Setting Camshafts

[David Morris]

Hi All,

When building up Vic Francis's Javelin engine a few weeks ago, we followed the instructions on setting the position of the camshaft. This was the later camshaft with the adjustable vernier and peg. All went well, and we encountered no problems. The lift on No.1 inlet valve was set to the book value of 0.014" and the camshaft was pegged.

Subsequent trials and now some many miles later, and the engine is going like a rocket. But, in hindsight, I have realised that I made a fundamental mistake! My dial gauge is metric and my setting of what I thought was 0.014" was in fact 0.14mm! Oh dear, this equates to 0.005", not the 0.014" as per the book! But, the engine is lively and running sweetly, and I have no inclination to strip her down to correct my schoolboy error.

It made me think about that book setting and our camshafts. I realise that the book setting is with nice new camshafts. Unless you have a new shaft, ( lucky you! ) what we have in our engines are pretty well worn shafts? Might even be 60 years old? I did a bit of work in the past with measuring up a range of camshafts, and they were a pretty mixed bunch. Those from an engine with hydraulic tappets are completely different to the later solid tappets. Did Jowett's ever modify the camshafts? Are those from Javelins the same as Jupiters? Perhaps more importantly, what tolerance should we be giving when installing and setting a used camshaft? Should we be measuring the lift profile and making downstream adjustments to that 0.014" lift? Just a thought?

Any ideas and your experiences please?

Meanwhile, stay safe and socially distanced!

All the best,

David

[Keith Clements]

It is not the distance which is important but the opening and closing points of the valves and the charge which will get into the cylinder.
As such there are many variables which affect this.
There are quite a lot of different camshafts out there now as some people had originals reground or stellite repaired.Others had new ones fitted with different profiles. And yes most will now be worn, as will all the drive gear that tries to get the valve opening at the optimal time.
In this case, the late opening of the valve , albeit by perhaps only a degree or two, MIGHT reduce the charge ingested, but not necessarily as it depends much on the gas flow and complex fluid dynamics.
Remember the setting was arrived at probably using limited test equipment (or none at all), and limited computational capability and engineering knowhow. It may also have been derived on a slightly different engine with smaller capacity (rebore), different carbs and distributor.
If you really want to set up the camshaft then perhaps do what Philip Dingle did and see the valve opening points of ALL the valves not just ONE.
I think you will find quite a variance and it would be good to record what that might be.
viewtopic.php?f=2&t=4231&p=23133&hilit= ... aft#p23133

[Srenner]

In extended discussions with Neil Moore, he offered that retarding the cam may result in a livelier motor. IIRC, Neil thought the factory set the racing cams at 8*, not the 12* as in the repair manual. I'm fairly certain that is how he set the timing in his Jupiter.

[Keith Clements]

Of course a later opening of the valve also means there is a corresponding later closing of the valve (assuming no other change). Since the air is flowing considerably quicker at race revs this might push in more charge. The exhaust overlap is the same so might push out more of the exhausted mixture.
It really is only something you could test on a dyno, although some clever software might be able to tell us more easily.

I have had a look at page 40 of Philip's pdf trying to ascertain what the lift difference would alter on the opening point.
14 thou is .35 mm so with a tappet max lift of 5mm it is about 5% of total lift.
This puts the setting point about two thirds way along the up ramp curve. The up ramp curve is before where the lift accelerates linearly. My very rough guess would be that setting at .14mm alters the opening point to about one third along the up ramp curve and by 34 degrees. A considerable amount!
I would suggest just checking where 14 thou lift is now actually occuring. Maybe the engine feels better but it actually might be being damaged. Valve burn out or maybe you have found a better sweet spot!

[David Morris]

Hi All,

It's very interesting to hear that the factory might have used 8 degrees as the setting point for their camshafts on racing Jupiters, instead of the recommended 12 degrees. Thanks for the comments.

My question was to ask what tolerance we should be allowing on the 0.014" lift, when setting a camshaft that has had some wear? Has someone measured the lift on a fresh camshaft, so that we can judge when to consider a camshaft has reached it's 'end of life' ? How much wear is too much? I know I have seen some shafts where the cam profile looks positively like the Matterhorn and others that resemble a molehill.

Perhaps JCS can advise, if they have original drawings or new camshafts that could be measured?

Meanwhile, enjoy the spell of sunshine and keep safe.

All the best,

David

[Keith Clements]

David,
I do not think there should be any tolerance or any compensation for wear. Please refer to Philip's diagram.
The push rod is accelerating rapidly so any variation will make a large difference on opening point but no difference in the opening duration.
Wear reduces the valve lift and has little effect on the opening point or duration.
Cam profile design is a compromise. Jowetts altered the profile to reduce valve noise. They also fitted double spring to reduce valve bounce.
High lift cams may give more charge intake but will also stress the valve gear. Different profile cams may give more power but in a narrow band and at high revs.
It is all a compromise.