Ok, this is matter of terminology
Rob your right, the "trough" on the power curve at around 3500 – 4000 that many L6 installations suffer from, feels more like an exaggerated "on/off cam" feel rather than a true flat spot, you may go from heavy breaking and back on throttle in between which can mask it.
This drop in the power curve is most often a result of inappropriate exhaust lengths, carb sizing, inlet restrictions, cam specs and port size.
Its quiet common, most people don’t even know its there for many reasons,
Such as they don’t drive that hard, they don’t “know” what a well sorted L6 should feel like, they have never been to a decent rolling road and had it explained, they have been to a decent rolling road but they just don’t get it or they just don’t care.
Torque versus HP
For the purposes of analysis it’s also important that for now we should practically ignore the HP curve and concentrate on the torque curve as it tells us far more about what is going on, such as cylinder fill and top end breathing where as the HP curve is a composite view of Torque x RPM / 5252 and as such engine and dyno related HP get confusing with real world HP.
Ideal power profile
A well engineered L6 will see a steep rise in the torque curve from 1500/2000 to 4000 as the velocity of the incoming fuel/air charge gets going and VE (volumetric efficiency) starts to get going (over 100% is not unusual) then a surprisingly flat(ish) torque curve from 4K right through to 6K plus with the odd "dip" of only 10 or so ft/lbs, then after 6-6500 the torque tails off as VE drops due to the super high velocity of the incoming charge exceeding the mach number of the port (a port ceases to be efficient past 0.55 the speed of sound)
With a profile like this, driving you will get a real kick in the pants feel as it comes “on cam” followed by a continual “feed” of power and as a result acceleration and then at the top end a fairly swift drop off which most people won’t get too unless they are pushing their engines to 7000+
Its all about how much air/fuel mix you can cram into the cylinder
The term “On Cam” is a bit of a misnomer on a single cam engine…. its mainly to do with cylinder fill and VE
Cylinder fill is when the piston is on its way down and sucking air in…..WRONG its atmospheric pressure that is causing this inrush, that is why engines work better on a high pressure day and when the air is cooler (more dense, higher pressure to fill the cylinder more).
Air has mass and air moving at speed has inertia due to this mass, if your in-rushing air is moving quick enough then this inertia will fill the cylinder past its static capacity,
Imagine a goods train moving towards a closed end tunnel at 50 mph, when the train hits the other end only some of the train has entered the tunnel, now imagine that same train traveling at 300 mph, guess what, all of the train entered the tunnel, compresses right up with room to spare.
couple this effect with a rapid closing inlet valve that is shutting right at peak cylinder fill before the air/fuel mixture can escape and hey presto you have a volumetric efficiency of over 100%....this is the key to getting big torque (and thus big HP) out of an L6.
OK so we all do this then
Where we have failed (in the UK) is to ignore this absolute science and blanket everything with a rather Neanderthal approach of “bigger is better” leading to inappropriately carb’d over ported and over cammed engines many of which are still out there today.
Some L6 tuners have ignored the science (some don’t even know it exists, others think its witchcraft) some have wiped a die grinder around the ports, bolted on a set of triple something’s and charged a fortune for it, ohhing and ahhhing about a mid 14 second ¼ mile car, a few examples of this came out of Silverstone in the early 70’s….emperors new clothes anyone.
In summary, without a decent rolling road or dyno graph you don’t have a clue what your engine is doing at any given moment in time, so how on earth can you bolt any bits on to make it better (or in many cases worse).
