Bikes Out-Teched By Cars?
BIKES OUT-TECHED BY CARS?
ROUNDUP
TIMES HAVE CERTAINLY changed. Only a decade ago, motorcyclists seemingly had it all over car guys. Even the lowliest sportbikes had four valves per cylinder and dual overhead camshafts, while four-wheelers often made do with bendy pushrods, a single cam buried somewhere near the crankshaft and one measly intake valve per pot.
Well, the automotive industry has played catch-up-big time. So well, in fact, that motorcycles have been left in its hightech dust. Now, not only do Ferraris sport Yamaha-esque
five-valve combustion chambers, so does the pedestrian Volkswagen Beetle.
Variable valve timing mechanisms, for example, are as ubiquitous as the sparkplug. Just try to buy a Honda without some form of the company's VTEC system. Ditto Toyota and its VVT-i. And while most work their cam-timing magic on the intake side of the equation, BMW's VANOS system alters cam phasing for both intake and exhaust.
But even that is becoming old hat. The German company’s latest 7-Series luxury sedan takes VVT to another level. In fact. Valvetronic’s control over the inlet valve is so complete it can control engine output by just manipulating inlet cant timing and lift, completely eliminating the throttle.
At slower engine speeds, the camshaft opens the intake valves a mere 0.3mm, allowing just enough air/fuel mixture into each cylinder to sustain the 640rpm idle. Then, when the driver asks for wide-open throttle, the cam generates 9.5mm of lift for maximum power. BMW engineers claim that by eliminating
turbulence created by the throttle plate, fuel economy is improved by 10 percent, with an attendant increase in power.
The big Beemer also takes another aspect of engine control a giant step forward. Many current autos use dual-stage intake manifolds-one set of long runners for low-speed operation and another shorter set for max rpm. While an improvement over bike-like fixed-length manifolds, these are still crude affairs that cannot hope to optimize the inlet manifold’s configuration for all engine rpm.
BMW’s system eliminates this compromise by continuously varying the length of the intake tract. Between idle and 3500 rpm, the manifold’s effective length varies over a 442mm span for optimum efficiency. At idle, the intake mixture must travel 673mm to the intake valve. Above 3500 rpm, the system is held in its wide-open position with 231mm runners.
It’s a very advanced system, previously used only on a few high-tech race cars. The closest bikes come to matching this is on Honda’s CBR929RR, where the HTEV exhaust valve effectively alters header design from a 4-into-2-into-l to a 4-into-l, depending on engine rpm.
Salvation is forthcoming, at least a trickle of it. Honda’s ’02 VFR800 gets a VTEC system to help broaden its powerband. It’s
no surprise that the first bigbore VVT-equipped bike will be a Honda. After all, Honda’s system is very compact, and an early ’80s home-market 400cc Four and a 250cc Single both sported crude versions of VVT. Imagine where we’d be now if the engineers hadn’t abandoned the concept then.
David Booth
