Big valves or small valves

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The old domni 500ss and 650ss offered the same size valves as Norton used in their 750s and 850s - 1-1/2 in and 1-5/16 ex. They just increased the bore and didn't bother enlarging the valves size to suit. See 500 and 850 pistons below.

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They should have upped the valve diameters - there's room for them and its what the bigger motors need.
 
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@jseng1
Thanks Jim,
That pretty much sums up what I always ramble at least about intake valve size (if alternative aggressive cam profiles are available).
As can be seen on the XR750 oval ports there are plenty other locations where one can raise the gas velocity in the ports besides in the valve curtain (of course certain other things need to respected and looked upon) area.

Happy new year
Christian
 
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1.41" in the 650ss.
They upped the size to 1.5" for 750 but left it the same for the 828.

Glen
 
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The pre-Commando Haynes manual says 1.5" for both the 500ss and 650ss - turns out to be a source of misinformation. I've had several customers order 1.5" inlet valves for their 500SS and 650 SS racers. Seems like a common conversion.

There was certainly enough room for larger valves in the 750/850 If you can run a 1.5" valve in a 66 to 68mm bore then you can go bigger with a 73 to 77mm bore. They never bothered to resize the combustion chamber from the 650 when boring up to 750 - choosing only to offset the pistons instead. Neither did they move the valves away from each other when they went to 1.5" - making clearance an issue when using hotter cams.

Customer's 500SS head shown with seats cut for 1.5" in valves - a work in progress.
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Interesting print of domni 650 head below. Compare it to the 750/850

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I've never seen this drawing, its much more detailed than the huge port 850 head drawing. This one says Dimensions Shown Are Actual Machining Sizes and has dimensions for intersect points of valve centerline to ports. I had to measure this myself on my 850 heads and the short side radius has about 1/16" less material than the 850 drawing shows. The last edit on this one says Valve Seat Location Dims Were something Planning Req. Not sure whats up there.
If 850 bores are moved forward of crank centerline that means more than 180 degrees from TDC to BDC and less than 180 going up? Like most motors are going to?
 
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Interesting that you can convert the intake valve from 1.4" to 1.5" on a 500SS and 650SS.
Are you still using the original seats, or do they need to be replaced??
Do you leave the exhaust valves size stock when you do this?
 

SteveA

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It is interesting Jim, but, how many run 750 shorts strokes? And how many run 750 shorts stroke with standard size valves?

I may be in a minority here.

I have run an 850 with big valves, on a head angled for them from the blank, that is, I used a short stroke head that had never been set up for standard valves. Cam similar to a 4S+ a little. The valves did eventually tangle at 7200.

Undoubtedly, this set up delivered real horsepower, probably around 80 at the crank. I think the valves sizes contributed, as did the large ports and 36mm carbs.

I now run a 750 shorts stroke with a standard Fullauto on standard valves, this is probably delivering up to 75 at the crank at 7000, probably, when dynoed it gave 60 rwhp at 6500, Cam similar to a PW3 and cam similar to a 2S+ a little. Valves have been spun up to over 8000 without tangling, so far!

Empirically, I would say the standard valves are indeed good up to 750, but above capacity that you should fit a larger valve!

Empirically, I would also say standard valves give a little more safety!
 
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The stock 650ss went 119 mph on test at MIRA in 1962, quite a bit faster than the Triumph Bonneville had managed.
I don't doubt that the 750 deserved 1.5 inch valves, but it's likely that the 1.41" size for the 650 was chosen by Doug Hele as it was the correct size for that head on that engine.
Bigger isn't always better.
Hele designed that head which was the main source of a 60% power increase vs the previous 600cc Model 99.

Glen
 

SteveA

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As a further point, we can all say when Norton should have improved something to meet the parameters of racing. The works team did their stuff with access to blank heads!

But Norton did what they did when in difficult financial times and to meet the requirement of selling road bikes, so they did as little as they needed, on purpose!
 

SteveA

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The stock 650ss went 119 mph on test at MIRA in 1962, quite a bit faster than the Triumph Bonneville had managed.
I don't doubt that the 750 deserved 1.5 inch valves, but it's likely that the 1.41" size for the 650 was chosen by Doug Hele as it was the correct size for that head on that engine.
Bigger isn't always better.
Hele designed that head which was the main source of a 60% power increase vs the previous 600cc Model 99.

Glen
Bigger isn't always better.

And indeed, John Hudsons tuning notes from the mid '70s says he did not believe in larger valves, but advocated getting the best possible flow around the standard ones....

Which of course as you point out were already larger than previous generations.
 
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If you read books from Bell, Robinson, Vizard, Yunick, Cameron they all say to avoid too large a throat under the intake valve head. 85% of valve diameter is the figure mentioned you don't want to go past as it causes flow seperation turbulance. The worst of it happens when the valve is just starting to open and Kevin Cameron describes it as a free jet, like trying to blow between two pieces of paper but the pressure outside the paper pinches the flow between. Putting a 1.56" intake in a 850 head gets it closer to 85% and should make it run all around better. I don't think flow benches at .4" lift or max horsepower tell the whole story
 
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The question is - do bigger valves flow more air and make more power (when there's room for them) - that's already been answered on the dyno tests. As for keeping valves from tangling - you have to compensate for a heavier valve with a better spring and adequate distance between the valves. Two solutions are the beehive springs and asymetrical cams that let the exhaust valve down more gentley so it won't bounce back up after closing at high RPM and interfere with the opening intake valve. BTW the PW3 cam produces some of the worst valve bounce of popular Norton performance cam choices.
 
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1.5" valves in a standard 650 might make less overall power. I suspect that if a larger valve was better for that engine for road use, Doug Hele would have insisted on it.
I don't doubt that you might get a higher number on a dyno right at the very top, if you rejigged the entire motor to run above its design RPM and it somehow held together. Motors like that tend to be not so great on the road and they sometimes blow up.
We always tend to go bigger is better with this stuff and I know from personal experience it doesn't always work out as planned!
On the other hand, had Doug Hele been at Norton when the Commando was bumped from 750 to 828, he likely would have insisted on a valve size increase from 1.5"
Glen
 
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b
1.5" valves in a standard 650 might make less overall power. I suspect that if a larger valve was better for that engine for road use, Doug Hele would have insisted on it.
I don't doubt that you might get a higher number on a dyno right at the very top, if you rejigged the entire motor to run above its design RPM and it somehow held together. Motors like that tend to be not so great on the road and they sometimes blow up.
We always tend to go bigger is better with this stuff and I know from personal experience it doesn't always work out as planned!
On the other hand, had Doug Hele been at Norton when the Commando was bumped from 750 to 828, he likely would have insisted on a valve size increase from 1.5"
Glen
One thing I have never done is build a bigger motor to go faster. The guys in our historic racing do it. CB750 Hondas are now 1100cc and methanol-fuelled. They are still not fast enough on smaller circuits. They cannot accelerate through corners, but are always out on the ripple strip at full lean. The problem is for a while they dominated their class, so nobody persevered much with any other type of bike. iMid-range power and good handling are better for road racing then heaps of top-end power. 'Torque wins races' - because you see a high figure on the dyno at high revs and full throttle, what does that tell you about the mid-range urge ?
 
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My friend has a 750cc Atlas which he raced in the 1960s. The crank is balanced to 79 oer cent, so it probably revs to 8000 RPM. And I have never raced against a 650SS. But I think a good guy on either of those would give me a hard time. Piston weight and the rev limit have a bearing on this matter. My 850 motor will rev easily past 7500 RPM, but I dare not do it.
 
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I would say Yes to bigger valves, intake mostly in a Norton with both valves inline and no problems with shrouding. A 2 valve motor with offset valves so they can be bigger and not tangle as easy, less angle like you do would have been on the development table if Norton had wanted to. The valve sizes could then start as the biggest that will fit an inline valve head. So I don't think you can put too big an intake valve in if you can deal with tangle and bounce. What can be done wrong is to arbitrarily open the port bigger because the valves bigger. Thats material to make a radius of the throat from valve seat to pocket above. This is where a smart flow bench guy can do good work getting the shape right to flow at all lifts. Not stall right when the valves opening. I know you know all this and more than me Jim, just adding to the conversation. My favorite car is an old bmw 2002 with m10 motor. It has an 89mm bore with 46mm intake and 38mm exhaust valves and makes 100 hp because the ports are so big it has asthma trying to inhale. They work fine if you build for 200 hp so whats the first thing folks ask when building a daily driver? How big can I open the intake throat?
 
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@edgefinder

Working now for about almost +20years on my flowbench and some of those mentioned guys 1. Refer mostly to V8 engines, or 2. In referral to the down under guy requote without citing (in his 2stroke book, which nonetheless is good, that borrows heavily) correctly others.
As mentioned before it depends on the package, cam lift curve, agressiveness of profile, intake closure etc etc. and where one wants to place certain crucial flow areas
There are certain numbers like flow capacity regarding to SQ/inch of valve area etc that need to be respected.

Kind regards

Christian

Ps: just read your comment on the 2002, eg a very well researched and developed combustion chamber design if you're referring to the double bowl chamber.
 
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Bigger-than-stock valves can make more power IF the rest of the engine dynamics support it - camshaft, intake/carburation and exhaust system. If you put larger valves on an otherwise stock setup you will typically lose low end and midrange AND have no noticeable improvement in the top end.

Believing that larger valves will increase power on their own is like believing a loud exhaust makes more power or that all those people on your Facebook site are really your "Friends." ;)
 

Dan1950

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Bigger-than-stock valves can make more power IF the rest of the engine dynamics support it - camshaft, intake/carburation and exhaust system. If you put larger valves on an otherwise stock setup you will typically lose low end and midrange AND have no noticeable improvement in the top end.

Believing that larger valves will increase power on their own is like believing a loud exhaust makes more power or that all those people on your Facebook site are really your "Friends." ;)
Everything has to work together.
 
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