The valve stem seal
In the quest to make engines as oil-tight as possible, we often forget the effect of the humble valve stem seal. Tucked away inside the cylinder head well out of sight, this small, upturned cup of a device grips onto the top of the valve guide and seals against the valve stem, enabling the valve to open and close without letting comparatively large quantities of oil get sucked down the clearance between the stem and guide and into the combustion chamber. With such a small component - and let's not forget there are generally four of them per cylinder these days - it is therefore very easy to underestimate the effect they can have on oil consumption.
In the past, when oil was cheap and the environment was simply a word in the dictionary - or indeed, if we just liked the aroma of Castrol 'R' - stem seals, particularly those of the exhaust valves, could be removed and the car would circulate just in front of an oil haze. Punctuated by an additional blast of blueness immediately after every gear change, we all thought the friction saved at the seal-stem surface was worth real power and was evidenced by the fact that once in front, no-one else could catch us. Little did we understand at the time that the real reason that no one could pass was simply because no-one could get anywhere near because of the oil haze!
In reality, however, the power consumed by a typical valve stem seal is measured in Watts rather than kW, and friction values in the region of 10 W per seal for some of the latest OE engine manufacturer designs are typical. And while reduced friction (maybe up to 10%) can be achieved using PTFE coatings around the lip of the seal, the real benefits are more to do with oil consumption, particularly at high turbo manifold pressures or vacuum levels at elevated temperature.
But no matter how effective they might be, the barrier to using more bespoke competition valve stem seals is often cost. Roadcar engines now use smaller valve stem diameters, and when oil consumption is as critical (as it is these days) to achieving zero oil top-up between servicing in passenger cars, emission-critical components like the stem seal have to be of the highest quality. And when made in thousands to the strictest quality control, it is easy to see why many race engine designers opt for this route.
It is therefore only when running conditions are outside traditional roadcar operating envelopes that bespoke seals may be needed. Roadcar oil seals may be made from any of the common elastomer compounds - nitrile (NBR), polyacrylic (ACM) or silicone (VMQ) - but are more likely these days to be made from any of the modern FKM/FFKM types of fluoroelastomer. Bespoke race engine stem seals, however, are more likely to be made from any of the PTFE products, which accounts for their additional stiffness and higher temperature performance characteristics.
And as for that oil haze of the past, much of it could have also come from the missing oil control rings, which were removed just to see how much more friction could be saved. If a two-stroke could get away with only one ring, then why not a four-stroke?
Well, you simply have to try these things!
Fig. 1 - Cross-section of a valve stem seal
Written by John Coxon