“I have a question”, writes our correspondent Demon Tweeks … “What’s a good all round valve timing for a Cooper JAP for sprints, hillclimbs and short circuit racing?”

 Good question! But perhaps a more important enquiry might be this one. How good are the standard JAP factory speedway cam, for those three forms of competition?

The answer is … VERY good!  Which begs the next question. What are the real valve timings of a Speedway JAP? 

Here’s where it gets interesting, because although we are given when we try to replicate the factory valve timing figures in JAP manuals, we seldom get even close. As Professor Sumner Miller famously used to say.’ Why is this so?’

Happily, there is a logical explanation for the variation between what the factory says the valves should be doing, and what our best investigative efforts sometime produce. This is because the manufacturer very seldom if ever, tells us what tappet clearances were used to establish their published valve timing figures!

That means if we want to replicate factory timings, we have to try to work out ourselves what tappet clearance they used, and because there are so many options, doing that can take up a lot of viewing time, so it’s easier to go back inside the house where it’s lots warmer.

So, we must ask … Is there a universally accepted tappet clearance that we can use for comparing valve timings. The answer is both YES and NO! 

YES, because most modern camshaft grinding firms quote valve timing figures based on 50 thou tappet clearances, (or lash as the Americans say). This makes direct timing and comparisons of camshafts easy……but 50 thou! That seems an awful lot! Why 50 thou?

The clearance of 50 thou is chosen because good camshafts are ground on the opening and closing sides of the lobe so that for up to 30 degrees of crank rotation, the valves are only GENTLY lifted a few thou off, and lowered onto their seats. You can’t see them but the cams have ‘quietening ramps’ both side of the lobe. This means if we check our valve timings with NO or little tappet clearance, we will get greatly extended opening periods, which will almost certainly make nonsense of any published timing figures we might be trying to replicate.

So, to eliminate this confusion, modern camshaft experts mostly use 50 thou tappet clearances for reference. But since we are working with camshafts that were ground 70 years or so ago when this technology wasn’t universally used, that’s not much use to us. Thus, the question remains unanswered. What checking tappet clearance did Mr. JAP use when he published valve timing figures for his Speedway engines in 1950 and will we ever know?

Perhaps we might! Because there is good news to hand! Working together and combining their genius, Miss Marple and Hercule Poirot discovered by trial and error that if we set our checking tappet clearances to 20 thou all is revealed! 

The following published valve timings for a Speedway JAP now make sense!      

INLET Opening 44 BTDC.  Closing 62 ATDC.
EXHAUST Opening 65 BBDC. Closing…34ATDC.  

This is a major breakthrough! Now we know what the manufacturer did. Their checking tappet clearance was 20 thou! Now we can do the same. We can set our JAP tappets at 20 thou. and play around with the various keyways until we get as close as we can to the factory JAP valve timings, which we know from experience give a really wide spread of power all the way up to 6000 RPM…… Bingo!

But the serious racer is never satisfied! Is there a better cam? Something with more overlap? More lift?  There is always the temptation to try something different.

As JAP speedway engines became less competitive internationally, JAP concessionaire George Greenwood introduced a new short-stroke engine with a camshaft which had been around for quite a while and had significantly broader valve timing. This was variously known as the Special or 84S cam and the temptation to try it out in our Cooper car was irresistible!

The published JAP timing figures for this camshaft were…

INLET Opening 60 BTDC. Closing 80ABDC.  
EXHAUST Opening 80 BBDC. Closing 50 ATDC.

To get a better visual idea of the value of these numbers, just draw a circle freehand and divide it in quarters with a horizontal and a vertical line. Now mark TDC at 12 o’clock and mark on the circle the approximate degree positions of the two sets of timings given above for comparison. We see immediately the 84S camshaft has 110 degrees valve ‘overlap’ at TDC against 78 degrees.

And how did these two quite different JAP camshafts compare in performance when installed in the same engine and in the same Cooper car by Demon Tweeks? 

The only really good thing we could say about the 84S cams was that they gave the engine another 500 maximum revs through the gears and in a straight line, but that was their only advantage. Missing was the easy starting and the beautiful flexibility of the early cams which allowed the engine to pull in any gear without the necessity of slipping a fragile clutch and /or changing down half-way through a corner.

On the race track there was little difference in lap times between the two camshafts as long as the track was dry. But if the track was wet, the later model’s peaky cams made survival much less predictable! This was demonstrated at a wet Wakefield Park, Australia meeting where Brian Simpson’s MK9 Cooper with standard factory cams had Chas McGurk’s 84S-engined Mk6 for lunch!

So there we are. Standard speedway cams are definitely the go.

Over the years that your correspondent was learning all this, your editor Terry Wright was also investigating the cams used in his ex Bruce Walton cars, blown and unblown, and can add more on the subject of camshafts and their individual characteristics –  the good, the bad and the ugly. He will do this in a future article.

In the meantime, ponder the fact that no JAP material appears to make any mention of a 20thou checking clearance!

Demon Tweeks

One thought on “CAMS”

Leave a comment or further information

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s