Big valves or small valves

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I had a 748 years back. It was, without doubt, the best handling bike I ever owned. Turned by telepathy. It was also, without doubt, the most uncomfortable. It revved up very nicely too.
By contrast, my Bevel SS turns like an oil tanker! I had a couple of 851s over the years and that was a very different animal
 
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@B+Bogus

Well let's say I'm somehow somewhat, even with the danger of getting off topic, slightly sceptic bout proclaimed +100hp!
In standard tune they have about +80 somewhat horses with in regard to bore diameter, if I remember right, smallish 44mm intake valves and an intake port that looks more like a swan neck (with lots of turbulent/thus throttled flow from certain velocities on) and a somewhat limited bowl area.
After a lot of fettling and some welding in the port walls and reshaping and installing 47mm intake valves in a modified intake port we found some very tractable 115-118horses on a (from what I got told) very reliable Dyno of a reputable Ducati tuner in Germany.
Same bat time same bat channel, similar story on the bevel engines just that on the ones i had in hand/work that the short side was next to non existent and in order to achieve higher somewhat less turbulent flow numbers one has to raise the SSR.
I'm not saying this (as an avid anachronistic Guzzi big block fan and user) to put Ducati down I'm just the opinion that the commando ports are not nearly as bad as some might think and with some TLC the head might give splendid results.
Imho it ain't too strange that both JS and HB as well as me (me poor sod that don't even own one) are telling that the commando head is capable of a lot more, darn I'd wish the Guzzi ports were as nice.

Kind regards

Christian
 
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If we are talking about racing a Norton twin against Manxes. That is a very difficult game. Tom Phillis raced the Domiracer, but he was a superb rider. It is difficult to get a featherbed twin handling as good as a Manx. Modern Manxes probably have 18n inch wheels and 60 degree rake and handle like Suzukis. The old type with the 19 inch wheels and 54.5 degree rake was much more nimble. And if you got offline, you just gave it more stick. Usually the twin cylinder motor is slightly further back when fitted into a featherbed frame. The bike cannot be ridden as hard. Jawa speedway motors are good in featherbed frames.
Many years ago (1963 ? ) I watched Jack Forrest beat all the A grade Manxes at Bathurst with a 650SS. But he used to be a works' rider for BMW.
Everyone who road races should have at least one ride on an old style Manx. The one I rode taught me why A grade riders were so good, back then.
 
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@acotrel
From what I saw and noticed, years ago, being in the same training group at a race with some of the short stroke Manx and G50 repro's it would seem quite hard to me that one could achieve that level with a vintage domiracer.

Kind regards

Christian
 
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Acotrel wrote "The old type with the 19 inch wheels and 54.5 degree rake was much more nimble."

I believe the original Manx headstock was set at 27 degrees from vertical. Measured from horizontal that is 63 degrees.

Glen
 

Time Warp

.......back to the 70's.
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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

On my own 905 VeeTwoAustralia welded the combustion chambers, changed the included valve angle pivoting between the opening and closing rockers to fit bigger valves to avoid drama at overlap (I guess) and excessive sinking as was done with Imola type camshafts.
That street engine was making power to 8800 rpm and would hold that to around 9200 rpm. (on their dyno)
The chambers, valve heads and piston tops were ceramic coated to run the high compression on any form of pump fuel which it would not do when leaded was phased out without pinging.
I seem to recall at the time (1990's) the head modifications were around $2200, the V2 camshafts were extra.
Pistons $800 a pair then the lightened (2.2kgs from the counterweight) crankshaft, straight cut primary, FCR's etc etc

I remember back in the 2000's a sweeping onramp with an overhead bridge a short distance up the motorway, on a constant 70 kmh then nail it at the same spot my (stock then) TL1000S would hit 185 by the bridge, the desmo around 160/165. (Speedometer era unknown)
Not bad for an old bus that had a top speed of around 190 stock and around 52 rwhp. (Comparable to a Commando ?) and archaic ports and combustion chambers.
A ride all day, any day 65 rwhp 850 Commando would be easy surely.
 
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@Time Warp
Ah sounds pretty much, without dwelling on it, somewhat like the program, that we did conduct, considering that the hi lift camshaft came also from down under from named WC company.
 

SteveA

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We really go round in circles, don't we?

'Seeley is better because it has no flex!'

'Rickman is too stiff!'

In my experience, neither of these statements is true!

The benefit of a Seeley has shown in tighter corners, the ability to change line at whim! How do you do that with no flex?

The benefit of a Rickman has shown in faster corners, the ability to hold a line at speed pays dividends! Too stiff for what?

The truth is, a rider who has just got off a Seeley thinks a Rickman is too stiff, a rider who has just got off a Rickman thinks a Seeley flexes too much.

It is all about your perspective and ability to adapt!

At some tracks a Norton can finish in front of a Ducati, at others it mostly can't....

BTW. This has very little to do with valve sizes!
 
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Am not sure which are the critical dimensions when considering valve size... My 71 mk3 fastback had as original an rm1 head and 30mm mk1 concentrics, Changing these to32mm whilst retainingthe 28.5 mm inlet choke made a substantial diffdefrence toperformance. Subsequently the inlets were opened up to 32 mm as per Dunstall tuning guide advice. Would there be be benefit from going up to say 34 mm foruse with standard cam?
 

Fast Eddie

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Am not sure which are the critical dimensions when considering valve size... My 71 mk3 fastback had as original an rm1 head and 30mm mk1 concentrics, Changing these to32mm whilst retainingthe 28.5 mm inlet choke made a substantial diffdefrence toperformance. Subsequently the inlets were opened up to 32 mm as per Dunstall tuning guide advice. Would there be be benefit from going up to say 34 mm foruse with standard cam?
You would have been better off keeping the ports small, having it flowed, and fitting bigger valves.
 
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Here's one of the fastest 750s ever - Leo Goffs dragster with 1/16" oversize valves. He designed his own cam and got 140mph in the 1/4 mile.

10409054_296454677207081_3948229493568282750_n.jpg

If I remember correctly he shaped the intake ports to maximize combustion chamber swirl. This works on a Norton because the ports are already angled to one side.
 
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We really go round in circles, don't we?

'Seeley is better because it has no flex!'

'Rickman is too stiff!'

In my experience, neither of these statements is true!

The benefit of a Seeley has shown in tighter corners, the ability to change line at whim! How do you do that with no flex?

The benefit of a Rickman has shown in faster corners, the ability to hold a line at speed pays dividends! Too stiff for what?

The truth is, a rider who has just got off a Seeley thinks a Rickman is too stiff, a rider who has just got off a Rickman thinks a Seeley flexes too much.

It is all about your perspective and ability to adapt!

At some tracks a Norton can finish in front of a Ducati, at others it mostly can't....

BTW. This has very little to do with valve sizes!
With more trail, your bike stays more upright in corners. If you are out on the ripple strip with all the other riders, you play THEIR power game. If you can turn earlier and ride under the other guys, you are much faster in corners. It is not how much power your motor has, it is getting it onto the ground which is more important. If you watch MotoGP, the bikes are very evenly matched and nobody has much advantage. To my knowledge, they has only been one rider who has been very different. It was John Kocinsky with the RVF750 Honda. If you watch his races on Youtube, he often comes from a very long way back and ends up in front. The bike must handle very differently from the others. There must obviously be a downside to having lots of trail in the steering. It might be the likelihood to high-side. But a Commando cannot do that unless you chuck a chain. From what I have read, apparently bikes which have too much trail oscillate when accelerating in a straight line. I have never experienced that.

The trouble is that if you are going to change the offset on your fork yokes to get more trail, how do you know what to change it to ? There would be very few people who own a Seeley and have tried different fork yokes. Even if you get an improvement, you do not know it until you ride the bike in a way in which you probably should not. I just happened to have a set of TZ350 fork yokes. THey solved my mishandling problem. But it took me a long time to discover what the bike would do if gassed hard halfway through a corner. Normally, you just do not do that. When you are out on the ripple strip with the others, you concentrate fully on being as smooth as possible, so you don't drop.
 
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baz

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Here's one of the fastest 750s ever - Leo Goffs dragster with 1/16" oversize valves. He designed his own cam and got 140mph in the 1/4 mile.

10409054_296454677207081_3948229493568282750_n.jpg

If I remember correctly he shaped the intake ports to maximize combustion chamber swirl. This works on a Norton because the ports are already angled to one side.
Is this the bike with the z1000 gearbox? I'm sure I read about it in one of the classic magazines years ago
 
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I am no genius motorcycle racer. The methanol-fuelled 1100cc CB750 Hondas I race against must have 100 BHP. My bike would be lucky to have 70 BHP. But in races, I am right up there with the leaders. If I play their power game, I am always behind them. But I don't. There is a better way.
Perhaps they are going slow in corners to make me feel good when I pass under them ?
 
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Chris

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Al you must lend your bike out to one of your A graders & then he can tell us how he gets on against the big boys. I have 4 bikes out on loan. There is no point having them sit af the back of the garage for a decade or more. Sponsor a young rider.
 

baz

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Al you must lend your bike out to one of your A graders & then he can tell us how he gets on against the big boys. I have 4 bikes out on loan. There is no point having them sit af the back of the garage for a decade or more. Sponsor a young rider.
I agree let a young bloke have a go with it
You can still get the joy of proving your theories ,and trackside involvement
 
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Here's one of the fastest 750s ever - Leo Goffs dragster with 1/16" oversize valves. He designed his own cam and got 140mph in the 1/4 mile.


If I remember correctly he shaped the intake ports to maximize combustion chamber swirl. This works on a Norton because the ports are already angled to one side.

Wow that is fast... I tried something similar on myhead but without such startling results . This was some 40 years ago mind but I suspect that th area round the valve itself on which I followed Dunstall's Tuning Notes was not the profile he or his engine builders actually used... maybe I should try oversize valves as Nigel suggested..
 
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I have a Triumph T120 with a Morgo kit, cams with 320 degrees duration and .400 lift 36mm carbs as well as lightened valve gear and heavy valve springs. It pulls from 2500 RPM and revs to 8000 RPM. The intake valves are 1 3/4"! This bike has never been on a dyno but it runs away from bikes that were dynoed at 55 HP. I think the engine might run pretty well with standard sized valves but all this stuff goes together to make it rev way higher than standard. The only trouble is if you rev it like that much it will blow up. I think Triumph engines like these work on a different principle than the Norton.
 
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Al you must lend your bike out to one of your A graders & then he can tell us how he gets on against the big boys. I have 4 bikes out on loan. There is no point having them sit af the back of the garage for a decade or more. Sponsor a young rider.
I have been talking to Bod Rosenthal and Andrew Oszko. Bob is an old A grader and still races. Andrew's father Steve was probably the best A grader on Manxes in the early 60s. Andrew has had a few rides on his father's 40 M, which was a works bike. I could race again myself, but I have deteriorated due to the pandemic. In a few months time, I will have the Seeley going again. I intend to do some on board video with the two guys I have mentioned and some others. Bob still races a G50 Matchless. He is the Period 4 Historic champion. I will see what is happening with the Austin 7 Historic meeting at Winton in May. - Wolfenden's Hondas are worth beating.
 
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