Airhead Cams

A comparative religious study

Within the cannon of Airhead religious thought, nought engenders more mystery than the holy bump-stick. I chanced to witness one spontaneously burst into flames. I really did. ‘It’s a miracle!’ I cried.

As the flames subsided, ecstasy gave way to the realisation that I was possessed of the Devil in the detail of yet another misleading cam specification.

To compare cam denominations is folly! They pray to different Gods who mete out judgment according to a confusing panoply of checking-height* scripture. Even within denominations, these scriptures can display great contradiction.

Encyclicals from the Church of Schleicher, for instance, proclaim that checking-heights exist at the end of cam lobe clearance ramps. Some of this denomination’s children share the same ramp and are therefore directly comparable. The others are orphaned in Limbo.

An Ecumenical movement has attempted to unify scripture across all denominations so the world’s rev-head flock may receive wisdom: A checking-height of 0.05” in the New World and 1mm in the Old**. Sadly, in acts of religious arrogance, many denominations resist.

Therefore, a quest came upon this author: To collate all insights gleaned by the Skrunkwerks Bump-Stick Diviner into a Holy Ecumenical Cam Table under a Unitarian checking-height of 1mm. Let all Airhead disciples rejoice!

* Checking height is the cam or valve lift at which various cam charateristics and specifications (duration, valve opening and closing timing, overlap etc) are measured. Comparison between cams is made difficult if not impossible by a confusing array of proprietary checking heights based on various criteria. Worse, some manufacturers persist in publishing cam specifications without any mention of checking height rendering their specifications utterly worthless!

** In an effort to unify checking heights so that meaningful comparisons can be made, the USA adopted 0.05″ (1.27mm) as the convention. This convention is often used in the metric world outside the States. In Europe, 1mm is also used.  Still, many manufacturers fail to use these conventions. The main supplier of after-market Airhead cams is one example.

The Skrunkwerks Bump-Stick Diviner:

Holy Ecumenical Cam Table

Accuracy


The Skrunkwerks bump stick deviner is set up with a stock lifter which is positioned on each lobe exactly as it would in a working engine. Therefore, the measured lift data is as the lifter sees it before the information is transmitted along its merry distorted way through the valve train. Due to the relatively small crown radius of the Airhead lifter, the lift curve at the lifter is not quite the same as at the cam lobe. It is unclear whether manufacturers such as Schleicher specify their cams at the lifter or at the lobe.  At any rate, the difference is small and is more than gobbled up by other noise.

The largest sources of inaccuracy are manufacturing tolerances, measurement tolerances of the cam analyser and the fact that many of the measured cams had seen service and were worn to varying extents. These factors can account for deviations of a few degrees between cams that should otherwise be the same. Lobes on a given cam that should be the same can show a fair measurement range so most of the measured quantities have been averaged between the four lobes (for single pattern cams). Quoting velocities and accelerations to three significant figures is rather optimistic on my part…

Observations


1. Are the BMW 336 ‘Sports’ and the Schleicher 332 the same cam?

As some have long suspected, the answer is yes. At least with regard to lobe profile. This is illustrated in the following plot which superimposes the lobe lift profile of each cam. Within the measurement tolerance (see accuracy note above), they are identical right down to the bumps and lumps in the acceleration and jerk curves.

Both cams have a Lobe Separation Angle (LSA) of 100 deg and being timed ‘straight up’, the Schleicher 332 is identical to the earlier BMW 336 Sports cam before it was tweaked with a 6 deg advance. In this, the later 6 deg BMW Sports cam offers a bit more low down tractability than the Schleicher 332 – a welcome thing on the street.

Lobe profile comparison between BMW 336 'Sports' and Schleicher 332 cams

2. What about similarities between the Schleicher 336 and the now discontinued Schleicher 330?

As is common practice with cam manufacturers, identical lobes are generously shared between cams that only differ in LSA and timing advance. The lobes on the Schleicher 330 and 336* are the same except for the clearance ramps: The 330 having constant acceleration ramps and the 336 having more forgiving constant velocity ramps.  From a performance perspective, the 336 can be considered a 330 with a 6 degree timing advance.

There is some anecdotal evidence suggesting that the 330 was prone to failure and discontinued as a result. If this is the case, would the 336 not suffer the same fate? The nose deceleration of the 330/336 lobe is relatively high and would require a higher spring force than even the Schleicher 340 for a given red line. For its lift, the 330/336 lobe also has a smaller nose radius than the 340. Higher spring forces at the contact point between a sharper nose and a relatively small lifter crown radius challenge the friendship between contacting parts – So called Herzian stresses can exceed the capacity of contacting surfaces to stay intact. The 336 would no doubt be more sensitive to raising the red line than the 340.

I have not had any experience with the Schleicher 336. It does not seem to be a commonly used cam. If you have experience with it, please leave your observations in the comments section below.

* Not to be confused with the BMW 336 ‘Sports’ cam.

Notes:


Ideal valve motion is to instantaneously slam the valve open,  leave it there for a prescribed period then instantaneously slam it shut. In other words, a valve lift ‘curve’ that is rectangular in shape – an impossibility in the real world unless you’re Christian Von Koenigsegg who gets darn close with his camless Freevalve System.

We in the cam driven world contend with lift curves the shape of mullock hills and quaintly proclaim a cam to be ‘aggressive’ as it’s lift curve vainly attempts an approach to anything that resembles a rectangle.

Note 1:  The sainted Harvey Crane was probably the first to quantify ‘aggression’ with his three measures of ‘Intensity’. These measure the difference in cam duration between two specified cam lifts. His ‘Major Intensity’ is stated in the table for each cam and is the difference in cam duration between cam lifts of 0.020″ and 0.050″ (0.51 – 1.27mm). The lower the number the more aggressive the cam. It’s not a bad measure of how aggressively a cam will accelerate off its clearance ramp into the opening flank but doesn’t provide such a clear picture of what might be happening above 0.050″. The KLD factor fills in that part of the story – see Note 3.

Note 2: Harvey’s ‘hydraulic intensity’ measured at cam lifts of 0.004″ and 0.05″ (0.10 – 1.27mm) is also included in the table as it considers more of the lash ramp in its calculation thereby giving some comparative idea of how fat and wide the ramps are.

Note 3:  Dr Gordon Blair’s KLD factor provides a broader overal picture of aggression. As illustrated in the pic below, it calculates the ratio of the actual lobe lift area to the ideal rectangular area thereby providing a means of comparing how a lift profile deviates from the ideal…which is generally considerable – most cams only make it a bit more than 50% of the way to the ideal as listed in the table. A relatively small difference in this ratio can amount to a noticable difference in engine behavior. It’s a useful metric in conjuction with the ‘Intensities’ per above.

Cam FactorNote 4: The Schleicher numbering system is comprised of two parts: The numbers before the decimal point denote a particular lobe profile. Many of these profiles are used in cams across the Airhead model range regardless of displacement and might only differ in Lobe Separation Angle (‘LSA’ – the angle in cam degrees between the inlet and exhaust lobe centerlines) which is denoted by the numbers after the decimal point. e.g. Ro214.0, Ro214.5 and Ro214.10 have exactly the same exhaust and inlet lobes but LSA’s of 100, 105 and 110 degrees respectively.

While lobes can often be the same, changing the LSA changes the valve event timings and the overlap angle. Everything else, including duration, lift, area etc remains the same.  All Schleicher airhead cams except for the Ro1145.1 (336) are timed ‘straight up’. That is, the centre line between the exhaust and inlet lobes co-incides with piston Top Dead Centre (TDC). i.e.: The inlet and exhaust valve events are symmetric about TDC as denoted, for example, by the typical form: 28/60/60/28

The Ro1145.1 is delivered with a key offset of three cam degrees which results in an advance of all valve events by six crankshaft degrees* to 36/64/76/24 at 1mm cam lift. If it were timed straight up like the rest of its brethren, its timings would be a symmetric looking 30/70/70/30.

The later BMW 308, BMW 336 Sports, and the Skrunkwerks Dual Pattern cams are similarly advanced by 6 crankshaft degrees.

* The camshaft turns at exactly half the rate of the crankshaft.

What do the different cam lobes look like?

To give an idea of cam lobe shape, the following figure illustrates three cams that span a decent range of what can be achieved within the Airhead architecture starting with the stock 308 cam.  The drawings are to scale and specify the maximum cam lift for each case.

The first two are with the stock 22mm lifter. The lifter diameter limits the opening speed of a particular cam lobe which in turn plays a role among other factors in limiting a cam’s aggression and maximum lift*. The 8.87mm lift of the Schleicher 340 Racing cam is pushing the limits of what can be achieved with the stock lifter. Beyond this, custom large diameter mushroom lifters are used or the more adventurous builders will  go for roller lifters with specially designed cams. Roller cams do not suffer the same speed restriction as flat tappet cams though they are acceleration limited – but that’s another story.. Their advantage is well illustrated below.

* The large cam lobe to lifter offset and small crown radius of the Airhead lifter are other factors that limit aggression and maximum lift.BMW Airhead cam lobe shape

A most excellent treatise on the mysteries of Cams by the sainted David Vizard may be had here and here


The Holy Cam Table is a continuing work of religious devotion for the benefit of all. As such it survives on alms. Please loan generously of cams not yet included in the Table (including those marked with a #) so that their secrets may be divined and disseminated to all. Shekels will be sent to cover postage. Cams will be promptly returned once communion with the Skrunkwerks Bump-Stick Diviner is complete.

Let us give thanks to the following Airhead disciples who have hitherto given generously:
Darren Craig
Rudi Gregor
Michel Schenk
Hans Poutsma

Please contact here

Also please leave comments below.

Adrian

The Gospel of Airhead Torque

(or Dante’s Eight Levels of Cam Hell)

Chapter 1 Verse 1: BMW 308.

Our Profit, beloved of many and rightly so. Revered and beautifully mannered gentleman who never misses a sabbath visit to the Church of the Sacred Airhead. Gives generously of torque to those of mid, low, and higher RPM alike. Thus arose the Gospel of Airhead Torque.

Schleicher 320:

New Testament mild sports cam. Sometimes speeds past the Church of the Sacred Airhead on the Sabbath but never forgets to leave a shekel in the bowl for those of lower RPM

Schleicher 324

Post Reformation but still tempted by the Gods of HP Mammon, guilt has it occasionally remembering its  religious observances, leaves half a shekel.

Schleicher 340

Completely lapsed, it prays to a pagan God. 9000rpm+ and matching HP from lift and duration of Occult proportions. One must first paint a hexagram on the workshop floor before debauching a motor to sate this abomination’s sinful appetite. Those of lowly RPM are truly forsaken.

Schleicher 336

Witnessing the excesses and fall of its pagan brother, the 336 strives vainly for perfection though doomed to purgatory as it struggles to retain power while bending to those of lower RPM.  Shuns any suggestion of religious lineage to its bronze age namesake – the BMW 336 ‘Sports’ cam.

Skrunkwerks Dual Pattern

Of the New Age prosperity gospel. Speaks in tongue of generously spreading increased bounty to those of all RPM’s. Claims Asymmetry spoken by others is a false God but struggles with Cartesian duality.

BMW 336 Sports Cam

Jealous Old Testament God of sports cams. Hypocritical in spreading the Gospel of RPM love. Its spectre is still sometimes seen angrily waving gilded crutches at New Testament upstarts who think they know better. The Schleicher 336 stands accused of using his name in vain.

Andrews No.2

Pilgrim Airhead Missionaries failed in having the Gospel of Airhead Torque take root in the New World. Materialism and avarice for HP instead spawned the Devil’s cam, luring the greedy into short lived gratification before revealing its true nature and devouring its host. Long banished and NLA for its sins.

Schleicher 330

A craven image of the Andrews No.2? Or is it the other way? Either way, they are very similar right down to their destructive nature. As a result, the 330 is also NLA having been banished from the Kingdom of Airheads.

Edelweiss EM2V1 and Siebenrock ‘Asymmetric’

Both created by the same vengeful God. Waring siblings that have been measured and studied, but to speak their names in the Holy Cam Table is to invite litigious wrath. The Holy Cam Diviner has shown Asymmetry – in its true meaning –  to be a false God

2 replies
  1. Lorne Wilkinson
    Lorne Wilkinson says:

    Adrian, your work and offerings here are invaluable to the airhead community. This information when sorted onto a level playing field (as opposed to the different standards and states measurements may be made at depending on the various approaches) helps those so inclined to change things up – to move forward from a good foundation.
    I have an interest in trying the 296, from a forced induction point of view – but until I get rich or inherit a treasure trove of spares, I will continue with a stock cam, and with lash settings increased (which has the effect of reducing overlap. I tried it last season and my butt dyno felt increases)
    Keep up the excellent work,
    Lorne

    Reply
    • adrian
      adrian says:

      Thanks Lorne, Been a fun project unlocking the mysteries of the Airhead cam world. I hope the table is useful beemerphiles everywhere.
      Good luck with your many and varied projects!

      Reply

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your email address will not be published.

fifty six ÷ seven =