The Sifton 480 - Monster of all Norton cams

Hortons Norton said:

A 650 Triumph below 6,300 rpm blowing off a built H2 750 triple in a straight????? You will never get me to believe that! lol lol

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I was on the bike and I did not believe it. It was actually that fast. My friend did not use to race-change up through the gears when he raced against me, but I could still never convincingly beat him. Back in those days we rode in Allpowers C grade against H2 and Z900 Kawasakis and were still fairly competitive, at least on shorter circuits.

yet another figment of the imagination . . . .

To go back to the part of this thread where an extended length of the rocker arm was being discussed, assuming the valve remained in it's original position, the contact position would still be on the top of the valve stem, but not necessarily central.

Whether the rocker arm is stock length or extended, a change in valve angle creates an interesting situation. Firstly, assuming the centre of the valve head remains in it's original position, then the top of the valve stem will move towards the rocker spindle. If the angle change is 1.5 degrees, and the length of the valve is 103 mm, the top of the valve stem will be moved approx. 2.6 mm towards the rocker pivot spindle.

This is a very significant amount, but with a stock rocker arm, the contact point it still on top of the valve stem. An extended arm will place the contact very much closer to the edge of the stem.

This is not particularly easy to describe, but when laid out on a drawing board it's much easier to follow.

There exists the possibility of modifying the head to facilitate the use of an eccentric rocker spindle, which I understand was a feature of the original Dommiracer engine. This would also enable the tappet adjusted to be disposed of and in it's place a radius pad, thus getting away from the point contact of the stock adjuster, and enable a small reduction of mass in a vital area.

I doubt JS will contemplate boosting the lift further of the Megacycle 480 by the use of a longer rocker arm, but as has been mentioned earlier, a similar valve lift could be obtained by using such an arm in conjunction with a lower lift cam profile.

These are some photos sent by Snotzo, they are of the timing sheets at Sulby for the 2017 TT and pictures of Steve Lindells bike in the 2018 paddock that achieved the 140.1MPH through the timing gun in 2017.






Its an instantaneous reading along the longest fastest straight on the 37.73 mile course, the straight is several miles after the last slow speed corner at Ballaugh Bridge which has to be taken at 35 to 45 mph. So different rules and context to Hillbilly's records, both are impressive achievements in their own right.

kommando
thanks for posting the photo's, but one small but important correction. The Enfield Bullet is not mine (I wish it was) but it belongs to Steve Linsdell who can be seen relaxing in the background.
This is a departure from the original thread for which I apologise, but by way of an excuse must admit that there is nothing inside the engine that is in any was connected with either Sifton or Megacycle.

That's a very fast Enfield.

Snotzo. Any Norton taken this far is going to have larger valves. The center of the valve head will be moved along with the valve seats. Re-angling makes the valve more vertical, extending the rocker arm helps restore the angle towards the original. The hard oversize tappet head for the extended rocker in the photo is radiused if that helps. Its just another option in the race tuners toolbox in case he needs it - and tuners are always looking for ways to find more performance - some have gone as far as making custom rockers.

The cam flex problem can be solved with a central 1/2 shell bearing to support the bottom of the cam with a machined area in the center of the cam acting as a journal. Herb Becker (& perhaps others) has done this for a 480 cam. Unfortunately I don't have a photo of the fabricated center bearing.

How much asymmetrical difference is there between the opening side and closing side of the original Sifton 480 cam? I'm not seeing much or any asymmetrical difference in the Megacycle 480.

JS
the asymmetry on the Sifton 480 profile that I measured is not noticeable by eye on the lift trace, and very little on the velocity trace, but very noticeable on the accelleration trace, having initially a much faster rate of closing after the peak than when compared with the same period leading up to the peak.

Velocity trace does reveal a difference, but it's not easy to understand why it should be different until the accelleration trace is examined.

Tom Sifton was in my considered opinion a pretty savvy individual where his valve lift design work was concerned, and that he was doing this work without computer aids of any kind is even more remarkable.

The camshaft flex is not a problem when making a calculation to examine the dynamics, but there are most likely issues to be overcome when the loadings become serious as one might expect with 125 lbs seating force for the valves. If a central support bearing is not possible, then the camshaft for optimum stiffness should be made without a thru hole.

Besides me, I know quite a few guys running pushrod engines in LSR...Triumphs, BSA, Harley and yes a few Nortons...These guys can be private about engine modifications but I do know they all the engines have modified heads to take advantage of higher lift ,longer duration cams..If ports can flow more air with better velocity, a less wild cam can be used and avoid potential problems from excessive spring pressure or exotic expensive valve materials..And getting the exhaust out and the timing for best combustion...

..A successful racer needs to pay attention to what the fastest guys are doing and apply some of that within the experience and budget of the builder...And always, the bike engine must be reliable because you can't win if the crankshaft falls out onto the track..

Theory can only take you so far, the bike needs to get on a chassis dyno or plenty of track time to sort out details.
This was the point about my Triumph LSR bike.....proven changes that worked out on the track, not theory..A Norton is not a Triumph but they both respond pretty much the same to modifications...

I don't know how anybody could get the best out of a LSR bike. The practice time is so limited. With a road racer, after you have done all the theory and have got the bike nailed together - if you are racing regularly, it usually takes several practice sessions and race meetings to get it going really fast. With a road race Commando, once you have enlarged the inlet ports to get more flow, you are done and there is no cheap way back. Good in theory, but bad in practice ?

Classic sprint and drag events are still popular here in the UK where a bike could be entered, this would build up knowledge and test time. Must be similar events in other parts of the globe. A season or two doing this would help immensely.

Fast Eddie said:

Perhaps the triple rider was 400lbs... and stuck in first gear... with a flat tyre...

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Prolly had oiled up plugs.

Getting there quicker is probably only part of it with an LSR bike. But torque at near peak revs, is probably important. Lowering the gearing on a Commando does not necessarily make you accelerate faster. If you practised at drag or sprint meetings, you might be deceiving yourself. When you are running at full speed on a very long circuit, it often takes very little to slow you. If your motor is all top end, it might not have the grunt to get you to peak revs. What I have found with a top end motor, is they are usually slow at first then accelerate forever, depending on the gearing. But I have never had one on the salt. If I was in America and intending to go to Bonneville, I'd take the bike to Daytona and run it into a head wind a few times and clock it at the end of the straight.

I don't know if a person can go to Daytona and run a bike there ??? Maybe they have track days? Bonneville is around 4200 feet elevation and can have an air density of over 5000 feet depending on weather.But the elevations offers a bit less wind resistance..The salt surface has a bit more drag than pavement .. Some guys who race at the paved 1-1/2 mile standing start track at Maine tell me they also race on the salt.. A race vehicle may go faster or slower at Bonneville depending on many factors like you mentioned. The longer run at Bonneville may help...or not...You also lose power at altitude ....

One of the issues is traction, the surface can be like sand. My brother would tell me that was something you can lose without warning. Even though I never rode a bike there I sure had great time around all those bikes. There were a lot of bikes that had done good in testing, but when they arrive on the salt their bikes just don't run quite right. Can be a tough place. But anyway does anyone know what became of the Triumph twin engine streamliner?

acotrel said:

From my own experience with playing with a top end motor, I have found it is possible to create a motor which will pull forever, but never get there quickly enough.

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Likely the compression ratio was too low for the cam duration.

Snotzo said:

JS
the Sifton 480 was already asymetrical before you started playing with the ramps, your post however does raise a question, just how much valve lift is really necessary for top performance at 9000 rpm with a 750 short stroke Commando at a land speed record attempt ?

Most will be aware that the lift is necessary in order to obtain a reasonable period where the valve is open for maximum flow potential, the most often used yardstick being 25% of the valve seat inner diameter.

The other problem is how to arrange the cam lobes for best results. Norton and BSA are both restricted by having all cams on one shaft, whereas Triumph, Matchless and Enfield have separate camshafts. Tom Sifton made his choice years ago, and most probably his choice would still be considered a good one, but without any means of adjustment, getting it wrong could be a disaster.

Others may have attempted to increase the valve lift beyond .480" in the past, and likely may have been sucessful, but to my knowledge only Tom Sifton's 480 cam has survived to still be considered for top performance events such as that which JS is preparing for

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25% of inner seat diameter as max lift seems overly conservative? 30% to 35% is in line with a lot of engine builders practice.

I possibly did not explain the 25% sufficiently well for your understanding.
This amount of lift calculated as a percentage of the valve seat inner diameter, affords a curtain area equal to the area of the port. It is not exact. but is considered near enough for all discussion purposes.

Simply, if maximum lift of the valve does not reach or exceed this figure, the valve/seat combination cannot flow all that the port could provide.
If valve lift reaches the 25% value and no more, then only for one brief moment is the flow at maximum potential.
To overcome this limitation, a valve may be lifted more than the 25% value, often to the 30 or 35% values as you suggest, thus affording a period when the valve flow potential is optimum relative to the port.

There are other reasons why a port may not flow at it's maximum potential, but with a valve lift of greater than 25% seat id, it will not be due to a period of insufficient flow area through the seat

Perhaps if the port size is too large, the gas velocity might be too low, so the amount of gas actually flowed might be smaller ? The port size probably depends on the rev range in which the motor is intended to be used, which is determined by mechanical limitations - loads, flex and material strengths. - 'How fast can you afford to go' ?

Perhaps if the port size is about right, the gas velocity might be optimized, so the amount of gas actually flowed might be greater?

It's not as simple as port size and must factor in port (intake tract) volume (ie integration of length X size) and harmonics.

Many people only consider flow when considering inlet and exhaust systems. If you can hear it, it is sonic - so harmonics are important. However it might be more difficult to establish a stable sound wave in a large diameter port or tube, than a small one ? One thing which worries me is my exhaust system is made up from welded bends. There must be welding flash on the inside. If the sound waves in side the pipe are sonic, the weld flash probably puts bumps in the gas flow which might slow extraction of the gases. It still seems to work well, but if it could be better, I would never know. I do know this - if the cross area of the tail pipe of a 2 into 1 exhaust system is less then the total cross areaof the two header pipes, you lose too much off the top end. So I use skinny header pipes which means I don't have to use a huge diameter tail pipe. If you have a look at what is being used on most of the Commando road racers these days, they have the same diameter header pipes and tail pipes.

See comparison of JS0 (stock Norton cam for radiused lifters) to JS4 on right (New asymmetrical Sifton 480 with smooth ramps)



.490" valve lift at 0 checking clearance. 292 deg duration at .040"
Not for the faint of heart.

The 1st one is going on a land speed racer.