. But Mussett's bikes wrre very modified. British production racing allowed a few more modifications, But the Six Hour was very strict. It got silly - somebody got pinged for taking an inch off the exhaust of a bevel Ducati. It lifted the pipe slightly to allow faster cornering
 
acotrel said:
At about that time Peter Allen was riding Frank Mussetts 750 triple and Bonneville 650. He would probably lapped slightly faster then the guys in that production race. But Mussett's bikes wrre very modified. British production racing allowed a few more modifications, But the Six Hour was very strict. It got silly - somebody got pinged for taking an inch off the exhaust of a bevel Ducati. It lifted the pipe slightly to allow faster cornering
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Al I know you have a problem with rules. But racing is based on rules. Getting collared for ‘cutting and shutting‘ the exhaust pipes in order to gain a competitive advantage on a PRODUCTION race machine is totally correct!

If a racer wants open ended rules… he should compete in an open class. If he wants to race production bikes, he should compete in the production class AND pay close attention it what is / is not allowed. It’s really not that complex. It’s all part of racing.
 
UK Production rules weren't the same as Aussie rules.

UK allowed cutting & shutting - for example... Malcolm Moffatt winning the NW 200 in 1976 with a heavily re-worked exhaust.
This bike is currently residing in my garage. The exhaust didn't survive.



Ducatis featured heavily in the Castrol 6-hour but never won. I seem to recall Hailwood rode one a couple of times prior to his TT win.

A Bonneville won the Avon Production Championship in 1978 - none of my non-Brit riding mates would believe me!
 
B+Bogus said:
UK Production rules weren't the same as Aussie rules.

UK allowed cutting & shutting - for example... Malcolm Moffatt winning the NW 200 in 1976 with a heavily re-worked exhaust.
This bike is currently residing in my garage. The exhaust didn't survive.



Ducatis featured heavily in the Castrol 6-hour but never won. I seem to recall Hailwood rode one a couple of times prior to his TT win.

A Bonneville won the Avon Production Championship in 1978 - none of my non-Brit riding mates would believe me!
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Wow what a cool thing to have in the garage!

If cutting and shutting is allowed, then it’s allowed… but if it’s not, then it’s not. That’s what I mean about it not being complex, just read the rule book.

But…are you sure it was allowed? Or did they just get away with it? I know of one prominent Norton racer (who shall remain nameless, but SteveA knows him) who told me they played around with different diameters and lengths and cut n shut them to tuck them in, he also gutted the peashooters to modify the internals, but by his own admission this was all “not quite strictly within the rules”!
 
To quote the Late, Great Cal Rayborn:
'Don't get ahead on your cheatin', but don't get behind on it either'

I seem to recall Jamie Witham's YZF 750 was well over 830ccs...
 
acotrel said:
In my opinion, people who own road-going Commandos are simply custodians, Commandos are like what genuine featherbed Manx Nortons are to the people who own them. And they suffer from the same problem - modification.
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That makes some sense.

Only issue I have with modifying a pre-Commando vintage Norton motorcycle, so that it actually makes a little more usable HP, handles and stops better is it makes the motorcycle really difficult to sell. My Norton doesn't suffer at all from the modifications. It suffers from the perception that modifications are not OK anymore. That and it can't be raced like it is. It is a rule breaker.
 
Nige
Slippery Sam had a raised frame? Well Bob Hollen rode an immaculate proddie Trident with the CRMC. Martin Rustler of Rustler racing (ex BSA frame man & triple racer) built Bob a replica frame. I saw this on Bob's garage wall. Turned out the race bike had a MK2 version & had been raised again!! Norman White would be able to tell us some storys about the Commandos.
 
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Schwany said:
That makes some sense.

Only issue I have with modifying a pre-Commando vintage Norton motorcycle, so that it actually makes a little more usable HP, handles and stops better is it makes the motorcycle really difficult to sell. My Norton doesn't suffer at all from the modifications. It suffers from the perception that modifications are not OK anymore. That and it can't be raced like it is. It is a rule breaker.
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Ok, who sells their Norton anyway... lol
 
Sometimes the rules in motorcycle racing are designed to help the bikes the rule-makers own. One of my friends was a Suxuki dealer. He had a letter which gave the engine numbers for the bikes in a shipment which were intended to be used ibn the Castrol 6 hour race. The race showcases road bikes. It was intended to gull the punters into buying bikes. I think the video I posted showed the limitations of big powerful bikes on tight race circuits. Oran Park was a bit better for big bikes.

 
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For everything you do to your bike, there are usually trade-offs. Especially if you want your bike to be a good ride everywhere. Bigger valves is more a racing thing, as is fitting race cams, megaphone exhausts and higher compression. Often the law of diminishing returns applies. If you spend beyond a certain figure, to get extra performance you do not get much for your money. Plenty of guys have tuned their bikes to a standstill.
 
t ingermanson beat me to it, this started ouy about valves and has already gone thru many topics other than..... Valve sizes
 
cjandme said:
t ingermanson beat me to it, this started ouy about valves and has already gone thru many topics other than..... Valve sizes
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True, but, even without Al‘s usual thread bombing, a thread about big valves IS about tuning, so is therefore always gonna stray into conversations about other parts of the ‘package’ necessary for big valves.

And such talk about tuning and big valves is inevitably gonna be related to racing.
 
Fast Eddie said:
Wow what a cool thing to have in the garage!

If cutting and shutting is allowed, then it’s allowed… but if it’s not, then it’s not. That’s what I mean about it not being complex, just read the rule book.

But…are you sure it was allowed? Or did they just get away with it? I know of one prominent Norton racer (who shall remain nameless, but SteveA knows him) who told me they played around with different diameters and lengths and cut n shut them to tuck them in, he also gutted the peashooters to modify the internals, but by his own admission this was all “not quite strictly within the rules”!
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They 'all' also ran 33mm carbs and 2S or 4S cams.....and a bit of compression, and started races with a dead engine, ask them why they all had long kickstarters!

(That all is except me, who ran 28.5mm ports and 30mm carbs, because I was young and naive and that is what I had!)

I don't actually know what size valves most favoured, but honestly I think mainly standard, or a fraction larger inlet, meaning they didn't all re angle the valves!

Nigel, that Procuction racer we both know, it has never ceased to amaze me that he isn't better known or talked about more, after all, he was the last to race a Cosworth for Thruxton in '76!
 
Time Warp said:
To much horsepower might be just enough to launch you to the scene of your next accident (Big valve input) so one more drum brake story can't hurt.
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Never have to worry about that with a classic Norton with small valves small ports and small carburetors as long as one has decent brakes.

A hot rodded up Norton doesn't make that much HP either. Just saying Nortons are a safe ride for the most part.
 
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When it comes to port size - smaller is not always better. My Norton started off with 28.5mm ports and it was a slow dog compared to my flat tracker style 650 BSA. There was no top end. I was so dissappointed that I decided something had to be done. So I worked on the ports. I didn't go too far. I opened them up to approx 30mm and improved the flow with a broader the curve down to the seats. I was amazed at the results. Now with a 19 tooth sprocket I was getting 2nd gear wheelies down the block. The bike was so fast that I decided to race it and it was one of the fastest Nortons on the track. Sure you can go too far and over port and lose all the mid range, but if done right you can get a lot more power. Later on when racing an 850 with the same head I went to slightly reangled 1mm oversize valves and long duration cam I got even more power.

Stripped down street racer below. Note the extended rubber manifolds, 32mm carbs, velo stacks under the air filters.

Big valves or small valves
 
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jseng1 said:
When it comes to port size - smaller is not always better. My Norton started off with 28.5mm ports and it was a slow dog compared to my flat tracker style 650 BSA. There was no top end. I was so dissappointed that I decided something had to be done. So I worked on the ports. I didn't go too far. I opened them up to approx 30mm and improved the flow with a broader the curve down to the seats. I was amazed at the results. Now with a 19 tooth sprocket I was getting 2nd gear wheelies down the block.

Stripped down street racer below. Note the extended rubber manifolds, 32mm carbs, velo stacks under the air filters.
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This is your Atlas motor you ported out to 30mm, and the same motor on the "street racer"?

Fast Eddie said:
A thread about big valves IS about tuning, so is therefore always gonna stray into conversations about other parts of the ‘package’ necessary for big valves.

And such talk about tuning and big valves is inevitably gonna be related to racing.
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Of course, but complaints of promoter conflicts of interest, counter steering, drum brakes, and fork yoke offset don't have much to do with anything close to valves, big or small, open or closed.

I enjoy all the contributions and contributors when they are even remotely pertinent to the thread, as do I think everyone here.
 
Years ago when I was racing my short stroke 500cc Triton, I had the valve seats cut with the three cutters. So the width of the seats were only about 3 mm. It definitely made a difference. But that motor used to rev to 10,500 RPM and had huge ports. With my Seeley, I have not bothered to do that. The motor is fast enough. I have just learned to use the bike better. The steering geometry was a critical factor. You might have a problem if you change that on a normal Commando, because of the isolastics. The forces on the frame and engine cradle become different.
With the motor, there are two things - gas speed in the port and valve area. But cutting the valve seats correctly is probably more important than fitting bigger valves. Bigger valves weigh more, so affect the rev limit. I have not lightened the valve gear in my motor.
With petrol motors jetting is much more critical than it is if you use methanol. The prescribed needle jets for the amal carbs on Commandos are 0.106 and 0.107 inch for petrol. That is a huge step. Even when using methanol, half a thou of an inch in needle jet internal diameter makes a very big difference
 
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