PW3 cam in 750 motor

Status
Not open for further replies.
If you raise the comp. ratio in many motors, it often involves using pistons with higher crowns. If you look at any used high comp. piston from a Triumph 650, the side of the crown away from the spark plug is always coked up. What does that tell you ? If you leaned the jetting off to efficiently use all the fuel in the combustion chamber, you'd probably burn the piston near the spark plug.
Many years ago, there was a fast guy called Peter Allen here in Australia. His 650 Triumph motor had two spark plugs per cylinder. It probably worked, but it is not a good answer. The other two plugs were 10mm and put in at weird angles.
 
Last edited:
Why is two plugs per cylinder not a good answer?
Most Mercedes high compression gasoline car engines use 2 plugs per cylinder to avoid pre-ignition.
Porsche also does this as part of the conversion on their factory hotrodded engines.

Glen
 
Two plugs is a big improvement. If you've ever taken a ride in a small plane - when warming up you check the magnetos by turning one or the other off at a time. You get a significant drop in RPM with only one set of plugs sparking.
 
I don’t think anything can be taken for granted with twin plugs...

A mate of mine used to race Velo’s. He twin plugged one and, just like Jim’s plane story, when he pulled one cap off the revs would drop. It seeemd obvious that this proved how great it was. Sadly however, on the dyno it produced LESS power with the twin plugs.

An engine designed for twin plugs is obviously a different matter. But we can’t just assume sticking an extra plug in any old head will automatically be better.
 
I don’t think anything can be taken for granted with twin plugs...

A mate of mine used to race Velo’s. He twin plugged one and, just like Jim’s plane story, when he pulled one cap off the revs would drop. It seeemd obvious that this proved how great it was. Sadly however, on the dyno it produced LESS power with the twin plugs.

An engine designed for twin plugs is obviously a different matter. But we can’t just assume sticking an extra plug in any old head will automatically be better.


Twin plugs generally require less ignition advance. It is conceivable that it would have made more power with a bit of tweaking.
 
I swapped from an Axtel 5S to a PW3 and to be honest, couldn't tell the difference. Maybe it would be measurable on a dyno, but not on my seat of the pants dyno.
 
pommie john, thanks for getting back on topic. How maladjusted twin plugged Triumph's can be considered relevant to this topic I fail to understand !
A correctly manufactured PW3 is an excellent cam, but niether the designer nor the manufacturer can have any control over who purchases one, or how it will be installed and/or used. Issues with wear and follower compatibility have recently caused much concern among users, but those concerns aside, the actual design, originated as it was some few decades ago, was then and still is a first class effort, and despite recent attempts to'improve' on it, is still able, when installed and used effectively, to provide a performance as good as, and often better than most other options.
 
Yes it is an excellent cam designed by the great Peter Williams and very popular . It works especially well - unless you rev it over 7000. Then it has "severe" valve bounce problems (even with high performance springs) as shown on the spintron.

Unfortunately Peter Williams didn't have the luxury of a spintron and slow motion cameras when he designed his cam. If he did he probably would have given it more gradual closing ramps and avoided the problem as shown in the critical video on page one of this thread. He did the best he could with the tools that he had on hand at the time and the PW3 sold like hotcakes (winning races) regardless of the valve bouncing.
 
Last edited:
Why is two plugs per cylinder not a good answer?
Most Mercedes high compression gasoline car engines use 2 plugs per cylinder to avoid pre-ignition.
Porsche also does this as part of the conversion on their factory hotrodded engines.

Glen


With two plugs per cylinder, it might be wise to use separate double-ended coils to fire both plugs in each cylinder. Otherwise you would never know if both plugs were firing. I believe the coking which occurs on the crowns of high comp. Triumph pistons, is an indication as to why hot Norton engines perform better - and low comp. engines usually perform almost as well as high comp. That coking is an indication of an incomplete combustion process. It does not usually occur with flat top pistons, in motors which are jetted fairly lean. With Triumph 650 motors, it is possible to raise the compression by increasing the height of the crown less, but making it a better fit at the sides of the combustion chamber - squish-band ?
 
Last edited:
One of my friends raced a 750 Triumph motor in a featherbed frame in historic races. The only Triumph part on the whole bike was the cylinder head casting. He won several championships in one particular class, He still said 'I cannot beat the Nortons'.
 
Yes it is an excellent cam designed by the great Peter Williams and very popular . It works especially well - unless you rev it over 7000. Then it has "severe" valve bounce problems (even with high performance springs) as shown on the spintron.

Unfortunately Peter Williams didn't have the luxury of a spintron and slow motion cameras when he designed his cam. If he did he probably would have given it more gradual closing ramps and avoided the problem as shown in the critical video on page one of this thread. He did the best he could with the tools that he had on hand at the time and the PW3 sold like hotcakes (winning races) regardless of the valve bouncing.

Maybe Peter simply thought 7,000rpm is enough for a Norton? If I recall correctly even he never revved above that.
 
Just a thought, but the PW3 was surely designed for the F750 racers , presumably with the so called big valve heads and short stroke cranks.
 
It occurs to me that one thing always seems to be forgotten in all these performance part discussions. Norton ( and any other manufacturer of the day) were only interested in results. They could give a fig if the camshafts only lasted one or two races as long as they got results. I bet Peter's brief never mentioned this camshaft needs to last a season. Race wins meant headlines, headlines kept Imperial Tobacco happy and JPN team funding coming, and most importantly headlines meant sales .....
 
One of my friends raced a 750 Triumph motor in a featherbed frame in historic races. The only Triumph part on the whole bike was the cylinder head casting. He won several championships in one particular class, He still said 'I cannot beat the Nortons'.
I wonder why he used/kept the worst part of a triumph engine? Is it because of the class he was racing in?
 
Maybe Peter simply thought 7,000rpm is enough for a Norton? If I recall correctly even he never revved above that.

Peter was using Factory supplied cranks and cases - neither survived very long at higher racing RPMs. So he had to limit his redline.

Ron Wood told me that his 8000 RPM short stroke would barely make it through a race before cases etc began to fail - cylinder through bolt threads pulling out and other problems. Ron used stock rods and milled everything to bring the head down for the short stroke (heavy bronze bushed steel rods shook too much). If the bike had to make too many runs to qualify etc - finishing became less likely. He used Axtell cams which would rev higher without valve bounce. Some factory sponsored dirt trackers used Sifton 460 cams with lighter BSA lifters.

Some other sponsored dirt trackers changed their factory cranks every race. Maney cases, billet cranks and lightweight pistons have changed all that.
 
Last edited:
I wonder why he used/kept the worst part of a triumph engine? Is it because of the class he was racing in?


Period 3 historic racing in Australia. It is a silhouette class. As are the rest. As long as the bike looks vaguely genuine, nothing else matters. The problem is nobody can remember what genuine looked like. My friend filed the front edge of the Puma crankcases to make them look as though they were made in the Triumph factory and not twice as thick.
I have rarely ever raced historic, however when I do, I ride in Period 4. My major competition are 1150cc methanol-fuelled CB750s. Their problem is that most of them run wide on corners, so I simply ride under them. They are very difficult to beat in a straight blast down a straight. But if you go under, you have the run on them coming out of corners. You don't need a million horsepower to win races. Historic racing is OK - it keeps the fellas amused. What turns me off, is that aircooled Ducati twins are in a different class (Period 5) and if I want to race against them I may not use methanol. And anyway most people don't race Ducatis in Period 5 because they cannot compete with the really fast four cylinder bikes and the two-strokes.
 
Last edited:
Peter was using Factory supplied cranks and cases - neither survived very long at higher racing RPMs. So he had to limit his redline.

Ron Wood told me that his 8000 RPM short stroke would barely make it through a race before cases etc began to fail - cylinder through bolt threads pulling out and other problems. Ron used stock rods and milled everything to bring the head down for the short stroke (heavy bronze bushed steel rods shook too much). If the bike had to make too many runs to qualify etc - finishing became less likely. He used Axtell cams which would rev higher without valve bounce. Some factory sponsored dirt trackers used Sifton 460 cams with lighter BSA lifters.

Some other sponsored dirt trackers changed their factory cranks every race. Maney cases, billet cranks and lightweight pistons have changed all that.


In road racing, it can become 'how fast can you afford to go ?' There are ways of winning other than going silly. The reason my Seeley sat unraced for about 20 years, is that one look inside the motor told me it could not cop high revs. What I did not know was that with the Seeley frame and Commando engine, there is no need to rev the motor so high. You don't need so much horsepower if you can outride most of your opposition. But stay away from big circuits where high power is a major advantage. If you outride your opposition there, you will be riding too fast. Then you need better tyres and suspension and the costs escalate.
 
Blue 750
are you beginning to wonder whatever has happenned to your topic ?
Perhaps you should have been warned, there are some notorious persistent thread high jackers on this forum.
I do seriously wonder whether it's worth posting any more, admin seem uninterested in doing anything to curb the practice
 
Two plugs is a big improvement. If you've ever taken a ride in a small plane - when warming up you check the magnetos by turning one or the other off at a time. You get a significant drop in RPM with only one set of plugs sparking.
yes that is how you check that both ignitions are working, but aircraft twin systems are designed for redundancy/reliability, not horsepower performance, you would be crazy to go flying without a back up system! Coasting to a halt does not kill you!
 
Status
Not open for further replies.
Back
Top