|A two valve versus four valve debate is certainly nothing new
for me. Anyone who has spent time talking with me on this
subject soon discovers I am not really a fan of four valve
engines for the true street power plant. But they are out there
in the millions and rather than fight the situation I thought it
might just be more practical to look for ways to make a four
valve engine that much better. So here is the patented PolyQuad
head design that David Martin and I worked on for a number of
years before taking out the patents in 1999 - 2003.
Granted there are plenty of variations on 4 valves per cylinder
and this begs the question as to what a PolyQuad head does
better than a current and typical four valve per cylinder head?
Answer - everything
Compared to Honda's VVT and other such variable valve timing
system costs almost nothing and can be produced in a typical
head shop! To best appreciate how the PolyQuad concept came into
being making a start at the two versus four valve debate would
be as good a place as any.
There have been some serious
debates concerning combustion efficiency going on in serious
hi-tech forums and from these it seems that it is well
understood that there is more to producing torque and power than
just filling cylinders with air and fuel. At low speed the
engines ability to breath is hardly a problem as relatively
speaking, there is plenty of time to fill the cylinder as full
as it is ever going to get. The key to making low speed torque
is two-fold a) how much charge is trapped at valve closure and
b) how effectively the charge is burned.
Aspect a) is governed by the reverse flow properties of the
intake valve seats and to a lesser extent the port area and the
cam event timing. Better anti-reversion at low lift and higher
port velocities are good here but at the end of the day it's
still largely down to the intake valves closing soon after BDC.
AS for point b) we find that how effectively a charge burns
is down to mixture preparation and in-cylinder motion at the
time of ignition. Assuming that mixture preparation is good
then, at this point, best results would be achieved by the
cylinder with the best combustion dynamics stemming from
appropriate in-cylinder charge motion. The bottom line here is
that a parallel valve two-valve bath-tub or wedge chamber design
has inherent swirl properties. This means a head designer would
have to work hard to eliminate the heads natural advantageous
tendency to swirl the mixture (and believe me some have managed
to do that). For a two valve head then the natural tendency is
to swirl the mixture and this, in part, contributes to a two
valve heads normally superior low speed torque.
other hand a conventional four valve head has no natural
tendency to swirl - but it does have a tendency to 'tumble' the
charge. The following illustration shows the difference between
Not unreasonably a first inclination would be to assume that
both types of mixture motion would work equally well. Up to a
point they do but as the piston nears the top of it's stroke
these two forms of mixture motion diverge in characteristics.
The tumbling charge motion breaks down to small tumbling eddies
where as the swirl, as it gets compacted in the chamber smaller
than the cylinder bore can speed up (conservation of angular
momentum) while still retaining it's swirling motion form. This
swirling motion in conjunction with a well placed spark does a
good job of effectively and rapidly burning the charge. On the
other hand the tumbling action seen most often in four valve
engines tends not to produce enough charge motion at low speed
to produce as good results as the two valve engine swirl. Also
tumble can be almost cancelled out when compression ratio's much
above 11/1 are used.
In addition to the lower combustion
speed the inclined valve four valve heads tendency to allow the
fresh incoming charge to cross flow out of the exhaust during
overlap also cuts the low speed torque as well as increasing
fuel consumption and emissions. With the appropriate
proportioning of valves and bore/stroke, a two valve engine,
with a well designed cylinder head, can out-perform a four valve
engine to about 4000 rpm.